This paper presents a time spectral method which is applied to study the unsteady effects on the aerodynamic coefficients of a stationary airfoil generated by the unsteadiness of the flow separations. Periodic variations of the aerodynamic coefficients of lift and drag were found at incidences larger than 8 degrees, due to the unsteady flow separation at low Reynolds numbers. This time spectral method considers truncated Fourier series expansions for the fluid variables and reduces the solution of the unsteady Navier-Stokes and continuity equations to the solution of several harmonic flow problems which are steady and solved sequentially. In this way, this time spectral method reduces considerably the computational time in comparison with the time accurate methods which have to compute several unsteady computational solutions until convergence. The obtained solutions are presented for unsteady lift and drag coefficients for several symmetric and cambered airfoils. The results obtained by this method are validated by comparison with previous numerical results obtained by a time accurate method. The influence of various flow and geometric parameters, such as Reynolds number and the airfoil relative thickness and camber on the unsteady aerodynamic coefficients are also presented in this paper.
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