Flow regime characterisation of an impinging axisymmetric synthetic jet

Impinging synthetic jets have excellent potential for energy-efficient local cooling in confined geometries. For a given geometry, synthetic jet flows are mainly characterised by the Reynolds number and the ratio of stroke length to a geometric length scale. The flow field of an impinging synthetic jet and the corresponding surface heat transfer distribution are strongly dependent on the dimensionless stroke length, yet few studies have investigated the flow field dependence for a wide range of stroke lengths. Therefore, the aim of this paper is to identify the various flow regimes as a function of stroke length. The experimental approach combines high speed particle image velocimetry and single point hot wire anemometry, and investigates an axisymmetric synthetic air jet impinging onto a smooth planar surface for a wide range of stroke length (3 < L0/D < 32) and nozzle-to-surface spacing (2 < H/D < 16). Since the Reynolds number effect is better understood, most of the presented results are for a single Reynolds number (Re = 1500). Four free synthetic jet flow morphology regimes are identified based on threshold values for the stroke length L0/D, which are in good agreement with previously published findings for an impulsively started jet flow. Furthermore, four impinging synthetic jet flow regimes are identified based on threshold values for the ratio of normalised stroke length to nozzle-to-surface spacing (L0 − 2D)/H, which are in good agreement with previously published thresholds for stagnation point heat transfer regimes.

► An impinging synthetic jet flow strongly dependent on the stroke length.
► High speed particle image velocimetry is combined with near-wall hot wire anemometry.
► The stroke length to nozzle diameter ratio (L0/D) determines the free jet flow morphology.
► The stroke length to surface distance ratio (L0/H) determines the impinging flow regime.
► Four impinging jet flow regimes found to agree well with heat transfer regimes.