Time-Resolved Measurements of Cellular Separation on a Stalling Airfoil

Abstract

The morphology of spanwise cellular patterns observed in the post-stall flow over wings with trailing-edge separation is the focus of the current investigation. Commonly termed stall cells, the time-averaged flow field takes on the appearance of a mushroom shape in the wall shear pattern. Much like the dynamics of a separation bubble, important unstable effects apparently underlie the formation of stall cells, given that they appear within a relatively narrow angle of attack range on wings. The phenomenon is explored in the shallow stall regime using a rectangular NACA 0015 airfoil of aspect ratio 2.78 at a chord Reynolds number of 560,000. Time-resolved, planar particle image velocimetry measurements acquired at 3.0 kHz are presented to characterize the cell shear layer dynamics. Surface oil particle visualizations using a high-speed camera are also presented to capture the evolution of the cell pattern in a blowdown wind tunnel from flow startup to steady-state. NomenclatureAR = wing aspect ratio; for rectangular wing, AR = b/c b = wing span c = airfoil chord C L = lift coefficient f = flow oscillation frequency  = frontal projected height of airfoil chord,  sin c M = Mach number n = number of stall cells Re c = Reynolds number based on chord,  / c V  St = Strouhal number,  V f /  T = time for surface pattern to reach steady-state t = time parameter u = streamwise velocity component v = cross-stream velocity component V = velocity vector x = spatial coordinate in chordwise direction y = spatial coordinate in cross-stream direction z = spatial coordinate in spanwise direction  = airfoil angle of attack  = kinematic viscosity  = free stream value  = vector magnitude  = average  = fluctuation about average

Investigation of the three-dimensional flow past a flatback wind turbine airfoil at high angles of attack

“…It is noted at this point that very low frequencies have been observed in 3D separated flow over airfoils and more specifically when stall cells appear. [59][60][61][62] This low frequency has been attributed to a wholesale expansion/contraction of the stall cells 61 and, when normalized with the projection of the airfoil chord (St c;proj ¼ fcsin a ð Þ U1 ), corresponds to Strouhal number of St c;proj $ Oð10 À2 Þ. This nondimensional quantity is also plotted in Fig.…”

Unsteady RANS Simulations of Strong and Weak 3D Stall Cells on a 2D Pitching Aerofoil

“…The exact mechanism of stall cell formation is yet to be determined. Weihs and Katz [41] ascribed the phenomenon to the Crow instability [42] while Disotell and Gregory [43] claimed that stall cell formation could be related to shear layer instability. A more convincing theory was recently proposed separately by Sparlart [44] and Gross et al [45], who analyzed the stall cell formation based on the lifting line theory.…”