INVESTIGATION ON THE TURBULENT CHARACTERISTICS OF THE JET INDUCED BY A PLASMA ACTUATOR
ZhangXin1,*,, HuangYong2, YangPengyu2, TangKun2, LiHuaxing1 1School of Aeronautics, Northwestern Polytechnical University, Xi’an 710072, China);2China Aerodynamics Research and Development Center, Mianyang 621000, Sichuan, China); 中图分类号:V211 文献标识码:A
关键词:湍流特性;射流;等离子体;介质阻挡放电;拟序结构 Abstract In order to understand the controlling mechanism of plasma actuator and develop the mathematical model of plasma actuator, the turbulent characteristics of the jet induced by a dielectric barrier discharge (DBD) plasma actuator in quiescent air was studied in a closed chamber using particle image velocimetry (PIV). Here, an asymmetrical DBD plasma actuator was mounted on the plate model. First, measured time-averaged velocity field induced by the DBD plasma actuator indicated that voltage amplitude is an important parameter and could affect the flow characteristics of the induced jet. When the plasma actuator was driven at low voltage, the induced jet was a laminar wall jet. On the other hand, the Reynolds number of induced jet was improved and the shear layer was instability when the plasma actuator was actuated at high voltage. Then the induced jet became a turbulent wall jet. Secondly, the results of transient flow field structure suggested that the induced turbulent wall jet had some coherent structures, such as rolling up vortex and secondary vortex in the near-wall region. And these structures were linked to a dominant frequency of Hz. The rolling up vortices had the process of formation, movement, merging and breakdown. Thirdly, the rolling vortex was stretched and collapsed due to the instability of flow field when the plasma actuator was actuated at high voltage. Then turbulence kinetic energy of induced flow filed was increased and the breakdown of rolling vortex was accelerated. The turbulent characteristics of the induced jet could enhance the entrainment effect of plasma actuator between the outside airflow and boundary layer flow, which is very important for flow control applications.
由瞬态流场结果可知(如图6(b)所示), 卷起涡及二次涡随时间不断向激励器下游发展. 因此, 通过频域分析, 可以获得卷起涡及二次涡的特征频率. 根据射流理论[39], 在起始在之间、发展在之间及主流三个阶段, 选取计算点进行分析. 其中起始阶段是指卷起涡、二次涡等拟序结构产生前的区域; 发展阶段主要包括了卷起涡的产生、演化及耗散过程; 主流阶段是指射流充分发展区. 具体计算位置如图7所示. 为了精确获得频谱, 在不同位置取切向速度均方根最大值所在位置为计算点坐标. 显示原图|下载原图ZIP|生成PPT 图7射流计算点的位置. -->Fig.7The locations of the calculation point A to C ||||; ; ) -->
将每一个点的法向脉动速度 进行傅里叶变换, 获得频谱图像(如图8所示). 从图上可以看出: (1)在起始阶段, 卷起涡的主频, 即卷起频率(rolling-up frequency) 为109 Hz; (2) 在射流发展阶段, 由于出现了涡的融合, 因此, 除了主频外, 还存在半频 ( Hz, 它表示主频的一半); 但在此工况下, 半频的峰值略低于主频峰值, 表明涡的融合过程在卷起涡发展过程中不占主导; (3)在射流主流阶段, 诱导射流发展为湍流射流, 因此, 频谱内没有特征频率出现. 显示原图|下载原图ZIP|生成PPT 图8不同位置的法向脉动速度功率谱. -->Fig.8Power spectra of the vertical fluctuating velocity at different locations -->
依据主频, 获得了卷起涡的运动周期. 图9给出了一个周期内, 旋涡强度分布随时间变化情况. 其中swirl 是速度梯度张量特征值的虚部, 代表旋涡强度. 不少研究人员常采用旋涡强度去探测及分析旋涡, 具体的计算方法请参照文献[40]. 由图9可知, 诱导流场在壁面附近出现了几个旋涡强度较高的离散区域; 其中紧贴壁面的区域代表二次涡, 而二次涡上方的集中区域代表了卷起涡. 显示原图|下载原图ZIP|生成PPT 图9一个周期内旋涡强度的演化过程(表示一个周期). -->Fig.9Evolution of swirling strength in one cycle |||||( stands for cycle time) -->
为了进一步理解等离子体激励器诱导湍流射流的产生机制, 对诱导射流的转捩过程进行初步分析. 图10给出了较高电压激励下, 旋涡强度分布随时间变化情况. 从图10可以看出: (1) 当 s时, 旋涡强度主要集中在上层电极边缘, 近壁区鲜有旋涡强度较高的离散区域; (2)当 s时, 激励器在壁面附近诱导产生了卷起涡、二次涡等拟序结构. 显示原图|下载原图ZIP|生成PPT 图10不同时刻下诱导射流旋涡强度分布 -->Fig.10The swirling strength distribution at different times -->
由图10可知, 拟序结构是在一定时刻才出现的, 表明诱导射流经历了从层流到湍流的转捩过程. 图11给出了不同切向位置的均方根脉动速度剖面. 代表诱导流场沿方向的脉动速度, 表示诱导流场沿方向的脉动速度. 由图可知, 刚开始时, 的增长速度较快在之间, 并且 的峰值大于的峰值, 表明T-S波(Tollmien-Schlichting)在壁面发展[42-43]. 随着方向距离的增大, 增长速度较快, 并且峰值与的峰值接近, 表明在这些区域出现了K-H (Kelvin-Helmholtz)不稳定[42-43]. 显示原图|下载原图ZIP|生成PPT 图11均方根脉动速度剖面沿方向变化情况 -->Fig.11Root-mean-squared (r.m.s.) velocity profiles over the surface of flat plate -->
显示原图|下载原图ZIP|生成PPT 图12最大切向速度、最大法向脉动及最大雷诺应力沿方向变化情况|||| (a) 最大切向脉动速度; (b) 最大法向脉动速度; (c) 最大雷诺应力. -->Fig.12Distributions of the maximum streamwise r.m.s. velocity, the maximum vertical r.m.s. velocity, and the maximum Reynolds shear stress over the surface of flat plate |||| (a) The maximum streamwise r.m.s. velocity ; (b) The maximum vertical r.m.s. velocity; (c) The maximum Reynolds shear stress -->
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