Investigation of under-expanded jet screech associated convective velocity based on high frequency sampled schlieren visualisations

The present study is based on schlieren visualisations constituted by hundreds of thousands frames filmed at a frame rate of 430 kHz of a screeching circular under-expanded jet operated at Mj=1.15. Two processing methods are introduced and point out a standing wave pattern. The features of this standing wave are used for the estimation of coherent structures convective velocity. It is found that convective velocity is varying along the jet, which is not consistent with commonly used screech frequency prediction models.