The effect of a single vortex generator jet on the characteristics of a turbulent boundary layer

A flat plate experiment was performed in a water tunnel to determine the effects of a vortex generator jet on the characteristics of a turbulent boundary layer at various wall normal locations. The results show that the characteristic distributions of the turbulent fluctuation quantities are nearly unaffected by the induced vortex structures neither in the steady nor in the dynamic blowing case. The shear layer interaction between the turbulent main flow and the jet flow produces less turbulent fluctuations than it is expected from a turbulent free jet flow. Thus, the mixing process of this flow control strategy is based only on a large-scale momentum transport superimposed by the turbulent fluctuation quantities. This allows a separation of scales for physical interpretation and numerical simulations.     

定常吹气对倾转旋翼机向下载荷的影响

为进一步提高倾转旋翼机悬停状态下的有效载重,开展了定常吹气流动控制对向下载荷的影响研究。首先应用延迟脱体涡模拟(DDES)方法对翼型-90°迎角下非定常大范围分离流动结构进行了数值分析;然后分别开展了前缘吹气、后缘吹气降载措施研究,揭示了吹气降载的机理,并对不同吹气口位置和吹气动量系数的影响进行了定量分析,最后开展了前、后缘同时吹气作用下降载数值模拟研究。计算结果表明:前缘最佳吹气位置在翼型的前缘点,而后缘吹气最佳位置位于襟翼弦长的15%处;前缘吹气的降载效果要优于后缘吹气,而且吹气动量系数对向下载荷的影响较小;相对于初始未施加流动控制构型,阻力系数减小量可达到32.72%。     

The Flow Structure Produced by Pulsed-jet Vortex Generators in a Turbulent Boundary Layer in an Adverse Pressure Gradient

The effect of pulsed jet vortex generators on the structure of an adverse pressure gradient turbulent boundary layer flow was investigated. Two geometrically optimised vortex generator configurations were used, co-rotating and counter-rotating. The duty cycle and pulse frequency were both varied and measurements of the skin friction (using hot films) and flow structure (using stereo PIV) were performed downstream of the actuators. The augmentation of the mean wall shear stress was found to be dependent on the net mass flow injected by the actuators. A quasi steady flow structure was found to develop far downstream of the injection location for the highest pulse frequency tested. The actuator near field flow structure was observed to respond very quickly to variations in the jet exit velocity.     

Vortex ring velocity and minimum separation in an infinite train of vortex rings generated by a fully pulsed jet

A pulsed jet with a period of no flow between pulses (i.e., a fully pulsed jet) produces a multiplicity of vortex rings whose characteristics are determined by the jet pulsing parameters. The present study analyzes the case of impulsively initiated and terminated jet pulses in the limit of equal pulse duration and period to determine the minimum possible vortex ring separation obtainable from a fully pulsed jet. The downstream character of the flow is modeled as an infinite train of thin, coaxial vortex rings. Assuming inviscid flow and matching the circulation, impulse, kinetic energy, and frequency of the jet and vortex ring train allow the properties of the vortex ring train to be determined in terms of the ratio of jet slug length-to-diameter ratio (L/D) used for each pulse. The results show the minimum ring separation may be made arbitrarily small as L/D is decreased and the corresponding total ring velocity remains close to half the jet velocity for L/D < 4, but the thin-ring assumption is violated for L/D > 1.5. The results are discussed in the context of models of pulsed-jet propulsion.     

Effect of novel synthetic jet on wake vortex shedding modes of a circular cylinder

A novel actuator signal achieved by changing the ratio of the suction duty cycle to the blowing duty cycle is adopted to enhance the control effect of the synthetic jet for the flow around a circular cylinder. The suction duty cycle factor k defined as the ratio between the time duration of the suction cycle and the blowing cycle and the equivalent momentum coefficient C_μ are introduced as the determining parameters. The synthetic jet is positioned at the rear stagnation point in order to introduce symmetric perturbations upon the flow field. The proper orthogonal decomposition (POD) technique is applied for the analysis of the spanwise vorticity field. Increasing the suction duty cycle factor, the momentum coefficient is enhanced, and thus a stronger and larger scale synthetic jet vortex pair with a higher convection velocity is generated. The synthetic jet vortex pair interacts with the spanwise vorticity shear layers behind both sides of the cylinder, resulting in the variations of the wake vortex shedding modes at Re=950: for k=0.25, C_μ=0.148, vortex synchronization at the subharmonic excitation frequency with antisymmetric shedding mode; for 0.50≤k≤1.00, 0.213≤C_μ≤0.378, vortex synchronization at the excitation frequency with the symmetric or antisymmetric shedding modes; for 2.00≤k≤4.00, 0.850≤C_μ≤2.362, vortex synchronization at the excitation frequency with symmetric shedding mode. Hence, the control effect of the synthetic jet upon the wake vortex of a circular cylinder can be enhanced by increasing the suction duty cycle factor so as to increase the momentum coefficient. This is also validated at a higher Reynolds number Re=1600.     

Application of a Synthetic Jet to Control Boundary Layer Separation under Ultra-High-Lift Turbine Pressure Distribution

The transition and separation processes of the boundary layer developing on a flat plate under a prescribed adverse pressure gradient typical of Ultra-High-Lift low-pressure turbine profiles have been investigated, with and without the application of a synthetic jet (zero net mass flow rate jet). A mechanical piston has been adopted to produce an intermittent flow with zero net mass flow rate. The capability of the device to suppress or reduce the large laminar separation bubble occurring under steady inflow condition at low Reynolds numbers has been experimentally investigated by means of hot-wire measurements. Wall static pressure measurements complement the hot-wire time-resolved velocity results. The paper reports the investigations performed for both steady and controlled conditions. The active device is able to control the laminar separation bubble induced at low Reynolds number conditions by the strong adverse pressure gradient. An overall view of the time-dependent evolution of the controlled boundary layer is provided by the phase-locked ensemble averaging technique, triggered at the synthetic jet frequency. The separated flow transition process, which is detected for the uncontrolled condition, is modified by the synthetic jet in different ways during the blowing and suction phases. Overall, the phase-locked velocity distributions show a reduced separated flow region for the whole jet cycle as compared to the uncontrolled condition. The phase-locked distributions of the random unsteadiness allow the identification of vortical structures growing along the shear layer mainly during the blowing phase.     

Flow analysis of two-dimensional pulsed jets by particle image velocimetry

A purely alternating jet without mean mass flux and a mixed pulsed jet containing an additional blowing component were investigated by particle image velocimetry (PIV). The jets issued from a two-dimensional slit connected to a converging nozzle, opening normally from a flat wall. The pulsation was driven by a loudspeaker. The mean velocity fields were characterized by the combination of downstream directional blowing and omni-directional suction. The velocity fluctuations were dominated by contra-rotating eddy pairs synchronized with the pulsation and formed at the jet edges during blowing. Phase-synchronized measurements permit the investigation of the averaged patterns and the cycle-to-cycle fluctuations of these vortices. The mean trajectories of vortex centers during a whole injection cycle show how large lateral jet expansions are achieved. For a purely alternating jet, the expansion takes place close to the slit. For a mixed pulsed jet, the vortices develop farther from the orifice. In addition, proper orthogonal mode decomposition demonstrates that only a few modes are required to represent the main events of the flow dynamics. Received: 10 August 1999 / Accepted: 10 January 2001     

On the possibility of blowing a gas jet into a supersonic flow without formation of a three-dimensional boundary-layer separation zone

We consider the flow formed by the interaction of a supersonic flow and a transverse sonic or supersonic jet blown at right angles to the direction of the main flow through a nozzle whose exit section is in a flat wall. When a gas jet is blown through a circular opening [1] the pressure rises in front of the jet because of the stagnation of the oncoming flow. This leads to separation of the boundary layer formed on the wall in front of the blowing nozzle. The resulting three-dimensional separation zone leads to a sharp increase in the pressure and the heat fluxes to the wall in front of the blowing nozzle, which is undesirable in many modern applications. The aim of the present investigation was to find a shape of the exit section of the blowing nozzle for which there is no three-dimensional separation zone of the boundary layer in front of the blowing nozzle.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 3, pp. 162–165, May–June, 1979.     

熔喷双槽形喷嘴气体射流流场初探

在熔喷非织造布加工中,气体射流作为工作介质使聚合物熔体实现拉伸,气体射流流场的研究对熔喷气流拉抻数学模型研究非常重要。熔喷双槽形喷嘴形成的流场可以看作两股平面射流的合成。从单个点涡的性质出发,研究了涡偶的性质和涡偶代替射流的可行性。研究表明,在喷丝孔轴线附近,涡偶和射流的速度分布趋势相同,且有比较相近的速度分布,从而说明以涡偶代替射流是可行的。在此基础上,用两个涡偶分别代替两股射流,然后进行合成,推导出两股射流合成后速度分布的理论公式,该公式的计算结果与实验结果吻合较好。将该公式引入熔喷气流拉伸数学模型,预测出的纤维直径与采用经验公式时的预测结果几乎完全相同。结果表明,应用涡偶代替射流推导出的气流速度分布公式能够较好地描述熔喷双槽形喷嘴气体射流流场,可以用于完善熔喷气流拉伸数学模型。     

Experimental time-resolved study of the interaction between a pulsating injectant and a steady cross-flow: aerodynamics of film cooling

A test rig incorporating the injection from a single cylindrical hole with an inclination of 30° to a thermally uniform mainstream flow was used for determining variations in flow structures due to injectant pulsation. The average blowing ratios ([`(M)] \overline{M} ) were 0.65, 1, and 1.25. The periodic variations in injectant flow were rendered by a loudspeaker-based pulsation system to nondimensionalized excitation frequency (St St ) of 0, 0.2, 0.3, and 0.5. Pulsation resulting in a close-wall orientation of injectant fluid compared with steady blowing bearing outward orientation was only observed in few cases. At [`(M)] \overline{M}  = 0.65, jet fluid remains aligned and covers a significant part of the wall under steady blowing. At higher blowing ratios, pulsation induces large spatial variations in the jet trajectory, collapsing of the jet body, and the shedding of wake structures due to the periodic variation of injection flow rate. It was found that the pulsation improves wall coverage of the injectant fluid under low frequency excitation as the separation of the jet from the wall becomes evident ([`(M)] \overline{M}  = 1 and 1.25).     

自激振荡脉冲射流喷嘴装置系统频率特性理论研究

根据相似系统原理和流体网络理论建立了自激振荡脉冲射流喷嘴装置的等效网络模型,利用系统传递函数推导了系统频率特性方程并进行了数值计算。结果表明:喷嘴装置的固有频率主要由喷嘴形状、结构参数、入口流速、射流中压力扰动波波速决定;自激振荡腔腔径、自激振荡腔腔长、上喷嘴直径、下喷嘴直径都对系统频率特性影响很大。提出了相应的自激振荡脉冲射流喷嘴设计准则,即喷嘴装置在最佳阻尼比下产生谐波共振。     

Separation control by vortex generator devices in a transonic channel flow

An experimental study was conducted in a transonic channel to control by mechanical vortex generator devices the strong interaction between a shock wave and a separated turbulent boundary layer. Control devices—co-rotating and counter-rotating vane-type vortex generators—were implemented upstream of the shock foot region and tested both on a steady shock wave and on a forced shock oscillation configurations. The spanwise spacing of vortex generator devices along the channel appeared to be an important parameter to control the flow separation region. When the distance between each device is decreased, the vortices merging is more efficient to reduce the separation. Their placement upstream of the shock wave is determinant to ensure that vortices have mixed momentum all spanwise long before they reach the separation line, so as to avoid separation cells. Then, vortex generators slightly reduced the amplitude of the forced shock wave oscillation by delaying the upstream displacement of the leading shock.     

Particle removal from a surface by a bounded vortex flow

A bounded vortex flow consists of an axisymmetric vortex that is confined top and bottom between two plates (the “confinement plate” and “impingement plate”, respectively) and surrounded laterally by a swirling annular slot jet. The bottom of the vortex terminates on the boundary layer along the impingement plate and the top of the vortex is drawn into a suction port positioned at the center of the confinement plate. The circumferential flow within the annular jet is important for supplying circulation to the central wall-normal vortex. This flow field is proposed as a method for mitigation of dust build-up on a surface, where the vortex–jet combination supplements the more traditional vacuum port by enhancing the surface shear stress and related particle transport rate. The paper reports on a computational study of the velocity field and particle transport by a bounded vortex flow. Fluid flow computations are performed using a finite-volume approach for an incompressible fluid and particle transport is simulated using a discrete-element method. Computations are performed for different values of two dimensionless parameters – the ratio of the plate separation distance and the average radial location of the jet inlet (the dimensionless confinement height) and the ratio of flow rate withdrawn at the suction outlet and that injected by the jet (the flow rate ratio). For small values of the flow rate ratio, the impinging jet streamlines pass down to the boundary layer along the bottom surface and then travel up the vortex core. By contrast, for large values of flow rate ratio, the annular jet is quickly entrained into the suction outlet and no wall-normal vortex is formed. Particles are observed to roll along the impingement surface in a direction determined by the fluid shear stress lines. Particles roll outward when they lie beyond a separatrix curve of the surface shear stress lines, where particles within this separatrix curve roll inward, piling up at the center of the flow field. A toroidal vortex ring forms for the small confinement height case with flow rate ratio equal to unity, which yields double separatrix curves in the shear stress lines. The inward rolling particles intermittently lift up due to collision forces and burst away from the impingement surface, eventually to become entrained into the flow out the suction port or resettling back onto the impingement surface.     

High-lift airfoil trailing edge separation control using a single dielectric barrier discharge plasma actuator

Control of flow separation from the deflected flap of a high-lift airfoil up to Reynolds numbers of 240,000 (15 m/s) is explored using a single dielectric barrier discharge (DBD) plasma actuator near the flap shoulder. Results show that the plasma discharge can increase or reduce the size of the time-averaged separated region over the flap depending on the frequency of actuation. High-frequency actuation, referred to here as quasi-steady forcing, slightly delays separation while lengthening and flattening the separated region without drastically increasing the measured lift. The actuator is found to be most effective for increasing lift when operated in an unsteady fashion at the natural oscillation frequency of the trailing edge flow field. Results indicate that the primary control mechanism in this configuration is an enhancement of the natural vortex shedding that promotes further momentum transfer between the freestream and separated region. Based on these results, different modulation waveforms for creating unsteady DBD plasma-induced flows are investigated in an effort to improve control authority. Subsequent measurements show that modulation using duty cycles of 50–70% generates stronger velocity perturbations than sinusoidal modulation in quiescent conditions at the expense of an increased power requirement. Investigation of these modulation waveforms for trailing edge separation control similarly shows that additional increases in lift can be obtained. The dependence of these results on the actuator carrier and modulation frequencies is discussed in detail.     

Effects of acoustic excitation at resonance Strouhal numbers on characteristics of an elevated transverse jet

The effects of acoustic excitation on the flow behavior, penetration, and spread of the stack-issued and wall-issued transverse jets were studied experimentally. The jet flow was periodically excited by a loudspeaker that was driven with a square wave at resonance Strouhal numbers. The pulsed transverse jet was characterized by jet Reynolds number 2000. Streak pictures of the smoke flow patterns illuminated by the laser-light sheet in the median plane were recorded with a high-speed digital camera to illustrate the evolution process of the characteristic flow behavior within one excitation cycle. The binary edge-detection technique was used to determine penetration height and spread width. The tracer-gas concentration measurement provided jet dispersion information. The evolution processes of both the stack-issued and wall-issued transverse jets were characterized by a leading vortex ring and swing motion of the jet column near the jet exit as the jets were forced at resonance Strouhal numbers. A leading vortex ring appeared near the jet exit during the leading phase of excitation cycle and evolved subsequently to puffs of jet fluids in the upwind shear layer of the deflected jet. The swinging motion of the near-tube tip jet column induced up/down oscillation of the deflected jet. The excited stack-issued transverse jet exhibited significantly larger penetration height and spread width than the excited wall-issued transverse jet. The tracer-gas detection experiment results showed that the excited transverse jet disperses significantly faster and wider than the non-excited transverse jet. Pulsating the transverse jet at low resonance Strouhal numbers produced higher mixing and dispersion effects than pulsating the transverse jet at high resonance Strouhal numbers.     

Control of the corner separation in a compressor cascade by steady and unsteady plasma aerodynamic actuation

This paper reports experimental results on using steady and unsteady plasma aerodynamic actuation to control the corner separation, which forms over the suction surface and end wall corner of a compressor cascade blade passage. Total pressure recovery coefficient distribution was adopted to evaluate the corner separation. Corner separation causes significant total pressure loss even when the angle of attack is 0°. Both steady and unsteady plasma aerodynamic actuations suppress the corner separation effectively. The control effect obtained by the electrode pair at 25% chord length is as effective as that obtained by all four electrode pairs. Increasing the applied voltage improves the control effect while it augments the power requirement. Increasing the Reynolds number or the angle of attack makes the corner separation more difficult to control. The unsteady actuation is much more effective and requires less power due to the coupling between the unsteady actuation and the separated flow. Duty cycle and excitation frequency are key parameters in unsteady plasma flow control. There are thresholds in both the duty cycle and the excitation frequency, above which the control effect saturates. The maximum relative reduction in total pressure loss coefficient achieved is up to 28% at 70% blade span. The obvious difference between steady and unsteady actuation may be that wall jet governs the flow control effect of steady actuation, while much more vortex induced by unsteady actuation is the reason for better control effect.     

SBLI control for wings and inlets

Flow control can be applied to shock wave/boundary layer interactions to achieve two different goals;the delay of shock-induced separation and/or the reduction ofstagnation pressure losses, which cause wave drag or inletinefficiencies. This paper introduces the principles and maintechniques for both approaches and assesses their relativesuitability for practical applications. While boundary layersuction is already in wide use for separation control, themost promising novel device is the micro-vortex generator,which can deliver similar benefits to traditional vortex generatorsat much reduced device drag. Shock control is notyet used on practical applications for a number of reasons,but recent research has focused on three-dimensional deviceswhich promise to deliver flow control with improved offdesignbehaviour. Furthermore, there are some indicationsthat a new generation of control devices may be able to combinethe benefits of shock and boundary layer control andreduce shock-induced stagnation pressure losses as well asdelay shock-induced separation.     

Experiments on jet/vortex interaction

An experimental study was performed to evaluate the effect of a cold jet on a single trailing vortex. Flow visualization and particle image velocimetry (PIV) measurements were conducted in wind and water tunnels. The main parameters were the ratio of jet-to-vortex strength, the jet-to-vortex distance, the jet inclination angle and the Reynolds number. It was shown that the jet turbulence is wrapped around the vortex and ingested into it. This takes place faster with decreasing jet-to-vortex distance and increasing jet strength. Both time-averaged and instantaneous flow fields showed that the trailing vortex became diffused with its rotational velocity and vorticity levels reduced when the jet is located close to the vortex. The mechanism with which the jet interacts with the vortex is a combination of vortices shed by the jet and the turbulence. No noticeable differences were found within the Reynolds number range tested. The effect of jet on the vortex is delayed when the jet is blowing at an angle to the free stream and away from the vortex such as during take-off.     

Aerodynamic flow vectoring of a wake using asymmetric synthetic jet actuation

This paper reports an experimental investigation on the wake of a blunt-based, flat plate subjected to aerodynamic flow vectoring using asymmetric synthetic jet actuation. Wake vectoring was achieved using a synthetic jet placed at the model base 2.5?mm from the upper corner. The wake Reynolds number based on the plate thickness was 7,200. The synthetic jet actuation frequency was selected to be about 75?% the vortex shedding frequency of the natural wake. At this actuation frequency, the synthetic jet delivered a periodic flow with a momentum coefficient, C _??, of up to 62?%. Simultaneous measurements of the streamwise and transverse components of the velocity were performed using particle image velocimetry (PIV) in the near wake. The results suggested that for significant wake vectoring, vortex shedding must be suppressed first. Under the flow conditions cited above, C _?? values in the range of 10?C20?% were required. The wake vectoring angle seemed to asymptote to a constant value of about 30° at downstream distances, x/h, larger than 4 for C _?? values ranging between 24 and 64?%. The phase-averaged vorticity contours and the phase-averaged normal lift force showed that most of the wake vectoring is produced during the suction phase of the actuation, while the blowing phase was mostly responsible for vortex shedding suppression.     

基于涡流发生器的翼型失速流动控制及雷诺数效应影响研究

针对所设计的三角形涡流发生器开展用于翼型失速流动控制的风洞实验研究,重点讨论涡流发生器几何参数、方向角、安装位置及实验雷诺数等因素对翼型失速流动控制的影响。实验结果表明:涡流发生器作用下,在干净翼失速迎角后能够形成一个升力几乎不随迎角变化的相对稳定的高升力状态,抑制了失速流动的发生,与此同时阻力大幅下降;本文所设计的涡流发生器方向角过大时会削弱翼型失速流动控制的效果;同一涡流发生器作用下雷诺数过大其失速流动控制效果会急剧恶化,第一种涡流发生器控制翼型失速的雷诺数有效范围略宽于第二种涡流发生器。