Interactions of vortical unsteady flows with structures are often encountered in several engineering applications. Such flow structure interactions (FSI) can be responsible for generating significant loads and can have many detrimental structural and acoustic side effects, such as structural fatigue, radiated noise and even catastrophic results. Amongst the different types of FSI, the parallel Blade-Vortex Interaction (BVI) is one of the most prominent. The authors in a previous work (Weiland and Vlachos, 2006) reported an active flow control technique that successfully minimizes the parallel BVI. This technique is based on disrupting the incident vortex using a jet issued via Leading Edge Blowing (LEB), hence, hereon we term the method LEB. The effectiveness of the method was experimentally analyzed using Time-Resolved Digital Particle Image Velocimetry (TRDPIV) recorded at a rate sufficient to fully resolve the spatio-temporal dynamics of the flow field combined with simultaneous accelerometer measurements of the structure. These measurements quantitatively document the FSI dynamics. While our results demonstrated that for the range of our experimental parameters the LEB is successful in dramatically modifying the BVI, the question still remains as to which physical processes are responsible for this reduction. This paper represents a continuation of our effort to further understand the dynamics of using active flow control to mitigate BVI. We present Proper Orthogonal Decomposition (POD) analysis of the temporally resolved planar flow fields for two extreme cases that were reported in the previous work. The two cases correspond to a large wake generator and a small wake generator. The POD technique was chosen specifically for its ability to reduce a complicated flow field into its optimal fundamental modes with a description of the energy contained in each mode, thereby simplifying the dynamics of a flow-field system for analysis. Results of the POD analysis for the small wake generator indicate that for no LEB, the fundamental (i.e. most energetic) mode is given by the vortex shedding of the circular cylinder upstream. The addition of LEB reduces the energy contained in this fundamental mode. Thus the LEB jet has the effect of reducing the flow field coherency; the structure of the exciting vortices is broken up into smaller vortices which have less or little effect on the transfer of energy from the wake to the airfoil. For the case of the large wake generator, the LEB jet has the opposite effect: the jet organizes the circular cylinder wake into flow structures that maintain their form, and thus the wake retains its ability to excite the airfoil into vibrations.
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