Study on the artificial control of circular pipe jets with turbulent initial condition.

本实验研究的是初始条件为充分发展湍流状态的圆形自由射流的主动控制。考虑了两种不同的射流结构：（1）从长直管中喷出的“管道射流”和（2）从收缩喷嘴中喷出但具有较厚初始边界层厚度的“伪喷嘴射流”。将使用以下符号。丁：管径Ue：射流出口速度，f：速度波动频率，fe：激励频率，x：a）Strouhal数在x/D=0.5 - 2.5处，速度脉动的主频（基于管道直径）随初始动量厚度的减小而增大，并随x/D的增大而减小。B）射流优选模式fnD/Ue，定义在x/D=3 - 4处，在所有的初始条件下，福尔斯的值在0.3 - 0.5之间，但当初始边界层厚度超过0.15 D时，它趋于减小。c）湍流管道射流的发展取决于激励的频率和水平。当feD/Ue=0.1 ~ 1.0时，射流扩展速率增大，最大激励效应出现在feD/Ue=0.45 ~ 0.5时。当feD/Ue=1.2 - 2.0时，出现所谓的“湍流抑制现象”，射流扩展略有减小。d）伪喷嘴射流对周期性强迫的响应比湍流管道射流敏感。e）在feD/Ue=0.55 - 0.65时，激励实现伪喷嘴射流的最大扩展。其高于管道射流的相应值。在这种激发条件下，在速度波动的频率为0.5fe附近的频谱内容显着放大，在未激发的情况下观察到的主导模式消失。

The present experimental study is concerned with active control of circular free jets, whose initial conditions are in the fully-developed turbulent state. Two different jet-flow configurations were considered; (1)"pipe jets" which discharge from a long straight pipe, and (2)"pseudo-nozzle jets" which discharge from a contraction nozzle but have thick initial boundary layer thickness.Main results of the research are summarized bellow. The following notations will be used. D: pipe diameter, Ue: jet exit speed, f: frequency of velocity fluctuations, fe: excitation frequency, x: axial distance from jet exit plane.a) The Strouhal number (based on the pipe diameter) of dominant frequency of velocity fluctuations at x/D=0.5 - 2.5 increases with decreasing the initial momentum thickness, and decreases with increasing x/D.b) The jet preferred mode fnD/Ue, defined at x/D=3 - 4, falls in the range of 0.3 - 0.5 for all the initial conditions tested, but tends to decrease when the initial boundary layer thickness exceeds about 0.15D.c) The development of turbulent pipe jets depends on the frequency and level of excitation. For feD/Ue=0.1 - 1.0, the jet spreading rate increases; the maximum effect of excitation occurs at feD/Ue=0.45 - 0.5. For feD/Ue=1.2 - 2.0, the so-called "turbulence suppression phenomenon" occurs, and the jet spread slightly decreases.d) The response of the psuedo-nozzle jet to periodic forcing is more sensitive than that of the turbulent pipe jet.e) The maximum spreading of the pseudo-nozzle jet is achieved by excitation at feD/Ue=0.55 - 0.65, which is higher than the corresponding value of the pipe jet. Under this excitation condition, spectral contents around the frequency of 0.5fe in velocity fluctuations are significantly amplified, and the predominant mode observed in the unexcited case disappears.