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On MHD Flows of Viscous Incompressible Fluids
(Banaras Hindu University, Varanasi, India, 1986-08) Rahman, Fouzia; Kishore, Dr. Newal
The presented thesis entitled “ON MHD FLOWS OF VISCOUS INCOMPRESSIBLE FLUIDS” is being presented for the award of the degree of Doctor of Philosophy in Mathematics. It is the outcome of my researches conducted in the Department of Mathematics, Banaras Hindu University during the years 1983-86 under the esteemed guidance of Dr. Newal Kishore, Reader in the Department of Mathematics, Banaras Hindu University, Varanasi, India. The whole thesis consists of six chapters. The first chapter is introductory, giving the general description and fundamental equations of magnetohydrodynamics, free conviction flow, flow through porous media, rotating fluid flow, oscillatory flow and flow with Hall currents. Lastly a brief review of the past researchers related to the thesis have been given. Throughout the work we are considering the flows of electrically conducting viscous and incompressible fluids. The magnetic Reynolds number is assumed small for all the problems except the problems discussed in chapter two. The second chapter has been divided into parts. Part A of this chapter deals with the flow between two infinite, non-conducting, parallel porous flat: plates, when the lower plate is injecting fluid and the upper one is absorbing it. The flow is subjected to a uniform transverse magnetic field and the magnetic Reynolds number of the flow is sufficiently large so as to include the effect of induced magnetic field. The expressions for the velocity and induced magnetic fields have been obtained by using Laplace transform technique. The effect of the magnetic parameter on the velocity and induced rnagnetic field has been studied. It is found that the velocity decreases with increase in M in the lower region between the plates and increases with increase in M in the upper region. The induced magnetic field decreases with increase in M. In part B of this chapter, the effect of uniform transverse magnetic field on unsteady MHD free convictive flow past an impulsively started infinite vertical non-conducting plate has been discussed. Here also, the magnetic Reynolds number is assumed to be sufficiently large to take account of the induced magnetic field. There is constant heat flux at the plate. Expressions for the velocity and induced magnetic have been obtained by Laplace transform technique. The effect of the different parameters on the flow have been discussed with the help of tables. In part A of the third chapter, the effect of a uniform transverse magnetic field on the steady free convective flow through a porous medium, occupying a semi-infinite region of space and bounded by a steadily moving vertical porous plate has been studied. The flow is subjected to constant suction. Approximate solutions to the equations relevant to the problem have been obtained. The influence of the different parameters on the velocity and temperature fields have been discussed with the help of graphs and tables. The problem considered in part 3 of this chapter is an extension of the problem considered in part A. Here we have taken into account the effect of rotation on the flow. Due to rotation the flow become three dimensional. Approximate solutions to equations relevant to the problem have been obtained. Effects of the various parameters on the primary velocity, secondary velocity, the components of skin friction and the temperature have been discussed. The fourth chapter is concerned with the unsteady free convective flow past on impulsively started infinite vertical porous plate in presence of a uniform transverse magnetic field. The free stream is assumed to oscillate in time about a constant mean. The flow is subjected to content suction velocity and there is constant heat flux at the plate. Approximate solutions for the mean flow and transient flow have been obtained and the results have been discussed with the help of tables and graphs. In the fifth chapter we have studied the effects of flail currents on the unsteady MHD free convective flow past an impulsively started infinite vertical porous plate in presence of a uniform transverse magnetic field. The p1ate temperature is assumed to oscillate in time about a constant mean and the flow is subjected to constant suction at the plate. Approximate solutions for the mean flow and transient flow have been obtained. The inf1uence of the various parameters on the mean and transient flows has been discussed with the help of tables and graphs. In the last chapter, an attempt has been made to study the effects of rotation and Hall currents on the unsteady MHD free convective flow through porous medium occupying a semi-infinite region of space and counted by an infinite vertical porous plate in presence of a transversely applied uniform magnetic field. The plate is assumed to oscillate in time about a constant mean and there is constant heat flux at the plate. Approximate solutions for the mean flow and transient flow have been obtained and the results have been discussed with the help of graphs and tables.
Study of Mechanical Behaviour of Jute Fibre Reinforced Plastics
(Bangladesh University of Engineering and Technology (BUET), Dhaka, Bangladesh., 1986-09) Kazim, K. A.; Ilahi, Prof. Dr. Muhammad Fazli
Some strength characteristics of jute fibres, yarns, fibre bundles, yarn bundles, mats, and jute mat reinforced plastics, have been determined. The hardness, fatigue life, flexural modulus and percentage increase in weight due to immersion in water of jute mat reinforced plastics have also been investigated. The mean experimental value of strength of jute fibre bundle correlates well with that obtained theoretically on the basis of Daniels' theory. It was observed that the mean experimental value of strength of jute fibre decreased with the increase of the gage length. There is a reduction of about 40% in strength when the gage length changes from 1 cm to 10 cm. The mean experimental values of strength of jute fibre for three different gage lengths (10 cm, cm,i cm) correlated well with that obtained theoretically on the basis of Coleman's theory. The mean experimental value of strength of jute mat was found to be about 1 times greater than the mean experimental value of strength of jute yarn bundle. It was Observed that the mean exPerimental values of strength and Young's modulus of jute mat reinforced plastics increased with the increase of volume fraction of jute in the composite. But the experimental Young's modulus values of Jute reim'orcea PaSt1CS Were found to he about 11/2 times greater than the theoretically calculated values on the basis of Netting-type analysis. It was also observed that the mean experimental values of flexural modulus, fatigue life, hardness, and Dercentare increase in weight due to immersion in water, of jute mat reinforced plastics increased almost linearly with the increase of volume fraction of jute in the composite.
Study of the Yaw Stability of a Downwind Horizontal Axis Wind Turbine
(Khulna University of Engineering & Technology (KUET), Khulna, Bangladesh, 1990-01) Hossain, Khandkar Aftab; Quamrul Islam, Dr. Md.
Present thesis discuss a method to calculate the overall design, performance and structural analysis of a horizontal axis wind turbine. For a particular wind velocity, the optimum rotor configuration for twist and chord is determined using the momentum theory and the blade element theory, assuming zero drag,no coning and no tilting angle. After determining the optimal aerodynamic shape of the blade, the forces and moments on blades and tower top are calculated. At this stage, the equations may be extended to include the effects of coning and tilting angles, and the effect of several wind conditions such as wind gradient, wind shift and tower shadow. In this thesis, effects of coning and tilting angles are not considered. But the analysis of stability by determining the forces and moments acting on a horizontal axis wind turbine when yawed to non-axial flow is presented. The effect of tower blockage on the stability at various yaw angle is also considered. Three types of blade shape such as, Optimum-chord Optimum-twist, Linear-chord Linear-twist and Linear-chord Zero-twist are considered.
Protection of the Inter-Tie between an Industrial Cogenerator and a Utility
(Robert Gordon's Institute of Technology, Aberdeen, Scotland, 1990-11) Mollah, Abdur Rahim; Edwards, David C.; Deans, Prof. N. D.
The work presented in this thesis is concerned with the development of a microprocessor based technique to detect and locate symmetrical and unsymmetrical faults on the interfacing network of a utility interconnected industrial cogenerator system; and to identify the type of the detected fault. In order to provide operational flexibility and supply security to such an interconnected industrial cogenerator plant, fault location is regarded as being of prime importance. Current protection practices using conventional technology for this sort of interconnected cogeneration system were reviewed. A relay coordination study was carried out on a simulated utility interconnected cogeneration system. This resulted in identification of the problems and limitations of the application of standard IDMT relays for overcurrent protection of such a system. A review of digital protection, and its various algorithms, was undertaken. This provided the basis for a rational choice of algorithm for the proposed new technique. The sequence of functions of the protection scheme's algorithm, based on the proposed technique, can be presented in brief as follows: First, phase currents at selected positions on the system are sampled. The positive phase sequence (pps) currents at those positions are then obtained in digitised form from the sampled phase currents. Then, these pps currents, containing fundamentals along with other harmonics and dc offsets, are used in a Fourier filtering algorithm to extract only fundamental components in rectangular forms. Signs of imaginary parts of these components are then used to determine the directions of pps currents at the selected positions. The industrial bus voltage is used as the polarising reference. Finally these directions are based in a fault detection aridlocat1on subroutine to determine the location of any fault under such conditions. After detecting a fault and locating its position, the type of fault is determined from off-line analysis of the postfault phase currents. The analysis is based on identifying the phases carrying the maximum fault current and checking the presence of zero sequence current. The proposed protection scheme has been tested on a computer simulated system which consists of a utility interconnected to an industrial plant with cogeneration. The performance has been found to be satisfactory for all kinds of solid and low are resistance faults anywhere on the interfacing network between the systems.
Some Studies on Heat Transfer in Circulating Fluidized Beds
(Indian Institute of Technology (IIT), Kharagpur, India, 1990-12) Moral, Md. Nawsher Ali; Nag, Prof. Dr. P. K.
Experimental investigations were made to study the effect of fin and vertical probe height on hydrodynamics and heat transfer in circulating fluidized beds (CFB). The experiments were conducted in a 100 mm I.D., 5.15 m tall CFB unit. Air was supplied by a high pressure centrifugal blower. The distributor plate used was straight hole onfice type having 12.4% open area. The test section was located at 2.75 in above the distributor and electric tape heater was used as the source of heat. The temperatures of the inside wall and the bed at about the midpoint in the test section were measured with copper constantan thermocouples. Five plain and five finned test sections have been examined, three of which had rectangular fins and two had pin fins. Local sand of mean diameter 310 μm was used as the bed material. Measurements covered a range of superficial air velocity from 5.6 to 12.5 m/s, suspension density from 18 to 76 kg/m3 and bed temperature from 330 to 365 K. Three bed inventories of 20, 26 and 32 kg and five heat fluxes in the range of 3580 to 7876 W/m2 were used. One empirical model was developed with the help of dimensional analysis to predict heat transfer in a hot CFB to bare tube surfaces. One analytical model has been developed for the prediction of heat transfer to finned surfaces in a CFB both for long and short fins. An empirical equation has been developed correlating the parameters Nup, Rep and Lh/D to estimate heat transfer from the probes of different vertical heights. In addition one expression for calculating particle residence time has also been derived. The results predicted from the models and correlations have been compared with the present experimental results as well as those of other investigators and good agreement is observed. With the use of fins, the heat transfer coefficient was found to decrease by a maximum of 32% but the total heat transfer got enhanced by about 103% due to the additional surface area provided by the fins. Heat transfer coefficient was found to decrease and particle residence time was found to increase with the increase of vertical height of the probe.
Parameter Adaptive Vector Controller for CSI-fed Induction Motor Drive and Generalized Approaches for Simulation of CSI-IM System
(Indian Institute of Technology (IIT), Kharagpur, India., 1992-07) Ghosh, Dr. Bashudeb Chandra; Bhadra, Prof. S. N.
The main subject matter of this dissertation is to develop parameter adaptive rotor flux oriented current fed induction motor drive systems for controlled speed operation. The parameter adaptive approaches do not require special test signals and softwares in a big way. Three easy-to-implement schemes are suggested. One scheme is based on airgap power estimation method and takes into account of the non-linear magnetic state while tracking the actual rotor resistance. The other two methods constitute model reference adaptive controllers. They are based on the inverter input voltage and torque producing component of the stator current. The effectiveness of these methods is examined by extensive simulation and experimentation with two laboratory type induction machines. The experimental set up makes use of two microcomputers (i-8006 with i-6007) and a PC/AT with Burr Brown's PCI system having multi-channel 12 bit data acquisition modules for implementing the control algorithms. Prior to the description of parameter corrective algorithms, a control system suitable for a CSI-fed induction motor under rotor flux orientation is designed and tested by suitable simulation model. Then the influence of rotor resistance variation in the presence of magnetizing flux saturation has been extensively investigated. A two-parameter saturation model (two-term quintic non-linearity) is used in all these studies. It is further shown how the machine performances under field oriented condition are affected inclusion of saturation characteristics. Analytic expressions as well as digital simulation procedures are presented to study the performance of the drive system. Robustness of the drive system has been studied applying the concept of Kharitonov's theorem and then justified from the simulation results of the system for both parametric and torque disturbances. 'Bond Graph' and 'Nodal method' provide an organised way to write equations for a dynamic system. Tedious way of writing equations and their arrangement and re-arrangement for framing state equations can be avoided in the proposed simulation techniques. Simulation studies based on systematic modelling and framing of the system equations using the above two methods for the operation of a speed controlled CSI-IM system under normal and abnormal conditions have also been provided.
A Study on Distributive Nearlattices
(Rajshahi University, Rajshahi, Bangladesh, 1994-08) Rahman, Md. Bazlar; Noor, Prof. Dr. A.S.A.
This thesis studies the nature of distributive nearlattices. By a nearlattice S we will always mean a (lower) semilattice which has the property that any two elements possessing a common upper bound, have a supremum, Cornish and Hickman in their paper [14], referred this property as the upper bound property, and a semilattice of this nature as a semilattice with the upper bound property. Cornish and Noor in [15] preferred to call these semilattices as nearlattices as the behaviour of such a semilattice is closer to that of a lattice than an ordiary semilattice. In this thesis we give several results on nearlattices which certainly extend and generalize many results in lattice theory. In chapter 1 we discuss ideals, congruences and other results which are basic to this thesis. We include some characterizations of distributive and modular nearlattices. We generalize the separation properties given by M. H. Stone for distributive lattices. We also show that the set of prime ideals of a nearlattice S is unordered if and only if S is semiboolean. Chapter 2 discusses the skeletal congruences of a distributive nearlattice. Skeletal congruences on distributive lattices have been studied extensively by Cornish in [11]. Here we extend several results of Cornish for nearlattices. We also introduce the notion of disjutive nearlattices. A distributive nearlattice S with 0 is called disjunctive if for 0 ≤ a < b there is an element x ϵ S such that x ˄ a = 0 and 0 < x ≤ b. Then we give several characterizations of disjunctive nearlattices and semiboolean algebras using skeletal congruences. Finally we show that a distributive naerlattice is semiboolean if and only if θ -----> ker θ is lattice isomorphism of Sc(S) onto KSc(S) whose inverse is the map J´ ---> θ (J). In chapter 3, we discuss on normal and n-normal nearlattices. Normal lattices have been studied by several authors including Cornish [8] and Monteiro [34]; while n-normal lattices have been studied by Cornish [9] and Davey [16]. In proving some of the results we have used Principle of Localization, which is an extension of lecture note of Dr. Noor on localization. This technique is very interesting and quite different from those of the previous authors. Chapter 4 studies the multiplier extension (meet translation) of a distributive nearlattjce. Previously multipliers on semilattices and lattices have been studied by several authors e.g, Szasz and Szendrje [54,55,56] Kolibiar [29],Cornjsh [101 and Niemenen [37] on a latice. In a more recent paper, Noor and Cornjsh in [39] studied them on nearlatticee. Here we extend some of their work. We also give a categorjcai result, where we see that the multiplier extension has a functorial character which is entirely different from that of the Lattice Theory, c.f. Cornish [10, theorem 2.41. In section 2 of this chapter we discuss multipliers on sectionally pseudocomplemented distributive nearlattices which are sectionally in B՛n, -1 ≤ n ≤ Ꞷ and generalize a number of results of [10]. We show that S is sectionally in Bn if and only if M(S), the lattice of multipliers is in Bn. Finally we show that for 1 ≤ n < Ꞷ, above conditions are also equivalent to the condition that S is sectionally pseudocomplemented and for any n+1 minimal prime ideals P1,P2,. ........... ,Pn+1, P1 V P2 V ........... V Pn+1 = S.
Laminar Slot Jet and Slot Jet Impingement Heat Transfer
(Indian Institute of Technology (IIT), Kharagpur, India., 1996) Hossain, Khandkar Aftab; Arora, Prof. R. C.
In this investigation, the numerical solutions of Navier-Stokes equations and energy equation are presented for (1) laminar free slot jet, (ii) laminar free axisymmetric jet, (iii) laminar jet impingement on a circular cylinder, (iv) vertically downward laminar slot jet impinging on a flat surface with and without buoyancy, and (v) offset slot jet in the range. of low Reynolds numbers for both, the uniform and the parabolic slot-exit velocity and temperature profiles. SIMPLE and SIMPLEC algorithm have been used for the numerical solutions. The velocity and temperature profiles are presented near the nozzle-exit and far away from the nozzle-exit plane for free slot jet and free axisymmetric jet. The centreline velocity and temperature become coincident for all Reynolds numbers when plotted against R= X/Re. The velocity profiles become coincident for all Reynolds numbers far away from nozzle-exit in Schlichting's variable ii and with half jet width as characteristic length. The results are in good agreement with the available experimental results. The jet diffuses at a fast rate at lower Reynolds numbers and the jet spread decreases at higher Reynolds numbers. Jet impingement over a cylinder has been investigated for low Reynolds numbers. The results for skin friction, pressure distribution and velocity profiles on cylinder surface, are presented. The results do not depend strongly on slot width. The effect of decrease in slot -to-cylinder spacing is equivalent to increase in Reynolds number. The flow separation occurs earlier at vi i higher Reynolds numbers. In case of vertically downward slot jet impinging on a flat surface, the buoyancy force significantly affects the flow field, the local Nusselt number, local friction factor and the pressure distribution along the impingement surface. The jet behaves almost like a non-buoyant jet for values of Richardson numbers less than 0.014 with uniform slot-exit profiles and Richardson number less than 0.007 with parabolic slot-exit profiles. The jet detaches from the surface and rises up like a plume for Ri 0.014 with uniform slot-exit profiles and for Ri 0.007 with parabolic slot-exit profiles. The jet does not come into contact with the surface at all for Ri 1.25 for both, the uniform and parabolic slot-exit profiles. Empirical correlations have been developed for determining the average Nusselt number, maximum Nusselt number and the stagnation point Nusselt number for both the types of velocity and temperature profiles in absence of buoyancy for laminar flow. The maximum Nusselt number occurs at the stagnation point for parabolic slot-exit profiles, whereas for uniform slot-exit profiles, it occurs slightly away from the stagnation point. The offset ratio significantly influences the 1ocalNusselt number, local friction factor and the pressure along the impingement surface for offset slot jet in the range of low Reynolds numbers. Secondary recirculation is observed at an offset ratio of 7 and Reynolds number of 80. The reattachment length is observed to be approximately constant at higher values of Reynolds numbers. Empirical correlations have been developed for the reattachment length, maximum Nusselt number and the average Nusselt number in the separated region.
Analysis of Soft Ground Reinforced by Columnar Inclusions
(Saga University, Saga, Japan, 1996-03) Alamgir, Muhammed; Miura, Prof. Norihiko
A general method of analysis to solve an important class of problems encountered in the field of geotechnical engineering is developed in this dissertation. The analysis is formulated from the fundamental equation of equilibrium. It leads to develop a simple integro-differential equation to characterize the overall behaviour of the system. The behaviour of rigid columns (e.g. concrete/timber piles, lime/cement columns), deformable columns (e.g. stone columns/granular piles, sand compaction piles), pile groups, pile-raft foundations are the type of situations that can be analyzed by the proposed method. A numerical scheme using finite difference method is proposed to solve the governing equation with the aid of the. Relevant boundary conditions. A systematic process of trial is proposed to identify the possible slip and its magnitude developed at the column-soil interface. The numerical scheme can be used for end bearing and floating columns and for the situation involving stratified layers. The behaviour of soft ground reinforced by a group of columnar inclusions such as stone columns/granular piles, sand compaction piles and lime or cement columns, are predicted using the proposed method of analysis. The reinforced ground is covered by a layer of granular fill. The response of reinforced ground ranges from flexible to rigid loading conditions depending on the magnitude of thickness and deformation modulus of overlaying granular fill. The compacted granular fill over the column reinforced ground is very effective in reducing both the overall and the differential settlements of the composite ground. The compressibility of S the granular fill has an appreciable influence on the settlement response of the composite ground as long as the modulus of granular fill is less than approximately fifty times that of the soft ground. The depth of slip zone at column-soil interface increases with the decreasing value of limiting shear stress. It also increases with the increase of degree of penetration of column. Slip situation predicts higher depth of neutral plane than its no slip counterpart and it increases with the decreasing value of limiting shear stress. In case of end hearing column, a good portion of column sustains little or no interface shear stress at all but for floating column, the whole length of column is subjected to shear stress either positive or negative. The stress in column increases with depth and attains it high value at the bottom for end hearing columns. But in case of floating column, it increases up to the depth of neutral plane beyond which decreases up to the bottom of column. In both the cases no slip situation predicts higher depth of neutral plane than that of possible slip. The differential settlement of the composite ground is noticeable for the case of uniform flexible loading acting over the entire area. The overall and the differential settlements are more for slip analysis than that of no slip case. As the differential settlement does not reduce in case of floating column, end bearing column is more effective due to giving less overall settlement. The influence of soil stratification is evident in the predictions. This is, of course, as expected; the present analysis quantifies it. The spacing, length to diameter ratio, the degree of penetration of columns, the relative stiffness of column and soil, and the angle of friction between column and soil have a significant influence on the mobilization of shear stress, variation of normal stresses in column and soil and the settlement of the treated ground. But the Poisson's ratio of soil has little influence on them. A simple uncoupled consolidation model is proposed to determine the time-dependent response soft ground reinforced by columnar inclusions. This method is simple compared to Biot's coupled consolidation theory. The radial inhomogeneity of the soil properties such as deformation modulus and shear modulus and also soil stratification can be handled easily by the proposed method. From predicted results, it is realized that to evaluate the subsequent response of reinforced ground, the value of degree of consolidation predicted at every nodal points should be used rather than using the average value. The mobilization of shear stress at column-soil interface, the stress in column and soil and the settlement profile with time can be evaluated easily and reasonably accurately by the proposed method. The proposed foundation model has been compared with the existing approaches and verified by the finite element analysis as well as experimental results both in the laboratory and in the field. The existing approaches can be used for rigid loading condition but their application is restricted for flexible loading. In both cases flexible and rigid loading, the proposed model offers better solution as it can be used by taking account the role of overlaying granular fill, soil stratification, slip and no slip situations, can be used for both end bearing and floating columns and also for time-dependent analysis. The comparison of results obtained from finite element method and those of by the proposed model, indicates that the proposed model can be used with a reasonable degree of accuracy to depict the settlement behaviour of end bearing or floating columns reinforced ground subjected to either flexible or rigid loading. The prediction obtained from the proposed model show also good agreement with test results both in laboratory and field.
Some Aspects of Generation Expansion Planning
(Indian Institute of Technology (IIT), Kharagpur, India., 1996-04) Azad, Dr. Md. Abul Kalam; Misra, Dr. R. B.
The primary objective of generation expansion planning is to meet the electrical energy needs of the customers as economically as possible with an acceptable degree of safety, reliability, and quality. Power system planning involves studies to determine the resources required to meet the growth in demand at the lowest possible cost considering environmental and financial constraints. A power utility should meet the demand under a wide range of normal, abnormal, and emergency conditions including the reasonable foreseeable failures and maintenance outages of facilities. This requires some generation system capacity reserve in excess of forecasted demand. The uncertainty associated with future demand projections could make the system facilities inadequate, or excessive and uneconomical, both cases being unacceptable. Purchasing cheaper power from other utility through interconnection is another alternative which improves system reliability. in this thesis, some studies have been made on generation expansion planning taking into account the above important criteria. The thesis contains a brief research background, motivation and objective to the modelling and analysis of some important requirements of generation expansion planning. The proposed techniques are implemented to an existing electric utility for generation expansion. The key to all generation expansion planning is a good forecast of load that reflects current and future trends, tempered with good judgment. This is quite important for financial success. Undoubtedly, the most obvious deficiency in any expansion plan is and eventually is seen to be, the accuracy of the demand forecasts. Four modules of Artificial Neural Network have been studied in detail, to forecast peak load of two distribution substations. It is observed from the sensitivity analysis that all modules are not valid for all systems. In other words, for a specific system only a particular module is suitable for load forecasting. It is also found that the predicted results of minimum mean absolute error (MAE) is dependent on learning rate and learning momentum. Using the findings of the sensitivity analysis, various modules of ANN are studied and the most suitable module found is used to forecast yearly peak load of an existing electric utility for fifteen years choosing an appropriate module for the system. Reliability criterion, the next aspect, is primarily used to determine the required S stein generating capacity reserve, to operate the system under equipment failures, equipment maintenance, and load variations. Probabilistic methods are used to evaluate reliability of generation expansion plans using stochastic representation of the generating unit failure-repair process, load variability and emergency help from interconnections. A modified FFT method has been proposed to evaluate the loss of load probability (LOLP) of a power generating system consisting of different types and sizes of unit. This approach uses hourly loads, or any suitable time interval for system demand, for a given period. Out of several properties of FFT scheme, some properties have been used for the reduction of computational complexity. The accuracy of the method has been illustrated using an example. This modified FFT algorithm is applied to IEEE Reliability Test System (IEEE-RTS). The modified approach improves the efficiency in comparison to the conventional FFT method. Further, another new approach has also been developed to evaluate the LOLP of a power generating system. This approach uses joint probability density function (PDF) concepts to convolve the unit outages and loads of the system. The reduction of computational effort for identical generating units is obtained using the binomial distribution. The method has been illustrated through an example and is applied to IEEE-RTS. This method is found efficient and easy to use as compared to other existing methods. In addition, the proposed approach can simulate multistate representations of generating units at less computational efforts. Exploiting these advantages of the proposed approach, generation expansion planning of an existing utility has been made based on levelized LOLP for projected future demand. In generation expansion planning, interconnected system may play an important role on system reliability. Reliability evaluation methodology of interconnected systems is different from that of a single area system. If the available capacity in one geographical region can be transmitted to other regions whenever it is needed without tie line restrictions then and only then, this system may be treated as a single area. Though it is possible to evaluate interconnected systems as a single system with some approximation, but, it is not in practice due to many obvious reasons. The improved modified FFT scheme, developed for single area, has been extended to evaluate LOLP of two area interconnected power systems. A stochastic procedure for interconnected systems is presented using improved two dimensional FF'T IMSL subroutine of Cyber 180/840A mainframe. This method can simulate multi-state generating units without affecting the computational complexity, whereas the computational complexity of other existing methods increases with the increase of number of outage states of generating units. The joint PDF approach, developed for single area, has also been applied to evaluate LOLP of two area interconnected power system consisting of different types and sizes of generating units considering independent as well as correlated system demands. The Probability Density Function (PDF) of equivalent load is obtained by convolving the PDF of generating unit outages with the PDF of system demands using the proposed approach. The LOLP values of each system are obtained from the PDF of equivalent load. The accuracy of the proposed method has been illustrated using a simple example. The results obtained for IEEE-RTS are compared with existing methods. In addition, the above approach is also implemented to an existing utility for which expansion planning studies were carried out. The system under study is considered to be interconnected with a hypothetical system. The benefits desired due to interconnection are evaluated and the impact of interconnection on expansion planning is studied. An efficient approach has been developed to evaluate the expected energy generation, expected unserved energy, production costs (in the thesis production cost is used for fuel cost) and loss of load probability of a power generating system. The expected energy generation of a given generating unit is obtained by evaluating the difference of unserved energy before and after the commitment of the unit. The method can evaluate expected energy generation and production costs of identical generating units at a time. This is not restricted to load duration curves and unit outage density function of any shapes, or size of the systems with a large number of generation units. Multiple generating units with same outage behavior can be committed with system demand efficiently. The new developed approach has been extended to evaluate the expected energy generation, expected unserved energy and production costs for two area interconnected power generating system. An example and IEEE-RTS have been used for illustration. Further, the proposed method has been used to evaluate expected energy generation, unserved energy and production cost of the generation expansion plan developed for existing utility under study. Based on the detailed study, addition of generating units for the existing utility has been recommended for the period under consideration. Reliability index is derived from the estimates of availabilities of generating units, forecasted loads and unit incremental costs. Uncertainty which results in an unacceptable estimates of system reliability is implicit in the estimates of availability of units and forecasted loads. The uncertainty in the availability of generating units is due to the variation of failure data for different reporting sources and fluctuation in environmental conditions. Fuzzy set theory provides optimistic and pessimistic values of the derived quantities, corresponding to the assumptions of the highest and lowest possibilities of concerned events. In many cases, these bounds provide excellent guidelines to the generation expansion planners. A model has been developed for fuzzy reliability to quantify the effect of uncertainty associated with unit capacity, FORs and the forecast loads on the LOLP of a power generating system. The model has been explained with the help of a simple generation system. Studies are also conducted on the IEEE-RTS to demonstrate effect of uncertainty on system parameters. Effects on the system reliability index due to variation in the amount of uncertainty of the parameters is studied. This proposed model has also been applied to a practical system to predict fuzzy LOLP.
Non-Linear Finite Element Analysis of Wall-Beam Structures
(Bangladesh University of Engineering and Technology (BUET), Dhaka, Bangladesh., 1997-05) Hossain, Dr. Md. Monjur; Rahman, Prof. M. Azadur; Ali, Prof. Sk. Sekender
The composite action between masonry wall and its supporting beam concentrates the vertical loading applied on the top of wall close to the beam supports. This produces bending moments much less than would be expected when the full load is acting directly on the beam. The study of this composite action is of economic importance since if the concept is utilised, a rational design of the beam will be achieved. This thesis presents a comprehensive material model and its incorporation into non-linear finite element computer model for the analysis of wall-beam structures made with brickwork of solid brick subjected to uniformly distributed load. The program is incremental in nature and capable of reproducing the non-linear behaviour caused by material non-linearity and progressive local cracking and crushing. The program is thus capable of modelling the behaviour of wall-beam structures subjected to non-linear load from first crack to final failure. The material model used in this program is derived from tests on representative samples of brick, mortar and small samples of brickwork. In the finite element model brick, mortar, concrete and steel are treated as separate materials along with the simulation of actual directional effects of mortar joints. A series of failure criteria have been adopted to model the different modes of failure experienced in the constituent materials. Due to the crack sensitive nature of the problem, particular emphasis has been given to the modelling of cracking and the post-cracking behaviour of the materials, especially the manner in which the local stresses in the fractured region are redistributed. The results of finite element model have been verified by comparison with experiments on brick masonry wall-beam structures subjected to uniformly distributed load applied at the top and extending over the full length and thickness of the wall. Comparison with published literature reveals that the present method can provide a more comprehensive prediction of behaviour of the wall-beam structure up to failure load. Sensitivity analyses of the various parameters defining the material model and the boundary conditions have been canied out. With the important parameters thus obtained, the finite element model has been used to carry out a comprehensive parametric study of the behaviour of storey height wall-beams subjected to uniformly distributed load. Based on the findings of this study, design recommendations have been proposed. As the computer program developed can handle general cases with arbitrary geometry, loading and boundary conditions, recommendations for investigations of various other wall-beam structures have been made.
Studies on Long Term Average Optical Efficiency for Principal Solar Collectors and Shading Factors for Windows
(Indian Institute of Technology (IIT), Kharagpur, India., 1997-09) Rahman, A. N. M. Mizanur; Satyamurty, Prof. V. V.
Assessment of techno-economic viabiity of solar energy systems is based on long term performance. The popular design methods for active as wel1 as passive solar energy systems require certain proprocessed parameters in addition to meteorological information on appropriate time scale. The present thesis deals with defining monthly average transmittance-absorptance product for flat plate collectors and optical efficiency for concentrating collectors consistent with the definition of monthly average daily utilizability. The procedure involved comparing the monthly average daily useful energy gain for flat plate collectors obtained by hour by hour summation procedure with the monthly average daily useful energy gain obtained as a product of heat removal factor, monthly average daily solar radiation on the collector aperture, monthly average daily utilizability and monthly average. Transmittance-absorptance product. In the second calculation transmission-absorptance product is an all day average value on the mean day of the month. The difference and an examination of the definition of monthly average daily utilizability established that the transmittance-absorptance product needs to be (defined as a weighted average of the instantaneous transmittance- absorptance I)product and solar radiation on the collector aperture above the critical radiation level. An eqaivalemit mean day (EMD) calculation has been proposed and validated to obtain the monthly average transmittance-absorptance product for flat plate collectors and the optical efficiency for concentrating collectors tracked in the five principal modes according to the above definition. A method to estimate the monthly average shading factors for recievers shaded by finite overhangs taking atmospheric transmittance into account has been developed and validated. The procedure involved developing sim pie equations relating finite shading factor values to the infinite values which are easily calculable in terms of the tilt factors for different planes. The present approach is valid for south as well as non-south facing receivers. Finally, expressions for the monthly average shading factors for infinite wingwalls, analogous to the expressions for infinite overhangs, have been developed validated. A feature of these algorithms is that they take into account the monthly average daily diffuse fraction and are valid for south facing as well as for non-south facing receivers.
Prediction of Formation and Tracks of Bay of Bengal Cyclones
(Dhaka University (DU), Dhaka, Bangladesh., 1998-12) Alam, Dr. Md. Mahbub; Shafee, Prof. Sultana
Rawindsonde data of 0000 UTC for standard isobaric surfaces at the surface, 850, 700, 500, 400, 300, 200, 150 and 100 hPa levels for the periods of different cyclones in the last decade were considered to study the different meteorological parameters. The vertical wind shear, the vertical variation of zonal wind speed and meridional wind speed and temperature were studied for different cyclones in the surroundings of the Bay of Bengal in relation to the movement of cyclones. The dry static energy, the latent heat energy, the moist static energy, the total energy, the meridional and zonal fluxes of moist static energy and their vertical distribution were studied in the surroundings of the Bay of Bengal in relation to the movement of cyclones and their ultimate landfall. A two dimensional model is developed for the prediction of cyclone tracks in the Bay of Bengal. The boundary value problem is then solved by using the complex potentials. The different cyclones are integrated in different time steps to find the relationship between the actual travel time and observed time by the models. The results obtained for cyclone tracks by this model are almost the same as the actual track. The cyclonic track data were collected from the Storm Warning Centre (SWC) of Bangladesh Meteorological Department (BMD). A simplified system of equations that can simulate the evolution and mature stages of tropical cyclones is presented. The study has been done using a three-dimensional numerical model constructed which produces circularly symmetric feature of the tropical cyclone. It has 5 layers in the vertical with the horizontal extent of 1200 km. The horizontal grid length of the model is 80 km. The model equations have been solved using the Leapfrog scheme. Numerical simulation shows that the model sensitivity to the vertical stability is similar to results obtained from other general models. Latent heat release associated with condensation is calculated from the moisture equation. The sensible heating parameter is added to the heating term. Initially the model atmosphere is assumed to be at rest and is then excited by a large scale heating to generate an initial vortex. The evolution of surface pressure drop, tangential wind, radial wind, heat released due to condensation, temperature anomaly, D-value, horizontal structure of vertical p-velocity, vertical structure of vertical p-velocity, vorticity and static stability parameter have been discussed based on the results obtained from the tropical cyclone modelling experiment.
Temperature Dependence of Magnetization and Induced Magnetic Anisotropy of Some Fe, Co and Ni-Based Amorphous Ribbons
(Bangladesh University of Engineering & Technology (BUET), Dhaka, Bangladesh, 1999-07) Sikder, Shibendra Shekher; Asgar, Prof. M. Ali
Ferromagnetic amorphous ribbons of Fe-Si-B, Ni-Fe-B and Co-Fe-B-Si series have been prepared by rapid quenching technique and the amorphousity of the samples has been confirmed by X-ray diffraction technique. The kinetics of glass formation and crystallization as affected by a slight change in composition is studied by differential thermal analysis (DTA). It is observed that the thermodynamics of the amorphous ribbons in respect of the formation of glassy state and its stability is affected by the complexity of the composition. The magnetic ordering of Fe-based, Ni-based and Co-based amorphous ribbons is studied by measuring A.C. and D.C. magnetization as functions of temperature. D.C. measurements are made by using a vibrating sample magnetometer and A.C. initial permeability is measured, using a furnace in which the heating wire is wound in accordance with the bifiller technique. Coercivity, remanence, saturation magnetization and maximum permeability have been determined as static magnetic properties for Fe-based, Ni-based and Co-based specimens from the hysteresis loops obtained by Ballistic method. The dynamic characteristic such as the real and imaginary components of the complex permeability in A.C. conditions have been measured as a function of the instantaneous value of a sine wave core current density by means of an adapted LCR bridge method. Initial permeability, frequency dependence of complex permeability and relative quality factor have been measured in the frequency range 0.5KHz to 13MHz for all the different compositions and for annealing effect of initial permeability. The measurements of saturation specific magnetization of Fe-based, Ni-based and Co-based amorphous ribbons have been measured by V.S.M. We have correlated the behaviour of magnetic moment in amorphous alloys containing 3d transition metals on the basis of rigid band model, assuming that the metalloid atoms contribute some of their S electrons and P electrons to fill the d band of the transition metal. This accounts for the reduction of magnetization of all the amorphous ribbons with increasing metalloid content. The temperature dependence of magnetization, of amorphous system decreases faster with increasing temperature as compared to crystalline materials. According to the mean field approximation, reduced magnetization versus reduced temperature graphs for different Fe-based, Ni-based and Co-based amorphous ribbons leads to fluctuation in the exchange interaction giving rise to a structural disorder. Induced magnetic anisotropy of amorphous ribbons having Fe-based, Ni-based and Co-based system have been measured for temperature and field dependence using torque magnetometer with proportional integrating differentiating compensation. The results of compositional dependence of initial permeability, coercive force and induced anisotropy are found to be mutually consistent for all the samples. The origin the uniaxial anisotropy in these amorphous materials is assumed to be the stress, induced during the preparation. The temperature dependence of uniaxial anisotropy is caused by stress, relieving and decreases with the increase of temperature. The corresponding curves of In [Ku(0)/ Ku(T)] versus In [σs(0)/ σs(T)] in amorphous ribbons with Fe-based, Ni-based and Co-based system follow straight lines arising from the disorder caused by randomly oriented easy axes. The values of the exponents are different for the different samples. All amorphous ribbons have been investigated using Mössbauer spectroscopy. The Mossbauer measurements were performed with a conventional constant acceleration spectrometer at room temperature in transmission geometry, using 57Co in a rhodium matrix. The velocity scale of a thin 57Fe sample and the isomer shifts are given relative to the centroid of the spectrum. The isomer shifts of Fe-atoms in all the Fe, Ni and Co-based samples behave as trivalent ions. The spectrum for the as prepared samples consists of six lines which are broadened as compered with the Mössbauer spectrum of a pure 57Fe sample in the form of a thin foil. The curve for the experimental spectrum is obtained by using the best fit of the distribution of hyperfine fields. The consistency between the average magnetization measured by V.S.M. and the magnetic moment of the Fe-atom as estimated from the hyperfine field distribution confirms the collinearity of the magnetic ordering of the sample.
Studies on Some Aspects of Electrochemical Grinding
(Indian Institute of Technology (IIT), Kharagpur, India, 1999-08) Bhowmick, Tarapada; Mishra, Profe. Dr. P. K.
Electrochemical grinding (ECG) has become more and more important for its industrial use, in machining problems associated with grinding hard and wear resisting and even very soft electrically conductive materials. In a survey on ECG process, little attempts on theoretical analysis of the process rather more experimental investigations have been made. The main objective of the present work was to develop theoretical models to explain joint phenomena, mechanism and electrochemistry i.e. (i) to determine metal removal rate due to mechanical and that of electrochemical action individually, (ii) to find out the feed force required in ECG process, and (iii) to corroborate the models with experimental results. For this, an industrial model for the electrochemical grinding machine with hydraulic feed control system has been developed retrofitting an existing obsolete manual feed surface grinder. Different stages of grinding action are thoroughly analyzed and corresponding force components are established in terms of chip thickness, stress and loading coefficients. An octagonal extended ring type grinding dynamometer of stainless steel has been designed, constructed and calibrated. A high gain operational amplifier has been built to record force components. Electrochemical grinding geometry and kinematics are analyzed, and electrochemical and mechanical aspects of the process have been extensively studied. Investigation on the different process parameters and their inter-relations are also made. Experiments have been conducted on stainless steel and tungsten carbide (GT20) to examine the validity of the theoretical works and other experimental investigations. Theoretical analyses have been carried out on the determination of MRR and feed force. Different stages of grinding actions viz, sliding, ploughing, cutting, rubbing, viscous drag due to electrolyte pool are considered in feed force analysis. The depth of cut is used for combined mechanical and electrochemical actions. They are experimentally verified and found within the closure range of acceptance for industrial exploitation. The results indicate that material removal rate due to electrochemical action can be achieved up to 90% of the total material removed, and as a result, the feed force in ECO is found very less compared to conventional grinding.
Impingement Heat Transfer Due to Circular Air Jet Over Rough Flat Surfaces
(Bangladesh Institute of Technology (BIT), Khulna, 2000-08) Islam, Md. Nurul; Ahmed, Prof. Dr. Naseem
An experimental investigation was carried out to investigate the pressure distribution and the local and average Nusselt number due to impinging of a circular air jet over uniformly heated rough flat surfaces. The present investigation shows the dependence of the pressure on jet exit Reynolds number, relative roughness of the surfaces and nozzle-to-surface spacings. It was observed that the overall pressure coefficient, cp increases with the increase of jet exit Reynolds number and decreases with the increase of surface roughness and nozzle-to-surface spacings. It also observed that the coefficient pressure at the stagnation point remains constant for the lower values of surface roughness but increases beyond a specific value of surface roughness. In this investigation the nature of dependence of heat transfer on various parameters namely, jet exit Reynolds number, relative roughness of the surface and nozzle-to-surface spacings are identified Jet exit Reynolds numbers of 6000. 8700. 16520 and 23400 , relative surface roughness of smooth. 0.01306, 0.01338, 0.01806. and 0.01952 and dimensionless nozzle-to-surface spacings of 1.61. 2.41, 3.22. and 4.03 are considered for the investigation. It was observed that the local Nusselt number increases with the increase of jet exit Reynolds number and surface roughness, but decreases with the increase of nozzle-to-surface spacings. It also observed that the stagnation point Nusselt number remains constant for the lower values of surface roughness but increases beyond a specific value of surface roughness, which predicts that there is a critical value of surface roughness. The average Nusselt number was calculated and a correlation developed in terms of jet Reynolds number, relative roughness of the surface and nozzle-to-surface spacings. The correlation yields ±10% accurately in context with experimental findings, however shows singularity for smooth surface. Experimental results provided useflul intbrmatiofl which have significant of potential industrial applications regarding the radius of the heat transfer area, nozzle-to-surface spacing atid surface roughness for maximizing the average Nusselt number.
The Effects of Car A-Pillar and Windshield Geometry on Local Flow and Noise
(RMIT University, Melbourne, Australia., 2000-09) Alam, Dr. Firoz; Watkins, Simon; Hurnphris, Clive
A desirable requirement in the production of modern vehicles world-wide is the provision of a high level of driving comfort. An important aspect of this is the minimisation of aerodynamic noise. As structure-borne, engine, tyre, and power-train noise sources have been reduced in recent years, the aerodynamic noise is significant, especially at driving speeds exceeding 100 km/h. Prior experimental studies have revealed that the flow around a passenger car's A-pillar region is a primary source of aerodynamic noise, since the highest pressure fluctuation occurs here. Also, this region is closest to the driver's ears. Whilst a small part of the noise can come from aerodynamic noise generated by the mirror as the flow is first incident on the A-pillar, this study will only be addressing flow around the A-pillar. It is known that the area and strength of the A-pillar flow separation depend mainly on the local A-pillar and windshield geometry and yaw angle. However, the effects of scaling, local radii and yaw angle on the potential for noise generation are not well understood. Scaling is important so that model-scale results can be translated to the full-scale. Computational Fluid Dynamics methods (CFD) are not sufficiently developed either to predict the surface pressure fluctuations or the resulting acoustic waves with the required degree of accuracy. The objectives of this work were to investigate the scale effects, the influence of the local A-pillar and windshield radii on the flow characteristics, and the influence of yaw angle. In order to address these objectives, a series of experimental investigations was conducted using five 40% scale generic models with different A-pillar and windshield geometries and using three production vehicles. One model had a sharp-edged vertical windscreen. The other four models had a 60° inclined windscreen, which is a typical slant angle for contemporary production passenger cars, and various degrees of edge rounding including a model with a sharp edge. These models were used to measure the surface mean and fluctuating pressures in the A-pillar region at different speeds and steady yaw angles. Production vehicles were used to evaluate the surface mean and fluctuating pressures close to the A-pillar region, and the 'in-cabin noise' as a function of steady yaw angle and increased rounding of the A-pillar. The production vehicle tests were performed at different speeds and yaw angles in wind-tunnels and on-road. Flow visualisation was used to supplement the pressure data. The surface mean and fluctuating pressures were converted to non-dimensional pressure coefficients and the frequency content of the fluctuating pressure was investigated via the normalised power spectral density. Generally the surface mean and fluctuating pressure coefficients were found to be independent of Reynolds numbers. However, when yawed, a slight dependency was found to occur on the leeward side. This minor dependency was noted in the separated regions, but was not evident in the re-attached areas. The amplitudes and frequencies of the fluctuating pressures scaled well with velocity head and Strouhal number. Therefore, a scale model can be used for the prediction of the surface hydrodynamic pressures in the Apillar region of a future vehicle when suitable scaling laws are used. The magnitudes of fluctuating pressures and the area of flow separation close to the A-pillar region depended largely on the local radii. Most energy from the fluctuating pressures in the A-pillar region was between Strouhal numbers 5 to 12. The maximum hydrodynamic pressure fluctuation was found to be between the separated and reattached areas rather than at the re-attachment points as has been proposed by other researchers. Yaw could increase the area and magnitude of the flow separation on the leeward side by an order of magnitude compared to the windward side for the slanted sharp-edged model. However, the model shape with no slant angle (i.e., a vertical windshield) produced an intense but relatively small flow separation on the windward side when yawed. Negligible flow separation was found on the models with corner rounding and increase of yaw angle did not increase the separation substantially, even on the leeward side. However, future work is recommended on an additional model incorporating a smaller corner radius. For the production vehicles an increased rounding of the A-pillar significantly reduced the magnitude of the external fluctuating pressures, although the 'in-cabin noise' typically reduced by 2-3 dB. The amplitudes and frequencies of the fluctuating pressures scaled well with velocity head and Strouhal number. Atmospheric turbulence, correlation between the external pressure fluctuations and in-cabin noise, and boundary layer characteristics in the A-pillar region were not included in this work but are thought to be worthy of further investigation.
Study of Artificial Neural Network and Observer-Based High Performance Induction Motor Drives
(Bangladesh Institute of Technology (BIT), Khulna, Bangladesh., 2001-03) Rafiq, Md. Abdur; Ghosh, Prof. Dr. Bashudeb Chandra
The main subject matter of this dissertation is to study the performance of artificial neural network and observer based high performance induction motor drive. Four suitable flux observers compatible with drive control law are discussed and flux estimation with these observers along with effectiveness is studied. Study of the artificial neural networks for flux estimation with baekpropagation training algorithm for simulation is presented in this dissertation. It also presents the general idea about feedforward neural networks, mapping and training of an artificial neural network. The direct and indirect field orientation control methods of induction motor For variable operating conditions are evaluated in this study. In the direct method, flux estimation is applied for vector rotators which controls drive current or voltage magnitude as well as position SO that the rotor flux can be kept constant. In the indirect method flux estimation is used for parameter compensation. Digital simulation procedures are presented to study the performance of these observerbased field oriented induction motor drives. Speed of an induction machine is also estimated with full order observer and parameter adaptation is also presented for sensorless field orientation control. 1'he main cirawbuck of indirect method of field orientation is due to Variation of rotor resistance that degrades performance and requires tuning. Observers are used for detecting the parameter mismatch condition and correcting the controller resistance. By flux feedback the rotor resistance is adapted and the effectiveness of observers is also examined. Reduced order observer in generalized form is used for parameter adaptation of current source inverter fed system. ix Flux estimation with artificial neural network has been carried out and extended to direct field orientation of voltage source inverter fed induction motor system. Finally, comparison with the results obtained by artificial neural network is given.
Study on Pile Cap Design
(Bangladesh Institute of Technology (BIT), Khulna, Bangladesh., 2001-11) Masum, Mir Md. Al; Hossain, Prof. Dr. Md. Monjur
Piles are used to transfer the loads from the superstructure down to the soil stratum. where the required resistance is available. Unless a single pile is used, a pile cap is necessary to spread the vertical and horizontal loads and any overturning moment to all the piles in the group. Unlike footing the base reaction of pile cap Consists of a number of concentrated load. Different methods of designing pile caps are in practice. Those are Broadly classified as ACI Building Code and strut-and-lie model (STM). In this project the cost incurred by different design approaches are compared. The cost of pile caps according to different approaches is compared considering 4 pile -cap with constant pile diameter and column size but with varying pile spacing. The comparison reveals that the pile cap designed by STM costs 5% to 20% lower than AC Building Code for pile spacing 2 to 3 times pile diameter within a range of 75 to 625 kip of column load. The experimental ultimate strength of the pile caps is compared with that obtained by different design approaches. The behavior of pile cap at loading stage is also observed and is seen to agree with other investigators. It is seen that the experimental strength of pile caps is higher than the strength predicted by the STM in comparison to ACI Building Code. Within the limitation of pile spacing up to 3 times pile diameter, modified STM is more rational than ACI Building Code in terms of cost and integrity.
Stress-Deformation Characteristics of Selected Coastal Soils of Bangladesh and their Sampling Effects
(Bangladesh University of Engineering & Technology (BUET), Dhaka, Bangladesh, 2002-03) Bashar, Md. Abul; Safiullah, Prof. Dr. A.M.M.
A considerable development activity within the Coastal Region of Bangladesh has necessitated an understanding of the geotechnical behaviour of soils from this region. With this objective in view a study into strength-deformation, compressibility and intrinsic properties of reconstituted samples of Chittagong coastal soils were undertaken. This thesis presents stress-deformation characteristics of three selected coastal soils and their sampling effects. The soils were collected from Banskhali, Anwara and Chandanaish in Chittagong coastal belt of Bangladesh. The soils are low to medium plasticity (Liquid limit = 34 to 45 and Plasticity index = 10 to 20). Reconstituted samples of the three soils were prepared in the laboratory by K0- consolidation of slurry in a large cylindrical consolidation cell using a consolidation pressure of 150 kN/m2. Overconsolidated samples were prepared in the triaxial cell by releasing the maximum isotropic consolidation pressure of 150 kN/m2 to appropriate values to achieve overconsolidation ratios (OCR) of 1.5, 2, 5, 10, 20 and 30. The stress-deformation-strength, stiffness and pore pressure characteristics of reconstituted isotropically normally consolidated and overconsolidated "block" samples of the three coastal soils were investigated in the laboratory by performing undrained triaxial compression tests. Models for the prediction of undrained shear strength of normally consolidated and overconsolidated samples have been developed. To develop intrinsic models of compressibility, intrinsic compression lines (ICL) for the three soils under K0 and isotropic stress conditions have been established which can be used to determine compressibility indices of soils at any depth of known overburden pressure. State boundary surfaces (Roscoe and Hvorslev state boundary surfaces) and critical state lines of the three coastal soils have been established. The critical state parameters of the soils have also been evaluated. Constitutive models relating critical state soil parameters and plasticity index of the soils have been proposed. Applications of these models to undisturbed naturaj clays will require further investigation. The experimentally observed stress-strain behaviour of reconstituted normally consolidated samples of the three coastal soils have been compared with those predicted using two critical state models, namely, "Cam clay model" and "Modified Cam clay model". It has been found that the results predicted by using "Modified Cam clay model" compared more favourably with the observed experimental results than "Cam clay model" for the three coastal soils. The present study has also been carried out to investigate the effects of "perfect" sampling disturbance and tube sampling disturbances on engineering properties of reconstituted normally consolidated samples of the three coastal soils. Undrained triaxial compression tests were carried out on "in situ", "perfect" and "tube" samples. "In situ" samples were prepared by consolidating reconstituted specimens of 38 mm diameter by 76 mm high under K0-condition in the triaxial cell to its in situ stress state. "Perfect" samples were prepared from "in situ" samples by undrained release of the total stresses in the triaxial cell. "Tube" samples were prepared from the large diameter consolidated samples by inserting samplers of different area ratios, external diameter to thickness ratio (De/t) but of constant outside cutting edge angle (OCA) and internal diameter (Di). Area ratio, De/t ratio, OCA and Di of the samplers were 16.4% to 73.1%, 27.3 to 8.3, 5° and 38 mm, respectively. Undrained triaxial compression tests were carried out on reconsolidated "perfect" and "tube" samples of the three coastal soils to assess the suitability of various reconsolidation techniques to minimize sampling disturbance effects. Experimental results indicate that disturbances due to perfect and tube sampling have significant influence on the mechanical properties of coastal soils. The nature of the effective stress paths and por' pressure responses of both "perfect" and "tube" samples are markedly different from those of the "in situ" samples. The "perfect" and "tube" samples adopted stress paths and showed pore pressure responses which are more typical of overconsolidated clays. Disturbances due to perfect sampling led to reduction in the values of undrained shear strength (se), Skempton's pore pressure parameter A at peak deviator stress (Ap), initial tangent modulus (Ei) and secant stiffiess at half the peak deviator stress (E50) whilr axial strain at peak deviator stress (εp) increased due to total stress relief. Due to total stress relief, the reduction Su, Ei and E50 increased with the decrease of plasticity while the increase in c. increased with the decrease of plasticity of the soils. It is also evident that the decrease in mean effective stress (p') due to perfect sampling increases with decreasing plasticity of the soils. The initial effective stress (σ՛i) of "tube" samples reduced considerably because of disturbance caused by penetration of tubes. Compared with the "in situ" samples, values of s, E1, E50 and A of the "tube" samples decreased while sp increased. The changes in measured soil parameters between the "in situ" and "tube" san )les have been found to depend significantly on the sampler characteristics, i.e., area ratio, De/t ratio, used for retrieving the "tube" samples. The values of σ՛i, su, Ei and E50 were decreased due to increase in area ratio (or reduction in De/t ratio). The values of εp, however, increased due to increasing area ratio. A quantitative increase in the degree of disturbance (Dd) has been obtained due to increase in area ratio, while the values of Dd increased with the decrease of De/t ratio of sampler. Disturbance due to tube sampling has been found to depend on the plasticity of the samples of the three coastal soils. The highest reductions in a', st,. Ei and E 0 occurred in the least plastic samples, whereas the minimum reduction in σ՛i, su, Ei and E50 occurred in the most plastic samples. Among the samples of the coastal soils, the least plastic sample produced higher degree of disturbance than the most plastic sample.