Measuring and modifying the near-wall behavior of wall-bounded turbulence

Abstract

Passive flow control can be applied to reduce the drag over ships and wings or increase the mixing in reacting flows simply through modifying the surface properties. Often it utilizes the dependence of turbulent flows on boundary conditions to manipulate momentum transport in the mean flow. To that end, the viscous sublayer has garnered significant research interests as any quantities transported from the wall to the mean flow (e.g. heat and momentum) must pass through it. However, our ability to design and implement passive control schemes is constrained by our inability to understand and predict turbulent flows as well as a lack of tools and means to interact with the flow. This is primarily because the flow encompasses a wide range of length and time scales that continues to expand with increasing Reynolds number. Difficulties in measuring or modifying the flow are exacerbated near walls where the scales of the flow are smallest. In this dissertation, I will present a series of findings and tools aimed at improving our ability to modify, understand, and measure near wall flows. First a novel method for passive drag reduction inspired by Nepenthes pitcher plants is proposed. A regime is found where both drag reduction and robustness characteristics of these surfaces are well predicted by laminar flow criteria. These surfaces modify the near wall flow with localized pockets of recirculating fluid, effectively lubricating the external flow, and exhibit behavior consistent with Townsend’s Reynolds number similarity hypothesis. Then, a novel nanoscale velocity sensor is deployed in a high Reynolds number boundary layer flow to obtain fully resolved measurements of the streamwise variance in the near wall region. To facilitate resolved measurements of the wall normal component of Reynolds stress, a novel strain-based sensing mode is developed and validated.