Wall-Bounded Turbulence: Structure and Passive Control

Abstract

The nature of wall-bounded turbulence is investigated with respect to spectral characteristics. As the Reynolds number approaches infinity, the $k_x^{-1}$ overlap region does not appear to be present, yet the $k_x^{-5/3}$ asymptotically takes shape in the logarithmic and wake regions. In terms of coherent structures, a new scaling is proposed that describes the size of the very-large-scale motions throughout the flow. The corresponding similarity to the large-scale motion scaling suggests that these two phenomena might be interrelated. These observations appear to be robust to assumptions of a wavelength-independent convection velocity, although it is clear that Taylor’s hypothesis indeed leads to some spectral distortion, especially close to the wall.

Passive techniques to control wall-bounded turbulence are investigated with a focus on frictional drag reduction. Surfaces coated with a thin layer of immiscible liquid can delay the transition of a flow to a turbulent state, thereby reducing the energy dissipation in the flow. It is hypothesized that interfacial tension between the two fluids acts to stabilize disturbances, helping to maintain a laminar flow. The friction reducing properties of superhydrophobic surfaces, in which micro-scale pockets of air are trapped within surface textures, are confirmed in turbulent Taylor-Couette flow, with drag reductions up to 11\% reported. The drag reduction on the surface decreases with increasing Reynolds number, that is, as the viscous length scale becomes smaller compared to the surface feature size. Further, turbulent drag reduction is measured over a textured surface infused with a second immiscible liquid, with values of 4\% in Taylor-Couette flow and 10\% in boundary layer flow.