等离子体激励器诱导射流的湍流特性研究

为了进一步掌握等离子体流动控制机理, 完善等离子体激励器数学模型, 提升等离子体激励器扰动能力, 采用粒子图像测速技术, 在静止空气下开展了介质阻挡放电等离子体激励器诱导射流特性研究. 实验时, 将非对称布局激励器布置在平板模型上, 随后将带有激励器的模型放置在有机玻璃箱内, 从而避免环境气流对测试结果的影响. 基于激励器诱导流场, 分析了激励电压对诱导射流特性的影响, 揭示了较高电压下诱导射流近壁区的拟序结构, 获得了卷起涡、二次涡等拟序结构的演化发展过程, 计算了卷起涡脱落频率, 阐述了卷起涡与启动涡的区别, 初步探索了卷起涡的耗散机制. 结果表明: (1)层流射流不能完全概括等离子体诱导射流特性, 激励电压是影响射流特性的重要参数. 当电压较低时, 诱导射流为层流射流; 当电压较高时, 诱导射流的雷诺数提高, 射流剪切层不稳定, 层流射流逐渐发展为湍流射流. (2)等离子体诱导湍流射流包含着卷起涡、二次涡等拟序结构; 在固定电压下, 这些涡结构存在恒定的卷起频率. (3)当激励电压较高时, 流动不稳定使得卷起涡发生了拉伸、变形, 引起了流场湍动能增大, 从而加速了卷起涡的耗散. 研究结果为全面认识激励器射流特性, 进一步挖掘激励器卷吸掺混能力, 提升激励器控制能力积累基础.      

等离子体激励器诱导射流的湍流特性研究

为了进一步掌握等离子体流动控制机理,完善等离子体激励器数学模型,提升等离子体激励器扰动能力,采用粒子图像测速技术,在静止空气下开展了介质阻挡放电等离子体激励器诱导射流特性研究.实验时,将非对称布局激励器布置在平板模型上,随后将带有激励器的模型放置在有机玻璃箱内,从而避免环境气流对测试结果的影响.基于激励器诱导流场,分析了激励电压对诱导射流特性的影响,揭示了较高电压下诱导射流近壁区的拟序结构,获得了卷起涡、二次涡等拟序结构的演化发展过程,计算了卷起涡脱落频率,阐述了卷起涡与启动涡的区别,初步探索了卷起涡的耗散机制.结果表明:(1)层流射流不能完全概括等离子体诱导射流特性,激励电压是影响射流特性的重要参数.当电压较低时,诱导射流为层流射流;当电压较高时,诱导射流的雷诺数提高,射流剪切层不稳定,层流射流逐渐发展为湍流射流.(2)等离子体诱导湍流射流包含着卷起涡、二次涡等拟序结构;在固定电压下,这些涡结构存在恒定的卷起频率.(3)当激励电压较高时,流动不稳定使得卷起涡发生了拉伸、变形,引起了流场湍动能增大,从而加速了卷起涡的耗散.研究结果为全面认识激励器射流特性,进一步挖掘激励器卷吸掺混能力,提升激励器控制能力积累基础.     

等离子体激励器控制圆柱绕流的实验研究

为了发展新型移动附面层控制技术,提升流动控制效率,采用粒子图像测速技术,开展了基于对称布局等离子体气动激励的圆柱绕流控制研究,获得了静止空气下,对称布局激励器诱导流场的演化过程,评估了来流条件下等离子体控制效果,通过等离子体诱导涡实现了虚拟移动附面层控制,分析了诱导涡随时间演化的过程,揭示了圆柱绕流等离子体控制机理.结果表明:(1)在静止空气下,对称布局激励器在刚启动瞬间,会在暴露电极两侧诱导产生一对旋转方向相反的启动涡;随着时间的推移,启动涡逐渐向远离壁面的方向运动;随后,激励器在暴露电极两侧产生了两股速度近似相等,方向相反的诱导射流,诱导射流在柯恩达效应的影响下,朝壁面方向发展.(2)当激励电压峰峰值为19.6 kV,激励频率3kHz时,施加等离子体气动激励后,圆柱脱落涡得到了较好抑制,圆柱阻力系数减小了21.8％;(3)在来流作用下,对称布局激励器在靠近来流一侧,诱导产生了较为稳定的涡结构.诱导涡通过旋转、运动,促进了壁面附近低能气流与主流之间的掺混,抑制了圆柱绕流流场分离,实现了"虚拟移动附面层控制"效果.与传统移动附面层控制技术相比,基于等离子体气动激励的新型移动附面层控制技术不需要复杂、笨重的机构,不会带来额外的阻力,具有潜在的应用前景.      

等离子体激励器控制圆柱绕流的实验研究

为了发展新型移动附面层控制技术,提升流动控制效率,采用粒子图像测速技术,开展了基于对称布局等离子体气动激励的圆柱绕流控制研究,获得了静止空气下,对称布局激励器诱导流场的演化过程,评估了来流条件下等离子体控制效果,通过等离子体诱导涡实现了虚拟移动附面层控制,分析了诱导涡随时间演化的过程,揭示了圆柱绕流等离子体控制机理.结果表明:(1)在静止空气下,对称布局激励器在刚启动瞬间,会在暴露电极两侧诱导产生一对旋转方向相反的启动涡;随着时间的推移,启动涡逐渐向远离壁面的方向运动;随后,激励器在暴露电极两侧产生了两股速度近似相等,方向相反的诱导射流,诱导射流在柯恩达效应的影响下,朝壁面方向发展.(2)当激励电压峰峰值为19.6 kV,激励频率3 kHz时,施加等离子体气动激励后,圆柱脱落涡得到了较好抑制,圆柱阻力系数减小了21.8%;(3)在来流作用下,对称布局激励器在靠近来流一侧,诱导产生了较为稳定的涡结构.诱导涡通过旋转、运动,促进了壁面附近低能气流与主流之间的掺混,抑制了圆柱绕流流场分离,实现了"虚拟移动附面层控制"效果.与传统移动附面层控制技术相比,基于等离子体气动激励的新型移动附面层控制技术不需要复杂、笨重的机构,不会带来额外的阻力,具有潜在的应用前景.     

零质量射流技术控制垂直轴风力机流动分离的数值研究

设计了一种斜出口零质量射流激励器并将其应用于垂直轴风力机。对施加零质量射流激励器的直线翼垂直轴风力机进行了数值研究。为适应垂直轴风力机运行工况,提出了一种冲程长度随方位角变化的激励器控制策略,验证其减小激励能耗及改善流场结构的特点;分析了射流孔数量及冲程长度对控制叶片流动分离及提升风力机气动性能的影响规律,并在最佳控制参数下对风力机流场结构进行了分析。结果表明:当激励器布置于叶片后部,最大射流吹气系数为0.0506时,采用该种激励器控制策略下的双射流孔风力机,风能利用系数相比未施加流动控制、定常吹气、定冲程长度最大分别提升21.31%、3.98%、0.06%,且射流孔数越多,提升效果越差。该种流动控制技术可抑制大涡的形成及发展,改善叶片周围流场结构。     

EXPERIMENTAL INVESTIGATION ON LIFT INCREMENT OF A PLASMA CIRCULATION CONTROL AIRFOIL

利用等离子体激励器发展了新型的环量增升技术,并对二维NACA0012翼型绕流实施控制。由于NACA0012翼型为尖后缘构型,环量增升装置由2个非对称型介质阻挡放电等离子体激励器构成。一个等离子体激励器贴附于翼型吸力面靠近后缘处,其诱导的壁面射流沿来流方向指向下游;另一个等离子体激励器贴附于翼型压力面靠近后缘处,其诱导的壁面射流与来流方向相反指向上游。在风洞中通过时间解析二维PIV系统对翼型绕流流场进行了测量,基于翼型弦长的雷诺数Re=20 000。结果表明在等离子体激励器的控制下,翼型压力面靠近后缘处可以形成一个定常回流区,从而起到虚拟气动外形的作用,因此翼型吸力面的流场得到加速,压力面的流场得到减速,使得翼型压力面的吸力以及压力面的压力都得到增加,进而增加了翼型的环量。风洞天平测力实验进一步验证了该环量增升技术的有效性。在整个攻角范围内,施加控制的翼型的升力系数相比没有控制的工况有明显的提高。     

等离子体环量控制翼型增升的实验研究

利用等离子体激励器发展了新型的环量增升技术,并对二维NACA0012翼型绕流实施控制.由于NACA0012翼型为尖后缘构型,环量增升装置由2个非对称型介质阻挡放电等离子体激励器构成.一个等离子体激励器贴附于翼型吸力面靠近后缘处,其诱导的壁面射流沿来流方向指向下游;另一个等离子体激励器贴附于翼型压力面靠近后缘处,其诱导的壁面射流与来流方向相反指向上游.在风洞中通过时间解析二维PIV系统对翼型绕流流场进行了测量,基于翼型弦长的雷诺数Re=20000.结果表明在等离子体激励器的控制下,翼型压力面靠近后缘处可以形成一个定常回流区,从而起到虚拟气动外形的作用,因此翼型吸力面的流场得到加速,压力面的流场得到减速,使得翼型压力面的吸力以及压力面的压力都得到增加,进而增加了翼型的环量.风洞天平测力实验进一步验证了该环量增升技术的有效性.在整个攻角范围内,施加控制的翼型的升力系数相比没有控制的工况有明显的提高.     

合成射流技术及其在流动控制中应用的进展

流动控制是流体技术最主要的研究领域,21世纪的空气动力学将在流动控制领域取得重大突破; 合成射流是一种基于旋涡运动的零质量射流,是流动控制领域近10年来最热门活跃的流动主动控制技术.首先介绍了合成射流激励器及激励器的基本类型,并概括了激励器的主要发展方向,即宽频域、高动量、``强壮'的合成射流激励器和微小型激励器.尔后,对合成射流激励器工作原理、合成射流结构以及合成射流独特的流场特征和合成射流技术的特点进行了综述.最后着重对合成射流技术主要和潜在应用,如流动分离及气动力控制、射流矢量控制、增强掺混及加强传热和传质、抑制噪声、微流体控制、飞行控制以及粒子的散布控制、合成射流陀螺仪技术等进行了介绍和综述,同时对其在各应用领域的控制机理进行了归纳总结.      

基于主动流动控制的射流矢量偏转技术

本文介绍了一种基于主动流动控制技术的射流矢量偏转新方法和控制思路。通过在主射流出口两侧加装斜置扩张固壁板来降低射流两侧与固壁边界间的流体压力,将射流偏转由"不敏感-难控"转变成"敏感-易控",再在固壁板布置自行研制的斜出口合成射流激励器对主射流进行比例偏转控制。实验结果表明,射流最大偏转角可达15°。此外还研究了激励位置角度、激励频率、激励电压不同工作参数对射流矢量偏转控制的影响,实现了主射流偏转角的比例控制。当合成射流与主射流动量比为1∶43时,主射流偏转角可达13°,合成射流激励器消耗的能量为1.5W,初步实现了以小的能量消耗获取高的控制效益。     

机翼尺度效应对等离子体分离流动控制特性的影响

等离子体流动控制技术是一种以等离子体气动激励为控制手段的主动流动控制技术. 为了进一步提高等离子体激励器可控机翼尺度, 以超临界机翼SC(2)-0714大迎角分离流为研究对象, 以对称布局介质阻挡放电等离子体为控制方式, 以测力、粒子图像测速仪为研究手段, 从等离子体激励器特性研究出发, 深入开展了机翼尺度效应对等离子体控制的影响研究, 提出了适用于分离流控制的能效比系数, 探索了分离流等离子体控制机理, 掌握了机翼尺度对分离流控制的影响规律. 结果表明: (1)随着机翼尺度的增大, 布置到机翼上的激励器电极长度会相应增加; 在本文的参数研究范围内, 激励器的平均消耗功率不会随电极长度的增加而线性增大; 当电极长度达到一定阈值时, 激励器的平均消耗功率趋于定值; (2)在固定雷诺数的情况下, 随着机翼尺度的增大, 等离子体的控制效果并未降低, 激励器能效比系数提高; (3)等离子体在主流区诱导的大尺度展向涡与在壁面附近产生的一系列拟序结构成为分离流控制的关键. 研究结果为实现真实飞机的等离子体分离流控制, 推动等离子体流动控制技术工程化应用提供了技术支撑.      

Experimental damping of boundary-layer oscillations using DBD plasma actuators

In the present work artificially excited Tollmien–Schlichting waves were cancelled using plasma actuators operated both in continuous and pulsed modes. To achieve this a vibrating surface, driven by an electromagnetic turbulator, was flush-mounted in a flat plate to excite the TS waves. These were amplified by an adverse pressure gradient induced by an insert on the upper wall of the test section. Control plasma actuators positioned downstream of the excitation actuator attenuate the waves by imparting a steady or unsteady force into the boundary-layer. In the case with steady actuation the two actuators change the velocity profile of the laminar boundary-layer, which then attenuates the waves by itself. In the case of pulsed actuation the actuator creates an unsteady body force to counteract directly the oscillation. As a result the amplitude of the velocity fluctuations at the excitation frequency is reduced significantly in both cases. The principles and the results of the two sets of experiments are presented and discussed.     

On factors influencing arc filament plasma actuator performance in control of high speed jets

Localized arc filament plasma actuators (LAFPAs) have been developed and used at The Gas Dynamics and Turbulence Laboratory for the purpose of controlling high-speed and high Reynolds number jets. The ability of LAFPAs for use in both subsonic and supersonic jets has been explored, and experiments to date have shown that these actuators have significant potential for mixing enhancement and noise control applications. While it has been established that the actuators manipulate instabilities of the jet, the exact nature of how the actuation couples to the flow is still unclear. All of the results previously reported have been based on a nozzle extension that has an azimuthal groove of 1 mm width and 0.5 mm depth along the inner surface approximately 1 mm upstream of nozzle extension exit. The ring groove was initially added to shield the plasma arcs from the high-momentum flow. However, the effect of the ring groove on the actuation mechanism is not known. To explore this effect, a new nozzle extension is designed, which relocates the actuators to the nozzle extension face and eliminates the ring groove. Schlieren images, particle image velocimetry and acoustic results of a Mach 0.9 jet of Reynolds number ~6.1 × 10⁵ show similar trends and magnitudes with and without a ring groove. Thus, it is concluded that the ring groove does not play a primary role in the LAFPAs’ control mechanism. Furthermore, the effect of the duty cycle of the actuator input pulse on the LAFPAs’ control authority is investigated. The results show that the minimum duty cycle that provides complete plasma formation has the largest control over the jet.     

Dielectric-barrier-discharge vortex generators: characterisation and optimisation for flow separation control

We investigated the use of dielectric-barrier-discharge plasma actuators as vortex generators for flow separation control applications. Plasma actuators were placed at a yaw angle to the oncoming flow, so that they produced a spanwise wall jet. Through interaction with the oncoming boundary layer, this created a streamwise longitudinal vortex. In this experimental investigation, the effect of yaw angle, actuator length and plasma-induced velocity ratio was studied. Particular attention was given to the vortex formation mechanism and its development downstream. The DBD plasma actuators were then applied in the form of co-rotating and counter-rotating vortex arrays to control flow separation over a trailing-edge ramp. It was found that the vortex generators were successful in reducing the separation region, even at plasma-to-free-stream velocity ratios of less than 10%.     

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.     

Control of diffuser jet flow: turbulent kinetic energy and jet spreading enhancements assisted by a non-thermal plasma discharge

An axisymmetric air jet exhausting from a 22-degree-angle diffuser is investigated experimentally by particle image velocimetry (PIV) and stereo-PIV measurements. Two opposite dielectric barrier discharge (DBD) actuators are placed along the lips of the diffuser in order to force the mixing by a co-flow actuation. The electrohydrodynamic forces generated by both actuators modify and excite the turbulent shear layer at the diffuser jet exit. Primary air jet velocities from 10 to 40 m/s are studied (Reynolds numbers ranging from 3.2 to 12.8 × 10⁴), and baseline and forced flows are compared by analysing streamwise and cross-stream PIV fields. The mixing enhancement in the near field region is characterized by the potential core length, the centreline turbulent kinetic energy (TKE), the integrated value of the TKE over various slices along the jet, the turbulent Reynolds stresses and the vorticity fields. The time-averaged fields demonstrate that an effective increase in mixing is achieved by a forced flow reattachment along the wall of the diffuser at 10 m/s, whereas mixing enhancement is realized by excitation of the coherent structures for a primary velocity of 20 and 30 m/s. The actuation introduces two pairs of contra-rotating vortices above each actuator. These structures entrain the higher speed core fluid toward the ambient air. Unsteady actuations over Strouhal numbers ranging from 0.08 to 1 are also studied. The results suggest that the excitation at a Strouhal number around 0.3 is more effective to enhance the turbulence kinetic energy in the near-field region for primary jet velocity up to 30 m/s.     

Development of localized arc filament RF plasma actuators for high-speed and high Reynolds number flow control

Recently developed localized arc filament plasma actuators (LAFPAs) have shown tremendous control authority in high-speed and high Reynolds number flow for mixing enhancement and noise mitigation. Previously, these actuators were powered by a high-voltage pulsed DC plasma generator with low energy coupling efficiency of 5–10%. In the present work, a new custom-designed 8-channel pulsed radio frequency (RF) plasma generator has been developed to power up to 8 plasma actuators operated over a wide range of forcing frequencies (up to 50 kHz) and duty cycles (1–50%), and at high energy coupling efficiency (up to 80–85%). This reduces input electrical power requirements by approximately an order of magnitude, down to 12 W per actuator operating at 10% duty cycle. The new pulsed RF plasma generator is scalable to a system with a large number of channels. Performance of pulsed RF plasma actuators used for flow control was studied in a Mach 0.9 circular jet with a Reynolds number of about 623,000 and compared with that of pulsed DC actuators. Eight actuators were distributed uniformly on the perimeter of a 2.54-cm diameter circular nozzle extension. Both types of actuators coupled approximately the same amount of power to the flow, but with drastically different electrical inputs to the power supplies. Particle image velocimetry measurements showed that jet centerline Mach number decay produced by DC and RF actuators operating at the same forcing frequencies and duty cycles is very similar. At a forcing Strouhal number near 0.3, close to the jet column instability frequency, well-organized periodic structures, with similar patterns and dimensions, were generated in the jets forced by both DC and RF actuators. Far-field acoustic measurements demonstrated similar trends in the overall sound pressure level (OASPL) change produced by both types of actuators, resulting in OASPL reduction up to 1.2–1.5 dB in both cases. We conclude that pulsed RF actuators demonstrate flow control authority similar to pulsed DC actuators, with a significantly reduced power budget.