Transient decay times and mean values of unsteady oscillations in transonic flow

Kerlick and Nixon (ibid., vol 19, p.1496, 1981) have made the important point that, when using a finite difference, time-marching computer code to investigate the lift on an oscillating airfoil in transonic flow, it is necessary to carry the solution sufficiently far forward in time that an essentially steady-state solution is obtained. Moreover, they offer a method for estimating the transient time before the steady state is reached. They note that, if one stops the time-marching solution before the transient is complete and the steady state is reached, then one may reach the incorrect conclusion that a change in the mean lift has occurred due to the oscillating motion of the airfoil when in fact no such change has occurred. The authors comment that for a narrow Mach number range the time for the transient to decay and a steady state to be reached is extraordinarily long. Moreover, for a very narrow range of Mach number a nonzero mean value of lift can occur for an airfoil of symmetrical profile oscillating about a zero angle of attack

Duke Scholars

Author Earl H. Dowell Thomas Lord Department of Mechanical Engineering and Materia ...