When using a collocated grid for the discretization of the unsteady Navier-Stokes equations special care has to be taken in the evaluation of the cell face velocity as if it is not properly calculated the resulting flow field may be dependent on the time step. This dependency increases as the time step is reduced so this problem can be of paramount importance in rapidly varying flows. As an illustration of the problem in a flow of industrial interest a synthetic jet has been chosen. Although the primary goal of the paper is not to compare computational and experimental results, the assessment with experimental data will highlight the discrepancies in the computational results with different time steps. For comparison purposes a well documented case was chosen: the first test case of the synthetic jet workshop organized by NASA in 2004, but with the new 2006 data. This flow is produced by a moving diaphragm at one of the sides of a cavity connected to an otherwise stagnant air through a slot. Near the slot exit the flow is almost bidimensional so in order to reduce computational time it has been modelled in a 2D domain with a transpiration velocity at the bottom boundary of a simplified cavity. This velocity tries to reproduce the waveform of the measurements at the slot exit with an appropriate combination of Fourier modes.
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