Experimental study on shear layer oscillations in the slat cavity of 30P30N multi-element high-lift airfoil

This paper investigates the relationship between the low-frequency multiple tonal noise of the slat and the flapping of the cavity shear layer using the 30P30N three-element high-lift airfoil as the experimental object. The experiment was conducted in the A3 acoustic wind tunnel of Southern University of Science and Technology. To overcome the limitation of the physical size of the cavity, a new wall pressure sensor based on the Micro-Electro-Mechanical System (MEMS) sensor combined with the Flexible Printed Circuit (FPC) was developed. Spectral results reveal multiple groups of discrete peaks near the first-order frequency in the Rossiser model. These peaks’ corresponding frequencies are closely related to the variation of the reattachment point caused by the flapping of the cavity shear layer. The flapping range of the cavity shear layer was further determined by the mean flow field obtained from the two-dimensional hot-wire measurement. Based on the parameters obtained from fitting flow field measurement results, multiple groups of frequency values caused by the change of the reattachment point were further calculated. These frequencies are highly consistent with the frequencies obtained by the MEMS microphone. Furthermore, coherence analysis of the synchronous measurement results of the MEMS microphone and the far-field microphone reveals that the variation of the dominant frequency of the cavity tonal noise is also closely related to the flapping of the cavity shear layer. In summary, a self-excited oscillation process similar to the square cavity flow is present in the slat cavity. Moreover, the shear layer of the slat cavity is flapping in a certain range, and the flapping process will further affect the change of the tonal frequency dominating in the slat cavity.