Interaction Of Synthetic Jet Research Articles

LES Investigation of a Piston-driven Synthetic Jet Actuator with Multiple Orifices

A piston-driven synthetic jet actuator has the potential for application in flow control and fundamental studies of turbulence, although the high-speed flow generated by this actuator is less investigated than a low-speed synthetic jet. The interaction of high-speed jets issued from a piston-driven synthetic jet actuator with multiple orifices is investigated with large eddy simulation (LES). The maximum jet Mach number is related to the maximum pressure inside the actuator regardless of the number of orifices. Temporal variations of the jet Mach number are almost identical for different cycles, and the jet formation in each cycle occurs under the same conditions despite the unsteady nature of the jet interaction. The phase-averaged statistics are used to examine the interaction of the synthetic jets. The converging, merging, and combined regions known for the interaction of continuous jets appear for the interaction of the high-speed synthetic jets slightly before the end of the blowing phase. However, the converging region is not clearly observed at the beginning of the blowing phase because the jets tend to be parallel to each other. Therefore, the combined region forms at a late stage of the blowing phase. Before the jets are combined, velocity fluctuations in the blowing phase become large near the furthest locations where the jets reach. Once the jets merge by their interaction, large velocity fluctuations are observed at the downstream end of the merging region. The probability density functions of velocity fluctuations in the blowing phase tend to deviate from a Gaussian distribution along the centerline of the jets. This deviation is more significant for the two-orifice model than for the four-orifice model under the same actuation frequency.

Interaction of synthetic jets with a massively separated three-dimensional flow field

The interaction of an array of finite span synthetic jet actuators with a massively separated flow was explored experimentally over a cantilevered, swept, and tapered model having a deflected control surface. The interaction of one synthetic jet with the local flow led to a flow field associated with formation and advection of vortex rings that bent into the flow, resulting in flow reattachment in their trajectory. Under specific conditions, when multiple jets were actuated, their interaction was either constructive or distractive, depending on the separation severity and spanwise flow magnitude. This work improves control of three-dimensional flow separation using distributed actuators.

Influence of synthetic jet in crossflow configuration on heat transfer enhancement

A two-dimensional computational model was developed to investigate the effect of synthetic jet interaction with crossflow in channel on the cooling of heat wall. A range of parametric studies by varying membrane oscillating amplitudes and inlet velocities of channel was conducted. The resulting complex, conjugate heat dissipation was analysed. The synthetic jet interacts with the channel flow and gives rise to heat dissipation enhancement at the heated wall. The upstream heat transfer deteriorates in the jet hole, and the heat transfer in the downstream region has improved.

Experimental and Computational Study on Separation Control Performance of Synthetic Jets with Circular Exit

This paper presents experimental and computational investigations of synthetic jets with a circular exit for improving flow control performance. First, the flow feature and vortex structure of a multiple serial circular exit were numerically analyzed from the view point of flow control effect under a cross flow condition. In order to improve separation control performance, experimental and numerical studies were conducted according to several key parameters, such as hole diameter, hole gap, the number of hole, jet array, and phase difference. Experiments were carried out in a quiescent condition and a forced separated flow condition using piezoelectrically driven synthetic jets. Jet characteristics were compared by measuring velocity profiles and pressure distributions. The interaction of synthetic jets with a freestream was examined by analyzing vortical structure characteristics. For separation control performance, separated flow over an airfoil at high angles of attack was employed and the flow control performance of the proposed synthetic jet was verified by measuring aerodynamic coefficient. The circular exit with a suitable hole parameter provides stable and persistent jet vortices that do beneficially affect separation control. This demonstrates the flow control performance of circular exit array could be remarkably improved by applying a set of suitable hole parameters.

Impact of orifice orientation on a finite-span synthetic jet interaction with a crossflow

The formation and evolution of flow structures associated with a finite-span synthetic jet issued into a zero-pressure gradient boundary layer were investigated experimentally using stereoscopic particle image velocimetry. A synthetic jet with an aspect ratio of AR = 18 was mounted on a flat plate and its interaction with a free stream, having a velocity of U∞ = 10 m/s (Reδ = 2000) at momentum coefficients of Cμ = 0.08, 0.33, and 0.75, was studied. The effect of the orifice pitch (α = 20∘–90∘) and skew (β = 0∘–90∘) angles on vortex formation as well as the global impact of the synthetic jet on the flow field was explored in detail. It was found that the orifice orientation had a significant impact on the steady and unsteady flow structures. Different orifice skew and pitch angles could result in several types of vortical structures downstream, including: no coherent vortex structure, a single (positive or negative) strong vortex, or a symmetric vortex pair. In all cases, the velocity near the wall was increased; however, cases with higher blockage (i.e., more wall-normal, transverse orifice) resulted in a strong velocity deficit in the free stream where orifices with lower pitch angles yielded in an increase in velocity throughout. The analysis is concluded with a summary of quantitative metrics that allude to flow control effectiveness.

Synthetic Jet Interaction With Approaching Turbulent Boundary Layer for Heat Transfer Enhancement

An experimental investigation of the interaction between a double-orifice synthetic jet with an approaching turbulent boundary layer developing on the bottom wall of square channel has been carried out. Schlieren visualization and velocity measurement using hotwire anemometry are carried out to characterize the synthetic jet actuators arrangement. Detailed heat transfer measurements are carried out using liquid crystal thermography. Hotwire measurements on the wall surface have been reported to understand the interaction between the approaching boundary layer and synthetic jet. Maximum overall heat transfer enhancement of 55.48% at an actuation voltage of 55 V is observed with entropy generation number equal to 0.649. The amplitude modulation is observed to increase the heat transfer enhancement in comparison to that without amplitude modulation.

English

This paper presents numerical simulations of synthetic jets in aerodynamic applications. We've analyzed the formation of isolated synthetic jets, the influence of nozzle geometry and the interaction of synthetic jets with a uniform flow on a flat plate. Also we've studied the influence of the active control in interaction with a stalled airfoil and the controllability of dynamic stall phenomenon. The results are obtained using a dedicated CFD solver. Appropriate comparisons are made with results from scientific literature; as well the numerical results are compared with a set of experimental images.

Flow Response of Active Flow Control Actuators

Numerical and experimental analysis of a synthetic jet actuator in quiescent air is reported. The study focuses on the actuator itself and on the vorticity field as well as on structures that are generated by the actuator. Large eddies simulation was used for numerical analysis and phase-locked, two-dimensional microscopic-particle image velocimetry technique was used in the experiment. In the numerical simulation, the equations were integrated using a nondissipative scheme that enforces discrete conservation of kinetic energy. The internal cavity flow of the actuator was part of the simulation. The actuator under consideration is of a unique design, fabricated to accommodate practical issues associated with helicopter rotor application. However, it has the same operating principles as a conventional synthetic jet actuator. Harmonics of the forcing frequency were traced in the velocity field. Insight into the evolution of the vortical structures, jet flapping, and onset of turbulence in the jet was retrieved. This is an important building block for better understanding of the interaction of synthetic jets with boundary layers.

An Assessment of Turbulence Models in Simulating a Synthetic Jet

This paper presents a two-dimensional numerical study on the interaction of synthetic jet and the cross flow inside a microchannel. Three different turbulence models namely the standard k-, Shear Stress Transport (SST) and Scale Adaptive Simulation Shear Stress Transport (SAS SST) were tested for their ability to predict the flow structure generated by a synthetic jet. The results are validated against existing experimental data. The SAS SST model was found to give the most realistic prediction of the fluid flow based on the good agreement with experimental data.

Interaction of a synthetic jet with the flow over a low aspect ratio cylinder

This paper discusses the interaction of a single synthetic jet with the flow over a finite span low aspect ratio cylinder. The synthetic jet was located at the mid-span of the cylinder, and was oriented such that the length of the jet orifice was parallel to the freestream direction. The investigation incorporated experimental techniques such as stereoscopic particle image velocimetry (SPIV) and hot-wire anemometry. The near wake of the cylinder was dominated by a non-negligible spanwise velocity, or downwash, from the cylinder's free-end that interacted with the near wake resulting in a highly three-dimensional flow field with a strong spanwise dependence. SPIV measurements within the near wake showed that, due to the actuation of the synthetic jet, the wake was vectored and narrowed, resulting in a redirection of the downwash behind the cylinder. Furthermore, the synthetic jet actuation resulted in a localized region of inward-directed flow at a spanwise location outboard of the jet orifice leading to a breakdown of the streamwise vorticity field within the near wake. Measurements obtained near the surface of the cylinder demonstrated that the synthetic jet created an elongated vortex ring that redirected the upwash flow away from the cylinder surface inboard of the jet orifice and entrained flow back towards the surface outboard of the orifice. The result was a three-dimensional separation line along the span of the cylinder that gave rise to the spanwise dependency seen in the near wake. Finally, it was shown, using a stability theory analysis that the near surface interaction of the synthetic jet with the separated mixing layer did not result in excitation of the mixing layer instability.

Synthetic jet actuation strategies for momentumless trailing edge wake

Periodic addition of momentum employing a Trailing Edge Synthetic Jet Actuation (TESJA) to control the wake of a blunt trailing edge plate was investigated experimentally at Reynolds number Reh=U∞h/ν=7200, based on the trailing edge thickness, h. A pair of synthetic jets were symmetrically placed at the model base. Two-component PIV measurements were reported for natural wake and actuated wakes under different actuation strategies namely asymmetric actuation, symmetric synchronous dual-actuation, and out-of-phase dual-actuation. A pair of synthetic jets were symmetrically placed at the model base. Two positions of the TESJA on the model base (peripheral or central position) were also studied and discussed. The forcing was applied at a frequency of 0.75 the natural vortex shedding frequency, while a high momentum addition coefficient of up to 24% was applied to balance the momentum deficit and to accomplish a near momentumless state. The measurements revealed that the application of TESJA reduced the extent of the separation bubble and redistributed energy among the Reynolds stresses. A perfect lock-in of vortex shedding in the wake was achieved. The actuation efficiency depended on both the applied control strategy and the actuators' positions. Forcing was found to be most effective in achieving a near momentumless state when applied at base model extremities (peripheral configuration) to permit the interaction of synthetic jet with the neighbouring shear layers and a large entrainment. The above strategies and positions led generally to three actuated wake states: (1) vortex pair shedding vectoring for asymmetric forcing, (2) symmetric vortex shedding under in-phase synchronous forcing, and (3) alternate vortex shedding with increased vorticity and turbulence, combined with a large momentum addition when out-of-phase forcing was applied.

Turbulent Flow Separation Control over a Two-Dimensional Ramp Using Synthetic Jets

An experiment study of flow control using an array of three circular synthetic jets has been carried out in a separated turbulent flow over a two-dimensional ramp in a boundary-layer tunnel. The freestream velocity is 6:5 m=s and the Reynolds number based on the momentum thickness at a reference location upstream of the synthetic jet array is Re 1150. The synthetic array is operated at mean jet-to-freestream velocity ratios ranging from 0.2 to 0.5 and a reduced frequency based on the ramp height of F 0:2 and 0.6. Two-dimensional particle image velocimetry is employed to evaluate the control effect of synthetic jets at different actuator operating conditions in both timeand phase-averaged senses. Stereo particle image velocimetry is used to reveal the vortical structures thatwere responsible for theflow separation delay. It is found that at both reduced frequencies an increase in velocity ratio results in a further reduction in the height of the reverse-flow region, and somedifferences exist in the way inwhich the reverseflowresponds to jet actuation of the twodifferent reduced frequencies. It is also found that at the conditions tested in the present study, the interaction of synthetic jets with the turbulent boundary layer results in formation of hairpin vortices, which induce a further pair of streamwise vortices of the opposite sign of rotation directly underneath. It is believed that it is the streamwise vortices that produce a downwash bringing highmomentum fluid toward the wall, hence reducing the flow separation.

Three-dimensional modelling of fluid flow and heat transfer in micro-channels with synthetic jet

A three-dimensional computational model was developed to investigate the effect of synthetic jet interaction with cross flow in micro-channel on the cooling of microchip. A range of parametric studies by varying heat fluxes at the surface of the top of the silicon wafer and membrane oscillating amplitudes was conducted. The resulting complex, conjugate heat transfer through the silicon substrate was analysed. When the actuator was switched on, noticeable temperature drop was observed at all points in the substrate. Quasi steady states have been reached for the presented results which indicated the available cooling potential of single synthetic jet actuator.

Secondary flow structures due to interaction between a finite-span synthetic jet and a 3-D cross flow

An experimental investigation was performed to study the formation of secondary flow structures due to the interaction of a finite-span synthetic jet with a three-dimensional boundary layer over a finite and swept-back wing configuration (cross-sectional profile of the NACA 4421, aspect ratio of 4 and sweep back angle of 30°). The investigation was carried out at a chord based Reynolds number of 105, two angles of attack of 9° and 15.5°, and two blowing ratios of 0.8 and 1.2. Stereoscopic particle image velocimetry (PIV) data were collected around a single synthetic jet actuator in the mid-span of the swept-back wing configuration. The effect of the blowing ratio was analyzed based on the three-dimensional flow field using time-averaged and phase-averaged statistics. Detailed boundary layer profiles show the actuation effects in cases where the jet is introduced to locally attached or separated flow fields, which are determined by the model’s angle of attack. Similar to the interaction of a synthetic jet with a flow over an unswept wing, the flow field in the vicinity of the synthetic jet orifice becomes highly three-dimensional with very distinct secondary flow structures. The flow field is governed by the superposition of two kinds of flow structures: (1) spanwise structures that are generated along the orifice’s span due to the synthetic jet’s vortex pairs and (2) streamwise structures that are associated with the finite span of the jet (edge vortices). These flow structures are formed almost immediately downstream of the synthetic jet orifice and evolve into a well-organized three-dimensional flow field farther downstream.

Particle image velocimetry measurements of the interaction of synthetic jets with a zero-pressure gradient laminar boundary layer

An experimental investigation of the interaction between a synthetic jet actuator and a zero-pressure gradient laminar boundary layer is reported. The aim of this study is to quantify the impact of synthetic jet vortical structures; namely, hairpin vortices, stretched vortex rings and tilted vortex rings on a boundary layer, and to assess their relative potential for flow separation control. Streamwise particle image velocimetry was employed in a water flume (free stream boundary layer thickness Reynolds number of 500 and boundary layer thickness-to-jet orifice diameter ratio of 4) to obtain phase- and time-averaged boundary layer profile information of the impact of synthetic jets near the wall. The potential for flow control was assessed by analyzing near wall fluid mixing, realized by the measure of increase in wall shear stress produced by a passing vortex. Hairpin vortices (produced at a jet-to-free stream velocity ratio, VR=0.32 and dimensionless stroke length, L=1.6) and stretched vortex rings (VR=0.27; L=2.7) exhibit characteristics akin to a streamwise vortex pair with a common upwash. Conversely, tilted vortex rings (VR=0.54; L=2.7) induce a streamwise vortex pair in the near wall region with a common downwash. Wall shear stress measurements show that synthetic jets composed of stretched vortex rings offer the best combination of near wall fluid mixing, persistency, and low rms fluctuations for potential applications of flow separation control.

Interaction of Synthetic Jet with Boundary Layer Using Microscopic Particle Image Velocimetry

The aerodynamic interaction between an unsteady, inclined synthetic jet and a crossflow boundary layer was studied as a precursor toward applying active flow control concepts for rotor applications, such as dynamic stall control and fuselage drag reduction. Because the flowfield offered numerous challenges from a measurement perspective, several experiments were carried out using a phase-locked, two-dimensional microscopic particle image velocity technique in a building block approach, by adding one complexity after another. The procedure began with boundary layer measurements made on a simple flat plate using the microscopic particle image velocity technique. Velocity measurements were made deep in the viscous sublayer, as close as 20μm from the surface. Following this, the synthetic jet actuator was characterized while operating in quiescent air as well as in crossflow. The results showed that the evolution of the synthetic jet in crossflow was substantially different from its evolution in quiescent air, suggesting that any flow physics or performance prediction (for example, the depth of penetration of the jet into the boundary layer) made based on the quiescent flow conditions may not be applicable in crossflow. All the momentum added to the boundary layer had its source from the synthetic jet actuator, and the penetration of the jet was limited to the viscous sublayer and log layer; the outer layer was unaffected, despite using a jet to freestream velocity ratio of four. Significant effort was also made to validate the microscopic particle image velocity technique and evaluate its capability to accurately resolve such a complex flowfield. To this end, microscopic particle image velocity measurements were compared with hot-wire measurements made on a simple steady jet, as well as an unsteady, periodic synthetic jet. Excellent correlation was found between the two techniques, validating microscopic particle image velocity measurements.

Interaction of synthetic jets with a fully developed turbulent channel flow

The effects of the interaction of a spanwise array of inclined circular synthetic jets with a turbulent channel flow have been experimentally studied. The array is arranged in a convergent couple configuration, where each single jet is oriented at ± 45° in the plane perpendicular to the mean channel flow. The actuator system is derived from a commercial engine for airplane models where the oscillating piston gives rise to the periodic blowing and suction phases needed for the generation of a synthetic jet. The interaction produces couples of counter-rotating large-scale longitudinal vortical structures organized periodically in the spanwise direction. These vortices persist along the channel up to long distances from the injection section and produce an attenuation of the wall turbulence. Reductions in the mean skin friction of up to 15% and in the turbulence fluctuations (wall shear stress and streamwise velocity) of up to 12% are obtained. The efficiency of the interaction depends on the piston oscillation frequency. The near-wall activity is influenced to a great extent by the action of the large-scale vortices. The variable interval time averaging (VITA) events are reduced and the integral time macroscales are increased while the probability density function (PDF) analysis shows higher levels of intermittency. The interpretation of these effects is given in terms of regeneration mechanisms of the near-wall structures. A comparison between the present control scheme and the forcing obtained using continuous jets has also been made. In the case of synthetic jets, the forcing effects appear weaker with respect to those obtained using continuous jets, at least for the tested conditions. Drag reduction and turbulence attenuation are in fact lower when synthetic jets are used as a forcing technique.

The behaviour of circular synthetic jets in a laminar boundary layer

AbstractDye flow visualisation of circular synthetic jets was carried out in laminar boundary layers developing over a flat plate at a range of actuator operating conditions and freestream velocities of 0·05 and 0·1ms–1. The purpose of this work was to study the interaction of synthetic jets with the boundary layer and the nature of vortical structures produced as a result of this interaction. The effects of Reynolds number (Re), velocity ratio (VR) and Strouhal number (St) on the behaviour of synthetic jets were studied. At low Re and VR, the vortical structures produced by synthetic jets appear as highly stretched hairpin vortices attached to the wall. At intermediate Re and VR, these structures roll up into vortex rings which experience a considerable amount of tilting and stretching as they enter the boundary layer. These vortex rings will eventually propagate outside the boundary layer hence the influence of the synthetic jets on the near wall flow will be confined in the near field of the jet exit. At high Re and VR, the vortex rings appear to experience a certain amount of tilting but no obvious stretching. They penetrate the edge of the boundary layer quickly, producing very limited impact on the near wall flow. Hence it is believed that the hairpin vortices produced at low Re and VR are likely to be the desirable structures for effective flow separation control. In this paper, a vortex model was also described to explain the mechanism of vortex tilting.

SYNTHETIC JETS

▪ Abstract The evolution of a synthetic (zero-net mass flux) jet and the flow mechanisms of its interaction with a cross flow are reviewed. An isolated synthetic jet is produced by the interactions of a train of vortices that are typically formed by alternating momentary ejection and suction of fluid across an orifice such that the net mass flux is zero. A unique feature of these jets is that they are formed entirely from the working fluid of the flow system in which they are deployed and, thus, can transfer linear momentum to the flow system without net mass injection across the flow boundary. Synthetic jets can be produced over a broad range of length and timescale, and their unique attributes make them attractive fluidic actuators for a number of flow control applications. The interaction of synthetic jets with an external cross flow over the surface in which they are mounted can displace the local streamlines and induce an apparent or virtual change in the shape of the surface, thereby effecting flow changes on length scales that are one to two orders of magnitude larger than the characteristic scale of the jets. This control approach emphasizes an actuation frequency that is high enough so that the interaction domain between the actuator and the cross flow is virtually invariant on the global timescale of the flow, and therefore, global effects such as changes in aerodynamic forces are effectively decoupled from the operating frequency of the actuators.