A STUDY OF THE MECHANISM OF VORTEX BREAKDOWN

涡旋击穿的特点是有三种类型的击穿，在涡旋轴上有一个滞止点，振荡速度频率显著。我们提出了一个解析模型来一致地解释这些特征，认为在一定区域内任何波长的小速度扰动都会被放大，而非定常速度会产生声流。对三种涡旋击穿的稳定性和速度场进行了分析，并将计算得到的条纹线与实测流型进行了比较。利用可视化技术观察了直管水流中气泡型、螺旋型和双螺旋型三种涡旋破裂。介绍了激光多普勒测速仪在气泡型击穿尾迹上游和尾迹中的测量结果。得到了脉动速度谱，以及环流区内外的时间平均旋流和轴向速度分布。对数据的频谱分析表明，在稳定再循环中，振荡速度的显著频率是恒定的。为了证实扰动的方位角波数决定击穿类型的观点，我们制作了自旋波发生器（SWG），它是一种在指定频率下产生沿周向自旋的正弦波扰动的装置。SWG由4个活塞组成，活塞在附在试验管壁上的气缸中工作，产生4种自旋正弦波扰动：4种方位波数m = 0、+1、-1、2。将四种方位角波数的正弦波施加到旋流中，研究了击穿类型和击穿位置的转变过程。结果表明，m = 0、+1和2次正弦波扰动分别产生气泡、螺旋型击穿和双尾气泡或扁平气泡。少

The features of the vortex breakdown are that there are three types of breakdown, and it has a stagnation point on the vortex axis and the prominent frequencies of oscillating velocities. We presented an analytical model to explain these all features consistently under the idea that the small velocity disturbances of any wave length lying within a certain region would be amplified and unsteady velocity would produce acoustic streaming. We analyzed the stability and the velocity fields, and compared the calculated streak lines with the observed flow pattern for the three types of vortex breakdown.The three types of vortex breakdown were observed in water flows through a straight pipe by a visualization technique : bubble, spiral and double-helix types. Laser-Doppler velocimeter measurements upstream and in the wakes of breakdown of bubble type are described. Spectra of the fluctuating velocities, as well as timeaveraged swirl and axial velocity profile both inside and outside the recirc … More ulation zone, have been obtained. Spectral analysis of the data indicates that the prominent frequencies of oscillating velocities are constant in the steady recirculation.We made a spinning wave generator(SWG), which is a device to produce, at a specified frequency, a sinusoidal wave disturbance spinning in the circumferential direction, in order to confirm the idea that the azimuthal wave number of the disturbance would determine the type of breakdown. SWG is composed of four pistons, which operate in the cylinders attached to the test pipe wall, and creates four kinds of spinning sinusoidal wave disturbances : four azimuthal wave numbers m = 0, +1, -1, 2. The four kinds of sinusoidal waves with azimuthal wave number were imposed to the swirl flow and the transition process of the breakdown type and location was investigated. It was shown that the m = 0, +1, and 2 sinusoidal wave disturbances produce the bubble, spiral type breakdown, and two-tailed bubble or flattened bubble respectively. Less