Manipulation of a turbulent boundary layer using active surface deformations

Abstract

We experimentally evaluate whether active wall-normal surface deformations are suitable for the targeted control of very-large-scale motions (VLSMs) in a turbulent boundary layer at a friction Reynolds number of $Re_\tau =2600$ . Circular surface deformations with a diameter $D$ roughly equal to the boundary layer thickness $\delta$ are generated periodically at a constant amplitude of $0.03\delta$ and at actuation frequencies of $St=0.05$ to 0.20, where $St$ is the Strouhal number based on $D$ and the free stream velocity $U_\infty$ . The resulting impact on the flow was captured using high-speed particle image velocimetry and analysed using a triple decomposition. We find that the active surface deformations produce high- and low-speed streamwise velocity fluctuations that are concentrated along the centreline of the actuator. These motions have a negligible impact on the mean velocity profile downstream, i.e. they are truly high and low speed with respect to the unactuated base flow. The motions produced at $St\lesssim 0.1$ are comparable to synthetic VLSMs in terms of their lengths and widths but with a reduced wall-normal extent and rapidly decaying strength. These synthetic motions produce a strong modulation of the turbulence similar to that of the naturally occurring VLSMs. Most notably, we observe that synthetic high-speed motions with magnitudes of the order of $0.05U_\infty$ cause up to a 30 % reduction in turbulence production within the logarithmic layer. The strength and turbulence-modulating characteristics of the synthetic motions appear well suited for targeting the naturally occurring VLSMs locally using a control scheme.

Reactive control of velocity fluctuations using an active deformable surface and real-time PIV

“…Additionally, the investigations of Kim et al. (2003) and Gibeau & Ghaemi (2022, 2023) used a circular membrane to replace a section of the wall and deformed the membrane at various frequencies using an electromagnetic actuator. They observed that high- and low-speed streamwise motions were produced by the downward and upward wall motions, respectively, in both laminar and turbulent boundary layers.…”

“…They observed that high- and low-speed streamwise motions were produced by the downward and upward wall motions, respectively, in both laminar and turbulent boundary layers. Gibeau & Ghaemi (2023) notes that the active wall motions induce streamwise velocity fluctuations that are stronger than the corresponding wall-normal velocity fluctuations. This suggests that the wall-normal actuators are also suitable for u -control strategies.…”