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Sweeping jets are oscillating jets generated by fluidic oscillators, i.e., devices designed to produce an oscillation of the flow without the use of any moving parts (Raghu, 2013). A typical configuration of such devices consists of an expansion chamber connected to a high-pressure supply via a converging nozzle and provided with feedback channels. The oscillating motion in the expansion chamber is triggered by an inherent flow instability and sustained by the flow rate across the feedback channels. Recently, sweeping jets have been studied in flow control applications for noise reduction, separation and circulation control over airfoils, control of resonant cavity oscillations and deflection of jets. The advantageous features of fluidic actuators, among which are the wide range of operating frequencies (up to kHz with meso-scale) and the distributed momentum addition, have also stimulated an increasing interest in their application to electronics cooling. Several recent studies on the convective heat transfer from impinging sweeping jets (e.g., Hossain et al., 2018; Park et al., 2018) have shown that, compared to conventional round jets, they offer higher cooling rates with better uniformity at least for small jet-to-plate spacings.

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.

Flow control via Synthetic Jets (SJs) is experimentally studied for a separated flow over a back-facing ramp in wind tunnel tests at height-based Reynolds number of 10700 using planar Particle Image Velocimetry. Two cases, i.e., either without control (baseline) or controlled by means of SJs, have been investigated. The effectiveness of the control is achieved through a momentum addition of cμ = 3.0 × 10^(-3) at a reduced frequency F+ = 0.34. Under the SJs control, the separated region undergoes a reduction in length of 62% of that observed for the baseline case.

The formation and evolution of quadruple synthetic jets issuing into quiescent air are experimentally investigated. Four different configurations (monopole-like, dipole-like, quadrupole-like and 90-degrees-circularly-shifted-jets) have been studied by varying the phase lags between the ejection strokes of the four jets, at constant values of the source Reynolds and Strouhal numbers, equal to 4000 and 0.2, respectively. For each configuration instantaneous three-component in-plane velocity fields in the most relevant streamwise planes are measured with stereoscopic particle image velocimetry. The evolution of the turbulent jet flow is analyzed by the velocity field triple decomposition, thus separating the time-averaged component from the forced periodic oscillation and the turbulent fluctuation. It is observed that the near region of the flow domain is dominated by the formation and advection of large-scale coherent vortex structures, which are originated by the interaction of the vortex rings inherent to the single synthetic jets. The vortex dynamics is captured by phase-averaged measurements and its role in determining the flow field structure of each of the investigated configurations is widely discussed. The vortex structures are observed to lose their coherence and break down into turbulence while propagating downstream. The decay of the large-scale coherent motions is accompanied by the establishment of a self-preserving behavior of both the mean flow and turbulent quantities. Nevertheless, it is shown that the far-field topology depends critically on the source conditions and near-field evolution.

This paper reports on an experimental study of the influence of the Strouhal number (0.011, 0.022 and 0.044) and orifice-to-plate distances (2, 4 and 6 orifice diameters) on the flow field of an impinging zero-net-mass-flux jet at a Reynolds number equal to 35 000. These jets are generated by a reciprocating piston that oscillates in a cavity behind a circular orifice. Instantaneous two-dimensional in-plane velocity fields are measured in a plane containing the orifice axis using multigrid/multipass cross-correlation digital particle image velocimetry. These measurements have been used to investigate the mean flow quantities and turbulent statistics of the impinging zero-net-mass-flux jets. In addition, the vortex ring behaviour is analysed via its trajectory and azimuthal vorticity as well as the saddle point excursion, the flow rate and entrainment. The behaviour of all these quantities depends on the Strouhal number and the orifice-to-plate distance because the former governs the presence and the relative importance of the vortex ring and the trailing jet on the flow field and the latter delimits the downstream evolution of these structures.

In this work, the heat transfer performance of impinging quadruple synthetic jets is experimentally investigated by means of infrared thermography and the heated thin foil technique. Four configurations (monopole-like, dipole-like, quadrupole-like and 90-degrees-circularly-shifted-jets), obtained by introducing different phase shifts between the single synthetic jets, are comparatively studied at constant values of the Reynolds and Strouhal numbers, equal to 4000 and 0.2, respectively. It is shown that the heat transfer behavior is strictly related to the dynamics of the coherent vortex structures that form after the interaction of the vortex rings inherent to the single jets. The signature of these vortical structures on the heat transfer distributions is evident at short impingement distances, where the most significant differences between the investigated configurations are indeed found. In the comparison, the 90-degrees-circularly-shifted-jets configuration is observed to offer the highest heat transfer rates, although with the lowest degree of uniformity, while the monopole-like jet is characterized by the lowest Nusselt number values and the highest uniformity. As the impingement distance increases, these differences are progressively smeared out and the four configurations exhibit equivalent performance from the viewpoints of both magnitude and uniformity of the heat transfer rate.

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.

This study focuses on the combined effect of the nozzle-to-plate distance and of the stroke length on the cooling performances of impinging synthetic jets. Infrared thermography is used as temperature transducer in conjunction with the heated thin foil heat transfer sensor to measure time- and phase-averaged convective heat transfer. All the experiments have been performed at a fixed Reynolds number equal to 5250, while different values of the dimensionless stroke length ( L 0 / D equal to 5, 10 and 20) and nozzle-to-plate distance ( H / D between 2 and 10) have been considered. At high L 0 / D , the heat transfer behaviour resembles that of a continuous impinging jet. It is characterized by a time-averaged stagnation Nusselt number maximum between H / D equal to 4 and 6 and inner and outer ring-shaped regions of Nusselt number maximum at short H / D . These two regions are replaced by a bell-shaped distribution at higher nozzle-to-plate distances. The existence of these regions is clearly observed through the phase-averaged heat transfer measurements. At short H / D , the heat transfer evolution reveals the simultaneous presence of two outer ring-shaped regions. The external outer region is ascribed to the strong coherence of the primary vortex ring, while the internal one is mainly due to the vortex rings generated by the Kelvin–Helmholtz instability along the trailing jet shear layer. At high H / D , the internal outer-ring shaped region disappears because of the weakening of the trailing jet Kelvin–Helmholtz vortex rings. In opposition, at short L 0 / D , the time-averaged stagnation point Nusselt number is found to have a maximum at lower values of H / D , and no inner ring-shaped region of heat transfer maximum is observed. This region is not present, at low dimensionless stroke lengths, because of the weakness and reduced extent of the trailing jet in the flow field. Indeed, the phase-averaged measurements mainly show the heat transfer caused by the impinging primary vortex ring. Despite the weakness of the trailing jet, the outer ring-shaped region of heat transfer maximum is observed at short H / D because of the presence of a strong primary vortex ring. In addition to that, the dimensionless stroke length and nozzle-to-plate distance also affect the heat transfer fluctuations, which decrease as L 0 / D decreases and/or H / D increases.

A comprehensive and detailed modelling to evaluate the efficiency of energy conversion of piezo-electric actuators driving synthetic jets is developed. The contribution is original because the analysis is based on the energy equations of the two coupled oscillators, the membrane and the acoustic one, which are directly derived from the corresponding motion equations. The modelling is validated against numerical as well as experimental investigations carried out on home-made actuators having brass and aluminum shims on which the piezo-disk is bonded. A major result is that for aluminum shim the global efficiency (representing the conversion of input Joule power to kinetic power) decreases with increasing the applied voltage. Another finding is that the conversion process of mechanical power transferred, by means of the driving membrane, to Helmholtz oscillator kinetic power scales dramatically with the coupling degree of the oscillators. The coupling degree influences the efficiency of two cavities actuators sharing the same piezo-diaphragm as well. Considerations are reported to relate the theoretical orifice efficiency to the practical jet efficiency issuing in the external field.

The temperature of a heated foil, exposed to a turbulent flow, changes continuously in time due to the fluctuation of the convective heat transfer coefficient. Measuring with enough temporal resolution the foil temperature map and solving the unsteady energy balance of the foil can allow restoring the instantaneous value of the convective heat transfer coefficient. Nevertheless, the high characteristic frequencies of turbulent flows (especially in air) result in very small temperature fluctuations, which are typically hard to measure with an infrared camera. This work presents a novel filtering approach based on Proper Orthogonal Decomposition which allows to dramatically reduce the measurement random noise, thus enabling the detection of small temperature fluctuations and, as a consequence, to improve the estimate of instantaneous distributions of the convective heat transfer coefficient. The proposed methodology is tested on synthetic jets impinging on a foil, actuated with a loudspeaker at various actuation frequencies. The measurement capabilities of the present technique are assessed through comparison with measurements obtained from raw data and phase averaged measurements. The authors wish to thank Mr. G. Sicardi for contributing the realization of the experimental setup. Marco Raiola, Carlo Salvatore Greco, Stefano Discetti, and Andrea Ianiro have been partially supported by grant DPI2016-79401-R of the Spanish Ministry of Economy and Competitiveness.

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.

This work presents an experimental investigation of impingement heat transfer in single circular synthetic jets and twin circular synthetic jets in phase opposition. All experiments have been performed at Reynolds number equal to 5100 and Strouhal number equal to 0.024 varying the jet axes distance and nozzle to plate distance. An IR camera is used as temperature transducer for both time average and phase average heat transfer measurements. Time average heat transfer maps show that single synthetic impinging jets have a behavior similar to that of continuous jets: at low nozzle to plate distance (up to 4 diameters) the heat transfer distribution shows an inner and an outer ring shaped region of maximum while for higher nozzle to plate distance such a feature disappears. While obviously the twin configurations produce an heat transfer enhancement due to the fact that two jets instead of one are impinging, the interaction is found in general to have a beneficial effect. The physical behavior is in common between single synthetic jets and twin configurations at jet axes distance equal to 3 and 5 diameters. The twin circular synthetic air jets, with jet axes distance equal to 1.1 diameter, shows a different behavior with respect to single synthetic jet for H/D equal to 2 but for values of H/D higher than 4 it starts acting like a single synthetic jet differently from the other twin configurations which behave as two separated synthetic jets. Phase averaged measurements allow for an accurate description of the heat transfer mechanism: at low nozzle to plate distances (2 and 4 diameters) the heat transfer is dominated by the unsteady impinging flow produced by the ring vortex that sweeps the wall and causes the formation of the inner and outer ring shaped regions. At higher nozzle to plate distance the heat transfer is due also to a steady and less coherent turbulent flow since the impingement occurs after the potential core and the saddle point.

In this work the flow features in the near field of single and twin synthetic jets are experimentally investigated. An acoustic resonator is designed as a cavity split in two equal sub-cavities by the membrane of a loudspeaker. Each sub-cavity is connected to the quiescent ambient by a cylindrical nozzle (a pipe) thus two circular synthetic jets in phase opposition are obtained. The influence of jets interaction has been evaluated by varying the jet-axes-distance (1.1, 3 and 5 nozzle diameters) at a fixed operating condition (Reynolds number equal to 6700 and Strouhal number equal to 0.046); the jets pulsating frequency is kept equal to the sub-cavities resonance frequency and the results are compared with those of the single circular synthetic jet in the same conditions. The flow is characterized with PIV phase averaged measurements. Results for the single synthetic jet are in good agreement with the ones present in the literature and a potential core region is defined for the phase averaged flow field of synthetic jets. Twin circular synthetic jets at jet-axes-distance equal to 3 and 5 diameters show the same behavior with respect to the single circular synthetic jet. Differently, strong interaction effects are seen at jet-axes-distance equal to 1.1 diameters: in particular a double vortex ring structure is visualized. This causes a different velocity distribution in the jet core, higher axial velocities and a lower jet width.

An experimental analysis of the flow field generated by an impinging jet equipped with a fractal grid insert located at the nozzle exit is carried out by means of Tomographic Particle Image Velocimetry. The Reynolds number based on the nozzle exit section diameter d is set to 15,000. The presence of the grid leads to a non uniform curvature of the jet shear layer. As a consequence, azimuthal instabilities are triggered in proximity of the nozzle exit section, causing the production of streamwise vortices. Furthermore, the organization of the coherent structures in proximity of the impinged wall is discussed and related to the convective heat transfer distribution. Owing to the presence of counter rotating wall vortices along the diagonals of the on-plate imprint of the fractal grid, a region of minimum in the scalar transfer map can be detected. In addition to that, similarly to the well known vortex rings that characterize the case of round jet without turbulence promoter, the presence of azimuthally coherent structures that might be generated by Kelvin-Helmholtz instability of the jet shear layer is presented and discussed.

The measurement of the convective wall heat flux in hypersonic flows may be particularly challenging in the presence of high-temperature gradients and when using high-thermal-conductivity materials. In this case, the solution of multidimensional problems is necessary, but it considerably increases the computational cost. In this paper, a low-computational-cost inverse data reduction technique is presented. It uses a recursive least-squares approach in combination with the trust-region-reflective algorithm as optimization procedure. The computational cost is reduced by performing the discrete Fourier transform on the discrete convective heat flux function and by identifying the most relevant coefficients as objects of the optimization algorithm. In the paper, the technique is validated by means of both synthetic data, built in order to reproduce physical conditions, and experimental data, carried out in the Hypersonic Test Facility Delft at Mach 7.5 on two wind tunnel models having different thermal properties.