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Wall effects on the transition to turbulence in low Reynolds number separated flows

机译：低雷诺数分离流动中壁对湍流过渡的影响

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For small scale flying vehicles, which operate at Re numbers less than 50,000, airfoil efficiency is greatly degraded and lift and drag become very sensitive to external turbulence levels, surface roughness and other disturbances. Separation is common in such flows, and laminar-to-turbulent transition is required to close the separated bubble. To improve airfoil performance and reliability at this low Re, transition to turbulence should be as fast as possible. The transition in a low-Re separation zone is very sensitive to both environmental conditions and the presence of the solid wall. Through the use of viscous linear stability theory and wind tunnel hot wire measurements, we characterize the transition process of a shear layer separating at Reynolds number (based on momentum thickness) ≈ 15. Although free shear layers are very unstable, even at Re_θ as low as 5, we find that the presence of a wall greatly reduces or eliminates shear layer instabilities and, depending on the distance from the shear layer to the wall, the space required for disturbance growth and laminar-to-turbulent transition can be greatly increased.
机译：对于Re数小于50,000的小型飞行器，机翼效率大大降低，升力和阻力对外部湍流度，表面粗糙度和其他干扰非常敏感。在这种流动中分离是常见的，并且需要层流到湍流的过渡来封闭分离的气泡。为了在低Re的情况下提高机翼性能和可靠性，应尽可能快地过渡到湍流。低Re分离区的过渡对环境条件和固体壁的存在都非常敏感。通过使用粘性线性稳定性理论和风洞热线测量，我们表征了以雷诺数（基于动量厚度）≈15分离的剪切层的过渡过程。尽管自由剪切层非常不稳定，即使在Re_θ低如图5所示，我们发现壁的存在极大地减少或消除了剪切层的不稳定性，并且根据从剪切层到壁的距离，扰动增长和层流到湍流过渡所需的空间会大大增加。

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