On the net forces produced by surface‐mounted obstacles

Numerical calculations of the forces involved in Ekman layer flow past three‐dimensional topography are presented. The forces are obtained from finite‐difference solutions of the Navier‐Stokes equations. The results confirm expectations from earlier two‐dimensional work that many flows produce no significant change in total momentum transfer between the fluid and the surface. Only flows generating some form of trailing vortex system appear capable of changing the total force on the lower boundary. The effects of Ekman boundary layer instabilities, and their interaction with topography, are also discussed.