Mixing of Saline Gravity Current Jet into Ambient Freshwater in Weakly Turbulent Regime

Abstract

This paper investigates mixing of a saline water jet into ambient freshwater at rest in a large basin. Reynolds-averaged Navier–Stokes (RANS) and diffusion-convection equations of the saline water volume fraction are used to model the mixing and the propagation of the saline gravity current jet. The comparison of the numerical and experimental front positions shows good agreement for Reynolds and Richardson numbers varying in the ranges 2,222<R<3,889 and .3<Ri<.1, respectively. The numerical simulations of the hydrodynamic fields show that the velocity maximum is located at .18 z.5, where z.5 is the height at which the mean velocity u¯ is equal to half the maximum velocity u¯max. The local gradient Richardson number Rig shows that maximum turbulent mixing occurs at z≈z.5 in the first stage of the gravity current close to the inlet, whereas far from the inlet, the turbulent mixing collapses. The entrainment depends both on the front position and on the values of the isodensity threshold, whereas it is independent of bulk Reynolds and Froude numbers in the narrow ranges (8<Rb<12; .3<Fb<.5).