Abstract We report an experimental investigation of free falling super-hydrophobic (SH) spheres in glycerine-water mixtures over a wide range of Reynolds number. SH coatings have the ability to reduce the contact area between the surrounding liquid and the solid surface by entrapping an air layer in the surface roughness. We investigate the effect of this air plastron on the hydrodynamic performance of spheres, focusing our attention on the onset of wake instabilities. Our results emphasise the key role of the surface roughness properties on the triggering of wake instabilities. It is shown that, unlike what was reported in previous numerical studies on SH bluff-bodies, local deformation of the interface may act as a by-pass mechanism promoting earlier transition, yielding a decrease of the critical Reynolds number at which the wake becomes unstable. A scenario coupling the hydrodynamic instabilities (scaling with the Reynolds number) and the interface deformation (scaling with the roughness-based Weber number) is proposed to describe the different transition mechanisms in presence of SH surfaces. It is found that the promotion of wake instabilities over SH surfaces occurs when the roughness-based Weber number is larger than a critical threshold. These findings are of primary importance for guiding the design of resilient and efficient SH surfaces.

中文翻译：

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关于超疏水球体后不稳定的开始

摘要 我们报告了在大雷诺数范围内甘油-水混合物中自由落体超疏水 (SH) 球体的实验研究。SH涂层能够通过在表面粗糙度中捕获空气层来减少周围液体和固体表面之间的接触面积。我们研究了这种空气腹甲对球体流体动力学性能的影响，将我们的注意力集中在尾流不稳定性的发生上。我们的结果强调了表面粗糙度特性对触发尾流不稳定性的关键作用。结果表明，与先前关于 SH 钝体的数值研究中报道的不同，界面的局部变形可能充当促进早期转变的旁路机制，导致尾流变得不稳定的临界雷诺数降低。提出了一种耦合流体动力学不稳定性（与雷诺数缩放）和界面变形（与基于粗糙度的韦伯数缩放）相耦合的场景，以描述存在 SH 表面时的不同过渡机制。发现当基于粗糙度的韦伯数大于临界阈值时，会促进 SH 表面上的尾流不稳定性。这些发现对于指导弹性和高效的 SH 表面的设计至关重要。发现当基于粗糙度的韦伯数大于临界阈值时，会促进 SH 表面上的尾流不稳定性。这些发现对于指导弹性和高效的 SH 表面的设计至关重要。发现当基于粗糙度的韦伯数大于临界阈值时，会促进 SH 表面上的尾流不稳定性。这些发现对于指导弹性和高效的 SH 表面的设计至关重要。