about LES/RANS hybrid methodology

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corresponding major papers

ᡒ Kawai, S., PhD dissertation, March 2005.	PhD(68MB)
ᡒ Kawai, S. and Fujii, K., AIAA J. in press.
ᡒ Kawai, S. and Fujii, K., AIAA Paper 2004-0068.	AIAA2004-0068

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In this research, LES/RANS hybrid methodology aims to develop for the improvement of the reliability to the simulation of massively-separated compressible turbulent flow at realistic Reynolds numbers as seen in many aerospace engineering problems within a practical computational cost. LES/RANS hybrid approach applies RANS computation based on the ensemble-averaged Navier-Stokes equations to the regions near solid surfaces, whereas LES computation based on the spatially-filtered Navier-Stokes equations to the other region that may include massively-separated flows within a single formulation. Thus, the advantages of the LES/RANS hybrid methodology are that it can alleviate immense grid requirements and severe time step limitation associated with the small eddy motions which need to be resolved in wall-bounded regions in LES or MILES. As a result, the hybrid method requires much less computational cost than that of LES and MILES approaches and is considered to have accuracy of similar level as LES and MILES.
There is a flexibility in the present hybrid method of connecting the solutions near the interface of the RANS and LES regions. In our LES/RANS hybrid methodology, the RANS and LES formulation sets are blended with buffer region by the following equation as shown in below figures. This buffer region has the effect of avoiding the discontinuity of the solution by an immediate connection at the interface.
where C1 and C2 are constants, and η is the ratio between the distance to the nearest wall and the distance from the wall to blending position.

Behavior of blending fuction.	Distribution of blending fuction around the base.

As we focus on the hybrid methodology to engineering problems within a practical computational cost, Smagorinsky and Baldwin-Lomax algebraic models are adopted for the LES-level and RANS-level turbulence model to close the system of governing equations. These models have the properties of robustness and low computational cost, and the properties often become significant advantages when the LES/RANS hybrid methodology is applied to engineering problems.
We already applied the LES/RANS hybrid methodology to some applications, such as a supersonic base flow and a thin-airfoil stall problem at subsonic Mach number. Both the reliability and capability of the LES/RANS hybrid methodology for the prediction of the flows at high Reynolds number are identified. More details are in corresponding major papers above or publication (with PDF library).

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