低速矩形射流近场压力结构大涡模拟研究

Large Eddy Simulation of Near-field Pressure Structure in the Low-speed Rectangular Jet

采用大涡模拟方法对宽高比为1.375、马赫数为0.2888的低速开放矩形射流进行了数值模拟,通过监测点压力信号的频谱信息与动力学模态分解(DMD)对近场压力脉动特性进行了分析。当矩形射流基于远场声速与喷管高度的雷诺数Re=5×10^(4)时,频谱分析结果表明近场压力脉动的主要频率处于5~10Hz,其主要与射流剪切层中剪切层Kelvin-Helmholtz(K-H)不稳定性和涡的非线性相互作用有关。DMD结果表明,低频压力脉动主要的空间相干结构呈沿流向分布且流向尺寸与喷管高度处于同一量级的环状结构。此外,分别对相同雷诺数的平板射流与相同宽高比下Re=5×10^(5)的矩形射流进行了数值模拟,进一步研究了矩形喷管宽高比和雷诺数对近场压力脉动的影响。研究发现,雷诺数的增加对射流近场的压力脉动频率分布影响较小,但平板射流中小于5Hz的低频成分相对更多,这是因为剪切层失稳过程相干性更强。不同雷诺数下矩形射流近场的压力脉动空间结构基本相同,近场的低频压力脉动为无黏机制主导。低频压力脉动易与风洞中的结构设备产生共振,对试验测量也有不利影响,了解射流的近场压力脉动特征对风洞的设计具有一定的参考价值。

The low-speed open rectangular jet with an aspect ratio of 1.375 and a Mach number of 0.2888 was simulated numerically through the large eddy simulation,and the near-field pressure fluctuation characteristics are analyzed through the spectrum information of the monitoring point pressure signal and the dynamic mode decomposition(DMD).When Reynolds number based on the far-field sound velocity and the nozzle height is 5×10^(4),the spectrum analysis results show that the dominated frequency of near-field pressure fluctuation is within 5~10Hz,which is mainly related to the Kelvin-Helmholtz instability in jet shear layer and the nonlinear interaction of vortex.DMD results show that the main spatial coherent structures of low-frequency pressure fluctuation are streamwise annular structures whose streamwise sizes are on the same order of the nozzle height.In addition,the numerical simulation was carried out on a planar jet with the same Reynolds number and a rectangular jet with the same nozzle under Re=5×10^(5),and the effect of nozzle aspect ratio and Reynolds number on the characteristics of pressure fluctuations in the near field are further studied.Results show that the increase of Reynolds number has little influence on the pressure fluctuation frequency distribution in the near field of the jet,but there are relatively more low-frequency components under 5Hz in planar jet,which is due to the stronger coherence of the shear layer instability process.The spatial structure of pressure fluctuation in the near field of rectangular jet is basically the same under different Reynolds numbers.The low-frequency pressure fluctuation in the near field is dominated by inviscid instability mechanism.Low-frequency pressure fluctuation tends to resonate with the structural equipment in the wind tunnel,and has adverse effects on experimental measurement.Understanding the characteristics of near-field pressure fluctuation is of certain reference value for wind tunnel design.