Browsing by Subject "Inhomogeneous"
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Item Anisotropic hybrid turbulence modeling with specific application to the simulation of pulse-actuated dynamic stall control(2015-12) Haering, Sigfried William; Moser, Robert deLancey; Murthy, Jayathi; Bogard, David G; Ezekoye, Ofodike A; Oliver, ToddExperimental studies have shown pulse actuated dynamic stall control may provide a simple means to significantly increase the performance of lifting surfaces and expand their flight envelope. However, precise information of the complex boundary layer reattachment mechanisms are inaccessible to experimental measurements. Therefore, simulations are necessary to fully understand, optimize, and apply this method. Due to the inherent shortcomings of RANS, computational expense of LES, and deficiencies in current hybrid modeling approaches, a new hybrid modeling framework has been developed. Based in using the two-point second-order structure function to drive a local equilibrium between resolved and modeled turbulence, the new approach addresses issues associated with inhomogeneous and anisotropic grids as well as the treatment of the RANS/LES interface in hybrid simulations. Numerical studies using hybrid RANS/LES modeling approaches of a stalled airfoil with spanwise-uniform actuation regions experiencing single pulse actuated flow reattachment have been performed. The mechanism responsible for reattachment has been identified as a repeating wall-vortex interaction process. The new hybrid framework and anisotropic SGS models developed here are anticipated to be of great benefit well beyond the focus of this work with application to many challenging flow situations of pressing engineering interest.Item Effect of Inhomogeneity and Unsteadiness on the Stability of High-Speed Shear Flows(2014-07-09) Bertsch, Rebecca LynneIn hypersonic flows, turbulence critically influences mass and momentum transport, mixing, heat transfer and acoustic noise generation. In contrast to incompressible flow, in high speed flows pressure is a true thermodynamic variable and flowthermodynamic interactions render the investigations extremely challenging. Most studies to date have been performed on steady, uniform or homogeneous shear flows leading to important insight on the flow physics. In most real world applications,flows of practical importance will exhibit unsteadiness and strong inhomogeneity. To date, investigations of unsteadiness and inhomogeneity in high-speed flows are rare. The goal of this dissertation is to study and understand these non-ideal effects when pertinent to shear flows. Towards this goal, we perform three distinct studies: (a) examination of time reversal characteristics of linear inviscid mass, momentum, energy and state equation in compressible flows; (b) Linear analysis (RDT) of compressibility effects on instabilities in temporally periodic (unsteady) homogeneous shear flow; and (c) Numerical investigation of small perturbation evolution in compressible Kolmogorov (inhomogeneous) shear flow. The first study shows that even with the additional governing equations required in the high-speed regime, the inviscid flow field is still reversible. This justifies the use of temporal periodicity to investigate the effect of unsteadiness. The second study presents a detailed analysis of the pressure equation in temporally periodic homogeneous shear flow. The analysis and numerical results show unsteady uniform shear exhibits two stages of evolution due to the changing behavior of pressure. These stages are analogous to the first two stages of evolution established in steady shear. The third stage seen in steady shear is not achieved by periodic shear flow. The final study shows that the evolution of small perturbations in spatially periodic Kolmogorov flow is influenced by: i) the initial compressibility parameter, M_(g0), ii) the initial perturbation orientation, and iii) the stream normal location. Ultimately, the final study supports the postulate that all shear flows exhibit perturbation stability boundary classifications seen in homogeneous shear flows. The findings of this research further our understanding of the effects of unsteadiness and inhomogeneity in realistic flows, which will aid in the development of improved computational tools.Item Mechanics of prestressed and inhomogeneous bodies(Texas A&M University, 2006-10-30) Umakanthan, SaravananIn finite elasticity, while developing representation for stress, it is customary to require the reference configuration to be stress free. This study relaxes this requirement and develops representations for stress from a stressed reference configuration. Using the fact that the value of Cauchy stress in the current configuration is independent of the choice of the reference configuration, even though the formula used to compute it depends on the choice of the reference configuration, the sought representation is obtained. It is then assumed that there exists a piecewise smooth mapping between a configuration with prestresses and a configuration that is stress free, and the representation obtained above is used to study the mechanical response of prestressed bodies. The prestress fields are obtained by directly integrating the balance of linear momentum along with the traction free boundary condition. Then, different classes of boundary value problems for the type of inhomogeneous and prestressed bodies of interest are formulated and studied. For the cases studied, it is found that even the global measures like axial-load required to engender a given stretch ratio for a prestressed body vary from the homogeneous stress free bodies, though not significantly. The local measures - stress and deformation - in a prestressed body differ considerably from their homogeneous stress free counterparts. The above gained knowledge is applied to understand the mechanics of circumflex arteries obtained from normotensive and hypertensive micro-mini pigs. It is found that the deformation of these arteries when subjected to inflation and axial extension is not of the form r = r(R), ???? = ????, z = Z. Comparison is also made between the response of an artery at various levels of smooth muscle activation and stretch ratio as well as normotensive and hypertensive specimens, using statistical methods.Item Stretch-induced wrinkling of thin sheets(2013-08) Nayyar, Vishal; Huang, Rui, doctor of civil and environmental engineering; Ravi-Chandar, K.Thin sheets and membrane structures are widely used in space applications such as solar sails, sunshields and membrane optics. Surface flatness over a large area is one of the key requirements for many applications using the flexible thin structures. However, wrinkles are commonly observed in thin sheets. It is thus important to understand the mechanics of thin sheets for practical applications that require reliable control of surface wrinkles. In this study, a model problem of stretch-induced wrinkling of thin sheets is considered. First, a two-dimensional (2-D) finite element model was developed to determine stretch-induced stress distribution patterns in hyperelastic thin sheets, assuming no wrinkles. As a prerequisite for wrinkling, development of compressive stresses in the transverse direction was found to depend on both the length-to-width aspect ratio of the sheet and the applied tensile strain. Next, an eigenvalue analysis was performed to predict the critical conditions for buckling of the elastic sheet under the prescribed boundary conditions, followed by a nonlinear post-buckling analysis to simulate evolution of stretch-induced wrinkles. Experiments were conducted to measure stretch-induced wrinkling of polyethylene thin sheets, using the three-dimensional digital image correlation (3D-DIC) technique. It was observed that the wrinkle amplitude first increased and then decreased with increasing nominal strain, in agreement with finite element simulations for a hyperelastic thin sheet. However, unlike the hyperelastic model, the stretch-induced wrinkles in the polyethylene sheet were not fully flattened at high strains (> 30%), with the residual wrinkle amplitude depending on the loading rate. The hyper-viscoelastic and the parallel network nonlinear viscoelastic material models were adopted for finite element simulations to improve the agreement with the experiments, including the wrinkle amplitude, residual wrinkles and rate dependence. Finally it is noted that wrinkling is sensitive to defects and material inhomogeneity in thin sheets. By varying the elastic stiffness in a narrow region, numerical simulations show drastically different wrinkling behavior, including the critical strain and evolution of wrinkle amplitude and wavelength. In conclusion, a comprehensive understanding of stretch-induced wrinkling is established, where geometry, material, and boundary conditions all play important roles.