Effect of streamwise vortices on wake properties associated with sound generation.

Abstract

T important aspects of the viscous wake leading to the generation of sound sources are: 1) unsteady twodimensional disturbances such as the Karman vortex street, and 2) unsteady forces generated by the passage of a rotor blade through the two-dimensional wake of a stator, or vice versa. The unsteady wake of an aircraft makes a significant contribution to the total noise during the landing approach, whereas the unsteady forces due to wake impingement on stator and rotor blades contribute significantly during takeoff and cruise. The latter effect, analyzed by Kemp and Sears, is responsible for about half of the pure-tone noise generated by turbomachines; the other half originates with the mutual interference between the rotor and stator flowfields. Wind-tunnel tests reported here show that the streamwise vortices shed from vortex generators near the trailing edges of a flat plate and an airfoil have a strong suppression effect on the formation of the Karman vortex street, and reduce the effective area of the velocity deficit in the wake, as seen by the following blade in a turbomachine. Both effects tend to suppress significantly the formation of sound sources.