On the total entrained flow rate of artificial downwelling

Abstract

Artificial downwelling potentially counteracts severe eutrophication and hypoxia in coastal regions. The entrainment that commonly occurs in the downwelling-induced plume can develop around a compensatory downward flow at a scale of kilometers, which favors below-pycnocline ventilation and thus mitigates hypoxia in bottom water. However, little discussion has been made on the entrainment of artificial downwelling. The paper focus on the total entrained (TE) flow rate that represents the magnitude of entrainment transport. Injection of a negatively buoyant jet from a round pipe into a stagnant and homogeneous ambient medium is numerically studied using a verified standard k- ε turbulence model, in which different initial pipe flow speeds, density differences and pipe radii are involved. Previous theories and experiments confirm the numerical model. An empirical formula is fitted to the simulation data to calculate the TE flow rate, which permit quantitative analysis of the capacity of ventilation for artificial downwelling. The efficiency index of ventilation (EIV) is also defined and applied to study a typical downwelling device, i.e., a tidal pump, to suggest optimal engineering parameters.