A Modal Description of Dynamic Wake Meandering

Scans from a nacelle-mounted lidar provide time series wake measurements during three time periods, from which we describe the coherent turbulent structures that contribute to wake meandering through the proper orthogonal decomposition. Subsets of modes are used to make low-order flow field reconstructions in a combinatorial sense, providing more than 30,000 sets of meandering statistics for each case. A regression test using the reconstructed flow statistics identified the modes that contribute most to the accurate description of wake meandering. Spectra were defined from each mode coefficient highlighting the dominant Strouhal number associated with each cohere turbulent structure. The lowest ranking modes do not necessarily contribute most to the accurate representation of wake meandering. Instead, some modes appear to have no influence on meandering dynamics, and still others consistently detract from wake meandering represented in low-dimensional flow reconstructions. No consistent relationship is revealed between characteristic frequencies for each mode and either the inflow or wake measurements, suggesting that a more complex relationship between wake and inflow turbulence may be needed to accurately describe meandering.