Adaptive large eddy simulation for complex unsteady flows

Abstract

Large eddy simulation (LES) is an attractive turbulence modeling approach due to thebalance it provides between computational cost and turbulence/scale-resolving capabilities. LES is shown to be robust and provides accurate predictions for complex flow problems including unsteady aerodynamic flows with spatiotemporal inhomogeneity. In this work, the specific problem of interest involves flow over surging airfoils, which arises in many aerodynamic applications. For example, for a rotorcraft in a forward flight, or for wind turbines in non-uniform flows (e.g., shear). Mesh resolution requirements for such complex and unsteady flow problems are notknown a priori. In this work, we develop an adaptive approach for LES where the mesh resolution is changed (refined or adapted) based on an a posteriori error estimator leading to error-driven/controlled adaptive LES. In addition, a flow feature-based adaptation criterion that can use the error estimator is developed. Both the LES methodology and the error estimator used here are based on the variational multiscale (VMS) framework. A range of Reynolds numbers and advance ratios is considered in adaptive LES forsurging airfoils. Three different adaptive strategies based on VMS error estimator are ex- plored: (i) zonal-based refinement/adaptation, (ii) nodal size field-based adaptation, and (iii) feature-based refinement/adaptation. The zonal-based strategy is found to be most effective. This strategy is applied for adaptive LES of flow over surging airfoils to construct a series of adapted meshes and to demonstrate mesh convergence. In particular, for quantities of interest including pressure coefficient and leading edge vortex (LEV) evolution. In addition, very high advance ratios are considered, where a massive trailing edge separation also occurs due to flow reversal. In these cases, LES includes active flow control in the form of active reflex camber that is applied to dynamically morph the airfoil shape during flow reversal (i.e., only for a part of the surging cycle). Active reflex camber results in a significant reduction in drag force and its fluctuations.