Unsteadiness of the flow rate of the gas passing through gas-dynamic tracts with contour discontinuities

Apart from usual noise caused by a free air stream, the operation of wind tunnels with an open test section is accompanied by the excitation of periodic fluctuations of pressure and flow rate, which increase with the flow velocity. To verify the fact that the vortices pass through the throat, Schlieren pictures of the flow in a plane model were taken. The vortices generated in the subsonic section of the channel come into the throat. For a symmetric inflow, the vortices pass from below and from above with an equal probability. Judging by the behavior of the characteristics passing from the channel generatrices, the velocity of vortices coincides with the flow velocity.