| The unsteady flows generated by the harmonically variable inflow velocities are studied in this thesis by using a numerical method for the time-accurate solution of the Navier-Stokes equations. This method, which has been developed by Mateescu and Venditti, uses an implicit three-point-backward scheme for the real time discretization, and a pseudo-time discretization based on a relaxation procedure using artificial compressibility. A special decoupling procedure is used to eliminate the pressure from the momentum equations with the aid of the continuity equation in pseudo-time, in order to reduce the problem to the efficient solution of scalar tridiagonal systems of equations. The method uses a finite difference formulation on a stretched staggered grid, which was validated for the steady incompressible flows in a duct with a downstream-facing step, by comparison with previous computational and experimental results.; This thesis first presents the solutions obtained for the unsteady flows with multiple separation regions in a duct with fixed geometry, which are generated by the variation in time of the inflow velocities. Then, the solutions for the unsteady flows generated by both an oscillating wall and the variation in time of the inflow velocity are also presented. The influence of the Reynolds number, of the inflow velocity amplitudes, and of the phase difference is also thoroughly studied. |
