VTechWorks Repository :: Browsing by Author "Ragab, Saad A."

Browsing by Author "Ragab, Saad A."

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Active/Passive Controls and Energy Harvesting from Vortex-Induced Vibrations
Mehmood, Arshad (Virginia Tech, 2013-10-17)
Fluid-structure interactions occur in many engineering and industrial applications. Such interactions may result in undesirable forces acting on the structure that may cause fatigue and degradation of the structural components. The purpose of this research is to develop a solver that simulates the fluid-structure interaction, assess tools that can be used to control the resulting motions and analyze a system that can be used to convert the structure's motion to a useful form of energy. For this purpose, we develop a code which encompasses three-dimensional numerical simulations of a flow interacting with a freely-oscillating cylinder. The solver is based on the accelerated reference frame technique (ARF), in which the momentum equations are directly coupled with the cylinder motion by adding a reference frame acceleration term; the outer boundary conditions of the flow domain are updated using the response of the cylinder. We develop active linear and nonlinear velocity feedback controllers that suppress VIV by directly controlling the cylinder's motion. We assess their effectiveness and compare their performance and required power levels to suppress the motion of the cylinder. Particularly, we determine the most effective control law that requires minimum power to achieve a desired controlled amplitude. Furthermore, we investigate, in detail, the feasibility of using a nonlinear energy sink to control the vortex-induced vibrations of a freely oscillating circular cylinder. It has been postulated that such a system, which consists of a nonlinear spring, can be used to control the motion over a wide range of frequencies. However, introducing an essential nonlinearity of the cubic order to a coupled system could lead to multiple stable solutions depending on the initial conditions, system's characteristics and parameters. Our investigation aims at determining the effects of the sink parameters on the response of the coupled system. We also investigate the extent of drag reduction that can be attained through rotational oscillations of the circular cylinder. An optimization is performed by combining the CFD solver with a global deterministic optimization algorithm. The use of this optimization tool allows for a rapid determination of the rotational amplitude and frequency domains that yield minimum drag. We also perform three-dimensional numerical simulations of an inline-vibrating cylinder over a range of amplitudes and frequencies with the objective of suppressing the lift force. We compare the amplitude-frequency response curves, levels of lift suppression, and synchronization maps for two- and three-dimensional flows. Finally, we evaluate the possibility of converting vortex-induced vibrations into a usable form of electric power. Different transduction mechanisms can be employed for converting these vibrations to electric power, including electrostatic, electromagnetic, and piezoelectric transduction. We consider the piezoelectric option because it can be used to harvest energy over a wide range of frequencies and can be easily implemented. We particularly investigate the conversion of vortex-induced vibrations to electric power under different operating conditions including the Reynolds number and load resistance.
Adaptation of Nontraditional Control Techniques to Nonlinear Micro and Macro Mechanical Systems
Daqaq, Mohammed F. (Virginia Tech, 2006-07-28)
We investigate the implementation of nontraditional open-loop and closed-loop control techniques to systems at the micro and macro scales. At the macro level, we consider a quay-side container crane. It is known that the United States relies on ocean transportation for 95% of cargo tonnage that moves in and out of the country. Each year over six million loaded marine containers enter U.S. ports. Current growth predictions indicate that container cargo will quadruple in the next twenty years. To cope with this rapid growth, we develop a novel open-loop input-shaping control technique to mitigate payload oscillations on quay-side container cranes. The proposed approach is suitable for automated crane operations, does not require any alterations to the existing crane structure, uses the maximum crane capabilities, and is based on an accurate two-dimensional four-bar-mechanism model of a container crane. The shaped commands are based on a nonlinear approximation of the two-dimensional model frequency and, unlike traditional input-shaping techniques, our approach can account for large hoisting operations. For operator-in-the-loop crane operations, we develop a closed-loop nonlinear delayed-position feedback controller. Key features of this controller are that it: does not require major modifications to the existing crane structure, accounts for motion inversion delays, rejects external disturbances, and is superimposed on the crane operator commands. To validate the controllers, we construct a 1:10 scale model of a 65-ton quay-side container crane. The facility consists of a 7-meter track, 3.5-meter hoisting cables, a trolley, a traverse motor, two hoisting motors, and a 50-pound payload. Using this setup, we demonstrated the effectiveness of the controllers in mitigating payload oscillations in both of the open-loop and closed-loop modes of operation. At the micro level, we consider a micro optical device known as the torsional micromirror. This device has a tremendous number of industrial and consumer market applications including optical switching, light scanning, digital displays, etc. To analyze this device, we develop a comprehensive model of an electrically actuated torsional mirror. Using a Galerkin expansion, we develop a reduced-order model of the mirror and verify it against experimental data. We investigate the accuracy of representing the mirror using a two-degrees-of-freedom lumped-mass model. We conclude that, under normal operating conditions, the statics and dynamics of the mirror can be accurately represented by the simplified lumped-mass system. We utilize the lumped-mass model to study and analyze the nonlinear dynamics of torsional micromirrors subjected to combined DC and resonant AC excitations. The analysis is aimed at enhancing the performance of micromirrors used for scanning applications by providing better insight into the effects of system parameters on the microscanner's optimal design and performance. Examining the characteristics of the mirror response, we found that, for a certain DC voltage range, a two-to-one internal resonance might be activated between the first two modes. Due to this internal resonance, the mirror exhibits complex dynamic behavior. This behavior results in undesirable vibrations that can be detrimental to the scanner performance. Torsional micromirrors are currently being implemented to provide all-optical switching in fiber optic networks. Traditional switching techniques are based on converting the optical signal into electrical signal and back into optical signal before it can be switched into another fiber. This reduces the rate of data transfer substantially. To realize fast all-optical switching, we enhance the transient dynamic characteristics and performance of torsional micromirrors by developing a novel technique for preshaping the voltage commands applied to activate the mirror. This new approach is the first to effectively account for inherent nonlinearities, damping effects, and the energy of the significant higher modes. Using this technique, we are able to realize very fast switching operations with minimal settling time and almost zero overshoot.
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.
Adaptive Process Control for Achieving Consistent Mean Particles' States in Atmospheric Plasma Spray Process
Guduri, Balachandar (Virginia Tech, 2022-02-08)
The coatings produced by an atmospheric plasma spray process (APSP) must be of uniform quality. However, the complexity of the process and the random introduction of noise variables such as fluctuations in the powder injection rate and the arc voltage make it difficult to control the coating quality that has been shown to depend upon mean values of powder particles' temperature and speed, collectively called mean particles' states (MPSs), just before they impact the substrate. Here we use a science-based methodology to develop an adaptive controller for achieving consistent MPSs. We first identify inputs into the APSP that significantly affect the MPSs, and then formulate a relationship between these two quantities. When the MPSs deviate from their desired values, the adaptive controller based on the model reference adaptive controller (MRAC) framework is shown to successfully adjust the input parameters to correct them. The performance of the controller is tested via numerical experiments using the software, LAVA-P, that has been shown to well simulate the APSP. The developed adaptive process controller is further refined by using sigma (Ïƒ) adaptive laws and including a low-pass filter that remove high-frequency oscillations in the output. The utility of the MRAC controller to achieve desired locations of NiCrAlY and zirconia powder particles for generating a 5-layered coating is demonstrated. In this case a pure NiCrAlY layer bonds to the substrate and a pure zirconia makes the coating top. The composition of the intermediate 3 layers is combination of the two powders of different mass fractions. By increasing the number of intermediate layers, one can achieve a continuous through-the-thickness variation of the coating composition and fabricate a functionally graded coating.
Aeroacoustic Study of a Model-Scale Landing Gear in a Semi-Anechoic Wind Tunnel
Remillieux, Marcel Christophe (Virginia Tech, 2007-03-19)
An aeroacoustic study was conducted on a 26%-scale Boeing 777 main landing gear in the Virginia Tech (VT) Anechoic Stability Wind Tunnel. The VT Anechoic Stability Wind Tunnel allowed noise measurements to be carried out using both a 63-elements microphone phased array and a linear array of 15 microphones. The noise sources were identified from the flyover view under various flow speeds and the phased array positioned in both the near and far-field. The directivity pattern of the landing gear was determined using the linear array of microphones. The effectiveness of 4 passive noise control devices was evaluated. The 26%-scale model tested was a faithful reproduction of the full-scale landing gear and included most of the full-scale details with accuracy down to 3 mm. The same landing gear model was previously tested in the original hard-walled configuration of the VT tunnel with the same phased array mounted on the wall of the test section, i.e. near-field position. Thus, the new anechoic configuration of the VT wind tunnel offered a unique opportunity to directly compare, using the same gear model and phased array instrumentation, data collected in hard-walled and semi-anechoic test sections. The main objectives of the present work were (i) to evaluate the validity of conducting aeroacoustic studies in non-acoustically treated, hard-walled wind tunnels, (ii) to test the effectiveness of various streamlining devices (passive noise control) at different flyover locations, and (iii) to assess if phased array measurements can be used to estimate noise reduction. As expected, the results from this work show that a reduction of the background noise (e.g. anechoic configuration) leads to significantly cleaner beamforming maps and allows one to locate noise sources that would not be identified otherwise. By using the integrated spectra for the baseline landing gear, it was found that in the hard-walled test section the levels of the landing gear noise were overestimated. Phased array measurements in the near and far-field positions were also compared in the anechoic configuration. The results showed that straight under the gear, near-field measurements located only the lower-truck noise sources, i.e. noise components located behind the truck were shielded. It was thus demonstrated that near-field, phased-array measurements of the landing gear noise straight under the gear are not suitable. The array was also placed in the far-field, on the rear-arc of the landing gear. From this position, other noise sources such as the strut could be identified. This result demonstrated that noise from the landing gear on the flyover path cannot be characterized by only taking phased array measurement right under the gear. The noise reduction potential of various streamlining devices was estimated from phased array measurements (by integrating the beamforming maps) and using the linear array of individually calibrated microphones. Comparison of the two approaches showed that the reductions estimated from the phased array and a single microphone were in good agreement in the far-field. However, it was found that in the near-field, straight under the gear, phased array measurements greatly overestimate the attenuation.
The Aerodynamics of Low Sweep Delta Wings
Rullan, Jose Miguel (Virginia Tech, 2008-04-21)
The aerodynamics of wings with moderately swept wings continues to be a challenging and important problem due to the current and future use in military aircraft. And yet, there is very little work devoted to the understanding of the aerodynamics of such wings. The problem is that such wings may be able to sustain attached flow next to broken-down delta-wing vortices, or stall like two-dimensional wings, while shedding vortices with generators parallel to their leading edge. To address this situation we studied the flow field over diamond-shaped planforms and sharp-edged finite wings. Possible mechanisms for flow control were identified and tested. We explored the aerodynamics of swept leading edges with no control. We presented velocity and vorticity distributions along planes normal and parallel to the free stream for wings with diamond shaped planform and sharp leading edges. We also presented pressure distributions over the suction side of the wing. Results indicated that in the inboard part of the wing, an attached vortex can be sustained, reminiscent of delta-wing type of a tip vortex, but further in the outboard region 2-D stall dominated even at 13° AOA and total stall at 21° AOA. To explore the unsteady flow field and the effectiveness of leading-edge control of the flow over a diamond-planform wing at 13° AOA, we employed Particle Image Velocimetry (PIV) at a Reynolds number of 43,000 in a water tunnel. Our results indicated that two-D-like vortices were periodically generated and shed. At the same time, an underline feature of the flow, a leading edge vortex was periodically activated, penetrating the separated flow, eventually emerging downstream of the trailing edge of the wing. To study the motion and its control at higher Reynolds numbers, namely 1.3 x 106 we conducted experiments in a wind tunnel. Three control mechanisms were employed, an oscillating mini-flap, a pulsed jet and spanwise continuous blowing. A finite wing with parallel leading and trailing edges and a rectangular tip was swept by 0°, 20°, and 40° and the pulsed jet employed as is control mechanism. A wing with a diamond-shaped-planform, with a leading edge sweep of 42°, was tested with the mini-flap. Surface pressure distributions were obtained and the control flow results were contrasted with the no-control cases. Our results indicated flow control was very effective at 20° sweep, but less so at 40° or 42°. It was found that steady spanwise blowing is much more effective at the higher sweep angle.
Analysis of surface pressure and velocity fluctuations in the flow over surface-mounted prisms
Ge, Zhongfu (Virginia Tech, 2004-12-14)
The full-scale value of the Reynolds number associated with wind loads on structures is of the order of 10^7. This is further complicated by the high levels of turbulence fluctuations associated with strong winds. On the other hand, numerical and wind tunnel simulations are usually carried out at smaller values of Re. Consequently, the validation of these simulations should only be based on physical phenomena derived with tools capable of their identification. In this work, two physical aspects related to extreme wind loads on low-rise structures are examined. The first includes the statistical properties and prediction of pressure peaks. The second involves the identification of linear and nonlinear relations between pressure peaks and associated velocity fluctuations. The first part of this thesis is concerned with the statistical properties of surface pressure time series and their variations under different incident flow conditions. Various statistical tools, including space-time correlation, conditional sampling, the probability plot and the probability plot correlation coefficient, are used to characterize pressure peaks measured on the top surface of a surface-mounted prism. The results show that the Gamma distribution provides generally the best statistical description for the pressure time series, and that the method of moments is sufficient for determining its parameters. Additionally, the shape parameter of the Gamma distribution can be directly related to the incident flow conditions. As for prediction of pressure peaks, the results show that the probability of non-exceedence can best be derived from the Gumbel distribution. Two approaches for peak prediction, based on analysis of the parent pressure time series and of observed peaks, are presented. The prediction based on the parent time series yields more conservative estimates of the probability of non-exceedence. The second part of this thesis is concerned with determining the linear and nonlinear relations between pressure peaks and the velocity field. Validated by analytical test signals, the wavelet-based analysis is proven to be effective and accurate in detecting intermittent linear and nonlinear relations between the pressure and velocity fluctuations. In particular, intermittent linear and nonlinear velocity pressure relations are observed over the nondimensional frequency range fH/U<0.32. These results provide the basis for flow parameters and characteristics required in the simulation of the wind loads on structures.
Analysis of Time-Varying Characteristics of Simulated Turbulence in Wind Tunnel
Tian, Lin (Virginia Tech, 1999-05-28)
Eight roughness configurations in Clemson boundary layer wind tunnel are presented. For these configurations, flow parameters such as turbulent intensities, integral length scales, large- and small- scale turbulence, and spectra of velocity components of the wind are obtained and studied to the simulated turbulence. At the same time, new analyzing tools, orthogonal wavelet techniques, are applied to provide additional information in time domain. This makes it possible to study the intermittency event, one important characteristic associated with pressure peak activities in turbulence. Three parameters, scale energy, intermittency factor and intermittency energy are defined. Variation of these quantities as a result of different configuration is discussed. Finally, the corresponding variations in measured pressure peaks in relation with the variations of configuration as well as with the intermittency parameters are investigated. The work here is of important significance for future wind tunnel and field data comparison, and this could help to find the best simulation among all configurations.
Analysis, Simulation and Control of Peak Pressure Loads on Low-Rise Structures
Ben Ayed, Samah (Virginia Tech, 2013-07-30)
Wind storms pose dangerous threats to human lives and are an enormous drain on the economy. Their damage to buildings usually starts with the failure of structural components that are subjected to excessive wind loads. In this dissertation, we investigate the characteristics of extreme loads on low-rise structures through analysis of full-scale and numerical data. We also use numerical simulations to evaluate different approaches to control the separated flow over a surface-mounted prism with the objective of reducing extreme pressure coefficients or loads on its surface. In the first part, we use a probabilistic approach to characterize peak loads as measured on a subject house during Hurricane Ivan on 2004. Time series of pressure coefficients collected on the roof of that house are analyzed. Rather than using peak values, which could vary due to the stochastic nature of the data, a probabilistic analysis is used to determine the probability of non-exceedence of specific values of pressure coefficients and associated wind loads. The results show that the time series of the pressure coefficients follow a three-parameter Gamma distribution, while the peak pressure follows a two-parameter Gumbel distribution. The results of the analysis are contrasted with the design values. In the second part, we perform numerical simulations of the flow over a surface-mounted prism as a simplified example for the flow over a low-rise structure. A Direct Numerical Simulation (DNS) code is developed to solve the unsteady two-dimensional incompressible Navier-Stokes equations of the flow past the prism. The pressure coefficients are then computed on the prism surface in order to assess the wind loads. The code is written on a parallel platform using the Message Passing Interface (MPI) library. We use the simulations to study the effects of inflow disturbances on the extreme loads on structures. The sensitivities of peak loads on a surface mounted prism to variations in incident gust parameters are determined. Latin Hypercube Sampling (LHS) is applied to obtain different combinations of inflow parameters. A non-intrusive polynomial chaos expansion is then applied to determine the sensitivities. The results show that the gust enhances the destabilization of the separation shear layer, forces it to break down and moves it closer to the roof of the prism. As for the sensitivities, the results show that the extreme loads are most sensitive to the transverse amplitude of the disturbance. Because the separated flow over sharp edges is responsible for the extreme pressure peaks, we investigate the use of active and passive control strategies to reduce wind loads. The studied active flow control strategies include blowing, suction, and synthetic jets. We implement them by using different flux injections, different slot locations and different angles. Investigation of the possible peak pressure reduction for two Reynolds numbers is performed. For Re = 1000, a reduction by nearly 50% of the peak pressure is obtained. For Re = 10, 000, the highest achieved reduction is nearly 25%. For passive control, we mount a flexible membrane on the top of the prism. In a two-dimensional framework, the membrane equation is modeled by a forced string equation. This mechanical equation is coupled with the DNS solver and integrated in time using a fourth order Hamming predictor corrector scheme. The results show that this strategy is as efficient as the active control approach, in terms of reducing extreme loads, for Re = 10, 000.
Analytical and Experimental Investigation of Insect Respiratory System Inspired Microfluidics
Chatterjee, Krishnashis (Virginia Tech, 2018-11-06)
Microfluidics has been the focal point of research in various disciplines due to its advantages of portability and cost effectiveness, and the ability to perform complex tasks with precision. In the past two decades microfluidic technology has been used to cool integrated circuits, for exoplanetary chemical analysis, for mimicking cellular environments, and in the design of specialized organ-on-a-chip devices. While there have been considerable advances in the complexity and miniaturization of microfluidic devices, particularly with the advent of microfluidic large-scale integration (mLSI) and microfluidic very-large-scale-integration (mVLSI), in which there are hundreds of thousands of flow channels per square centimeter on a microfluidic chip, there remains an actuation overhead problem: these small, complex microfluidic devices are tethered to extensive off-chip actuation machinery that limit their portability and efficiency. Insects, in contrast, actively and efficiently handle their respiratory air flows in complex networks consisting of thousands of microscale tracheal pathways. This work analytically and experimentally investigates the viability of incorporating some of the essential kinematics and actuation strategies of insect respiratory systems in microfluidic devices. Mathematical models of simplified individual tracheal pathways were derived and analyzed, and insect-mimetic PDMS-based valveless microfluidic devices were fabricated and tested. It was found that not only are these devices are capable of pumping fluids very efficiently using insect-mimetic actuation techniques, but also that the fluid flow direction and magnitude could be controlled via the actuation frequency alone, a feature never before realized in microfluidic devices. These results suggest that insect-mimicry may be a promising direction for designing more efficient microfluidic devices.
The Application of CFD to Building Analysis and Design: A Combined Approach of an Immersive Case Study and Wind Tunnel Testing
Kim, Daeung (Virginia Tech, 2014-01-23)
Computational Fluid Dynamics (CFD) can play an important role in building design. For all aspects and stages of building design, CFD can be used to provide more accurate and rapid predictions of building performance with regard to air flow, pressure, temperature, and similar parameters. Generally, the process involved in conducting CFD analyses is relatively complex and requires a good understanding of how best to utilize computational numerical methods. Moreover, the level of skill required to perform an accurate CFD analysis remains a challenge for many professionals particularly architects. In addition, the user needs to input a number of different items of information and parameters into the CFD program in order to obtain a successful and credible solution. This research seeks to improve the general understanding of how CFD can best be used as a design assistance tool. While there have been a number of quantitative studies suggesting CFD may be a useful tool for building related airflow assessment, few researchers have explored the more qualitative aspects of CFD, in particular developing a better understanding of the procedures required for the proper application of CFD to whole building analysis. This study therefore adopted a combined qualitative and quantitative methodology, with the researcher immersing himself into a case study approach and defining several lessons-learned that are documented and shared. This research will assist practicing architects and architecture students to better understand the application of CFD to building analysis and design.
Approaches to Simulation of an Underground Longwall Mine and Implications for Ventilation System Analysis
Zhang, Hongbin (Virginia Tech, 2015-06-19)
Carefully engineered mine ventilation is critical to the safe operation of underground longwall mines. Currently, there are several options for simulation of mine ventilation. This research was conducted to rapidly simulate an underground longwall mine, especially for the use of tracer gas in an emergency situation. In an emergency situation, limited information about the state of mine ventilation system is known, and it is difficult to make informed decisions about safety of the mine for rescue personnel. With careful planning, tracer gases can be used to remotely ascertain changes in the ventilation system. In the meantime, simulation of the tracer gas can be conducted to understand the airflow behavior for improvements during normal operation. Better informed decisions can be made with the help of both tracer gas technique and different modeling approaches. This research was made up of two main parts. One was a field study conducted in an underground longwall mine in the western U.S. The other one was a simulation of the underground longwall mine with different approaches, such as network modeling and Computational Fluid Dynamics (CFD) models. Networking modeling is the most prevalent modeling technique in the mining industry. However, a gob area, which is a void zone filled with broken rocks after the longwall mining, cannot be simulated in an accurate way with networking modeling. CFD is a powerful tool for modeling different kinds of flows under various situations. However, it requires a significant time investment for the expert user as well as considerable computing power. To take advantage of both network modeling and CFD, the hybrid approach, which is a combination of network modeling and CFD was established. Since tracer gas was released and collected in the field study, the tracer gas concentration profile was separately simulated in network modeling, CFD model, and hybrid model in this study. The simulated results of airflow and tracer gas flow were analyzed and compared with the experimental results from the field study. Two commercial network modeling software packages were analyzed in this study. One of the network modeling software also has the capability to couple with CFD. A two-dimensional (2D) CFD model without gob was built to first analyze the accuracy of CFD. More 2D CFD models with gob were generated to determine how much detail was necessary for the gob model. Several three-dimensional (3D) CFD models with gob were then created. A mesh independence study and a sensitivity study for the porosity and permeability values were created to determine the optimal mesh size, porosity and permeability values for the 3D CFD model, and steady-state simulation and transient simulations were conducted in the 3D CFD models. In the steady-state simulation, a comparison was made between the 3D CFD models with and without taking the diffusivity of SF6 in air into account. Finally, the different simulation techniques were compared to measured field data, and assessed to determine if the hybrid approach was considerably simpler, while also providing results superior to a simple network model.
An architecture of a wall
Latulippe, Michael II (Virginia Tech, 1998-09-14)
A wall is a primordial architectural artifact. The power and potential of a wall lies in its ability to transcend the necessities of construction and become a generator of architecture. A wall can be more than a plane in space, it can sculpt light and provide housing for various activities within its tectonic dimension. These additional functions can create an experience of both solidity of a wall as well as the possibility of inhabiting a wall. The creation of a "place". A wall also possesses the ability to create a sense of place. This can occur not only through the provision of habitable space, but also through the provision of bearing points for other structural members, illuminable surfaces, or zones of both visual and physical penetration. Within this thesis project, the wall generates a stair, and together, they begin to produce the rest of the architecture. At first there is a wall. Then there is the apartment.
A Basic Three-Dimensional Turbulent Boundary Layer Experiment To Test Second-Moment Closure Models
Sadek, Shereef Aly (Virginia Tech, 2008-09-10)
In this work, a three-dimensional turbulent boundary layer experiment was set up with alternating stream-wise and span-wise pressure gradients. The pressure gradients are generated as a result of the test section wavy side wall shape. Each side had six sine waves with a trough to peak magnitude to wavelength ratio of 0.25. Boundary layer control was used so that the flow over the side walls remains attached. The mean flow velocity components, static and total pressures were measured at six plane along the stream-wise direction. The alternating mean span-wise and stream-wise pressure gradients created alternating stream-wise and span-wise vorticity fluxes, respectively, along the test section. As the flow developed downstream the vorticity created at the tunnel floor and ceiling diffused away from the wall. The vorticity components in the stream-wise and span-wise directions are strengthened due to stretching and tilting terms in the vorticity transport equations. The positive-z half of the test section contains large areas that generate positive vorticity flux in the trough region and smaller areas generating negative vorticity around the wave peak. The opposite is true for the negative-z half of the test-section. This results in a large positive stream-wise vorticity in the positive-z half and negative stream-wise vorticity in the negative-z half of the test-section. The smaller regions of opposite sign vorticity in each half tend to mix the flow such that as they diffuse away from the wall, the turbulent stresses are more uniform. Turbulent fluctuating velocity components were measured using Laser Doppler Velocimetery. Mean velocities as well as Reynolds stresses and triple velocity component correlations were measured at thirty stations along the last wave in the test section. Profiles at the center of the test section showed three dimensionality, but exhibited high turbulence intensities in the outer layer. Profiles off the test section centerline are highly three dimensional with multiple peaks in the normal stress profiles. The flow also reaches a state where all the normal stresses have equal magnitudes while the shear stresses are non-zero. Flow angles, flow gradient angles and shear stress angles show very large differences between wall values and outer layer vlaues. The shear stress angle lagged the flow gradient angle indicating non-equilibrium. A turbulent kinetic energy transport budget is performed for all profiles and the turbulence kinetic energy dissipation rate is estimated. Spectral measurements were also made and an independent estimate of the kinetic energy dissipation rate is made. These estimates agree very well with those estimates made by balancing the turbulence kinetic energy transport equation. Multiple turbulent diffusion models are compared to measured quantities. The models varied in agreement with experimental data. However, fair agreement with turbulence kinetic energy turbulent diffusion is observed. A model for the dissipation rate tensor anisotropy is used to extract estimates of the pressure-strain tensor from the Reynolds stress transport equations. The pressure-strain estimates are compared with some of the models in the literature. The comparison showed poor agreement with estimated pressure-strain values extracted from experimental data. A tentative model for the turbulent Reynolds shear stress angle is developed that captures the shear stress angle near wall behavior to a very good extent. The model contains one constant that is related to mean flow variables. However, the developed expression needs modification so that the prediction is improved along the entire boundary layer thickness.
Characterization of Carbon Mat Thermoplastic Composites: Flow and Mechanical Properties
Caba, Aaron C. (Virginia Tech, 2005-09-14)
Carbon mat thermoplastics (CMT) consisting of 12.7 mm or 25.4 mm long, 7.2 micrometer diameter, chopped carbon fibers in a polypropylene (PP) or poly(ethylene terephthalate) (PET) thermoplastic matrix were manufactured using the wetlay technique. This produces a porous mat with the carbon fibers well dispersed and randomly oriented in a plane. CMT composites offer substantial cost and weight savings over typical steel construction in new automotive applications. In production vehicles, automotive manufacturers have already begun to use glass mat thermoplastic (GMT) materials that use glass fiber as the reinforcement and polypropylene as the matrix. GMT parts have limitations due to the maximum achievable strength and stiffness of the material. In this study the glass fibers of traditional GMT are replaced with higher strength and higher stiffness carbon fibers. The tensile strength and modulus and the flexural strength and modulus of the CMT materials were calculated for fiber volume fractions of 10-25%. Additionally, the length of the fiber (12.7 mm or 25.4 mm) was varied and four different fiber treatments designed to improve the bond between the fiber and the matrix were tested. It was found that the fiber length had no effect on the mechanical properties of the material since these lengths are above the critical fiber length. The tensile and the flexural moduli of the CMTs were found to increase linearly with the FVF up to 25% FVF for some treatments of the fibers. For the other treatments the linearly increasing trend was valid up to 20% FVF, then stiffness either stayed constant or decreased as the FVF was increased from 20% to 25% . The strength versus FVF curves showed trends similar to those of the modulus versus FVF curves. It is shown that choosing an appropriate sizing can extend the usable FVF range of the CMT by at least 5%. Published micromechanical relations over-predicted the tensile modulus of the composite by 20-60%. An empirical fiber efficiency relation was fit to the experimental data for the tensile modulus and the tensile strength giving excellent agreement with the experimental results. Flow tests simulating the compression molding process were conducted on the CMT to determine what factors affect the flow viscosity of the CMT. The melt viscosity of the neat PP was measured using cone and plate rheometry at temperatures between 180°C–210°C and was fit with the Carreau relation. The through thickness packing stress of the CMT mat was measured for FVFs of 8-40% and was found to follow a power law behavior based on the local bending of fibers up to a FVF of 20.9%. Above this FVF the power law exponent decreases, and this is attributed to fracture of some of the fibers. Heated platens were used to isothermally squeeze the CMT at axial strain rates of 0.02-6 s^-1. The plot of the load-displacement behavior for the 10% FVF CMT was similar in shape to that for a fluid with a yield stress. For FVFs of 15-25% the load-displacement curves showed a load spike at the beginning of the flow, then followed the curve for a fluid with a yield stress. The matrix was burned off the squeezed samples, and the remaining carbon mat was dissected and visually inspected. It was found that fiber breakage increased and fiber length decreased as the FVF of the sample was increased.
Characterization of damage mechanisms and behavior in two-dimensionally braided composites subjected to static and fatigue loading
Burr, Scott T. (Virginia Tech, 1996-05-20)
In the present research project, four braided composite architectures consisting of graphite fibers in an epoxy matrix were tested under static and fatigue loading conditions to determine damage mechanism types and progressions. The braided architectures consisted of straight axial fiber bundles, which were surrounded by braider fiber bundles oriented at ±a 0 with respect to the axial fiber bundles. Static tension and compression testing was completed first to determine material strengths and basic damage modes for each of the architectures. Under static tension loading, cracking in the braider fiber bundles occurred first, and was followed by splitting in curved regions of the axial fiber bundles. Matrix cracking and kink band formation were found to occur under static compression loading.
Compaction and Cure of Resin Film Infusion Prepregs
Thompson, Joseph E. (Virginia Tech, 2004-12-22)
Gutowski et al.'s model has been employed to describe the cure and consolidation of prepregs used for resin film infusion. Resin kinetics, rheology, flow and fiber deformation are considered. Resin kinetics are simulated with an isothermal autocatalytic-1 type relation. The non-Newtonian viscosity of the Cytec™ 754 resin is represented with a gel type expression. The one dimensional flow of resin through a deformable, partially saturated porous medium is studied. A nonlinear partial differential equation describing the spatial and temporal variation of the fiber volume fraction combining the continuity equation, Darcy's Law, and mat compressibility has been derived and solved numerically. Resin is assumed to be incompressible and inertial effects are neglected. Based on the resin content of regions where resin and fiber coexist, expressions for tracking resin flow through fully and partially saturated regions of fiber are given. Values of material parameters for the E-QX 3600-5 glass fabric are estimated from literature data involving compression of similar dry fabrics and through comparison of computed results with the experimental data. Results for the final thickness of the consolidated part agree with the experimental values, but those for the mass loss do not.
Comparison of Paul and Morlet Wavelets for Measuring the Characteristic Scale of Peak Pressure Events on Low-Rise Structures
Chabalko, Christopher Carter (Virginia Tech, 2001-08-16)
A methodology to measure a characteristic time scale (duration) of peaks in pressure and velocity data is presented. This methodology is based on the use of the Morlet and Paul wavelets. Detailed descriptions of these wavelets and their implementation procedures are given. The results show that similar time scales or durations can be measured using either Morlet or Paul wavelets. To obtain consistent results data windowing might need to be applied. Using the Paul wavelet, durations of events measured in different wind tunnel simulations are obtained and discussed.
Composite circular braid mechanics
Hopper, Robert Huston (Virginia Tech, 1991-08-15)
Braided composites find many diverse applications in modern technology and tailoring the mechanical properties of these structures has become increasingly important. This thesis will examine one class of circular braids encompassing an elastic core. By hypothesizing four modes of operation and incorporating primary influences, the mechanical response of the composite is predicted based on its initial parameters and material properties. The ability to model the yarns that constitute the braid as nonlinear materials enables the simulated response to span finite deformations. A scheme for nondimensionalizing the model parameters and governing equations for each mode of operation is also proposed and implemented. In an effort to validate the assumptions underlying the model's formulation, a series of experimental trials are documented that verify the fundamental braid mechanics. A wide variety of analytical cases are also introduced to investigate the influences of various model parameters. Possible extensions for the existing model are also noted.
Computation of Reynolds stresses in axisymmetric vortices and jets using a second order closure model
Jiang, Min (Virginia Tech, 1994-08-15)
Donaldson's single-point second-order model [13] is used to close the Reynolds stress transport equations in cylindrical coordinates. A reduced set of equations are then solved for the decay of axisymmetric vortices and jets. A self-similar solution to the axisymmetric vortices is obtained numerically. The characteristics of the mean flow variables as well as the Reynolds stresses in this solution are discussed. Comparisons of the current results with Donaldson[13J and Donaldson and Sullivan[16] are also presented. The results show that the vortex core is free from turbulent shear stresses. The turbulent kinetic energy is also found to be relatively weak within the core region. The overshoot of the circulation is found to be 5% of the circulation at infinity over a wide range of Reynolds numbers. The effects of Reynolds number on the decay of the vortices are computed and discussed. Some of the quantities, such as mean flow circulation and turbulent kinetic energy, are found to be sensitive to the Reynolds number. However, the overshoot is found to be insensitive to the Reynolds number but its location does. A set of suitable model constants for the axisymmetric jets is also found and a self similar solution for the jet case is obtained. Comparisons of the computed results with some recent experimental data are presented.

Browsing by Author "Ragab, Saad A."

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