Modeling unsteady turbulent flows over ripples: Reynolds‐averaged Navier‐Stokes equations (RANS) versus large‐eddy simulation (LES)

Abstract

In this paper we consider the problem of modeling a turbulent pulsating boundary layer over ripples. We compare the results of two modeling strategies, Wilcox's k − ω Reynolds‐Averaged Navier‐Stokes equations (RANS) and large‐eddy simulation (LES) employing the Lagrangian dynamic eddy viscosity model. The geometry and parameters employed are relevant to nearshore oceanic flows, and the results are discussed in relation to the problem of sediment transport. Generally, RANS and LES agree well only with regard to the vertical profiles of the streamwise component of the velocity. Large discrepancies were found in all the other quantities considered (e.g., vertical velocity, turbulent kinetic energy, and Reynolds stress). In particular, RANS severely underpredicted the magnitude of the Reynolds stress and overpredicted the amplitude of the oscillations in the vertical velocity. We also found that often the trends exhibited by RANS and LES when the frequency and/or amplitude of the driving conditions was varied were at odds. Since comparison with available experiments indicates that LES is able to accurately model this kind of flows, we conclude that the RANS model is not appropriate to model the suspension and transport of sediment under conditions similar to the ones presented in this study.