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This article proposes a Reynolds number scaling of the required grid pointsto perform wall-modeled LES of turbulent flows encountering separation off asolid surface. Based on comparisons between the various time scales in anon-equilibrium (due to the action of an external pressure gradient) turbulentboundary layer, a simple definition of the near-wall ``under-equilibrium" and``out-of-equilibrium" scales is put forward (where ``under-equilibrium" refersto scales governed by a quasi-balance between the viscous and the pressuregradient terms). It is shown that the former length scale varies with Reynoldsnumber as lp Re^(-2/3). The same scaling is obtained from a simplified Green'sfunction solution of the Poisson equation in the vicinity of the separationpoint. A-priori analysis demonstrates that the resolution required toreasonably predict the wall-shear stress (for example, errors lower thanapproximately 10-15% in the entire domain) in several nonequilibrium flows isat least O(10) lp irrespective of the Reynolds number and the Clauserparameter. Further, a series of a-posteriori validation studies are performedto determine the accuracy of this scaling including the flow over the Boeingspeed bump, Song-Eaton diffuser, Notre-Dame Ramp, and the backward-facing step.The results suggest that for these flows, scaling the computational grids ()such that / lp is independent of the Reynolds number results in accuratepredictions of flow separation at the same ``nominal" grid resolution acrossdifferent Reynolds numbers. Finally, it is suggested that in the vicinity ofthe separation and reattachment points, the grid-point requirements forwall-modeled large eddy simulations may scale as Re^4/3, which is morerestrictive than the previously proposed flat-plate boundary layer-basedestimates (Re1) of Choi and Moin (Phys. Fluids, 2012) and Yang and Griffin(Phys. Fluids, 2021).

Reynolds number dependence of length scales governing turbulent flow separation with application to wall-modeled large-eddy simulations