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Experimental and numerical studies are carried out to validate the potential of opposing Plasma Synthetic Jet (PSJ) for drag reduction for a hemisphere. Firstly, flow field changes of opposing PSJ are analyzed by comparing the experimental schlieren images and simulation results in a supersonic free stream of Mach number 3. As PSJ is a kind of unsteady pulsed jet, the shock standoff distance increases initially and then decreases under the control of PSJ, which corresponds to the change of the strength of PSJ. Accordingly, the amount of drag reduction of the hemisphere increases initially and then decreases. It is found that there is a short period of “drag rise” during the formation of PSJ before the drag reduction, which is induced by the generation of normal shock waves and the area difference of the cavity wall of PSJ Actuator (PSJA). Secondly, the effects of five parameters, including exit diameter, discharge energy of PSJA, Mach number, static pressure of incoming flow and angle of attack, on drag reduction of opposing PSJ were studied in detail by using numerical method. It is found that the Maximum Pressure Ratio (MPR) has a significant impact on the average drag reduction for a configuration-determined PSJA. For the configuration selected in this study, the flow field of opposing PSJ shows typical Short Penetration Mode (SPM) in a control cycle of PSJ when the MPR is less than 0.89. However, the flow field shows typical Long Penetration Mode (LPM) at some time when the MPR is bigger than 0.89. Relatively better drag reduction is achieved in this case.

The autonomous and controllable Dual Synthetic Jet Actuator (DSJA) is firstly integrated into the Unmanned Aerial Vehicle (UAV), and flight tests without the deflection of rudders are carried out to verify the viability of DSJA to control the attitudes of UAV during cruising. DSJA is improved into an actuator with two diaphragms and three cavities, which has higher energy levels. Actuators, differentially distributed on both sides of the wings, are installed on the trailing edge close to the wing tips. Flight tests, containing Differential Circulation Control (DCC) using double-side actuators, Positive Circulation Control (PCC) using left-side actuators and Negative Circulation Control (NCC) using right-side actuators, are implemented at cruising speed of 25 m/s. Results show that roll attitude control without rudders could be realized by DSJAs. DCC and NCC can generate the rightward roll and yaw angular velocity, prompting UAV to turn right. The stronger controlling ability can be achieved by DCC, with the maximum roll angular velocity of 15.62 (°)/s. PCC can generate a rightward roll moment, but a leftward yaw moment will be produced at the same time. Leftward yaw induces the leftward rolling moment, which weakens the roll control effect, making UAV keep to yaw to the left with a small slope.

Aircraft icing has long been a plague to aviation for its serious threat to flight safety. Recently, researches about a newly proposed deicing method based on Plasma Synthetic Jet Actuator (PSJA) have just started. To meet the requirements of in-flight deicing, structure of PSJA needs to be adjusted. This paper completed the detailed design and experimental validation of a novel plasma striker, which was a modified version of PSJA. Influences of mass of the moving part and rod shapes on the ice-breaking performances were also studied. Besides, a “conical-nosed rod configuration” was proposed. Its purpose was to ensure a good ice-breaking performance of the plasma striker on long ice, by generating a splitting failure. Results show that, though mass of the moving part was just several grams, ice-breaking performance was better when the mass was lighter. The rectangular rod could generate an elliptical circumferential crack, whose major axis was parallel to the direction of the long side of the rectangular rod. And the “conical-nosed rod” concept was verified to be able to generate a splitting crack which can spread completely to the far end of long ice, and the crack direction was parallel to edge line of the cone. In general, the plasma striker has the advantages of simple structure, low energy consumption, little harm to the flow field and the aircraft skin. Simulations will be carried out in future works to study in detail the working process of the plasma striker.

SparkJet (SPJ) actuator was used to control shock and shock-on-shock interaction in high-enthalpy hypersonic flow with a total temperature of 4490 K and Mach number 6.9 for the first time. Aiming at the characteristics of rarefied gas in hypersonic flight environment, the control authority of SPJ actuator was improved by air supply method. The process of SPJ interacting with high-enthalpy hypersonic crossflow near a ramp was obtained by using high-speed schlieren camera. When pressurized cavity pressure was 133 kPa, ramp distance was 22 mm and ramp angle was 50, SPJ can eliminate 68.3 % of the whole ramp shock. Results of different experiment cases showed that, within a certain range, the control effect of SPJ on ramp shock improves with the increase of pressurized cavity pressure, ramp distance and the decrease of ramp angle. The elimination of shock by SPJ can be explained by the upward motion of the sonic line in the boundary layer due to the formation of SPJ shock, the thickening of boundary layer and the heating of local flow field. Except for the elimination of shock, the shock intersection point moves upstream and downstream corresponding to the strength changes of SPJ and SPJ shock. Accordingly, local high heat flow around the shock intersection point can be decreased. The control effect of SPJ on shock-on-shock interaction can be improved by properly increasing discharge energy.

For lessening the weight and volume of flow control system, enlarging the circulation control applying area of angle of attack (AOA), and achieving nice controlling characteristics, a novel lift enhancement method based on dual synthetic jet actuators (DSJAs) and synthetic jet actuator (SJA), and an adaptive proportional integral and differential (PID) algorithm based on radial basis function neural network are introduced. DSJAs are uniformly located along the chord to suppress the separation and trailing-edge SJA is applied to achieve the high circulation. Velocities of actuators are modulated to realize the real speed profile and on–off controlling laws of DSJAs are designed. Numerical simulations show that DSJAs and SJA could suppress the separation completely and move leading-edge stagnation point and trailing-edge separation point downstream even at AOA of 19°, hence achieve the highest lift and nose-down moment augmentation ( $${\Delta {Cl}}_{\text{max}}$$ = 0.92, $${\Delta {Cm}}_{\text{max}}$$ = 0.02534), and $$\Delta L/D$$ can reach 11.39 at AOA of 18°. Stalling is delayed to more than 19°. Linear lift area and pitch-break angle are both increased to 16°. $$\Delta {Cl}/{C}_{\mu }$$ can reach 76.7, indicating the greatest control efficiency. The results of adaptive PID control, whose controlling effects are proved better than PID, indicate that lift could track the objective with the rise time of 0.0325 s and finally keep steady, suggesting the nice stability and rapidness.

The excellent vectoring characteristic of Dual Synthetic Jet (DSJ) provides a new control strategy for the active flow control, such as thrust vectoring control, large area cooling, separated flow control and so on. For incompressible flow, the influence relation of source variables, such as structure parameters of actuators, driving parameters and material attributes of piezoelectric vibrating diaphragm, on the vectoring DSJ and a theoretical model are established based on theoretical and regression analysis, which are all verified by numerical simulations. The two synthetic jets can be deemed as a main flow with a higher jet velocity and a disturbing flow with a lower jet velocity. The results indicate that the influence factors contain the low-pressure area formed at the exit of the disturbing flow, which could promote the vectoring deflection, and the impact effect of the disturbing flow and the suppressive effect of the main flow with the effect of restraining the vectoring deflection. The vectoring angle is a complex parameter coupled by all source variables. The detailed theoretical model, whose error is controlled within 3.6 degrees, can be used to quantitatively assess the vectoring feature of DSJ and thus to provide a guidance for designing the control law applied in the active flow control.

A new skill of modulating driving signals of a synthetic jet with low frequency is introduced to enhance the control authority. Based on dynamic mesh technique, the effect of flow control over a NACA0015 airfoil with a Synthetic Jet Actuator (SJA) has been investigated. The SJA is located at x/c = 10%, and two jet injection angles of 90° and 30° have been considered. Flow structures indicate that modulation with low frequency can reduce the dissipation rate of vortices after they shed off the surface. Thus aerodynamic performance of the airfoil could be improved. For the injection angle of 90°, modulation can increase the control authority of the SJA. Compared to the unmodulated case, increment of lift-to-drag ratio is up to 15%. The modulating frequency corresponding to the largest increment is F M + ≈ 1 . For the injection angle of 30°, improvement of aerodynamic forces positively relates to momentum coefficient of the SJA. Due to the decrease of effective momentum, modulation degrades the effect of flow control. The opposite responses to modulation are believed to have a relationship with the variation of primary control mechanism. When jet injection angle varies from 90° to 30°, the primary mechanism changes from the interaction of vortices and waves to the direct momentum addition. The skill of modulation of driving signals is very useful for applications of SJA.

Shock wave/boundary layer interaction (SWBLI) is still one of the unresolved bottlenecks that restrict the development of more advanced flight vehicles. Supersonic forward-facing step (FFS), an extreme case of compression ramp, often occurs severe SWBLIs with a large separation bubble. In this paper, experimental investigations on vortical structures and density fluctuations characteristics of supersonic FFS controlled by self-sustaining dual synthetic jets (SDSJ) are carried out in a Mach number 2.95 wind tunnel. High spatial–temporal resolution flowfield images of FFS without/with active flow control are captured by adopting nano-particle-based planar laser scattering technique. The control effects of the distance between the actuator and the step are mainly compared. The paper finds that the SDSJ can effectively change the feature of flowfield, eliminate the separation shock and the reattachment shock, compel the original shock induced by the step leading edge to distort and reduce its intensity finally. Density fluctuations analysis demonstrates that the whole flows seem to move upstream with the increase of distance (dS-J). Discrete Fourier transformation spectrums results reveal that the fluctuations are mainly located in the low-frequency region at first. High-frequency components and frequency bandwidth increase slightly after the SDSJ are applied.

A simple estimation method of momentum coefficient of synthetic jet actuator has been proposed. Flow separation over an airfoil can be suppressed sufficiently with the estimated momentum coefficient. At a certain angle of attack (AOA), momentum deficit of boundary layer at front of separation point can be quantified based on Bernoulli's theorem. The function of synthetic jet actuator is deemed to be momentum addition and two stages of the addition corresponding to blowing and suction phases are considered respectively. The required momentum coefficient as well as the associated injection angle is determined preliminarily with the assumption of synthetic jet actuator at front of separation point. Since synthetic jet actuator is usually installed near the leading edge, a modification is introduced to consider the influence of excitation location. Using this method, flow separation over a NACA0015 airfoil is suppressed nearly completely with AOA = 15° and synthetic jet actuator at x/ c = 10%. Different angle of attacks corresponding to pre-stall angle, post-stall angle, and deep stall angle have been examined, respectively. Results indicate that the method is effective at pre-stall angle and post-stall angle. However, momentum coefficient will be underestimated when the separation point is upstream to the location of the airfoil's maximum thickness. At deep stall angle, the method is invalid. The estimation method is useful for practices of flow separation control with synthetic jet actuator.

In order to improve the control ability of synthetic jets in compressible boundary layer, a novel control method based on dual synthetic cold/hot jets coupled control of velocity profile and temperature profile was proposed. As fundamental investigations on the effects of synthetic jet temperature on the jet behavior and flow field characteristics were essentially necessary, preliminary numerical simulations were conducted to study the influence of temperature (200 K and 400 K) on the flow field characteristics of synthetic jets using Large Eddy Simulations (LES) model. Time-averaged flow fields showed that different temperatures led to variable behavior of two strands of jets. For dual synthetic cold jets, a potential-core arose apparently with its height ranging from 0.01 to 0.03 m, while for dual synthetic hot jets, two strands of jets emerged downstream. The modal decomposition of instantaneous flow fields had been done using both Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD). Various modes showed different characteristics of the flow fields. As the POD method focuses on the energy of flow while the DMD method focuses on the frequency, the first two modes had many similarities, but the third and fourth modes demonstrated completely different vortex structures. The current researches play a role of preliminary investigations for further and comprehensive exploration of novel flow control measures in global velocity field. Keywords: Dual synthetic cold/hot jets, Dynamic mode decomposition, Jet behaviors, Proper orthogonal decomposition, Synthetic jet, Vortex structures

In this paper, the Eulerian Stochastic Field (ESF) model in the Transported Probability Density Function (TPDF) class model is combined with the Flamelet Generated Manifolds (FGM) model. This method solves the joint probability density function transport equation by ESF method that considers the interaction mechanism between flame and turbulence with high precision. At the same time, by making use of the advantage of the FGM model, this model is able to incorporate the detailed chemical reaction mechanism (GRI 3.0) with acceptable computational cost. The new model has been implemented in the open source CFD suite-OpenFOAM. Validation of the model has been carried out by simulating the Sandia flame series (three turbulent piloted methane jet flames) issued by the National Laboratory of the United States. The accuracy and advancement of the ESF/FGM turbulent combustion model are verified by comparing the LES results of the new model with the rich experimental data as well as the RANS results. The results demonstrate that the model has a strong ability in capturing combustion phenomena such as extinction and re-ignition in turbulent flame, which is essential in the accurate prediction of the combustion process in real combustion devices, for example, aircraft engines. Keywords: Eulerian Stochastic Field (ESF), Flamelet Generated Manifolds (FGM), Large Eddy Simulation (LES), OpenFOAM, Turbulent combustion

To solve the heat dissipation problem of electronic equipment, a novel spray cooling device based on a dual synthetic jet actuator integrated with a piezoelectric atomizer (DSJAPA) is proposed and designed. The flow field, spray characteristics and cooling performance of DSJAPA are investigated experimentally. The experimental results show that the spray angle increases with the increase of driving voltage, and it can achieve the maximum of 74°. Under a certain driving voltage, there exists an optimum driving frequency making the spray angle reach to the maximum. The spray flow field is investigated by droplet image velocimetry (PIV) system. The results show that the spray droplet velocity can be accelerated to about 22.5 m/s by the high-speed ejection of the jet. With the increase of dual synthetic jet velocity, the dense spray with low velocity changes into the dilute spray with high velocity. A dual synthetic jet actuator can improve the cooling capability of spray, reduce the temperature non-uniformity of surface (from 32 °C in spray cooling to 18 °C in DSJAPA cooling) and enlarges the direct impingement range (from (−30 mm, 25 mm) in spray cooling to (−60 mm, 57 mm) in DSJAPA cooling).

For realizing the nice aerodynamic maneuverability of flying wing aircrafts (FWAs), a longitudinal aerodynamic control technology based on circulation control (CC) using trailing-edge synthetic jet actuators was proposed. Influences on longitudinal characteristics of a FWA were investigated. Results show that synthetic jets could improve the lift, drag, and nose-down moment, having potential of flight control at entire area of attack of angles (AOAs). Cl increment dips and then rises with the growth of AOAs, reaching the minimum at AOA of 12°. The maximum percentage of Cl enhancement and ΔCl/ Cμ is separately 64.5% and 50.74%, respectively. Before 12°, synthetic jets could narrow the area of “dead zone,” improve flow velocities along the upper surface, and then move the trailing-edge separation point and the leading-edge (LE) stagnation point downward, enhancing the circulation. Moreover, synthetic jets grow rapidly through entrainment of the local flow, leading to the improvement of valid camber and, hence, the increase in Cl. CC efficiency decreases with the augmentation of AOAs, and leading-edge vortex (LEV) is weakened, causing the drop of Cl increment. After 12°, synthetic jets could enhance the longitudinal velocity of LEV and reduce the swirling number, improving the strength and stability of LEV, which results in larger suction near the leading edge. Moreover, the strengthened LEV could promote flow mixing and then weaken reverse pressure gradients along the wing section, thus improving flow velocities and CC efficiency at the wing section. It is above two factors that make Cl increment rise after 12°.

A dual synthetic jet (DSJ) actuator has two chambers sharing the same wall equipped with a single oscillatory diaphragm. The novel structure doubles the function of synthetic jet (SJ) actuator and brings it merits that a traditional synthetic jet actuator does not have like resolving the problems of pressure loading and energy inefficiency. However, self-support, which is unique to DSJ, may result in weak jets or even no jets. In order to effectively solve the self-support problem of DSJ and further improve its performance, this paper proposes a novel DSJ exit structure with a penetrating channel. The experimental and numerical investigations prove that the jet penetration depth and streamwise momentum flux of DSJ are increased by nearly 25%–50%. Furthermore, through the analysis of a large number of simulation data, it is found that the streamwise momentum flux distributions of DSJ under random driving frequency and driving amplitude can be expressed by a unified mathematical fitting formula, that is, the streamwise momentum flux has self-similarity. This criterion can efficiently and quickly calculate the SJ/DSJ driving parameters required for effectively controlling the flow field.

The interaction between self-sustaining dual jets and supersonic laminar boundary layer is experimentally studied using the nanoparticle-based planar laser scattering in a Ma 2.95 supersonic low-turbulence wind tunnel for the first time. The typical fine flow structures, such as separation shock, “W” bow shock, barrel shock, horseshoe vortex, large-scale vortex structures and counter-rotating vortex pairs, are clearly identified by streamwise and spanwise flowfield images. The curves of jet penetration depth are fitted at the momentum ratio of 0.55 between self-sustaining dual jets and supersonic crossflow. The results play a significant role in further and comprehensive exploration of the active flow control for a supersonic flow.

Gas discharge plasma techniques have been applied successfully in a wide range of fields. As one of these techniques, plasma synthetic jet (PSJ) actuator shows promising application in high-speed flow control. However, for practical application to aircraft, the capacity of the power source is very limited. So the energy efficiency optimization of PSJ actuator is vital to its longtime effective performance. The overall efficiency of PSJ actuator can be divided into three parts: discharge efficiency, electrothermal efficiency and exhaust efficiency. In this paper, discharge and electrothermal efficiencies of a home-designed and manufactured two-electrode PSJ actuator working in single-shot mode and normal atmosphere are evaluated experimentally. The influence of circuit parameters and actuator configurations are investigated and analyzed as a reference for optimization. The results show that, discharge efficiency is basically determined by the resistance ratio of the RLC circuit. The parasitic resistance of the circuit has a great influence on discharge efficiency due to the very short length and low resistance of the plasma arc (O (100 mΩ)). Electrothermal efficiency seems to be less affected by the circuit parameters and actuator configurations compared to discharge efficiency. The variation extent of electrothermal efficiency is around 50–70% for the tested conditions. Smaller discharge chamber volume produces PSJ with higher speed, but will reduce electrothermal efficiency. For a certain overall input energy required, a design of smaller capacitance and wider anode-to-cathode distance is highly recommended to increase both discharge efficiency and electrothermal efficiency.

Water seepage from a tunnel may induce the drawdown of the groundwater table and destroy vegetation and the ecological environment in the tunnel area. One of the primary objectives in tunnel engineering is to develop effective methods to estimate the amount of the limiting drainage of a mountain tunnel. Based on the area-well theory, a groundwater seepage model of a tunnel is established. Due to the model and groundwater dynamics, the equation describing the relationship between the amount of water inflow and drawdown of the groundwater table for a tunnel is derived under unsteady flow conditions. After the ecological groundwater table for vegetation is introduced, a method of calculating the amount of the limiting drainage for a mountain tunnel is proposed to prevent the drawdown of groundwater table from exceeding the ecological groundwater table required to support vegetation. According to the proposed method, the three-dimensional form of the groundwater drawdown funnels and the influence scope of tunnel water inflow are determined. The rainfall recharge amount Wt within the influence scope is obtained based on the rainfall infiltration coefficient method, and the value is compared to the total groundwater discharge Q. These values Wt and Q are then equated through adjusting the unit groundwater inflow q0. The values less than or equal to q0 can be determined as the limiting drainage value of groundwater for the tunnel, and the groundwater table will not drop and balances the groundwater and the vegetation ecological environment. The results and case study show that the proposed method exhibits good agreement with actual conditions and is suitable for use in tunnel construction.

Plasma Synthetic Jet (PSJ) actuators have shown wide and promising application prospects in high-speed flow control, due to their advantages including high exhaust speed, wide frequency band, rapid response, and non-moving components. Although previous studies on PSJ actuators are abundant, most of them have focused on the performance of a single actuator. However, in practice, an actuator array is very necessary for large-scale aerodynamic actuation on account of the small affected area of a single PSJ. In this paper, the characteristics of a two-electrode plasma synthetic jet actuator array in serial are investigated experimentally. Compared to a parallel actuator array, the serial actuator array requires simpler power supply design and is much easier to realize. High-speed photography of the discharge evolution, voltage-current measurement, and shadowgraphy visualization are used in the investigation. Experimental results show that, for the serial actuator array, weak discharges happen firstly between energized and suspending electrodes, and then a strong pulse arc discharge is triggered. The breakdown voltage in serial is irrelevant to such factors as the number of actuators, the maximum or minimum gap in serial, the connection sequence, etc. It is mainly determined by the sum of gaps. For serial actuators with the same anode-to-cathode spacing, the energy deposition is the same, and the jet is synchronous and similar. Because of the entrainment and merging of adjacent jet vortices, the jet front speed of an aligned synchronous jet array increases as the orifice distance decreases. To achieve the highest jet front velocity, the orifice of the actuator has an optimal diameter. Keywords: Capacitive power supply, Flow control, Plasma synthetic jet, Pulse arc discharge, Serial actuator array, Two-electrode

A velocity-temperature coupled control method is proposed for turbulence drag reduction of supersonic boundary layers. Direct numerical simulations indicate that heated wall blowing achieves the best drag reduction, whereas cooled wall blowing leads to a drag increase. The reduction of mean viscous shear stress is mainly responsible for the drag reduction mechanism, though there is a substantial increase in Reynolds stresses. Nevertheless, the control efficiency of heated wall blowing is low due to high energy consumption, but further investigation is needed.

The control of a shock wave produced by a ramp (ramp shock) in Ma3 supersonic flow using a two-electrode SparkJet (SPJ) actuator in a single-pulse mode is studied experimentally. Except for schlieren images of the interaction process of SPJ with the flow field, a dynamic pressure measurement method is also used in the analysis of shock wave control. In a typical experimental case, under the control of single-pulsed SPJ, the characteristic of ramp shock changes from &ldquo, short-term local upstream motion&rdquo, in the initial stage to &ldquo, long-term whole downstream motion&rdquo, in the later stage. The angle and position of the ramp shock changes significantly in the whole control process. In addition, the dynamic pressure measurement result shows that the ramp pressure is reduced by a maximum of 79% compared to that in the base flow field, which indicates that the ramp shock is significantly weakened by SPJ. The effects of some parameters on the control effect of SPJ on the ramp shock are investigated and analyzed in detail. The increase in discharge capacitance helps to improve the control effect of SPJ on the ramp shock. However, the control effect of the SPJ actuator with medium exit diameter is better than that with a too small or too large one. In addition, when the SPJ exit is located in the separation zone and outside, the change in the ramp shock shows significant differences, but the control effect in the case of medium ramp distance is better when the SPJ exit is located outside the separation zone.

In this paper, we combine the Eulerian stochastic field (ESF) model with flamelet generated manifolds (FGM) model to achieve a new type of turbulent combustion flamelet model with high accuracy under consideration of the detailed chemical reaction mechanism. It solves the limitations of using the presumed PDF model in the original FGM model and the problems of the FGM model in the promotion of the use of non-applicability and other issues. Using OpenFOAM software and Sandia Flame D experimental data to complete the model to achieve and verify. The results show: the simulation results of the new model are good, which can accurately simulate the axial and radial temperature and the distribution of the mass fraction of the main components, and the accuracy is better than the original FGM model; with the increase of the number of stochastic fields in the ESF model, the accuracy of simulation is continuously improved; when the number of stochastic fields increases to a certain value, the simulation results are stable within a certain range of variation. The above results show that the model is effective for this type of flame simulation and has good prospects for simulating a more complex flame.

As a promising high-speed flow control technique, plasma synthetic jet actuator (PSJA) has the superiorities of requiring no moving parts or flow supplies, extremely fast response, wide frequency band and high efflux speed. However, it has limitations for application: in near space, the air in the cavity which is used to generate the puled plasma jet becomes rare, and the low refill rate often leads to insufficient recovery which limits the working frequency. In order to overcome these limitations, a novel actuator called ram-air plasma synthetic jet actuator (RPSJA) is proposed. Inspired by the ramjet, the principle of this actuator is to take advantage of the tremendous dynamic pressure of the high-speed inflow using an added ram-air inlet. Numerical investigations were conducted to demonstrate the feasibility of such an actuator. The results show that, compared with PSJA, the air in the chamber becomes denser and the refill rate is notably increased owing to the “ram-air effect” of RPSJA. Based on the flow characteristic analysis, a revised actuator with a stepped ram-air inlet is proposed and investigated as well, and the results show that the performance is improved as the stepped height rises.

In order to better solve the thermal management of high-power LED and facilitate its miniaturization, an active-passive combined heat dissipation method based on a dual synthetic jet actuator (DSJA) is proposed. Comparative analyses on heat dissipation characteristics and installation-space reductions of nine thermal management schemes are carried out. The results indicate that the temperature rapidly decreases and stabilizes once the actuator is activated. The heat dissipation effect based on DSJA is better than that based on synthetic jet actuator (SJA). The combination method with the vertical layout of DSJA makes the stable temperature of the LED and the installation space decrease 16 °C and 72.3% with respect to the commercial heat sink. Furthermore, a closed-loop system based on the combination method is developed to realize the autonomous control of the LED temperature within a setting temperature range. It is concluded that the operation duty cycle (ODC) reduces with the increase of the upper limit temperature, and there is an optimal impingement distance where the ODC is lowest of 29.1%. The closed-loop control will be greatly useful for the adaptive thermal management of electronic devices.

A three-electrode high-energy plasma synthetic jet (PSJ) actuator was used for shock wave control. This actuator is an enhanced version of the two-electrode actuator as a high-voltage trigger electrode is added to increase the cavity volume and the input energy while retaining a relatively low disruptive voltage. The electrical properties were studied using current-voltage measurements, and the energy consumption was calculated. To assess the jet strength, the penetration of PSJ was compared with empirical values, and the results show that the momentum flux ratio of PSJ for a capacitance of 0.96, 1.6, and 3 μF was approximately equal to 0.6, 1.0, and 1.3, respectively. The interaction of PSJ with shock waves was acquired using high-speed shadowgraph imaging. The shock was generated by a 25° compression ramp in Mach 2 flow, and PSJ actuator was placed up-stream of the compression ramp. Under the action of PSJ, the strength of the shock was notably weakened, and the near-wall part of the shock was entirely eliminated. The results show the good control effect of the three-electrode high-energy PSJ in high-speed flow.

Direct numerical simulations of a spatially developing Ma 2.25 supersonic turbulent boundary layer with streamwise-striped wall blowing (SSB) are performed for the purpose of turbulence drag reduction. The SSB is designed to blow through long and straight slots in the flow direction. The slots are equally distributed in spanwise direction, with different stripe spacing and width. Blowing amplitudes of 0.1% and 0.2% are investigated. For comparison, uniform blowing (UB) of 0.1% is also simulated. It is found that in spite of weak control amplitudes, 7.4% and 14.5% drag reduction can be achieved in the case of 6SSB 0.1% and 6SSB 0.2%, respectively. Similar to UB, the boundary layer displacement thickness is thickened by SSB, with a thicker viscous sub-layer and a logarithmic zone which moving away from the wall. Turbulence intensities and turbulent coherent structures are enhanced, although the streamwise scales of the latter are attenuated. Turbulent kinetic energy analysis presents that both the production and dissipation are increased in the near wall. Compressible Renard-Deck decomposition of C f shows that it is the spatially growth term that mainly responsible for the turbulence drag reduction.

This paper presents an active flow control method for flow around a finite-length square cylinder using dual synthetic jet (DSJ). A piezoelectric dual synthetic jet actuator (DSJA) was installed at the leading edge of the free end for generating the DSJ. The amplitude and frequency of DSJ can be adjusted through the driving voltage and driving frequency. In our research, the pressure measurements and flow visualization are conducted to test the control effectiveness of dual synthetic jet. According to the aerodynamic results, there is a correlation between the control efficiency and the amplitude and frequency of DSJ, which are determined by jet momentum coefficient and driving frequency. After the application of the free-end DSJ with moderate amplitude and frequency, the maximum reduction of total mean drag coefficients, fluctuating drag coefficients and fluctuating lift coefficients respectively reached 4.3%, 18.0% and 30.8%. Moreover, the smoke-wire and PIV results indicate that there is a remarkable suppression of the free-end separated shear flow and a prominent increase in the TKE in the area near the free end. All results point to that the aerodynamic forces and wakes of a surface-mounted finite-length square cylinder can be effectively controlled using dual synthetic jet at free end.

An innovative cooling technology based on a vectoring dual synthetic jet (DSJ) actuator is proposed, and then flow field and heat transfer characteristics are experimentally investigated. The deflection angle of vectoring DSJ can vary bilaterally and linearly with the movement of the adjustable slide. This novel feature makes the installation more flexible to obtain a maximum heat transfer coefficient and the effective heat transfer area is greatly extended, comparing to a conventional DSJ. The sweeping-flow capability can prevent the marginal region from working continuously at a high temperature. This cooling technology is expected to have important practical implications in space-constrained and large-area electronic cooling. Moreover, a model predicting the heat-transfer influence domain is established based on the vectoring angle of DSJ. The predicted results are validated to be in good agreement with the experimental results in the normal impingement region and the cross-flow region. It can provide a guide for properly arranging electric devices to improve the convective heat transfer.

In order to realize large-area and high-temperature plate heat dissipation in a restricted space, an innovative vectoring spray cooling device based on a novel synthetic jet actuator is proposed. Depending on the different response characteristics of different piezoelectric diaphragms to the same electric signals, this device doesn't need transmission or rotating parts, but is able to change its spray impingement direction. It effectively saves installation space and improves the spray cooling efficiency. The research shows that the visualized spray vectoring angle is basically consistent with the vectoring angle based on its potential-core momentum, which indicated that the distribution of spray particles is closely related to the velocity field of synthetic jets. Both of them can be used as effective methods to evaluate the spray vectoring characteristic. Using the Coanda effect can amplify the spray vectoring deflection angle from 10° to 16°, and the enhancement effect reaches more than 60% in this paper. The greater the thickness differences of the diaphragms, the better the vectoring characteristic of the spray. The maximum vectoring deflection angle from −25° to 24° is realized in this paper. However, the heat dissipation capacity depends on the vectoring spray velocity. The use of a diaphragm combination with a greater difference in thickness results in a greater loss of velocity with the variation of driving frequency. Finally, the vectoring angle based on its potential-core momentum is also basically consistent with the vectoring angle based on its temperature field, which indicates that the spray vectoring angle can be directly controlled to accurately remove the high temperature point or district.

To promote high-speed boundary layer transition, this paper proposes an active self-sustaining dual jets (SDJ) actuator utilizing the energy of supersonic mainflow. Employing the nanoparticle-based planar laser scattering (NPLS), supersonic flat-plate boundary layer transition induced by SDJ is experimentally investigated in an Ma-2.95 low-turbulence wind tunnel. Streamwise and spanwise NPLS images are obtained to analyze fine flow structures of the whole transition process. The results reveal the transition control mechanisms that on the one hand, the jet-induced shear layer produces unstable Kelvin–Helmholtz instabilities in the wake flow, on the other hand, the jets also generates an adverse pressure gradient in the boundary layer and induce unstable streak structures, which gradually break down into turbulence downstream. The paper provides a new method for transition control of high-speed boundary layer, and have prospect both in theory and engineering application.