Browsing by Subject "LES"

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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.
Extending the active model split to compressible flows
(2022-08-11) Pederson, Clark Curtis; Moser, Robert deLancey; Oliver, Todd; Goldstein, David; Bogard, David; Ezekoye, Ofodike
Hybrid RANS/LES models have consistently shown superior accuracy over RANS in predictions of massively separated flows. However, these models have experienced difficulties with modeled stress depletion, smooth-wall separated flow, and shock-separated flows. The Active Model Split (AMS) hybridization was created to address some of the fundamental shortcomings of traditional hybrid RANS/LES models. While previous work considered incompressible flows, this thesis demonstrates how the AMS framework can be extended to address compressible flows. AMS-based hybrid models for the turbulent heat flux and turbulent transport terms in the total energy equation are presented. This compressible hybrid RANS/LES framework is demonstrated on a subsonic channel, a supersonic channel, an axisymmetric transonic bump, and an impinging shock boundary-layer interaction. Some features, such as shock location and separation length, are seen to improve with the AMS framework. The AMS framework is shown to be resistant to modeled stress depletion, though modeling errors in the mean stress can still occur. In general, skin friction is predicted less accurately with the AMS framework than with steady RANS models. Possible explanations for this are given, and model corrections and future lines of research are suggested.
Large eddy simulation analysis of non-reacting sprays inside a high-g combustor
(2012-08) Martinez, Jaime, master of science in engineering; Raman, Venkat; Clemens, Noel T
Inter-turbine burners are useful devices for increasing engine power. To reduce the size of these combustion devices, ultra-compact combustor (UCC) concepts are necessary. One such UCC concept is the centrifugal-force based high-g combustor design. Here, a model ultra-compact combustor (UCC) with fuel spray injection is simulated using large eddy simulation (LES) and Reynolds-Averaged Navier-Stokes (RANS) methodologies to understand mixing and spray dispersion inside centrifugal-based combustion systems. Both non-evaporating and evaporating droplet simulations were carried, as well as the tracking of a passive scalar, to explore this multiphase system. Simulation results show that mixing of fuel and oxidizer is based on a jet-in-crossflow system, with the fuel jet issuing into a circulating oxidizer flow stream. It is seen that a a high velocity vortex-like ring develops in the inner core of the combustor, which has enough momentum to obstruct the path of combustion products. There is minimal fuel droplet and vapor segregation inside the combustor and enhanced turbulent mixing is seen at mid-radius.