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The flow field of a chevron synthetic jet in impinging configuration at Reynolds number Re = 4500 , dimensionless stroke length L 0 / D = 28 and nozzle-to-plate distance H / D = 2 is experimentally investigated by means of Stereoscopic Particle Image Velocimetry. This study is motivated by the need of explaining the heat transfer behaviour of the chevron synthetic jet highlighting its differences with respect to the circular synthetic jet. Two different experiments are carried out. The first experiment is devoted to the characterisation of the flow field evolution on the impingement plate. The second experiment is a three-dimensional reconstruction of the entire flow field at three characteristic phases. The results show that the circular synthetic jet presents a circular velocity pattern on the plate, while the chevron exit leads to the acceleration of the fluid, thus to a larger impinging velocity with a star-shaped pattern. Furthermore, in both configurations a secondary vortex ring is formed on the wall at a radial position of about 2 diameters. For the circular synthetic jet, the intense turbulence levels are located in two annular regions: the inner region is due to the turbulent wall jet generated by the impinging trailing jet and the outer region is due to the effect of the generation of the secondary vortex ring. For the chevron synthetic jet, the intense turbulence level is produced by the turbulent wall jet along preferential paths. These preferential paths are caused by the star-shaped distribution of impinging velocity and the impinging streamwise coherent vortices. These features characterizing the impinging flow field are fundamental to understand and explain the mechanisms behind the heat transfer behaviour of such devices.

An experimental investigation on the heat transfer enhancement achieved by impinging synthetic jets with vortex generators, in the form of chevron elements at the nozzle exit, is carried out. The heated thin foil heat transfer sensor is used in conjunction with the infrared thermography to measure the spatial distribution of the Nusselt number on the target plate. The heat transfer rates of impinging circular and chevron synthetic jets are compared under the same condition of cavity pressure. A parametric study on the effect of the dimensionless stroke length and the nozzle-to-plate distance on the heat transfer rates is carried out. For increasing dimensionless stroke length, at short nozzle-to-plate distances, the chevron synthetic jet reveals a star-shaped heat transfer pattern similar to that observed for an impinging continuous one. The results show that a chevron exit geometry can provide a significant heat transfer enhancement for relatively small nozzle-to-plate distances, up to a 20% increase with respect to the circular synthetic jet. At small dimensionless stroke lengths, such an enhancement is observed for a wide range of nozzle-to-plate distances.

Synthetic jets are largely used in the electronic cooling field; indeed their heat transfer performances have been widely investigated. The heat transfer performances have been enhanced through the design of innovative synthetic jet devices, as the twin synthetic jets device. Obviously the heat transfer performances of the classic and innovative synthetic jet devices are strictly related to their impinging flow field. Therefore the behavior of impinging single and twin circular synthetic jets in phase opposition is experimentally investigated by using Particle Image Velocimetry (PIV) at Reynolds and Strouhal numbers equal to 5100 and 0.024, respectively. Several nozzle-to-plate distances (H), ranging between 2 and 10 nozzle diameters (D), have been investigated. The time-averaged behavior of the velocity components has been reported and discussed. Their distributions, near the impinging plate, have been described. For the single jet, at short nozzle-to-plate distances (H/D6), the axial velocity profile is bell-shaped. This is ascribed to the adverse pressure gradient strength and the potential core-like region extension. External oscillations are observed in all the flow field quantities near the impinging plate at 2 diameters from the stagnation point due to a secondary counter rotating vortex ring generation. The presence of such a counter rotating vortex ring decreases as the nozzle-to-plate distance increases. Comparing the two synthetic jet configurations, higher axial velocity and turbulence level but lower axial phase-correlated organized contribution to velocity have been found for the twin case because of the jets interaction. The evolution of the flow field for both configurations has been explained through phase-averaged measurements. High turbulence is observed along the shear layer emanated by the nozzle edge and in the vortex ring core. During the suction phase the saddle point shows a different behavior in the two configurations. In the single case, the saddle point reaches the impinging plate causing injection of air from the plate into the device. Differently the twin configuration generates two saddle points which do not reach the impinging plate because of the presence of the other impinging synthetic jet.

The behaviour of a chevron syntethic jet in impinging configuration is experimentally investigated by using Stereoscopic Particle Image Velocimetry at Reynolds number equals to 4500, dimensionless stroke length (i.e. nverse of Strouhal number) equals to 28 and at a nozzle-to-plate distance of 2 diameters. The effect of the presence of the impinging plate on the flow field organisation is discussed. The characterisation of the flow field evolution on the impingement plate is studied. Then, a complete three-dimensional reconstruction of the entire flow field at the phases, chosen by analysing the results obtained through the previous analysis, is carried out. Three phases are reconstructed corresponding to three synthetic jet conditions: approaching vortex, impinging vortex and flow developing on the plate. The chevron synthetic jet is characterised by two main features: the cross-shaped exit leads to the acceleration of the fluid, thus to a larger impinging velocity; second one, the presence of streamwise structures are responsible forthe generation of preferential paths along which the turbulent wall jet develops modelling the convecting primary coherent vortex structure in a hexagonal shape.

In the present study, the flow fields generated by two synthetic jets with a chevron and a conventional circular nozzle exits are studied and compared. For both configurations, the devices are operated at the same input electrical power, thus leading to Reynolds and Strouhal numbers equal to 5600 and 0.115 (for the circular exit) and 6000 and 0.106 (for the chevron exit). Phase-locked stereoscopic particle image velocimetry measurements are used to reconstruct the three-dimensional coherent vortex structures. Time-averaged and phase-averaged mean and turbulent statistics are analysed and discussed. The flow field strongly depends on the exit geometry. In presence of the chevron exit, the conventional vortex ring issued through the circular nozzle exit, is replaced by a non-circular vortex ring with additional streamwise vortices. The mutual interaction between these structures prevents the axis-switching of the non-circular vortex ring during its convection. These streamwise vortices disappear convecting downstream and the vortex ring assumes a circular shape. Comparing the two configurations, the chevron exit generates a larger time-averaged streamwise velocity along the centreline but with lower turbulent kinetic energy intensity. Differences are also present between the notch and the apex planes of the chevron exit. In the notch plane, both the time-averaged axial velocity component profile in the spanwise direction and the shearlayer width are wider than in the apex plane. Furthermore, the presence of the streamwise vortices causes a flow motion towards the jet axis in the apex plane and an opposite motion in the notch plane.