past efforts: why LES/RANS hybrid methodology

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NACA64A006 airfoil has a thin-airfoil stall characteristic, where laminar flow separation occurs at the leading edge and turbulent transition makes the turbulent reattachment. The reattachment point gradually moves rearward with increasing angles of attack. Flows over a thin-airfoil at the angle of attack beyond 5.0 degrees include small vortices shedding from the leading edge, which produces strong unsteadiness in the flow. This is considered to be the main reason that RANS simulations in which we deal with time-averaged quantities using turbulence model do not give satisfactory results. The limitation of RANS simulations led to an interest in the methods such as LES that may improve the prediction accuracy of such flows. However, considering the feasibility of LES under the current computer environment, it still remains difficult to apply LES to complex flows at high Reynolds numbers as seen in many engineering problems.

In fact, in the best prediction by LES of Mary and Sagaut in LESFOIL project, the spanwise extent of the computational domain is limited to 1.2% of the chord length and the three-dimensional computational domain is only located near solid surface.

Mary et al. AIAA J. Vol. 40, No. 6.

To overcome these difficulties, our choice is LES/RANS hybrid methodology and the reliability of the hybrid methodology to the flows near stall conditions is focused on.

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