Experimental Time Resolved Flow Features of Separation Over an Elliptic Leading Edge

Abstract

This study investigates the flow over a flat airfoil of low aspect ratio using experimental, time-resolved particle image velocimetry methods. The flow conditions are characterized by low Reynolds flow at two relatively high angles of attack, 15° and 20°. A high pass Gaussian filter was employed to help identify structures within the flow field. Also, a velocity gradient tensor method, λ 2 , was used to visualize vortical structures within the flow. The Proper Orthogonal Decomposition was applied to analyze primary features within the flow. A few, high energy modes are found which recreate the process of vortex shedding for both angles of attack. Nomenclature a k (t) = amplitude time series function for mode k d(x,t) = function representing all velocity data points over all time steps D = NxM matrix of velocity at N locations and M times f k (x) = spatial eigenfunction of velocity for mode k G = 2x2 matrix of 2D velocity vector gradient α = angle of attack λ 2 = vortex detection parameter, second eigenvalue of G λ k = total variance of amplitude time series for mode k Σλ k = cumulative modal variance up through mode k Σλ tot = cumulative modal variance of all modes