Understanding how to decrease the friction drag exerted by a fluid on a solid surface is becoming increasingly important to address key societal challenges, such as decreasing the carbon footprint of transport. Well-established techniques are not yet available for friction drag reduction. Direct numerical simulation results obtained by Józsa et al. (2019) previously indicated that a passive compliant wall can decrease friction drag by sustaining the drag reduction mechanism of an active control strategy. The proposed compliant wall is driven by wall shear stress fluctuations and responds with streamwise wall velocity fluctuations. The present study aims to clarify the underlying physical mechanism enabling the drag reduction of these active and passive control techniques. Analysis of turbulence statistics and flow fields reveals that both compliant wall and active control amplify streamwise velocity streaks in the viscous sublayer. By doing so, these control methods counteract dominant spanwise vorticity fluctuations in the near-wall region. The lowered vorticity fluctuations lead to an overall weakening of vortical structures which then mitigates momentum transfer and results in lower friction drag. These results might underpin the further development and practical implementation of these control strategies.
|Number of pages||11|
|Journal||International Journal of Heat and Fluid Flow|
|Early online date||1 Sept 2020|
|Publication status||Published - Dec 2020|
Bibliographical noteThe authors are grateful to AkzoNobel’s Marine Coatings business (International Paint Ltd), UK and the Energy Technology Partnership, UK [ETP106] for financial support. We would like to thank the UK Turbulence Consortium and Dr. Sylvain Laizet of Imperial College London for supporting the research with computational resources, and Kirsty Jean Grant for proofreading the manuscript. DNS computations were carried out on the ARCHER UK National Supercomputing Service (http://www.archer.ac.uk).
- Active flow control
- Compliant surface
- Compliant wall
- Drag reduction
- Passive flow control
- Turbulent channel flow