Velocity structures associated with large streamwise density gradients in an incompressible turbulent boundary layer (with air as the working fluid, Pr = 0.71) are analysed experimentally using planar image velocimetry and aero-optic measurements. The resulting flow topologies for the velocity fluctuations associated with large negative and positive density gradients are in excellent agreement with results for coolings and heatings in time, respectively (Antonia & Fulachier, J. Fluid Mech., vol. 198, 1989, pp. 429–451). The current results are complimentary to those from Saxton-Fox et al. (AIAA J., vol. 57 (7), 2019, pp. 2828–2839), on the signature of the vertical velocity structures associated with large density gradients. In the present work, these structures are shown to exhibit a sign change, consistent with the scalar gradient, and are localised in the wall-normal direction with an average height of approximately 0.1δ, almost constant for increasing distance from the wall. The corresponding small-scale streamwise fluctuations also exhibit a consistent sign change, which is found to originate, on average, from upstream leaning structures. The emerging picture for the velocity field is then that of a multiscale phenomenon, where small-scale structures, responsible for large optical aberrations, are superimposed on the back of large-scale bulge-like structures that are known to populate the outer layers. The proposed conceptual model is consistent with early ideas of ‘typical’ eddies (Falco, Phys. Fluids, vol. 20 (10), 1977, pp. S124–S132), the hairpin vortex model and associated shear layers (Adrian et al., J. Fluid Mech., vol. 422, 2000, pp.1–54), as well as with notions of multiscale velocity organisation in shear layers (Klewicki & Hirschi, Phys. Fluids, vol. 16 (11), 2004, pp. 4163–4176; Saxton-Fox et al. 2019), and it provides new insight into the geometry of the small-scale velocity structures.

中文翻译：

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大被动标量梯度的速度结构的空间组织

与不可压缩湍流边界层（空气作为工作流体，Pr = 0.71）中的大流向密度梯度相关的速度结构使用平面图像测速和航空光学测量进行实验分析。与大的负密度梯度和正密度梯度相关的速度波动的结果流动拓扑分别与冷却和加热的结果非常一致（Antonia & Fulachier, J. Fluid Mech., vol. 198, 1989, pp. 429 –451）。目前的结果与 Saxton-Fox 等人的结果是互补的。（AIAA J., vol. 57 (7), 2019, pp. 2828–2839），关于与大密度梯度相关的垂直速度结构的特征。在目前的工作中，这些结构显示出符号变化，与标量梯度一致，并且都位于壁法线方向，平均高度约为 0.1δ，随着与壁的距离增加，几乎恒定。相应的小尺度流向波动也表现出一致的符号变化，平均而言，它起源于上游倾斜结构。速度场的新兴图像是多尺度现象，其中导致大光学像差的小尺度结构叠加在已知填充外层的大型凸起状结构的背面。提出的概念模型与“典型”涡流的早期想法（Falco，Phys. Fluids，vol. 20 (10), 1977, pp. S124–S132）、发夹涡模型和相关的剪切层（Adrian et al. , J. Fluid Mech., vol. 422, 2000, pp.1–54),