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Improving the measurement accuracy of PIV in a synthetic jet flow

Impinging synthetic jets have been identified as a promising technique for obtaining high convective heat transfer rates in applications with confined geometries such as electronics cooling. Using a partially enclosed cavity with orifice, alternating fluid suction and ejection generate a periodic vortex train. This flow creates stronger entrainment of surrounding air and more vigorous mixing near the heat transfer surface compared to continuous impinging jets of comparable Reynolds number. A better understanding of the flow field is needed to identify the governing convective heat transfer mechanisms and optimise the heat transfer to a synthetic jet. Particle image velocimetry is the preferred technique to quantify the whole flow field. However, a synthetic jet flow is characterised by large velocity gradients. For round jets in particular, the maximum velocity in the free jet region differs significantly from the velocity in the wall jet region. A multi-double- frame (MDF) PIV technique has been developed which determines the local optimal pulse separation, based on the maximum value of the correlation peak ratio weighted with the estimated relative velocity accuracy. The technique is used in conjunction with state-of-the-art multipass cross-correlation PIV algorithms with window shifting and deformation. Using MDF-PIV, a higher accuracy has been obtained for the round synthetic jet flow field compared to standard PIV with a single pulse separation, particularly in low velocity regions. A much higher percentage of velocity vectors correspond to particle displacements sufficiently greater than the uncertainty level. The dynamic velocity range increases proportionally to the ratio of applied pulse separations. Results using MDF-PIV are presented for the flow field of an impinging synthetic jet with conditions typical for heat transfer applications. An improved accuracy is notable for time-averaged streamlines, phase- resolved vorticity and turbulence intensity distributions.