This paper investigates the unsteady characteristics of flow in a specific type of microvalve with sudden expansion shape. The geometry of the channel is such that the flow resistance caused by vortex structures is different in forward and backward flow directions. This introduces the geometry as a good nominee as a microfluidic rectifier with application in micropump systems and MEMS-based devices. A time-varying sinusoidal pressure was set at the inlet of the microchannel to produce unsteadiness and simulate the pumping action. The existence of block obstacle and expansion shoulders leads to various sizes of vortex structures in each flow direction. All simulation results are based on the numerical simulation of two-dimensional, unsteady, incompressible and laminar Navier-Stokes equations using finite element algorithm. Two fundamental parameters are varied to investigate the vortices growth throughout the time: the frequency of the inlet actuating mechanism and the amplitude of the inlet pressure. The frequency of the inlet pressure was varied in the range of 1 Hz to 1000 Hz to cover the frequency range in many micropump applications. In this way, one can see the effect of actuation mechanism on onset of separation and follow the size and duration of the vortex growth. In order to better understand the effect of geometry and frequency on flow field, the pressure contours are studied through one cycle. Finally, Strouhal number is calculated for frequency to obtain a measure of unsteadiness of the flow. A critical value of f = 250Hz is found for St = 1. The obtained results give a deep insight into the physics of unsteady flow in valveless microvalves and lead to better use of current design as a part of microfluidic system.

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