在全消声室进行超声速欠膨胀射流冲击实验,收缩喷口直径D为56 mm,马赫数Ma为1.23,斜板边长600 mm,冲击角度β为65°。利用远场传声器弧阵列和高频粒子图像测速(PIV),对不同冲击距离噪声和相关流动结构进行观测,其中弧阵列位于射流水平轴线侧面,远场距离64.3D,PIV采样频率足够高,可以观测到大尺度涡结构的脱落过程。粒子灰度图显示自由射流核心区大约延伸9D,非对称模态的大尺度涡结构占据统治地位。系统分析了冲击斜板距离对总声压级和湍流混合噪声、宽带激波相关噪声、激波啸叫等不同噪声成分的影响规律。其中冲击距离(L)大于核心区(L＞9D),宽带激波相关噪声几乎没有变化;冲击斜板进入核心区,宽带激波相关噪声下降,冲击噪声在上游频谱占据统治地位,但后者对边线的辐射很弱,导致上游噪声增加而边线总声压级大幅降低。L＞5D时射流冲击使啸叫出现微小频移,由于剪切层不稳定波无法正常向下游发展,观测到L=7D时大尺度涡结构会在非对称模态、轴对称模态和过渡模态之间切换,而L=4D时监测到更高频率的啸叫,其无量纲波长对应轴对称模态。激波啸叫幅值的定量分析表明,L＞5D时冲击斜板主要影响上游啸叫强度,就边线而言,啸叫二次谐频强度的稳定性较高,但冲击斜板使边线平均声压级下降。
An experiment of supersonic under-expanded jet impingement was carried out in a full anechoic chamber, in which a jet of Mach number is 1.23 from a convergent nozzle with the diameter D of 56 mm impinged on an inclined plate with the length of 600 mm, and the impingement angle is β of 65°. Arc array of microphones and high frequency Particle Image Velocimetry (PIV) were used to observe the far-field sound and flow structure at different impact distances L, respectively. Microphones were located on the side of the horizontal axis of the jet with the far-field distance of 64.3D. The sampling frequency of PIV was high enough to capture the shedding frequency of large-scale vortex structure. The gray scale image of PIV snapshots shows that the potential flow core of the free jet extends about 9D, and the large-scale vortex structure is dominated by asymmetric mode. The effects of the impact distance of nozzle to inclined plate on the overall sound pressure level (OASPL), turbulence mixing noise, impingement noise, broadband shock-associated noise and shock screech were carefully analyzed. It is found that broadband shock-associated noise has almost no change if the impact distance is larger than that of the length of potential flow core (L > 9D). For L ＜ 9D, the aeroacoustic results show that the broadband shock-associated noise decreases, and the radiation ability of impact noise to the side line is very weak while the impingement noise dominates the spectrum of upstream, resulting in the increase of upstream OASPL and the sharp decrease of side line OASPL. The spectrums for L > 5D indicated that the jet impingement causes a small frequency shift of screech. Due to the limitation of inclined plate, the shear layer instability wave cannot develop downstream normally. It is indicated from the flow visualization of L = 7D that the large-scale vortex structure of shear layer switches between asymmetric mode, axisymmetric mode and transition mode. A higher frequency screech is detected when L = 4D, and its dimensionless wavelength corresponds to axisymmetric mode. The quantitative analysis of the shock screech amplitude for L > 5D shows that the inclined plate mainly affects the upstream screech intensity. As for side line direction, the intensity of the second harmonic of screech is stable, while the impingement on the inclined plate reduces the average sound pressure level.
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