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A flow visualization study has shown that strong Karman vortices a r e developed behind the blunt trailing edge of a plate when the free s t r e a m velocities over both surfaces are equal. These vortices tend to disappear when the surface velocities a r e unequal. This observation provided a n explanation for the occurrence and disappearance of the lip noise often present in coaxial jets. Vortex formation and lip noise occurred at a Strouhal number of about 0.2 based on the lip thickness and the average of the external steady-state velocities. Results from theoretical calculations of the vortex formation, based on an inviscid, incompressible analysis of the motion of point vortices, were in good agreement with the experimental observations. Results from theoretical calculations of the vortex formation, based on an inviscid, incompressible analysis of the motion of point vortices, were in good agreement with the experimental observations.

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