Observation of traveling waves in the three-dimensional boundary layer along a yawed cylinder

Experimental investigations are made of crossflow instability of the three-dimensional boundary layer along a yawed circular cylinder. To explore the effect of environmental disturbances on transition, comparative experiments are performed in high- and low-turbulence wind tunnels. Independent of turbulence level, unsteady disturbances with definite frequencies instead of stationary vortices are observed in hot-wire surveys of the unstable flow filed. Detailed measurements with the aid of hot-wire arrays have confirmed that the unsteady disturbances are identical with the so-called crossflow traveling waves predicted by linear stability theory as the most unstable disturbances, although some discrepancies still remain between experimental results and theoretical predictions.