14th Int Symp on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 07-10 July, 2008 - 1 - Improving the measurement accuracy of PIV in a synthetic jet flow Tim Persoons 1 , Tadhg S. O’Donovan 2 , Darina B. Murray 3 1: Mechanical Engineering Dept., Trinity College, Dublin 2, Ireland, tim.persoons@tcd.ie 2: School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh, EH14 4AS, U.K., T.S.O'Donovan@hw.ac.uk 3: Mechanical Engineering Dept., Trinity College, Dublin 2, Ireland, dmurray@tcd.ie Abstract 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. 1. Introduction A synthetic jet flow is generated by periodic pressure variations in a partially enclosed cavity, forcing fluid through an orifice. The successive suction and ejection of fluid causes vortices to form at the exit of the orifice. If the oscillation amplitude is sufficiently large, the vortices detach and propagate away from the orifice. Synthetic jets have been studied extensively for applications in active flow control (Glezer and Amitay 2002). Furthermore, initial studies by Kercher et al. (2003) and Pavlova and Amitay (2006) have shown the potential of impinging synthetic jets to increase convective surface heat transfer rates in applications with confined geometries such as electronics cooling. An unconfined synthetic jet flow is characterised by two parameters: the dimensionless stroke length L 0 /D and the Reynolds number Re = U 0 D/ν, where D is the orifice hydraulic diameter (see Fig. 1), 2 0 0 ( )d T L Ut t =  , T is the oscillation period, U 0 = L 0 /(T/2) and U(t) is the mean orifice velocity. The peak velocity U max = (π/2)U 0 . L 0 /D is inversely proportional to a Strouhal number, since L 0 /D = ½(f D/U 0 ) -1 . An impinging synthetic jet is further characterised by the spacing H/D between orifice and surface.