Direct numerical simulation of a high-Froude-number turbulent open-channel flow

Abstract

In this study, we successfully conducted a direct numerical simulation of a high-Fr turbulent open-channel flow at a Froude number of 1.8 and a Reynolds number of 2325 based on bulk velocity, gravitational acceleration, water depth, and kinetic viscosity, using the multi-interface advection and reconstruction solver (MARS). We confirmed that typical wall-bounded turbulent structures were observed in this high-Fr open-channel flow. On the other hand, near free-surface, surface deformations are constituted of large-scale gentle bumpy waves with the maximum wave height corresponding to approximately 4% of the water depth and small-scale isotropic waves on the large-scale waves. The large-scale waves would generate the high-speed streaky structures near the free-surface and the scale of the small waves would be equivalent to the near free-surface turbulent kinetic energy. Near the free-surface, wall-normal turbulent intensity and energy-dissipation rates increase toward the free-surface, and the tendencies of turbulent statistical quantities near the free-surface are in good agreement with the experimental measurements obtained using a laser Doppler anemometer (LDA) (Nakayama and Nezu, “Bursts near the free-surface in open-channel flows and their relationships with turbulence structures,” J. Hydraulic, Coastal and Environmental Eng. JSCE 635/II, 31 1999). As well as the rigid-lid turbulent open-channel flow, turbulent intensity distribution from the vertical component to the streamwise one is predominant beneath the free-surface.