Turbulent structures in planar gravity currents and their influence on the flow dynamics

Abstract

Direct numerical simulations (DNS) of planar gravity current in the Boussinesq limit have been conducted with the objective of identifying, visualizing, and describing turbulent structures and their influence on the flow dynamics. The simulations are performed for Reynolds numbers of Re = 8950 and Re = 15,000 with 31‐ and 131‐million grid point resolutions, respectively. This range of Reynolds numbers ensures fully developed turbulent gravity currents, which have never been simulated before using DNS. The flow develops zones with different turbulence characteristics, which eventually interact with each other. The near‐wall bottom flow resembles boundary layer flow with several hairpin‐like vortices oriented in the direction of the flow and preferential patterns of low‐ and high‐speed streaks. The separation between low‐speed streaks at the front scales with the lobe size, which is about 200 wall units for Re = 15,000. Upstream of the front, the separation between low‐speed streaks scales with the well‐accepted value of 100 wall units for turbulent boundary layers. These patterns have associated regions of low and high bottom shear stresses with implications for sediment erosion and bed load transport. Most of the erosive power of the flow is found in the gravity current front. The interface between heavy and light fluids rolls up by baroclinic generation of Kelvin‐Helmholtz vortices, which undergo sudden breakup and decay to small‐scale turbulence. The effect of turbulence and three‐dimensionality on the flow dynamics is addressed by comparing two‐ and three‐dimensional simulations. Three‐dimensional simulations present active mechanisms that undermine the strong flow coherence, comparing well with experimental observations.