Engineered bio-inspired coating for passive flow control

Abstract

Flow separation and vortex shedding are some of the most common phenomena experienced by bluff bodies under relative motion with the surrounding medium. They often result in a recirculation bubble in regions with adverse pressure gradient, which typically reduces efficiency in vehicles and increases loading on structures. Here, the ability of an engineered coating to manipulate the large-scale recirculation region was tested in a separated flow at moderate momentum thickness Reynolds number, Re θ = 1,200. We show that the coating, composed of uniformly distributed cylindrical pillars with diverging tips, successfully reduces the size of, and shifts downstream, the separation bubble. Despite the so-called roughness parameter, k + ≈ 1, falling within the hydrodynamic smooth regime, the coating is able to modulate the large-scale recirculating motion. Remarkably, this modulation does not induce noticeable changes in the near-wall turbulence levels. Supported with experimental data and theoretical arguments based on the averaged equations of motion, we suggest that the inherent mechanism responsible for the bubble modulation is essentially unsteady suction and blowing controlled by the increasing cross-section of the tips. The coating can be easily fabricated and installed and works under dry and wet conditions, increasing its potential impact on a diverse range of applications.flow control | bio-inspired surface | engineered surface | flow separation | adverse pressure gradient D uring the past few decades, considerable effort has been placed on controlling flow separation (1-4). This phenomenon is usually responsible for increased vibration and drag on bluff bodies as well as higher energy consumption in vehicles. The drag experienced by a body under subsonic motion mostly embodies viscous and pressure (form) effects. The former is a result of friction induced by the near-wall fluid, and the latter is a result of pressure imbalance around the surface of the body. The separation phenomenon is well exemplified in the canonic case of flow around a foil at a sufficiently high angle of attack. There, the adverse pressure gradient (APG) in the suction side leads to flow deceleration and eventually flow detachment. The direct consequence of this process is a change of the aerodynamic force components, namely lift and drag.Surface roughness plays a significant role in the turbulence dynamics near the wall and, in particular, in the separation regions (5-8). Evidence suggests that randomly distributed roughness, e.g., sand grain roughness, may move the separation point against the flow direction in the case of foils (9, 10); this shift results in drag increase and lift decrease. Experiments by Song and Eaton (11) showed an upstream shift of the separation point in a channel expansion with rough walls. However, various studies have shown that triggering transition to turbulence may reduce separation (12). These findings have motivated the use of flow control strategies such as vortex generators (13) and synthetic jets (3) t...

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