The effect of LES mode in hybrid RANS-LES modelling was examined on the mitigation of the so-called "grey area" by exploring the LES length scale adaptation to local grid resolution. An algebraic hybrid RANS-LES model was taken as the platform for the testing, with or without an additional Leonard term incorporated for inducing instantaneous energy backscatter and, consequently, to enhance resolved turbulent fluctuations. The modelling was tested in computations of a turbulent mixing layer, where the development of LES-resolved turbulent contents at the initial stage is a significant indicator of grey-area mitigation effectiveness. The results were verified in comparison with available experimental data. It is demonstrated that the resolved mixing layer in its initial formation may respond to a reduced LES length scale with relatively extensive shear-layer instabilities, which are however not sustainable with a coarse grid resolution. The LES length scale adaptation must be supported by a properly refined grid in mitigating the grey area for an effective re-establishment of resolved turbulent properties.
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