Large eddy simulations of unsteady flows over a stationary airfoil

摘要

Highlights?Airfoil flows with oscillating freestream are computed with large eddy simulation.?Harmonically varying flows delay mean separation and advances mean reattachment.?Oscillating freestream speed suppresses fluctuation of some velocity components.?Computed airfoil forces are compared to predictions by inviscid theories.AbstractTwo groups of unsteady flows over a stationary SD7003 airfoil are studied with the large eddy simulation method. In the first group, the angle of attack (AoA) is fixed, while the freestream velocity magnitude varies harmonically with various frequencies and amplitudes. In the second group, the freestream velocity magnitude is fixed but its direction, therefore the AoA varies harmonically. Over the range of parameters considered in this study the mean lift and drag coefficients of the unsteady flows with oscillating freestream velocity magnitude are found to be nearly the same as those calculated for steady flows. However, there are significant phase shifts between the aerodynamic forces and the unsteady freestream velocity. The phase shift for drag force is larger than that for lift force, even though both increase as the frequency of freestream velocity oscillations increases. Furthermore, the computed lift amplitudes are found to be noticeably higher than those predicted by Greenberg's inviscid theory, while the lift phase shifts are in better agreement with the theory. For flows with oscillating freestream AoA, there is little change in the mean lift, while the mean drag is reduced by oscillations in AoA due to Katzmayr effect. As the frequency of oscillations in AoA increases, the phase shift for lift increases while that for drag decreases. Our results also indicate that the mean separation point moves downstream and the mean reattachment point moves upstream when the freestream velocity magnitude or the freestream flow direction oscillates with respect to the airfoil.]]>