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Large Eddy Simulation Study for Fluid Disintegration and Mixing
 Created: Monday, 01 August 2011
This work is directly applicable to simulations of gas turbine engines and rocket engines.
A new modeling approach is based on the concept of large eddy simulation (LES) within which the large scales are computed and the small scales are modeled. The new approach is expected to retain the fidelity of the physics while also being computationally efficient. Typically, only models for the smallscale fluxes of momentum, species, and enthalpy are used to reintroduce in the simulation the physics lost because the computation only resolves the large scales. These models are called subgrid (SGS) models because they operate at a scale smaller than the LES grid.
In a previous study of thermodynamically supercritical fluid disintegration and mixing, additional smallscale terms, one in the momentum and one in the energy conservation equations, were identified as requiring modeling. These additional terms were due to the tight coupling between dynamics and realgas thermodynamics. It was inferred that if these terms would not be modeled, the high densitygradient magnitude regions, experimentally identified as a characteristic feature of these flows, would not be accurately predicted without the additional term in the momentum equation; these high densitygradient magnitude regions were experimentally shown to redistribute turbulence in the flow. And it was also inferred that without the additional term in the energy equation, the heat flux magnitude could not be accurately predicted; the heat flux to the wall of combustion devices is a crucial quantity that determined necessary wall material properties.
The present work involves situations where only the term in the momentum equation is important. Without this additional term in the momentum equation, neither the SGSflux constantcoefficient Smagorinsky model nor the SGSflux constantcoefficient Gradient model could reproduce in LES the pressure field or the high densitygradient magnitude regions; the SGSflux constantcoefficient ScaleSimilarity model was the most successful in this endeavor although not totally satisfactory. With a model for the additional term in the momentum equation, the predictions of the constantcoefficient Smagorinsky and constantcoefficient ScaleSimilarity models were improved to a certain extent; however, most of the improvement was obtained for the Gradient model. The previously derived model and a newly developed model for the additional term in the momentum equation were both tested, with the new model proving even more successful than the previous model at reproducing the high densitygradient magnitude regions. Several dynamic SGSflux models, in which the SGSflux model coefficient is computed as part of the simulation, were tested in conjunction with the new model for this additional term in the momentum equation. The most successful dynamic model was a “mixed” model combining the Smagorinsky and Gradient models.
This work is directly applicable to simulations of gas turbine engines (aeronautics) and rocket engines (astronautics).
This work was done by Josette Bellan and Ezgi Taşkinoğlu of Caltech for NASA’s Jet Propulsion Laboratory. For more information, contact This email address is being protected from spambots. You need JavaScript enabled to view it.. NPO47040
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