Turbulent structure of subcritical and supercritical gravity and turbidity currents *

We study the turbulent structure of sub- and supercritical gravity currents through high-fidelity simulations that employ up to a billion grid points. Regarding supercritical currents, three families of hairpin vortices appear in the near-bed and interface layers, with their generation mechanisms dictating their shape and orientation. The interaction of near-bed and lower-interface hairpins explains the weak inviscid lid-like behavior of the streamwise velocity maximum, while the upper-interface hairpins are responsible for ambient fluid entrainment. On the other hand, only one family of hairpins can be seen in subcritical currents. An intermediate layer of counter-gradient transport of momentum caps the near-bed layer populated by hairpins. We also present energy consistent depth-averaged mass, momentum, and energy conservation equations. We compute shape factors for currents in the sub- and super-critical regimes. Finally, we discuss the energy restrictions on ambient fluid entrainment and the contributions from turbulent kinetic energy production, buoyancy, and non-self-similarity.