LXXVI. Wave motions and the aerodynamic drag on a free oil surface

Summary Laboratory experiments are described in which air is blown over a viscous oil in a wind tunnel. At low windspeeds no waves at all appear on the surface, but at a sharply denned critical windspeed small ripples appear, some of which are unstable and grow quickly. The phenomenon is thought to be the instability predicted by Kelvin, which is not observed clearly on the sea. The sort of waves which grow by Jeffreys' ‘sheltering’ process do not appear until a higher windspeed than the critical one. The aerodynamic drag on the oil surface was also measured, and it appears that once the critical windspeed has been exceeded, the oil acts rather similarly to water, so far as drag forces are concerned.