Low drag surface

A low drag surface having an upstream edge and a downstream edge and having a flow of fluid across the same in a direction from the upstream edge toward the downstream edge, said low drag surface comprising an intermittent linear vortex producing means spaced downstream of the upstream edge and extending transverse to the direction of said flow of fluid across the surface for producing a succession of rolling vortices extending transverse to the flow of fluid, said intermittent linear vortex producing means including a re-entrant discontinuity in the surface, the discontinuity extending transverse to said flow and providing a sheltered leeward face against which each of said vortices may be successively formed;

• and
• means spaced downstream from said discontinuity and defining a ramp parallel therewith, said ramp being inclined outwardly from the surface and causing each of said vortices, when formed against said sheltered leeward face, to be dislodged from the sheltered leeward face to permit another vortex to be formed, the successively formed vortices being allowed to roll in a downstream direction across the surface in succession to reduce the drag on the surface.