Turbulence Effect on Frequency Characteristics of Unsteady Motions in Wake of Wing

Abstract

The ine uences of the freestream turbulence intensity on the frequency characteristics of the instability wave and vortex shedding in the wake of a NACA 0012 wing model are studied experimentally. The functional relationships among the Strouhal, Roshko, and Reynolds numbers in the viscous-effect-dominated and the inertialeffect-dominated regimesareobtained. Thefreestream turbulence hasapparenteffect on theStrouhal and Roshko numbers in the regimes of instability wave and laminar and subcritical vortex-shedding modes that are obtained at low Reynolds numbers. However, at large Reynolds numbers in the supercritical vortex-shedding mode, particularly at large angles of attack, the effect of freestream turbulence intensity on the frequency selection is not signie cant. For the instability wave, the Roshko number decreases with the increase of freestream turbulence and then approaches constant values when the freestream turbulence intensity is larger than a critical value between 0.45 and 0.5%. However, the Strouhal number of the vortex shedding of laminar and subcritical modes increases signie cantly with an increase of turbulence intensity and then decreases when the freestream turbulence intensity is larger than a critical value between 0.45 and 0.5%. The change of the characteristic surface e ow modes may contribute to the dominant mechanism of the existence of critical freestream turbulence intensity. Nomenclature C = chord length of wing, 6 cm d = length of wing-section projection on cross-stream plane f = frequency of instabilities in wake region, Hz ` = distance in X direction measured from mesh screen M = mesh size of turbulence generation screen Rec = Reynolds number based on chord length of wing, uwC/m Red = Reynolds number based on cross-stream projection of wing section, uwd/m Rod = Roshko number of vortex shedding, f d 2 /m Srd = Strouhal number of vortex shedding, f d/uw T = freestream turbulence intensity u = x component of local instantaneous velocity uw = average freestream velocity X = streamwise coordinate, originated from leading edge of wing model on root-plane Y = spanwise coordinate, originated from leading edge of wing model on root-plane Z = crosse ow coordinate, originated from leading edge of wing model on root-plane a = root angle of attack m = kinetic viscosity of air stream q = density of air stream U = power spectrum of e uctuating velocities