Experimental and Numerical Investigations on the Asymmetric Wake Vortex Structures of an Oscillating Airfoil

Abstract

Wake structures behind a sinusoidally pitching NACA0012 have been studied with both experimental and numerical approaches. The results from Particle Image Velocimetry (PIV) measurements and those from the high-order unstructured dynamic grid based spectral difference (SD) compressible Navier-Stokes (NS) solver agree well with each other. Two types of wake transition processes, namely the transition from a drag-indicative wake to a thrust-indicative wake and that from the symmetric wake to the asymmetric wake are experimentally investigated. Asymmetric wake phenomena have been emphasized in the study. The deflected wake is found to appear at approximately Strouhal number 0.31 and reduced frequency 15.1 for the pitching amplitude ૞ ࢕ . As the Strouhal number increases, the dipole mode of the vortex pair becomes more apparent, which is considered to be a vital element to form the asymmetric wake. Besides, the dependency of the deflective directions of the asymmetric wake on other parameters, e. g. initial phase angle, reduced frequency, has been analyzed both experimentally and numerically.