Effect of trailing edge shape on the flow characteristics around an airfoil at low Reynolds number

Abstract

DNS of the flow fi eld around a NACA 0012 airfoil at Re = 50,000 and angle of attack 5 degrees with 3 different trailing edge shapes (straight, blunt and serrated) have been performed. Initially the time-averaged flow characteristics of the 3 trailing edges are studied. The presence of serrations in the trailing edge is found to create a spanwise pressure gradient, which is responsible for the development of a secondary flow pattern in the spanwise direction, and for the reduction of the wake de cit downstream of the troughs. The dominant flow structures and their frequencies are investigated for the straight and blunt trailing edges, using the Dynamic Mode Decomposition method (DMD) and resolvent analysis. Using these techniques we are able to identify and analyse the interactions between the modes which characterize the dynamic behaviour of the flow fi eld. It is shown that for the straight trailing edge airfoil, the DMD can capture the fundamental as well as the subharmonic of the Kelvin-Helmholtz instability that develops naturally in the separating shear layer. The blunt trailing edge results in periodic vortex shedding, with frequency close to the subharmonic of the natural shear layer frequency, which is found to have an upstream effect and force the separating shear layer into a lock-in state. A set of optimal responses, calculated to maximize the energy of the response, were used to accurately estimate the spectra in the most energetic points in the flow field. Lastly, a tripping method is applied which keeps the boundary layer attached. The pressure and skin-friction coefficients as well as the boundary layer pro files were significantly modified. Furthermore, the selection of the domain used to optimise the response, used in the resolvent analysis, was found to have an important effect on the determination of the optimal modes.