Slot Synthetic Jet Research Articles

Experimental investigation of thermal characteristics of slot synthetic jet impingement on a circular cylinder: Free and constrained environment

The present study aimed to comprehensively investigate the thermal behavior of a slot synthetic jet (SJ) interacting with a circular cylinder in both free and constrained environments through experimental analysis. By traversing the cylinder along the jet centerline across a range of non-dimensional distances (H/w = 5-50), spanning from the near-field to the fully developed region, the research aimed to elucidate the factors governing heat transfer performance. Key parameters such as Reynolds number, jet-cylinder separation distance, and excitation frequency were scrutinized to understand their influence on thermal performance. The SJ was found to perform better in the constrained environment, attributed to relatively higher flow fluctuations developed by the complex interaction of the vortex with the sidewall boundary layer and the cylinder. Almost 12% higher average heat transfer was observed in the case of the constrained environment over the range of parameters employed in the current work. Moreover, a strong dependence of heat transfer on the jet cylinder separation distance was also found. In contrast to the SJ impinging on flat target surfaces where the maximum heat transfer was attained in the intermediate field, at H/w = 14 to 18, for SJ impingement on the circular cylinder, however, this was consistently achieved in the near field, i.e., H/w = 5. Also, the thermal performance as a function of the flow Reynolds number was comparable to the uniform flow case when the Reynolds number was based on the approach flow velocity (i.e., local velocity based on the cylinder location) instead of the velocity at the slot exit.

Flow characteristics and separation control in a transitional twin air-intake

Ambient air is ingested into the engine of military aircrafts using the air-intake system within a limited space. The flow non-uniformity produced in the air-intake creates flow separation and total pressure loss due to the centerline curvature and area diffusion which eventually affects its performance. The current work discusses the performance of a transitional twin air-intake without and in presence of slotted synthetic jets. The synthetic jet is employed just before the inflection at a range of velocity ratio from 1 to 10 and found that for a velocity ratio of 4.0, the pressure distortion at the aerodynamic interface plane is found minimum. Hence, the other performance parameters are computed at that optimized flow condition. The outcome of the study reveal that the static pressure is recovered by 4.61%–6.64% while the total pressure loss is decreased by 59.28%–89.95% using optimized synthetic jets for a range of Reynolds numbers.

Simplified Steady-State Representation of Slot Synthetic Jet Actuators to Enable Numerical Optimization With Steady RANS Simulations

To address the growing energy use of data centers, waste heat recuperation offers a solution to better integrate these facilities into the broader energy system, thus facilitating a transition to the decarbonization of the energy system. The use of liquid coolants for full immersion cooling or local heat extraction from high power density components is considered for this purpose. However, heat can also be extracted by novel air cooling approaches, perhaps in combination with localized liquid cooling. To optimize heat extraction from air-cooled systems and maximize the heat grade, synthetic jets (SJs) can be used for targeted adaptive cooling in conjunction with air ducting to facilitate maximum heat recuperation potential in rack or server-mounted air-to-liquid heat exchangers. Internal server layouts can be optimized numerically, e.g., using a multiple objective genetic algorithm approach based on minimization of entropy generation rates. However, since SJs are inherently transient flow phenomena, this would require transient flow simulations, which forms a bottleneck in a numerical optimization loop. This research aims to develop a simplified steady-state representation of a synthetic jet actuator (SJA) with slot orifice using a localized body force to generate a similar time-averaged flow field to a real SJA, suitable for steady Reynolds-averaged Navier–Stokes simulations within a numerical optimization loop. Both flow fields are compared in terms of the mean flow field, jet spreading rate, and turbulence intensity distributions.

Aerodynamic Performance Improvement of a Wing Model Using an Array of Slotted Synthetic Jets

Abstract This work deals with the improvement in aerodynamic performance of a NACA0025 wing model using an array of slotted synthetic jets (SJs). A novel SJ actuator was designed and located at 30% of the chord from the leading edge (LE). Time-resolved particle image velocimetry (TR-PIV), force balance, static pressure distribution, and hotwire measurements were carried out in a subsonic wind tunnel to assess the performance enhancement due to the slotted SJ array. Initially, the SJ velocity was measured in quiescent flow condition at different actuation frequencies and amplifier voltages. Actuation at 1000 Hz and 200 V resulted in the highest blowing velocity of 10.5 m/s. Experiments were performed at various actuation frequencies, namely, 200, 600, and 1000 Hz. It was observed that actuation at 1000 Hz led to the highest increase in lift coefficient by 35.6% and reduction in average drag coefficient by 33%. TR-PIV measurements showed flow separation with flow reversal in the baseline case. After switching on the SJ array at 1000 Hz, the flow separation was completely eliminated. The momentum transfer from the highenergy primary flow to the retarding boundarylayer flow and actuation of SJ in a particular frequency range was observed to be the mechanisms for the flow separation control. Subsequently, fast Fourier transform (FFT) power spectra of hotwire data were computed from 40% to 80% of the chord. The FFT power spectra showed the successful stabilization of the flow field at the actuation of 1000 Hz.

A Comparison of Circular and Slotted Synthetic Jets for Flow Control in a Twin Air Intake

The performance of an aircraft engine depends on air flow quality at the engine face / the exit of the air-intake also known as aerodynamic inlet plane (AIP). A single-engine aircraft has complex Y-shaped twin air-intake which causes severe flow separation, distortion and flow non-uniformity at the AIP. The present study compares the efficacy of slotted synthetic jet and a row of four circular synthetic jets attached to inner faces of a twin air-intake to improve aerodynamic performance at the AIP. The results are obtained using computational fluid dynamics. The velocity and vorticity plots show that lateral spread of the circular jets is limited as compared to the slotted jet. The circular jets are found to be weak as compared to slotted jet to prevent separation of main flow occurring in the twin air-intake. The various aerodynamic performance parameters, such as static pressure recovery coefficient, total pressure loss coefficient, distortion coefficient and secondary flow uniformity are compared for both the cases, exhibiting marked improvement in all these parameters. The study demonstrates that the slotted synthetic jets is a better option for controlling flow in twin air-intake as compared to a row of circular synthetic jets.

Effects of flow control in the presence of asymmetric inflows in twin air-intake

PurposeThe purpose of air-intake duct used in combat aircrafts is to decelerate the inlet flow and concurrently raise the static pressure recovery at the compressor inlet. Because of side-slip movement during sharp maneuvers of the aircrafts, the airflows ingested into twin air-intake ducts are not same and symmetric at its two inlets but are asymmetric in nature. The asymmetric inlet flow conditions at the twin air-intakes thus caused instabilities and deteriorated aerodynamic performance of aircraft components such as compressors and other downstream components. This study aims to investigate the flow control in a twin air-intake with asymmetric inflows.Design/methodology/approachThe continuity and momentum equations are solved with second-order upwind scheme for computing finite-volume method-based unsteady computational fluid dynamics simulation.FindingsPerformance parameters are deteriorated with the increase of inflow asymmetry in the twin air-intake duct. Slotted synthetic jets are used to manage flow separation, thereby increasing aerodynamic performance of the air-intake. A variety of vortical structures are generated from the rectangular slots, convected downstream of the twin air-intake. The use of slotted synthetic jets increases static pressure recovery by 64 per cent whereas reducing total pressure loss coefficient by 63 per cent, distortion coefficient by 58 per cent and swirl coefficient by 55 per cent which is an indicative of better aerodynamic performance of twin air-intake.Originality/valueThe study stresses the need of robust flow control technique to improve the performance of combat air-intake system under extreme maneuvering conditions. The results can be useful in designing air-intake satisfying the stealth features for modern combat aircrafts.

Comparison of the flow-field characteristics of a slot synthetic jet with and without sidewalls

In this paper, experimental investigation of the flow-field of a slot synthetic jet (SJ) with and without sidewalls, issuing into a quiescent environment is systematically reported. Two parallel sidewalls were mounted along the shorter side of the slot extending in the streamwise direction to constrain the flow along the slot span. Hot-wire anemometry was used to explore the flow-field characteristics of both configurations at a Reynolds number of 4000 based on the slot-width and slot average exit velocity during the ejection phase. The present work is a first step towards the investigation of the SJ flow-field characteristics in a bounded region. In a number of generic situations, this work is of high importance as the SJ is deployed in constrained environments (e.g., in cooling applications) where sidewalls may be present. The relative difference in the magnitude of the distinct peaks in the near-field spanwise velocity profiles for both configurations reveals that the vortex does not get curled up towards the centerline in the case of the synthetic jet with sidewalls due to the presence of the no-slip walls. Spectral analysis in the near-wall region further confirms the absence of the phenomenon of axis-switching in the case of the synthetic jet with sidewalls. This behavior is also demonstrated with the help of the relative spreading rate in both configurations, where the unbounded synthetic jet spreads rapidly compared to the bounded one, due to greater entrainment of the surrounding fluid. The statistically two-dimensional region for a synthetic jet with sidewalls is found to extend over a longer axial distance in the downstream. The other jet properties such as turbulence intensity, skewness, and flatness factors further reveal the differences in the flow-field of the two configurations. The results show that the presence of the sidewalls strongly influences the SJ flow-field and hence, it would significantly impact the heat transfer capacity of the SJ.

Effect of excitation frequency on flow characteristics around a square cylinder with a synthetic jet positioned at front surface

The flow over a square cylinder controlled by a slot synthetic jet positioned at the front surface is investigated experimentally at different excitation frequencies. The Reynolds number based on the free-stream velocity and the side length of the square cylinder is 1000. The flow visualization was conducted using the laser-induced fluorescence technique. The velocity fields upstream and downstream of the square cylinder were measured synchronously with the two-dimensional time-resolved particle image velocimetry technique. Both the evolution of vortex structures and the characteristic frequencies of upstream and downstream flow fields are presented. The flow dynamics vary significantly with the excitation frequency at a fixed stroke length. During one excitation cycle, the synthetic jet vortex pair deflects to one side and later swings to the other side at a quite small excitation frequency of $f_{e}/f_{0}=0.6$, while it only deflects toward one side and does not turn to the other side at $f_{e}/f_{0}=1.0$. Compared with the natural case, the wake characteristics for the above two cases are not changed much by the synthetic jet adopted. At a moderate excitation frequency of $f_{e}/f_{0}=2.0$, the synthetic jet deflects upwards and downwards alternatively. The upstream flow field has a dominant frequency identical to half of the excitation frequency. Under the perturbations of the synthetic jet, two wake vortex pairs are formed per shedding cycle with a shedding frequency equal to that of the square cylinder without control. At a higher excitation frequency of $f_{e}/f_{0}=3.4$, the synthetic jet keeps deflecting to one side, and the upstream flow field is governed by the excitation frequency. The flow separation on the deflected side is suppressed effectively, and no periodic vortex shedding can be observed in the wake. Statistically, the velocity profiles also change with control. The recirculation bubble length in the wake is shortened, and the time-averaged velocity fluctuation is weakened remarkably. The control effects of the synthetic jet and the continuous jet are compared in this paper when placed at the front surface of a square cylinder.

Flow Management in a Double-Offset, Transitional Twin Air-Intake at Different Inflow Conditions

Background: Transitional twin air-intake is a vital component of the air-induction system of single-engine combat aircraft. Combat aircraft do not always fly at steady, uniform flow conditions. But in some cases, it operates at different asymmetric flow conditions, which cause a change in aerodynamic performance of aircraft components like compressor and combustor. Objective: In order to improve the air quality at the outlet of air-intake- called Aerodynamic Inlet Plane (AIP) of this twin air-intake and to improve its aerodynamic performance for wide ranges of inflow conditions, slotted synthetic jets are used. Methods: Computational studies are carried out using Computational Fluid Dynamics (CFD) software for various types of skewed turbulent velocity profiles at inlet-2 with skewness number (ξ = 0, 0.3, 0.5, 0.7), while an average uniform velocity of 20m/s at inlet-1. Based on this analysis, worst case is selected and a pair of slotted synthetic jets is used just before the inflexion plane of the twin air-intake using transition SST turbulence model. Results: The flow behaviour of transitional twin air-intake becomes more complex with the increase in skewness number, thereby decreasing the aerodynamic performance of the air-intakes. With the use of slotted synthetic jets, an improvement in static pressure recovery and decrement in total pressure loss coefficient, distortion in coeffient swirl coefficient and secondary flow non-uniformity are observed which is a great sign of improved aerodynamic performance for the twin air-intakes. Conclusion: It is proved in this study that synthetic jet can be used effectively in twin air-intake to control the flow features leading to better flow uniformity and increased overall performance at the AIP without increasing the net-mass flow rate, thereby reducing the chance of stall/surge in the aeroengines. Hybrid flow control technique (synthetic jet coupled with vortex generator array) or newer flow control technique (plasma jet) are being explored for its possible use in engine air-intakes as revealed from recent patents filed/published in this area.

Wake vortex evolution of square cylinder with a slot synthetic jet positioned at the rear surface

Wake vortex evolution of a square cylinder with a slot synthetic jet issuing from the cylinder’s rear surface has been experimentally investigated using the time-resolved particle image velocimetry technique. The Reynolds number based on the side length of the square cylinder is $Re=836$. The excitation frequency normalized by the natural shedding frequency $f_{e}/f_{0}$ varies from 0 to 6 at the dimensionless stroke length $L_{0}/w=72.6$. The distributions of the time-averaged Reynolds stresses present significant differences as the excitation frequency increases. With control, the mean streamwise velocity deficit of the wake recovers more quickly in comparison with the natural case, and the vertical velocity fluctuation intensity becomes weaker. Moreover, a drag reduction can be achieved for the control cases, especially, for $f_{e}/f_{0}=4$ and $f_{e}/f_{0}=6$, a thrust instead of drag reduction can be obtained. The profiles of the mean streamwise velocity tend to have jet-like distributions. The wake vortex dynamics and its evolution with the excitation frequency are revealed. (i) For the low excitation frequency cases ($f_{e}/f_{0}=0.5$, 1, 2), no significant changes in the dominant frequency and the spanwise vortex structures are observed in comparison with the natural case. (ii) For the moderate excitation frequency case ($f_{e}/f_{0}=3$), the wake vortex shedding frequency is locked on half of the control frequency. In this case, the shear layer is divided into two parts by the synthetic jet vortex, and the wake vortices with smaller scales still shed asymmetrically and appear closer to the square cylinder. (iii) For the high excitation frequency case ($f_{e}/f_{0}=6$), the flow is governed by the synthetic jet. As a result of strong perturbations of the synthetic jet, the wake vortex shedding becomes symmetric with the shedding frequency consistent with the control frequency. And the separation is suppressed effectively. The different control effects of the slot synthetic jet on a square cylinder and a circular cylinder are also compared in detail. Generally speaking, the circular cylinder is easier to be controlled due to its non-fixed separation points.

Flat plate boundary layer transition induced by a controlled near-wall circular cylinder wake

The flat plate boundary layer transition induced by the wake of a circular cylinder close to the wall is experimentally investigated using particle image velocimetry (PIV) and hydrogen bubble visualization techniques. The wake of the circular cylinder is controlled by a slot synthetic jet at the rear stagnation point of the circular cylinder. It is found that when the synthetic jet is actuated, the wake can be greatly modified. When the excitation frequency of the synthetic jet is set at the natural shedding frequency of the cylinder wake, the symmetrical shedding pattern can be observed. While the excitation frequency increases to be twice of the natural shedding frequency, the wake appears to be antisymmetrical again, but with the shedding frequency locked onto half of the excitation frequency. Flow visualizations show that spanwise secondary vortices can be induced in the near wall region by these large scale vortices in the wake. It is found that the secondary vortices destabilize into streamwise stretched Λ vortices as they convect downstream. After the introduction of the synthetic jet, the destabilization process is promoted. By investigating the disturbance growth inside the boundary layer, it reveals that the synthetic jet can cause earlier initialization of the disturbance growth, thus promoting the transition process of the boundary layer. An explanation is provided that the low frequency components of the wake disturbances, which interact with the boundary layer, are enhanced by the introduction of the synthetic jet. Therefore, the destabilization of the secondary vortices is promoted, and disturbance growth in the boundary layer initiates earlier.

Laser Doppler vibrometry experiment on a piezo-driven slot synthetic jet in water

The present study deals with a slot synthetic jet (SJ) issuing from an actuator into quiescent surroundings and driven by a piezoceramic transducer. The actuator slot width was 0.36 mm, with a drive frequency proposed near the theoretical natural frequency of the actuator. The working fluid was water at room temperature. The present experiments used flow visualization (a laser-induced fluorescence technique) and laser Doppler vibrometry methods. Flow visualization was used to identify SJ formation, to demonstrate its function, and to estimate SJ velocity. Laser Doppler vibrometry was used to quantify diaphragm displacement and refine operating parameters. Phase averaging yielded a spatial and temporal diaphragm deflection during the actuation period. Taking incompressibility and continuity into consideration, the velocity in the actuator slot and the Reynolds number of the SJ were evaluated as 0.21 m/s and 157, respectively. The present results confirmed a SJ actuator function at the resonance frequency of approximately 46 Hz, which corresponds closely with the theoretical evaluation. The laser Doppler vibrometry results corresponded closely with an estimation of SJ velocity by the present flow visualization.

Elaboration of Compact Synthetic Micro-jets Based on Micro Magneto-mechanical Systems for Aerodynamic Flow Control

This paper described an optimized and compact lateral slot synthetic jet based on magneto-electro-mechanical actuation. The main objective in designing this device is to meet the performances required for flow control applications on the square back body. First, we present the synthetic jet design and the result of numerical simulations based on finite element method showing the formation of the vortex rings. Secondly, the fabrication process and the elaborated devices were described. Finally, fluidic characterisations were performed using one component calibrated hot wire. The results show that for an operation frequency close to 330Hz and a bias power less than 1W, the output velocity reaches 40m/s at 2mm from the slot exit. The design enables us to choose the range of operation frequency in the frequency range 100Hz-700Hz by varying the thickness of the elastomeric membrane between 100μm and 600μm.