Modal analysis of compressor cascades with sweeping jet actuator and pulsed jet for active flow control based on large Eddy simulation

Abstract

This study utilized large eddy simulation (LES) to explore the mitigation effects of endwall pulsed jet (PJ) and suction-surface sweeping jet actuator (SJA) controls on corner separation. Two modal analysis methods, proper orthogonal decomposition (POD) and dynamic modal decomposition (DMD), were applied to extract the main structures in the flow field and to analyze the mechanism of the effect of an unsteady jet on flow separation. First, LES was first performed in a linear cascade to capture the pulsating information in the flow field. The spectral analysis shows the dominance of the shedding vortex dynamic feature in the measured position of the original scheme. Furthermore, the dominant flow pattern in the flow field was obtained using POD. Both unsteady jet schemes made the energy distribution in the decomposed reconstructed flow field of the POD more dispersed than that in the origin. However, POD loses the actual physical meaning of the eigenvalues and corresponding modes of that order when finding the optimal orthogonal basis. Therefore, this study used DMD to retain information reflecting the time-evolution process. The magnitude of each order of energy corresponded to the characteristic frequency of the flow field. The highest energy modal frequency was consistent with the jet frequency for both jet schemes. The results of the modal analysis showed that the PJ and its induced small-scale flow structures dominate most of the higher energy modes. These small-scale flow structures inhibit the development of transverse secondary flows at the endwall. In comparison, the dominant flow-field structure using the SJA is a large-scale streamwise vortex. The sweeping jet accelerates the corner low-energy fluid over a broader area by the induced streamwise vortex itself, thus controlling the corner separation.