TU Bergakademie Freiberg, Freiberg, Germany.
J Phys Condens Matter. 2011 May 11;23(18):184120. doi: 10.1088/0953-8984/23/18/184120. Epub 2011 Apr 20.
The interaction of near-wall turbulence with hairy surfaces is investigated in a turbulent boundary layer flow along a flat plate in an oil channel at Re = 1.2 × 10⁶. The plate is covered locally with a dense carpet of elastomeric micro-hairs (length L = 1 mm, length in viscous units L( + ) = 30) which are arranged in a regular grid (60 × 30 hairs with a streamwise spacing Δx( + )≈15 and a spanwise spacing Δy( + )≈30). Instead of the micro-structures used in previous studies for sensory applications, the surface hairs are considerably larger and much more densely distributed with a spacing of S/D < 5 such that they interact with each other by flow coupling. The non-fluctuating mean part of the flow forces a substantial pre-bending in the streamwise direction (reconfiguration). As a consequence, the hairs align with the streamwise direction, thus imposing anisotropic damping characteristics with regard to flow fluctuations in streamwise and spanwise or wall-normal directions. Near-wall high-frequency disturbances excited by the passage of turbulent sweeps are dampened over their course along the carpet. The cooperative action of the hairs leads to an energy transfer from small-scale motion to larger scales, thus increasing the coherence of the motion pattern in streamwise and spanwise directions. As a consequence of the specific arrangement of the micro-hairs in streamwise columns a reduced spanwise meandering and stabilization of the streamwise velocity streaks is achieved by promoting varicose waves and inhibiting sinusoidal waves. Streak stabilization is known to be a major contributor to turbulent drag reduction. Thus it is concluded that hairy surfaces may be of benefit for turbulent drag reduction as hypothesized by Bartenwerfer and Bechert (1991 Z. Flugwiss. Weltraumforsch. 15 19-26).
在油槽中沿平板流动的湍流边界层内，对近壁湍流与多毛表面的相互作用进行了研究，Re = 1.2×10⁶。平板局部覆盖有密集的弹性微绒毛地毯（长度 L = 1mm，粘性单位长度 L(+) = 30），这些绒毛以规则的网格排列（60×30 根，流向间距Δx(+)≈15，展向间距Δy(+)≈30）。与以前用于传感应用的微结构不同，这些表面绒毛大得多，且分布密度大得多，间距 S/D < 5，从而通过流动耦合相互作用。流动的非脉动均值部分使绒毛在流向方向上发生很大的预弯曲（重新配置）。因此，绒毛与流向对齐，从而对流向和展向或法向的流动脉动施加各向异性的阻尼特性。由湍流扫掠通过激发的近壁高频干扰在沿地毯的传播过程中被阻尼。由于绒毛的协同作用，能量从小尺度运动传递到较大尺度，从而增加了流向和展向方向运动模式的相干性。由于微绒毛在流向列中的特定布置，通过促进曲折波并抑制正弦波，实现了展向涡动的减少和流向速度条纹的稳定。条纹稳定是众所周知的湍流减阻的主要贡献者。因此，可以得出结论，正如 Bartenwerfer 和 Bechert（1991 Z. Flugwiss. Weltraumforsch. 15 19-26）所假设的那样，多毛表面可能有益于湍流减阻。
