Nonhydrostatic mixing and three-dimensionality of gravity currents under turbulent pulsating flow

Direct numerical simulations (DNS) are used to investigate the impact of flow instabilities, nonhydrostatic mixing, and phase-dependent flow behavior on gravity currents propagating against a pulsating flow in a lock-exchange configuration. Two regimes are considered: (i) laminar pulsating flow, with two-dimensional (2D) and three-dimensional (3D) DNS at Re=3000 using eight combinations of Froude numbers Frm and Fro, representing dimensionless velocity amplitudes of the opposing mean flow and the oscillatory component, respectively; and (ii) turbulent pulsating flow, with 3D DNS at Re=1.2×104, Frm=0.46, and Fro=0.5. Across both regimes, the gravity current evolution is strongly phase dependent and shaped by nonhydrostatic processes: Kelvin–Helmholtz instabilities, lobe-cleft structures, lifting of the gravity current, Rayleigh–Taylor-like instabilities, and their impact on 3D mixing. We show that long-term processes and large-scale phenomena can be captured with 2D DNS. Other properties, related to nonhydrostatic processes, such as instabilities and lifting, active on shorter time scales (smaller than the oscillation period) or small-scale flow phenomena, require full 3D DNS. Turbulent ambient flow conditions modulate these short-time and small-scale processes significantly, thus always requiring full 3D DNS (or large-eddy simulation) and having a clear impact over the full oscillation cycle. Together, the presented results clarify which aspects of evolving gravity currents under pulsating flow conditions can be captured with 2D models and identify the 3D mechanisms that must be represented when parameterizing mixing and entrainment in estuarine salt-intrusion applications.