Evolution of a wave packet into vortex loops in a laminar separation bubble
Evolution of a wave packet into vortex loops in a laminar separation bubble
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层流分离泡中波包演化为涡环
DOI：
10.1017/s0022112099006138
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发表时间：
1999
影响因子：
3.7
通讯作者：
J. Watmuff
中科院分区：
文献类型：
--
作者：
J. Watmuff
层流边界层在有利的压力梯度中发展，其中速度剖面渐近线到Falkner & Skan相似解。飞热线测量表明，该层分离下游的一个随后的区域的不利压力梯度，导致形成一个薄的分离气泡。为了深入了解不稳定机制的性质，在最小压力下通过测试表面上的孔引入了小幅度的脉冲扰动。该设施和所有操作程序都是完全自动化的，并且在前所未有的大空间密度测量网格上获得相位平均数据。扰动的演变一直跟踪到再附着区，并超出完全湍流边界层。数据的空间分辨率提供了通常与计算相关联的详细程度。最初，一个波包的发展，保持相同的有界的形状和形式，而振幅随流向距离呈指数衰减。分离后，衰减率减小，达到最小振幅点，波包开始表现出色散特性。然后，振幅呈指数增长，并且包内的波的数量增加。该地区导致和包括再附着已被测量与十字线探针和轮廓展向涡在中心线平面清楚地表明，波包与猫眼图案，这是一个特征的开尔文-亥姆霍兹不稳定性。进一步的流向发展导致卷起的形成，涡量等值面表明它们是三维的。超过这一点，行为是非线性的，卷起演变成一组大规模的涡环附近的再附着。在下游湍流边界层中继续进行密集的十字线测量，并将泰勒假设应用于展向平面上的数据，以产生三维速度场。导出的涡量分布表明，第二涡环，出现在再附着区，保持其身份在湍流边界层，它一直持续到试验段结束。
A laminar boundary layer develops in a favourable pressure gradient where the velocity profiles asymptote to the Falkner & Skan similarity solution. Flying-hot-wire measurements show that the layer separates just downstream of a subsequent region of adverse pressure gradient, leading to the formation of a thin separation bubble. In an effort to gain insight into the nature of the instability mechanisms, a small-magnitude impulsive disturbance is introduced through a hole in the test surface at the pressure minimum. The facility and all operating procedures are totally automated and phase-averaged data are acquired on unprecedently large and spatially dense measurement grids. The evolution of the disturbance is tracked all the way into the reattachment region and beyond into the fully turbulent boundary layer. The spatial resolution of the data provides a level of detail that is usually associated with computations. Initially, a wave packet develops which maintains the same bounded shape and form, while the amplitude decays exponentially with streamwise distance. Following separation, the rate of decay diminishes and a point of minimum amplitude is reached, where the wave packet begins to exhibit dispersive characteristics. The amplitude then grows exponentially and there is an increase in the number of waves within the packet. The region leading up to and including the reattachment has been measured with a cross-wire probe and contours of spanwise vorticity in the centreline plane clearly show that the wave packet is associated with the cat's eye pattern that is a characteristic of Kelvin–Helmholtz instability. Further streamwise development leads to the formation of roll-ups and contour surfaces of vorticity magnitude show that they are three-dimensional. Beyond this point, the behaviour is nonlinear and the roll-ups evolve into a group of large-scale vortex loops in the vicinity of the reattachment. Closely spaced cross-wire measurements are continued in the downstream turbulent boundary layer and Taylor's hypothesis is applied to data on spanwise planes to generate three-dimensional velocity fields. The derived vorticity magnitude distribution demonstrates that the second vortex loop, which emerges in the reattachment region, retains its identity in the turbulent boundary layer and it persists until the end of the test section.