Instantaneous wave structure of localized secondary instabilities of a swept-wing boundary layer

A detailed systematic experimental study of four types of high-frequency and medium-frequency secondary instability of a swept-wing boundary layer has been carried out under conditions of predominance of the stationary primary cross-flow instability modes. The measurements were carried out with a hot-wire anemometer in a low-turbulence wind tunnel at a low subsonic velocity of the incident flow under conditions of excitation of controlled primary stationary disturbances and controlled non-stationary secondary disturbances. The structure of wave fronts of several types of secondary disturbances in a frequency range from 100 Hz to several kilohertz has been studied in detail. A new type of medium-frequency secondary instability, called type IV, has been found. The wavenumbers, wavelengths, phase velocities, and propagation angles of all four types of secondary disturbances are obtained along three spatial coordinates. The evolution in time of the instantaneous spatial structure of secondary disturbances in a broad frequency range is visualized.