VTechWorks Repository :: Browsing by Author "Reddy, Junuthula N."

Browsing by Author "Reddy, Junuthula N."

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Accuracy analysis of the semi-analytical method for shape sensitivity analysis
Barthelemy, Bruno (Virginia Polytechnic Institute and State University, 1987)
The semi-analytical method, widely used for calculating derivatives of static response with respect to design variables for structures modeled by finite elements, is studied in this research. The research shows that the method can have serious accuracy problems for shape design variables in structures modeled by beam, plate, truss, frame, and solid elements. Local and global indices are developed to test the accuracy of the semi-analytical method. The local indices provide insight into the problem of large errors for the semi-analytical method. Local error magnification indices are developed for beam and plane truss structures, and several examples showing the severity of the problem are presented. The global index provides us with a general method for checking the accuracy of the semi-analytical method for any type of model. It characterizes the difference in errors between a general finite-difference method and the semi-analytical method. Moreover, a method improving the accuracy of the semi-analytical method (when possible) is provided. Examples are presented showing the use of the global index.
Adaptive finite element simulation of incompressible viscous flow
Fithen, Robert Miller (Virginia Tech, 1993-08-05)
A finite element method is employed for solving two- and three-dimensional incompressible flows. The formulation is based on a segregated solution method. In this segregated formulation, the velocities and pressures are uncoupled and the equations for each are solved one after the other. This segregated solution method is numerically compared to the penalty method and to previous reported data to determine its validity. Next an iterative solution method which employs an element by - element data structure of the finite element method is developed. Two types of iterative methods are used. For a symmetric stiffness matrix, the conjugate gradient method is used. For an unsymmetric stiffness matrix, the bi-conjugate gradient method is used. Both iterative solution methods make use of a diagonal preconditioning method (Jacobi preconditioning). Several problems are solved using this segregated method. In two-dimensions, flow over a backward facing step and flow in a cavity are investigated. In three-dimensions, the problems include flow in a cavity at Reynolds number 100 and 1000, and flow in a curved duct. The simulation compares very well with previously reported data, where available.
An analysis of interlaminar stresses in unsymmetrically laminated plates
Norwood, Donald Scott (Virginia Tech, 1990)
The results of a numerical study of interlaminar stresses within unsymmetrically laminated plates is presented. The focus of the study is upon the linear thermoelastic response of thin square laminated composite plates subjected to extensional, compressive, or thermal loading. Symmetric and unsymmetric 0/90, +45/-45, and 0/+45 laminate stacking sequences are examined to determine the effects of mismatch between adjacent layers in Poisson’s ratio, coefficient of mutual influence, and coefficients of thermal expansion. Since the out-of-plane (transverse) deflections of unsymmetric laminates are typically large, a geometrically nonlinear kinematic description is used to account for the large displacements and rotations. The geometrically nonlinear three-dimensional boundary value problems are formulated from nonlinear elasticity theory and approximate solutions are determined using the finite element method. A total Lagrangian, displacement-based, incremental finite element formulation is implemented using Newton’s method. Geometrically nonlinear global/local finite element analysis is used to obtain improved free edge stress predictions. For laminates subjected to external loading, the mismatch in material properties between adjacent layers causes interlaminar stresses to arise near the free edges. For unsymmetric laminates under external loading, the mismatch in material properties about the geometric midplane causes out-of-plane deflections. For the laminates and loading conditions considered, the results of this study show that the out-of- plane deflections of unsymmetric laminates reduce interlaminar shear stresses. In addition, the out-of-plane deflections reduce interlaminar normal stresses for some laminates and increase these stresses for others. For the two-layer unsymmetric laminates considered, the effect of out-of-plane deflections upon interlaminar normal stress was shown to be dependent upon the type of in-plane strain mismatch (i.e., normal and/or shear) caused by the dissimilar material properties. The results also show that as the out-of-plane deflections become large, the effects of geometric nonlinearity upon this stress-deformational response become important. These conclusions apply to extensional, compressive (prior to a change in mode shape), and thermal loading. The numerical results include interlaminar stresses for laminated plates which have buckled as a wide column under compressive loading.
Analysis of metal matrix composite structures using a micromechanical constitutive theory
Arenburg, Robert Thomas (Virginia Polytechnic Institute and State University, 1988)
The nonlinear behavior of continuous-fiber-reinforced metal-matrix composite structures is examined using a micromechanical constitutive theory. Effective lamina and laminate constitutive relations based on the Aboudi micromechanics theory are presented. The inelastic matrix behavior is modeled by the unified viscoplasticity theory of Bodner and Partom. The laminate constitutive relations are incorporated into a first-order shear deformation plate theory. The resulting boundary value problem is solved by utilizing the finite element method. · Computational aspects of the numerical solution, such as the temporal integration of the inelastic strains and the spatial integration of bending moments are addressed. Numerical results are presented which illustrate the nonlinear response of metal matrix composites subjected to extensional and bending loads. Experimental data from available literature are in good agreement with the numerical results.
Analysis of the wake behind a propeller using the finite element method with a two-equation turbulence model
Kim, Seung J. (Virginia Polytechnic Institute and State University, 1988)
The finite element method in the form of the weak Galerkin formulation with the penalty function method was applied to several problems of axisymmetric turbulent flows including flow through a sudden pipe expansion, the stern region flow of a slender body, and flows past ducted and nonducted propellers in action. The coupled set of the Reynolds time-averaged Navier-Stokes equations and two turbulence transport equations for the turbulent kinetic energy and its rate of dissipation was solved by L/U decomposition and successive substitution with relaxation. An existing finite element code was modified with a low Reynolds number form for an appropriate treatment of wall influences on turbulence transport, which produces a better solution and provides an easier imposition of boundary conditions by solving up to wall with no slip boundary conditions. The two-equation turbulence model with the wall modification was first successfully tested by solving the turbulent flow through a sudden pipe expansion. The numerical simulation of the stern region flow of a streamlined body resulted in an excellent agreement with the measured data in terms of the mean-flow and turbulence quantities. Turbulent shear flows past a propeller at the rear end of the same slender body, modeled by an actuator disk, were successfully solved at two rotational speeds, self-propelled and 100% over-thrusted, using the same two-equation model. And finally, comparisons of the wake behind a propeller were made for the self-propelled conditions of a ducted and nonducted propeller on the same streamlined body.
Analytical solutions for the statics and dynamics of rectangular laminated composite plates using shearing deformation theories
Khdeir, Ahmed Adel (Virginia Polytechnic Institute and State University, 1986)
The Levy-type analytical solutions in conjunction with the state-space concept are developed for symmetric laminated composite rectangular plates. Combinations of simply-supported, free and clamped boundary conditions are considered. The solutions are obtained for the first-order and higher-order theories in predicting the transverse deflections and stresses. Numerical results are presented for various boundary conditions, aspect ratios, lamination schemes and different loadings. The developments of these theories accomplished in general coordinates allow one to fulfill both the invariance requirements and to derive the relevant equations in any convenient planar systems of coordinates. The dynamic response problems are analyzed in the framework of higher order theories where the effects of transverse normal stress and rotary inertia forces are evaluated. The comparison between the theories as well as previously reported results is reported.
Application of panel methods for subsonic aerodynamics
Kim, Meung Jung (Virginia Polytechnic Institute and State University, 1985)
Several panel methods are developed to model subsonic aerodynamics. The vorticity panel method for two-dimensional problems is capable of handling general unsteady, potential, lifting flows. The lifting surface is modelled with a vortex sheet and the wakes by discrete vortices. As an imitation of the conditions at the trailing edge, stagnation conditions on both surfaces are used. The over-determined system is solved by an optimization scheme. The present predictions are in good agreement with experimental data and other computations. Moreover the present approach provides an attractive alternative to those developed earlier. Two panel methods for three-dimensional nonlifting problems are developed. One uses source distributions over curved elements and the other vorticity distributions over flat elements. For the source formulation, the effect of weakly nonlinear geometry on the numerical results is shown to accelerate the convergence of numerical values in general. In addition, the extensive comparisons between two formulations reveal that the voticity panel method is even more stable and accurate than the curved source panel method. Another vorticity panel method is developed to study the lifting l flows past three-dimensional bodies with sharp edges. The body is modelled by single vortex sheet for thin bodies and two vortex sheets for thick bodies while the wakes are modelled with a number of strings of discrete vortices. The flows are assumed to separate along the the sharp edges. The combination of continuous vorticity on the lifting surface and discrete vortices in the wakes yields excellent versatility and the capability of handling the tightly rolled wakes and predicting continuous pressure distributions on the lifting surface. The method is applied to thin and thick low-aspect-ratio delta wings and rectangular wings. The computed aerodynamic forces and wake shapes are in quantitative agreement with experimental data and other computational results.
Calculation of skin-stiffener interface stresses in stiffened composite panels
Cohen, David (Virginia Polytechnic Institute and State University, 1987)
A method for computing the skin-stiffener interface stresses in stiffened composite panels is developed. Both geometrically linear and nonlinear analyses are considered. Particular attention is given to the flange termination region where stresses are expected to exhibit unbounded characteristics. The method is based on a finite-element analysis and an elasticity solution. The finite-element analysis is standard, while the elasticity solution is based on an eigenvalue expansion of the stress functions. The eigenvalue expansion is assumed to be valid in the local flange termination region and is coupled with the finite-element analysis using collocation of stresses on the local region boundaries. In the first part of the investigation the accuracy and convergence of the local elasticity solution are assessed using a geometrically linear analysis. It is found that the finite-element/local elasticity solution scheme produce a very accurate interface stress representation in the local flange termination region. The use of 10 to 15 eigenvalues, in the eigenvalue expansion series, and 100 collocation points results in a converged local elasticity solution. In the second part of the investigation, the local elasticity solution is extended to include geometric nonlinearities. Using this analysis procedure, the influence of geometric nonlinearities on skin-stiffener interface stresses is evaluated. It is found that in flexible stiffened skin structures, which exhibit out-of-plane deformation on the order of 2 to 4 times the skin thickness, inclusion of geometrically nonlinear effects in the calculation of interface stresses is very important. Thus, the use of a geometrically linear analysis, rather than a nonlinear analysis, can lead to considerable error in the computation of the interface stresses. Finally, using the analytical tool developed in this investigation, it is possible to study the influence of stiffener parameters on the state of interface stresses.
Comparison Of Flow Birefringence Data with A Numerical-Simulation Of The Hole Pressure
Baird, Donald G.; Read, M. D.; Reddy, Junuthula N. (AIP Publishing, 1988-08-01)
The penalty_Galerkin finite_element method is used to simulate the flow of a polystyrene melt over a rectangular slot placed perpendicular to the flow direction. The White_Metzner constitutive equation is used with a Carreau model viscosity function and a shear rate_dependent relaxation time defined so that the primary normal stress difference is exactly reproduced by the model in simple shear flow. Values of the stress field predicted by the simulation are compared with those obtained experimentally by means of flowbirefringence. As observed by others, the limiting elasticity value as determined by the Weissenberg number (We) for convergence of the algorithm decreased with increased refinement of the mesh. However, good agreement is still found between predicted values of stress using the coarse mesh and those measured by means of flowbirefringence. This work suggests that there may be an optimum mesh for a given flow and constitutive equation which will still give physically realistic results. The Weissenberg number for the melt used in the experimental study asymptotically approached a value of about 1.5 with increasing shear stress, suggesting that it may not be necessary to reach excessively high values of We for simulations involving some polymer melts.
Computational aspects of sensitivity calculations in linear transient structural analysis
Greene, William H. (Virginia Polytechnic Institute and State University, 1989)
A study has been performed focusing on the calculation of sensitivities of displacements, velocities, accelerations, and stresses in linear, structural, transient response problems. One significant goal of the study was to develop and evaluate sensitivity calculation techniques suitable for large-order finite element analyses. Accordingly, approximation vectors such as vibration mode shapes are used to reduce the dimensionality of the finite element model. Much of the research focused on the accuracy of both response quantities and sensitivities as a function of number of vectors used. Two types of sensitivity calculation techniques were developed and evaluated. The first type of technique is an overall finite difference method where the analysis is repeated for perturbed designs. The second type of technique is termed semianalytical because it involves direct, analytical differentiation of the equations of motion with finite difference approximation of the coefficient matrices. To be computationally practical in large-order problems, the overall finite difference methods must use the approximation vectors from the original design in the analyses of the perturbed models. In several cases this fixed mode approach resulted in very poor approximations of the stress sensitivities. Almost all of the original modes were required for an accurate sensitivity and for small numbers of modes, the accuracy was extremely poor. To overcome this poor accuracy, two semi-analytical techniques were developed. The first technique accounts for the change in eigenvectors through approximate eigenvector derivatives. The second technique applies the mode acceleration method of transient analysis to the sensitivity calculations. Both result in accurate values of the stress sensitivities with a small number of modes. In both techniques the computational cost is much less than would result if the vibration modes were recalculated and then used in an overall finite difference method.
Crack growth in unidirectional composites using singular finite elements and interactive computer graphics
Choksi, Gaurang Nalin (Virginia Polytechnic Institute and State University, 1988)
Graphical simulation of crack growth using singular finite elements and interactive computer graphics is presented. The study consists of two main parts : (i) the formulation and application of an anisotropic singular element (ASE) for analyzing homogeneous anisotropic materials with cracks and, (ii) graphical simulation of crack growth in unidirectional composites. Lekhnitskii’s stress function method is used to formulate the traction-free crack boundary value problem with the stress function expressed in a Laurent series. The geometry of the element is arbitrary. The development of the stiffness matrix for general anisotropic materials is presented and it is shown how the singular element can be incorporated into a conventional displacement based finite element program. The anisotropic singular element (ASE) developed is implemented to analyze cracked anisotropic materials subjected to inplane loading. A 2-D, displacement based linite element code is used and center cracked on- and off-axis coupons under tensile loading are analyzed using the element developed. A general, interactive menu driven program is developed to track crack growth in composite materials. PHIGS (Programmers Hierarchical Interactive Graphics System) is used as the application program interface to integrate the finite element program with interactive graphics. Simulation studies are performed for center cracked on- and off-axis Iaminae using the normal stress ratio theory as the crack propagation criterion. The direction of crack propagation and values of the crack initiation stresses predicted are in reasonable agreement with the experimental values for the cases analyzed.
Delamination initiation in postbuckled dropped-ply laminates
Dávila, Carlos G. (Virginia Tech, 1991-04-06)
The compression strength of dropped-ply, graphite-epoxy laminated plates for the delamination mode of failure is studied by analysis and corroborated with experiments. The nonlinear response of the test specimens is modeled by a geometrically nonlinear finite element analysis. The methodology for predicting delamination is based on a quadratic interlaminar stress criterion evaluated at a characteristic distance from the ply drop-off. The details of the complex state of stress in the region of the thickness discontinuity are studied using three-dimensional solid elements, while the uniform sections of the plate are modeled with quadrilateral shell elements. A geometrically nonlinear transition element was developed to couple the shell elements to the solid elements. The analysis was performed using the COmputational MEchanics Testbed (COMET), an advanced structural analysis software environment developed at the NASA Langley Research Center to provide a framework for research in structural analysis methods. Uniaxial compression testing of dropped-ply, graphite-epoxy laminated plates has confirmed that delamination along the interfaces above and/or below the dropped plies is a common mode of failure initiation. The compression strength of specimens exhibiting a linear response is greater than the compression strength of specimens with the same layup exhibiting geometrically nonlinear response. Experimental and analytical results also show a decrease in laminate strength with increasing number of dropped plies. For linear response there is a large decrease in compression strength with increasing number of dropped plies. For nonlinear response there is less of a reduction in compression strength with increasing number of dropped plies because the nonlinear response causes a redistribution and concentration of interlaminar stresses toward the unloaded edges of the laminate.
Dynamic response of cross-ply laminated shallow shells according to a refined shear deformation theory
Reddy, Junuthula N.; Khdeir, Ahmed A. (Acoustical Society of America, 1989-06-01)
The dynamic response of cross-ply laminated shallow shells is investigated using the third-order shear deformation shell theory of Reddy [J. Appl. Mech. 4 1, 47 (1984)]. The theory accounts for cubic variation of the in-plane displacements through the thickness and does not require shear correction coefficients. The state-space approach is used to develop the analytical solutions of simply supported, cross-ply shells using the classical, first-order, and higher-order theories. The use of the separation of variables technique for the higher-order theory is also presented. Numerical results of the higher-order theory for center deflection and normal stresses of spherical shells under various loadings are compared with those obtained using the classical and first-order [or Sanders, Q. Appl. Math. 2 1, 21-36 (1963)] shell theories. Copyright 1989 Acoustical Society of America
The effect of moisture gradients on the stiffness and strength of yellow-poplar
Conners, Terrance E. (Virginia Polytechnic Institute and State University, 1985)
Wood with a uniform moisture distribution is known to have different mechanical properties compared to wood with a non-uniform moisture distribution. Moisture gradients are likely to develop in full-size members tested in the In-Grade Testing Program and might therefore affect the test results. The purpose of this study was to mathematically model the effect of desorption moisture gradients on the stiffness and strength of yellow-poplar beams. An additional objective was to experimentally determine gradient effects in yellow-poplar beams. Three-dimensional finite-element modeling was employed and several subsidiary models were developed. Among these was a three-parameter segmented model for fitting digitized tension and compression stress-strain curves. Unlike previous models (such as the Ramberg-Osgood model), this model has a linear slope up to the point approximately corresponding to the proportional limit. A methodology was also devised whereby most hardwood and softwood elastic constants can be estimated at any moisture content. Data are required at one moisture content. Equilibrated uniaxial testing was conducted at four moisture contents to acquire data for the finite-element model. It was found that the longitudinal Young's moduli in tension and compression were approximately equal at 6% and 18% moisture content; the compression modulus was greater at 12%, but the tension modulus was greater for green specimens. Intersection points for tension and compression mechanical properties may be different. Tests of small clear yellow-poplar beams indicated that moisture gradients induced at 12% equilibrium moisture content had little effect on the modulus of rupture up to 19% average moisture content. At higher moisture contents, gradient-containing beams were significantly stronger than equilibrated beams when comparisons were made at identical moisture contents. Modulus of elasticity data exhibited a similar trend, although differences between equilibrated and non-equilibrated beams were observed below 19% moisture content. The finite-element program was moderately successful in predicting the effects of moisture gradients on the strength and stiffness of yellow-poplar beams. Computer time and storage constraints limited the accuracy of the solutions. Predicted trends were verified by the experimental data. Modeling of full-size lumber indicated that significant moisture gradients will likely influence the stiffness and strength of higher quality lumber.
Effects of plasticizers on extrusion of PVC: an experimental & numerical study
Datta, Arindam (Virginia Polytechnic Institute and State University, 1989)
Plasticizers are often interchanged with the idea that they will not affect the processing behavior of Polyvinyl Chloride (PVC). However, when the plasticizer type is changed, various complaints are made by the processors that the material no longer processes the same. This research was concerned with the effect of three different plasticizers on the plasticating extrusion behavior of PVC. Di-isodecyl pthalate (DIDP), di-hexyl pthalate (DHP) and 2-ethyl hexyl pthalate (DOP) are the three plasticizers used in this study. First some differences in the extrusion performance of the three differently plasticized PVC compounds were identified. In particular, it was observed that pressure build-up, flow rate and power requirement were affected by the plasticizer type with the DIDP plasticized materials generating higher pressures and requiring more power than the other two plasticized materials. The differences in extrusion characteristics have been observed for two different dies (1/8 and 1/16 inch diameter) attached to the extruder. The differences were most significant between the DIDP and the DHP plasticized mixes. Factors which could influence the processing behavior of plasticized PVC include viscosity, compaction, thermal conductivity, specific heat, and friction coefficient. It was found that all other properties other than the viscosity were unaffected by the plasticizer type. On the other hand, viscosities were significantly affected by the plasticizer type with the DIDP plasticized materials displaying higher values between 160 and 190 °C. The difference in viscosity was larger between the DIDP and DHP plasticized materials than between DIDP and DOP plasticized materials. The differences in viscosity between DIDP and DOP plasticized materials tend to diminish considerably at 190 °C. Two flow regions characterized by different degrees of fusion above and below 165 °C were identified for the plasticized PVC compounds. Plasticized PVC exhibited yield stresses with the DIDP plasticized materials having higher values. The yield stresses were responsible for the significant difference in viscosity at lower shear rates. The yield stress was a more dominant feature at temperatures below 160 °C and this fact was made use of in modeling the solids conveying zone as a fluid with yield stress. Correlation was established between the viscosities and the extrusion behavior of the plasticized PVC compounds. It was observed that the DIDP plasticized mixes had higher viscosities, fused earlier in the screw channel, gave rise to higher pressures, required more power and in general exhibited higher flow rates at the same screw speed. The finite element method was used for the numerical simulations. Based on the experimental results, the numerical modeling of the melt zone was performed in order to predict the differences in the extrusion characteristics. The melt zones were modeled as a temperature dependent power law fluid having two different viscosity expressions above and below 165 °C. The numerical predictions for pressures and flow rates in the extruder with the 1/8 inch diameter die were in good agreement with the experimental results. For the case of the 1/16 inch diameter die attached to the extruder, the numerical and experimental flow rates were in good agreement but the pressure predictions, although indicating the correct trends, were off by 15 to 20% from the experimental results. In general the differences in the physical properties, viz. viscosities, were used to predict the differences in the pressure build-ups and flow rates. Also the solid conveying zone was modeled using a Herschel Bulkley model. It was possible to match the experimental and numerical results for the solids conveying zone by using an average density value for the entire solids conveying zone, but more work needs to be done in order to establish greater validity and applicability of this model.
An evaluation of classical and refined equivalent-single-layer laminate theories
Bose, Partha (Virginia Tech, 1995-12-18)
In this thesis, we study the static and free vibration response of symmetric and antisymmetric cross-ply laminated plates using different plate theories. Governing equations for two displacement-based third-order equivalent-single-layer theories have been developed. The first one is called the General Third-Order Theory (GTOT), and the second one is called the General Third-Order Theory of Reddy (GTTR). The displacement field of the second theory can be obtained from the first by imposing the condition of zero shear stresses at the bounding planes of the plate. The governing equations, analytical solutions, and finite element model of GTTR have been obtained in terms of tracers. Proceeding in this manner, the governing equations, analytical solutions, and finite element models of some lower-order plate theories fall out by just assigning appropriate values to the tracers (typically 1 or 0). While analytical and finite element solutions have been obtained for GTTR and its derivative cases, only finite element solutions have been obtained for GTOT. The analytical solutions are of two types. The Navier-type solution is for rectangular plates simply supported on all four edges. In the Levy-type solution, two sides of the plate have to be simply-supported, while the remaining two sides can have any combination of free, clamped, or simply-supported boundary conditions. The results obtained from the different theories have been compared with exact solutions from existing literature . The response characteristics of the plates, like deflections, stresses, and frequencies, as well as the parameters affecting them have been studied. Some of the parameters investigated are span-to-thickness ratios, boundary conditions, loadings, and lamination schemes. The performance of the different theories in predicting plate responses have been evaluated.
An experimental and analytic study of earth loads on rigid retaining walls
Filz, George M. (Virginia Tech, 1992-04-01)
Experimental and analytic investigations were performed to examine the influences of wall height, backfill behavior, and compaction on the magnitudes of backfill loads on rigid retaining walls. Measurements of lateral and vertical backfill loads were made during tests using the Virginia Tech instrumented retaining wall facility. The tests were performed with two soils, moist Yatesville silty sand and dry Light Castle sand. Two hand-operated compactors, a vibrating plate compactor and a rammer compactor, were used to compact the backfill. The backfill height was 6.5 feet in all of the tests. Analyses of backfill loads were made using a compaction- induced lateral earth pressure theory and a vertical shear force theory. The compaction-induced lateral earth pressure theory was revised from a previous theory. The revisions improved the accuracy with which the theory models the hysteretic stress behavior of the backfill during compaction. The theory was also extended to include the pore pressure response of moist backfill in a rational manner. A vertical shear force theory was also developed during this research. The theory is based on consideration of backfill compressibility and mobilization of interface shear strength at the contact between the backfill and the wall. The theory provides a useful basis for understanding how wall height, backfill compressibility, wall-backfill interface behavior, and compaction-induced lateral pressures affect the vertical shear forces on rigid walls. Studies were also made to investigate the cause of erratic pressure cell readings. An important cause of drift in pressure cell readings was found to be moisture changes in the concrete in which the pressure cells were mounted. It was found that this problem could be mitigated by applying a water-seal treatment to the face of the wall. Both the vibrating plate compactor and the rammer compactor were instrumented to measure dynamic forces and energy transfer during compaction. The force applied by the vibrating plate compactor was about one-quarter of the manufacturer’s rated force. The force applied by the rammer compactor was about twice the manufacturer’s rated force. The transferred energy measurements provided a basis for relating laboratory and field compaction procedures.
Experimental and finite-element investigation of flow past single and multiple cylinders
Dhaubhadel, Manoranjan N. (Virginia Polytechnic Institute and State University, 1986)
Fluid flows past single and multiple cylinders in different configurations are investigated both experimentally and numerically. Three and five in-line cylinders and in-line and staggered bundles of cylinders with different pitch-to-diameter ratios are considered. Experimental work comprises of laser-Doppler velocimetry and flow visualization obtained in a water tunnel and skin friction, pressure, lift, drag and hot-wire measurements obtained in a wind tunnel. Both steady and pulsed flows are considered. Numerical work consists of finite element analysis of Navier-Stokes and energy equations governing viscous fluid flow past single and multiple cylinders. Detailed measurements of the fluid dynamic quantities for flow past cylinders reveal that flow pulsation at frequencies which induce lock-on increases the organization of the flow in gaps between cylinders. A new pattern of flow field is found for flow past a triad and a pentad of cylinders with a pitch-to-diameter ratio of l.8. The numerical analysis generates important integral characteristics like flow resistance and heat transfer. A staggered square arrangement of finite bundle of cylinders is found to have better heat transfer characteristics compared to the in-line or staggered equilateral-triangular arrangements.
An experimental study of frictional phenomena around the pin joints of plates using moire interferometry
Joh, Duksung (Virginia Polytechnic Institute and State University, 1986)
Although contact problems with friction have received considerable attention in recent years, analytical as well as experimental limitations have tended to obscure some of their essential features. All the experimental techniques employed in the past lacked either required sensitivity or adequate spatial resolution for local measurements of deformation near the contact surface. Further, most techniques also did not allow the use of prototype material which is crucial for investigation of contact stress problems with friction. In the present study, a relatively new experimental method, which has been developed at VPI & SU, is employed: high-sensitivity moiré interferometry. Using a clearance pin-joint model made of prototype structural material, Aluminum 7071-T6, studies on frictional phenomena between the pin and plate are conducted to provide a comprehensive treatment of the following subjects: slip-stick phenomena, variation of contact zone, distribution of frictional force at the contact region, effects of frictional shear stress on stress concentration, and identification of slip amplitude. Experimental techniques and algorithms of analysis necessary for the research are further developed. The results showed a strong influence of friction, including significant differences in the load-increasing and load-decreasing phases.
Failure processes in unidirectional composite materials
Sundaresan, Mannur J. (Virginia Polytechnic Institute and State University, 1988)
Failure processes in unidirectional composite materials subjected to quasi-static tensile load along the fiber direction are investigated. The emphasis in this investigation is to identify the physical processes taking place during the evolution of failure in these materials. An extensive literature review is conducted and the information relevant to the present topic is summarized. The nature of damage growth in five different commercially available composite systems are studied. In-situ scanning electron microscopy is employed for identifying the failure events taking place at the microscopic level. Acoustic emission monitoring is used for estimating the rate of damage growth on a global scale and determining the size of individual failure events. The results of this study have shown the important roles of the matrix material and the interphase in determining the tensile strength of unidirectional composite materials. Several failure modes occurring at the microscopic scale are revealed for the first time. Further, the results indicate that dynamic fracture participates to a significant extent in determining the failure process in these materials. Based on the results of this study the influence of various parameters in determining the composite strength is described.

Browsing by Author "Reddy, Junuthula N."

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