Turbulent flow regimes behind a coastal cape in a stratified and rotating environment

A numerical study aimed at investigating the roles of both the stratification and topographic slope in generation of turbulent coherent structures in the lee of capes is presented. We consider a steady barotropic current impinging on an obstacle in a rotating and linearly stratified environment. The obstacle is a triangular prism and represents an idealized headland extending from the coast. Numerical experiments are conducted at constant Rossby number Ro = 0.06, varying the Burger number, Bu, and the obstacle slope, alpha. Flow regime diagrams in the Bu-alpha space are determined. For Bu < 0.1, vertical movement over the obstacle is enhanced and a fully attached regime with pronounced internal waves is established. For 0.1 =< Bu < 1, fluid parcels flow more around the obstacle than over it. Flow separation occurs and small tip eddies start to shed. For Bu >= 1, tip eddies merge to form larger eddies in the lee of the cape. We find that previous laboratory results cannot be used for gentler slopes, since bottom flow regimes are strongly dependent on alpha when Bu >= 1. The form drag coefficient exerted by the cape is at least two orders of magnitude larger than the one due to skin friction. It increases with increasing Burger numbers and decreasing slopes. When no separation occurs (low Bu), the increase with decreasing slopes is the result of the mixing associated with hydraulic phenomena. For intermediate and high Bu, form drag coefficients reach larger values as a result of the boundary layer mixing associated with flow separation. We put forth an empirical parametrization of form drag in the Bu-alpha space.