In this research, we study the dynamics of vortex pairs both in and out of ground effect. Trailing vortices are generated by towing a 75 $$^\circ$$ leading-edge sweep-angle delta wing in an XY-towing tank, at 15 $$^\circ$$ angle of attack. As the delta wing is towed, it generates a spatially developing vortex pair with the axial (streamwise) flow in the vortex cores. Particle image velocimetry is used to determine the transverse velocity field, and the vortex pair is found to be well-characterized by the superposition of two Lamb–Oseen vortices. The axial flow is captured using a longitudinal light sheet. To ensure we capture the vortex core, a technique is used wherein the light sheet is set such that it is slightly oblique to the length of the vortex. Using this technique allows the axial flow to be measured in the vortex core more than 20 chord-lengths downstream of the delta wing, capturing data at a resolution as close as 0.03 chord-lengths apart. This technique is unaffected by displacements of the vortex core, allowing us to analyze the axial flow profile both in and out of ground effect. A Gaussian velocity profile with a wake-like velocity deficit (flow upstream) is observed in the far-wake of the delta wing in both cases. We also trigger the long-wavelength instability (Crow AIAA J 8:2172–2179, 1970) in the vortices, and compare the measured growth rate of the instability to theoretical growth rate predictions using the theory of Widnall and Bliss (J Fluid Mech 50:335–353, 1971) in a similar manner to Leweke et al. (Annu Rev Fluid Mech 48:1–35, 2016) and Fabre (Ph.D Thesis, 2000). This work is limited to a chord-based Reynolds number of 5000.

中文翻译：

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三角翼后尾涡的空间发展，在地面效应内外

在这项研究中，我们研究了地面效应内外涡对的动力学。拖曳涡流是通过在 XY 拖曳油箱中以 15 $$^\circ$$ 迎角拖曳 75 $$^\circ$$ 前缘后掠角三角翼而产生的。当三角翼被拖曳时，它会产生一个空间发展的涡旋对，涡核中的轴向（流向）流。粒子图像测速仪用于确定横向速度场，发现涡旋对通过两个 Lamb-Oseen 涡旋的叠加得到了很好的表征。使用纵向光片捕获轴向流。为了确保我们捕捉到涡流核心，使用了一种技术，其中光片被设置为稍微倾斜于涡流的长度。使用这种技术可以在三角翼下游超过 20 弦长的涡流核心中测量轴向流，以接近 0.03 弦长的分辨率捕获数据。这种技术不受涡核位移的影响，使我们能够分析地效内外的轴流剖面。在这两种情况下，在三角翼的远尾流中都观察到了具有尾流状速度缺陷（上游流）的高斯速度剖面。我们还触发了涡流中的长波长不稳定性 (Crow AIAA J 8:2172–2179, 1970)，并使用 Widnall 和 Bliss (J Fluid Mech 50) 理论将测得的不稳定性增长率与理论增长率预测进行比较:335–353, 1971) 以与 Leweke 等人类似的方式。(Annu Rev Fluid Mech 48:1–35, 2016) 和 Fabre (Ph.D Thesis, 2000)。