Browsing by Subject "Inhomogeneous"

Now showing 1 - 3 of 3

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, Todd
Experimental 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.
Inhomogeneous Shrinkage of Polymer Materials in Selective Laser Sintering 298
(2000) Steinberger, J.; Shen, J.; Göpfert, J.; Gerner, R.; Daiber, F.; Manetsberger, K.; Fersti, S.
It is well known that the laser beam of an SLS machine can be controlled almost exactly. The inaccuracy of the mechanical movements of the machines is also much lower than the actual errors of the SLS parts. How can we explain this discrepancy? One answer is the temperature inhomogeneity in the build field and in the part bed. In this article the effect of temperature dependent volume relaxation of pre-sintered polymer parts on the inaccuracy of the SLS process will be discussed. The investigation shows that it depends on the temperature, pressure and time. Measurements of the temperature distribution in an SLS part bed were carried out. By determining coordinate-dependent scaling factors, an empirical method to compensate this nonlinear shrinkage is presented in this article.
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.