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Browsing by Author "Swift, George W."

Now showing 1 - 14 of 14
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    Compressive strength of lamina reinforced and fiber reinforced composite materials
    Davis, John G. (Virginia Tech, 1973-05-15)
    Results are presented from a theoretical and experimental investigation on the compressive strength of lamina reinforced and fiber reinforced composite materials when loaded parallel to the reinforcement. An analytical model which replaces the fiber reinforced composite with a laminate containing initially curved laminae has been proposed. By applying the Timoshenko beam equations to each layer of the laminate, an interlaminar shear stress analysis which can be used to predict the behavior of the laminate under compressive loading was developed. Two modes of failure are considered in the analysis, delamination and shear instability, and nonlinear shear stress-strain behavior of the laminae is included. Axial compression tests were performed on aluminum-wax laminates, boron-epoxy tubes and S-glass-epoxy tubes. In addition, torsion tests and combined compression and torsion tests were conducted on the fiber reinforced tubes. Coordinates of fibers in a boron-epoxy laminate were measured. Experimental results indicate that the aluminum-wax laminates failed by delamination and that failure of the boron-epoxy composite in compression is most likely due to shear instabIlity. In addition it was shown that the shear modulus of boron-epoxy is a function of axial compressive stress and that the fibers in a boron-epoxy composite are not parallel but contain initial curvature. Adequate correlation between theory and experiment was obtained for both lamina and fiber reinforced test results.
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    The constitutive modeling of shape memory alloys
    Liang, Chen (Virginia Tech, 1990-08-15)
    This dissertation presents a one-dimensional thermomechanical constitutive model for shape memory alloys based on basic concepts of thermodynamics and phase transformation kinetics. Compared with other developed constitutive relations, this thermomechanical constitutive relation not only reflects the physical essence of shape memory alloys, i.e., the martensitic phase transformation involved, but also provides an easy-to-use design tool for engineers. It can predict and describe the behavior of SMA quantitatively. A multi-dimensional constitutive relation for shape memory alloys is further developed based on the one-dimensional model. It can be used to study the mechanical behavior including shape memory effect of complex SMA structures that have never been analytically studied, and provide quantitative analysis for many diverse applications of shape memory alloys. A general design method for shape memory alloy actuators has also been developed based on the developed constitutive relation and transient thermal considerations. The design methodology provides a quantitative approach to determine the design parameters of shape memory alloy force actuators, including both bias spring SMA force actuators and differential SMA force actuators.
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    Effective methods of controlling a junction vortex system in an incompressible, three-dimensional, turbulent flow
    Shin, Jaiwon (Virginia Polytechnic Institute and State University, 1989)
    An experimental investigation was conducted to find effective methods of controlling a junction vortex system around and in the wake of a NACA 0020 body with a 1.5:1 elliptical nose mounted normal to the flat surface. Control of the junction vortex system was investigated using more slender noses and leading edge fillets. The study includes surface flow visualizations, and extensive mean flow measurements. The mean flow measurements consist of velocity, total pressure, and static pressure distributions. The results of the mean flow measurements around and in the wake of a NACA 0020 body with a 1.5:1 elliptical nose show the presence of a large, dominant vortex, or a primary horseshoe vortex, in the junction between the body and the flat surface. A smaller, co-rotating vortex relative to the primary horseshoe vortex was observed at the 100 percent chord position, which is believed to be identified with the corner separation at the junction near the trailing edge. Mean flow measurements of the flow field around and in the wake of a NACA 0020 body with more slender noses were acquired. The results indicate that the more slender nose generated a weaker primary horseshoe vortex. The results also suggest that the weak primary horseshoe vortex developed by the more slender nose was dissipated alongside the body to the point where it had a very small effect on the flow field at the 100 percent chord position. The results of the mean flow measurements around and in the wake of a NACA 0020 body with a 1.5:1 elliptical nose with leading edge fillets indicate that the leading edge fillets were very effective in reducing the strength and size of the primary horseshoe vortex. The results suggest that an effective fillet should be about one boundary layer thickness high and two body thicknesses long. The vorticity identified with the corner separation vortex was observed in the wake for both leading edge shape changes and fillets. The results show that the corner separation vortex dominates the flow in the wake due to the weak effects of the primary horseshoe vortex. A trailing edge fillet was used to investigate its effects on the corner separation vortex in the wake of the NACA 0020 body with the 1.5:1 elliptical nose with a leading edge fillet. The results indicate a small decrease in the strength of the comer separation vortex at the 150 percent chord position, but the effects on the wake pattern were small.
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    An experimental and analytical investigation of liquid moisture distribution in roof insulating systems
    Woodbury, Keith Auburn (Virginia Polytechnic Institute and State University, 1984)
    An experimental investigation was carried out to determine the feasibility of using thermal conductivity measurements to detect moisture concentrations in a highly porous glass fiber insulation. A new technique employing thermistor probes was used to measure thermal conductivity over a range of low moisture contents. The results indicate that the material's thermal conductivity is a strong nonlinear function of the moisture concentration. The sensitivity of the moisture content to thermal conductivity is greatest for moisture contents less than 25 per cent for the material tested. A numerical procedure for predicting the temperature and moisture distributions in a highly porous material is detailed.
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    Extension of the finite volume method to laminar and turbulent flow
    Nicholson, Stephen (Virginia Polytechnic Institute and State University, 1986)
    A method has been developed which calculates two-dimensional, transonic, viscous flow in ducts. The finite volume, time marching formulation is used to obtain steady flow solutions of the Reynolds-averaged form of the Navier Stokes equations. The entire calculation is performed in the physical domain. The method is currently limited to the calculation of attached flows. The features of the current method can be summarized as follows. Control volumes are chosen so that smoothing of flow properties, typically required for stability, is not needed. Different time steps are used in the different governing equations to improve the convergence speed of the viscous calculations. A new pressure interpolation scheme is introduced which improves the shock capturing ability of the method. A multi-volume method for pressure changes in the boundary layer allows calculations which use very long and thin control volumes (length/height ≅ 1000). A special discretization technique is also used to stabilize these calculations which use long and thin control volumes. A special formulation of the energy equation is used to provide improved transient behavior of solutions which use the full energy equation. The method is then compared with a wide variety of test cases. The freestream Mach numbers range from 0.075 to 2.8 in the calculations. Transonic viscous flow in a converging diverging nozzle is calculated with the method; the Mach number upstream of the shock is approximately 1.25. The agreement between the calculated and measured shock strength and total pressure losses is good. Essentially incompressible turbulent boundary layer flow in an adverse pressure gradient is calculated and the computed distribution of mean velocity and shear stress are in good agreement with the measurements. At the other end of the Mach number range, a flat plate turbulent boundary layer with a freestream Mach number of 2.8 is calculated using the full energy equation; the computed total temperature distribution and recovery factor agree well with the measurements when a variable Prandtl number is used through the boundary layer.
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    Freeze-thaw durability of high strength silica fume concrete
    Kashi, Mohsen Gholam-Reza (Virginia Polytechnic Institute and State University, 1988)
    Specimens from 27 batches of concrete with water to cementitious (cement plus silica fume) ratio of 0.25 to 0.32, with and without entrained air, were tested for freeze-thaw durability in accordance with ASTM C666, procedure A (freezing and thawing in water). In addition, another set of similar specimens were moist cured for 28 days instead of 14 days and tested in accordance with ASTM C666 , Procedure A to determine the effect of curing time on the freeze-thaw durability of high strength concrete. Results show that non air-entrained high strength concrete with water cementitious ratio of less than 0.30, regardless of the length of curing time, is frost resistant. Non-air-entrained concrete with water-cement ratio of 0.32 is also durable if silica fume is not used.
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    In-plane vibration of a plate having an elliptical hole of arbitrary eccentricity
    Cooke, Robert Field (Virginia Tech, 1975-04-05)
    The in-plane vibration of a plate with an elliptic hole is studied. It is shown that standing waves whose wavelength is the same order of magnitude as the size of the hole are theoretically capable of causing microcracks which have been observed experimentally. Several approaches were used including reduction of the mixed boundary value problem to a Fredholm equation, and to a matrix eigenvalue problem. Contour curves of various stresses and displacements were obtained numerically. A new technique was developed for the solution of the wave equation appropriate for boundary conditions on an elliptical surface.
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    The reflection method in the bending of beams and plates
    Eskridge, Charles DeWitt (Virginia Tech, 1964-05-05)
    The problem of determining the deflection and stress in a plate under transverse loading can be approached by first considering the plate to be a portion of an infinite plate, ignoring the prescribed boundary conditions. The known loads are then applied to the infinite plate and their effects are calculated at those points which correspond to the boundary of the original plate. A system of suitably chosen loads and moments is then applied on the infinite plate at points beyond the boundary of the original plate such that the prescribed boundary conditions are satisfied. For an exact solution, the number of external loads and moments would have to be infinite. However, in order to deal with the problem numerically, only a finite number of each are considered. Thus, solutions are obtained by satisfying the boundary conditions at only a finite number of points. The method is illustrated for beams and then extended to plates. Several problems with known solutions are solved and the results compared with the exact values. Also, plots of the deflection and moment along the centerline of a cantilevered triangular plate are presented. Discussions of the problem of plates with holes and the effect on the solution of various placements balancing loads are also presented. An IBM 1620 digital computer is used to facilitate calculations
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    A response surface for the complex modulus of composite materials
    Arthur, Charles E. (Virginia Polytechnic Institute and State University, 1974)
    The significance of the role of advanced composite materials in many engineering applications is increasing.Environmental effects on the material properties of composites is an important aspect of design considerations . To determine the long term influence of the individual contributions of time, temperature, and humidity on composite materials would require extensive numbers of specimens an:f tests. In order to isolate significant variables experiments have been designed for maximum utilization of specimens. Due to wide variation in the information obtained from experimentation, a statistical analysis of the data was conducted. Multiple regression techniques were employed and the significance of the individual variables was tested. This experimental program resulted in the development of response surfaces for the complex moduli of composite materials.
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    Spectral estimates and flow characteristics from non-uniformly sampled LDV data in a turbulent junction vortex
    Nath, Subhra K. (Virginia Polytechnic Institute and State University, 1989)
    The strongly time variant flow in an incompressible, turbulent junction vortex formed at the base of a streamlined cylinder with a circular leading edge placed normal to a flat surface was investigated. The investigation centered around spectral analysis and time resolved measurements of the velocity fluctuations to characterize the time variant flow on the plane of symmetry. All the measurements were performed with a two-color, two-component, frequency shifted laser Doppler velocimeter. Spectral analysis methods for randomly sampled data occurring from the LDV were evaluated under various simulated and real flow situations. The real flow situations studied were the vortex shedding flow behind a cylinder and the two-dimensional turbulent boundary layer. The spectral estimates obtained from the discretized lag product method were found to be better than those obtained from the direct transform method. It was found that the exact lag product method does not offer significant improvements in the spectral estimates to offset its computational slowness. The mean velocity vectors in the junction vortex showed a single vortex on the plane of symmetry and a singular separation point upstream of the cylinder. The time resolved measurements showed the instantaneous separation point on the plane of symmetry to be randomly oscillating between two limits. Maximum possible excursions of the junction vortex position and size were also obtained form the time resolved measurements. The turbulence intensities in the junction vortex were found to be at least two to three times higher than typical two-dimensional boundary layer values. The histograms of instantaneous velocity fluctuations deviated from the expected Gaussian distributions and were found to have multiple peaks. The spectral content of the junction vortex flow was investigated. The overall character of the junction vortex flow was found to be similar to a two-dimensional turbulent boundary layer, with greater amplification perceived in the lower frequencies relative to the higher frequencies. The spectra at locations above the time mean center of the junction vortex showed distinct peaks around 20-30 Hz, unlike boundary layer flows.
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    A study of active control techniques for noise reduction in an aircraft fuselage model
    Jones, James D. (Virginia Polytechnic Institute and State University, 1987)
    A simplified cylindrical model is used to investigate the elementary mechanisms of control of sound transmission into aircraft cabins by two active control techniques: propeller synchrophasing and active vibration control. Propeller synchrophasing involves controlling the relative rotational phase of the engines to achieve maximum cabin noise reduction. Active vibration control involves structurally controlling the vibrational response of the cabin wall to reduce the important modes which transmit their energy into the cabin. Noise reductions for harmonic excitation at acoustic cavity resonance are shown to be in excess of 20 dB throughout most of the cavity whether synchrophasing or active vibration control is used. Off-resonance reductions are substantially less due to increased modal density requiring a larger number of actuators for effective control of the complex sound field. Additional studies were performed using synchrophasing in conjunction with active vibration control to study their joint capabilities in controlling complex sound fields. The dual control system displayed improved control performance with noise reductions on the order of 25-35 dB and a more uniform sound field. Also, the complementary control characteristics of the system clearly demonstrated effective control of orthogonal acoustic modes of the cavity. However, the improved effectiveness of the control system was dependent upon judiciously positioning the actuators for optimal control of the sound field. An independent study was performed to identify the effects of a complex geometry on sound transmission into an aircraft fuselage model interior. For this study, a geometrically scaled cabin floor was installed in the unstiffened test cylinder to investigate the structural and acoustic influence of the simulated cabin floor. Results indicated that the stiffening of the cylindrical model associated with insertion of the floor strongly influenced the structural response of the cylinder but generally had little effect on the coupled pressure response. Conversely, the modification of the interior acoustic cavity tended to have little influence on the cylinder response but substantially reduced the coupled pressure response. Thus, this investigation identified the fundamental mechanisms of control of sound transmission into simplified models of aircraft fuselages by active control techniques.
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    Turbomachinery cascade and wake calculation for two-dimensional compressible laminar and turbulent flow
    Micklow, Gerald J. (Virginia Polytechnic Institute and State University, 1989)
    A method is presented for the efficient analytical prediction of the two dimensional laminar or turbulent compressor or turbine cascade blade-to-blade flow field and wake. The scheme utilizes a viscous-inviscid interaction routine combining a two-dimensional full potential cascade flow solver with a two-dimensional compressible boundary layer analysis. The boundary layer analysis can compute in the direct mode with pressure gradient specified or in the inverse mode with "boundary layer mass flux" specified. When calculating with the inverse mode, flow separation can be handled easily. Turbulent flow is treated using an algebraic eddy viscosity model with the modified Levy—Lees transformation applied to capture the growth of laminar and turbulent boundary layers. The boundary layer solution is fully implicit and formally second order accurate. The viscous inviscid coupling is performed utilizing thin airfoil theory. Numerical solutions are presented for several numerical test cases and compared with published test data.
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    A viscoelastic model for the response of a step change in velocity of the human otolith organs
    Cotton, John R. (Virginia Tech, 1989-11-15)
    The otolith organs are housed in the inner ear and are responsible for sensing accelerations and gravity. These inertial sensing systems are modeled in this thesis as a three material system consisting of a rigid otoconial plate attached to the skull by a gel layer, surrounded by a viscous endolymph fluid. The gel layer is considered to be a viscoelastic solid, and modeled as a simple Kelvin element. The governing differential equations are derived and nondimensionalized, yielding three nondimensional parameters: nondimensional density, R, nondimensional viscosity, M, and nondimensional elasticity, 6. The equations are solved using uÌ nite difference techniques on a digital computer. By comparing the modelâ s response with previous biological research, values for the nondimensional parameters are found. The value of R is 0.75 and the value of 6 is between 0.3 and 0.075. While the value of M is placed between 5. and 10., results indicate that to properly model the long time response of the otolith, a single and constant value for viscosity is not feasible.