An experimental study of coherent structures in shock boundary layer interaction at Mach 3
The flow structure and unsteadiness of Shockwave-Boundary Layer Interaction (SWBLI) has been studied using Rainbow Schlieren Deflectometry (RSD), Ensemble Averaging, Fast Fourier Transform (FFT), snapshot Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD) techniques. The Mach number of the approach free-stream was Mach = 3.1. The shockwave was generated with a 12° isosceles triangular prism attached to the roof of the test section. The RSD pictures were obtained with high speed cameras both at 5,000 frames per second (fps) and 30,000 fps; RSD images were used to determine the transverse ray deflections at each pixel location of the pictures. The objectives of this research were a) to identify the flow structures of SWBLI, b) to identify the frequencies of the unsteady flow in the SWBLI and c) infer the link between different regions of the flow using the obtained characteristic frequencies. The interaction region flow structure was described statistically with the ensemble average and, root mean square deflections. FFT technique was used to determine the frequency content of the flow field. The frequencies and associated spectral power were used in studying the flow unsteadiness and the energy scales of the flow were studied to obtain the dominant frequencies. Coherence function was used to identify coherent structures present in the flow field and their corresponding frequencies. The POD technique was used to determine the dominant flow structures in accordance with their energy content. POD results complement the findings of the ensemble averaging technique and show distinct regions that contain most of the energy in the flow field. Dynamic Mode Decomposition (DMD) study was used to identify the complex nature of the interactions. Utilization of DMD gave frequency content of the flow field which was missing from snapshot POD. The analysis was concentrated on the regions of interest which were in the vicinity of the separation bubble including the incident, transmitted, separation and reflected shock regions. Stable modes were obtained, and based on the peaks of global energy norm, respective frequencies were obtained. Dominant flow fields were reconstructed based on the obtained frequency peaks. The dominant frequencies and coherent structures obtained using FFT, coherence, snapshot POD and DMD were compared to yield common set of information.