Opposition flow control for reducing skin-friction drag of a turbulent boundary layer

Abstract

This work explores the dynamic response of a turbulent boundary layer to large-scale reactive opposition control, at a friction Reynolds number of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:msub><a:mtext>Re</a:mtext><a:mi>τ</a:mi></a:msub><a:mo>≈</a:mo><a:mn>2</a:mn><a:mspace width="0.16em"/><a:mn>240</a:mn></a:mrow></a:math>. A surface-mounted hot-film is employed as the input sensor, capturing large-scale fluctuations in the wall-shear stress, and actuation is performed with a single on/off wall-normal blowing jet positioned <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:mrow><c:mn>2.4</c:mn><c:mi>δ</c:mi></c:mrow></c:math> downstream of the input sensor, operating with an exit velocity of <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"><d:mrow><d:msub><d:mi>v</d:mi><d:mi mathvariant="normal">j</d:mi></d:msub><d:mo>=</d:mo><d:mn>0.4</d:mn><d:msub><d:mi>U</d:mi><d:mi>∞</d:mi></d:msub></d:mrow></d:math>. Our study builds upon the work of Abbassi [] and includes a control-calibration experiment and a performance assessment using PIV- and PTV-based flow field analyses. With the control-off calibration-experiment conducted , a transfer kernel is identified so that the velocity fluctuations that are to-be-controlled can be estimated. The controller targets large-scale high-speed zones in an “opposing” mode and low-speed zones in a “reinforcing” mode. A desynchronized mode was tested for reference and consisted of a statistically similar control mode, but without synchronization to the incoming velocity fluctuations. An energy-attenuation of about 40 % is observed for the opposing control mode in the frequency band corresponding to the passage of large-scale motions. This proves the effectiveness of the control in targeting large-scale motions: an energy-intensification of roughly 45% occurs for the reinforcing control mode instead, while no change in energy, within the wall-normal range targeted, appears with the desynchronized control mode. Moreover, direct measures of the skin-friction drag are inferred from PTV data. Results indicate that the opposing control logic yields the lowest wall-shear stress (3% lower than the desynchronized control, and 10% lower than the uncontrolled flow). Finally, a FIK-decomposition of the skin-friction coefficient revealed that the off-the-wall turbulence follows a consistent trend with the PTV-based wall-shear stress measurements, although biased by an increased shear in the wake of the boundary layer given the formation of a plume due to the jet-in-crossflow actuation. Published by the American Physical Society 2024

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