Directivity and intermittency in the nearfield of a Mach 1.3 jet
Directivity and intermittency in the nearfield of a Mach 1.3 jet
复制标题
DOI：
10.1177/1475472x17709926
复制
发表时间：
2017-04
影响因子：
1
通讯作者：
S. Unnikrishnan;D. Gaitonde;L. Agostini
中科院分区：
文献类型：
--
作者：
S. Unnikrishnan;D. Gaitonde;L. Agostini
跟踪了1.3马赫冷射流的局部波动，以了解近场指向性和间断性的成因。采用一种利用两个同步大涡模拟的新方法来求解非定常基流演变的线性化的强迫Navier-Stokes方程。通过揭示两个声学上重要的湍流区域：唇线和核心崩塌位置的影响，总结了结果。近声场显示了这两个区域的清晰特征。然而，对于这两个区域，扰动场的近场演化特征是产生间歇性波包，这些波包传播到近声场，并分别在副线角和下游角逐渐获得预期的宽带和窄带特性。模拟结果阐明了低频从强迫波动中滤除后，如何在副线方向获得高频。同样，浅角声信号是通过对该方向的高频内容进行滤波而产生的。方向性和间断性与经验模态分解射流湍流对尺度的滤波有关。观测结果强调了看似随机的核心湍流逐渐演变成射流近场中定义明确的间歇性波包。中心线波动分为上游、副线和下游分量的方式通过窄带相关性进行检验。由于剪切层中较大的结构，对唇线贡献的类似分析显示主要是上游和下游模式。
Local fluctuations in a Mach 1.3 cold jet are tracked to understand the genesis of nearfield directivity and intermittency. A newly developed approach leveraging two synchronized large-eddy simulations is employed to solve the forced Navier–Stokes equations, linearized about the evolving unsteady base flow. The results are summarized by exposing the effect of two acoustically significant turbulent regions: the lip-line and core collapse location. The near-acoustic field displays the clear signature of the two regions. However, for both regions, the nearfield evolution of the perturbation field is characterized by generation of intermittent wavepackets, which propagate into the near-acoustic field and gradually acquire their expected broadband and narrowband characteristics at sideline and downstream angles respectively. The simulations elucidate how higher frequencies are obtained in the sideline directions as lower frequencies are filtered out of the forcing fluctuations. Likewise, shallow-angle acoustic signals arise through filtering of high frequency content in that direction. The directivity and intermittency are connected to the filtering of scales by jet turbulence with empirical mode decomposition. The observations highlight the gradual evolution of seemingly random core turbulence into well-defined intermittent wavepackets in the nearfield of the jet. The manner in which centerline fluctuations are segregated into upstream, sideline, and downstream components is examined through narrowband correlations. A similar analysis for the lipline contribution shows primarily upstream and downstream patterns because of the larger structures in the shear layer.