Near wall coherence in wall-bounded flows and implications for flow control

Samie, M, Baars, WJ, Rouhi, A ORCID: https://orcid.org/0000-0002-7837-418X, Schlatter, P, Örlü, R, Marusic, I and Hutchins, N, 2020. Near wall coherence in wall-bounded flows and implications for flow control. International Journal of Heat and Fluid Flow, 86: 108683. ISSN 0142-727X

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Abstract

Opposition-control of the energetic cycle of near wall streaks in wall-bounded turbulence, using numerical approaches, has shown promise for drag reduction. For practical implementation, real-time opposition control is only realizable if there is a degree of coherence between the turbulent velocities passing a sensor and the target point within the flow; for practicality, a sensor (and actuator) should be wall-based to avoid parasitic drag. As such, we here inspect the feasibility of real-time control of the near wall cycle, by considering the coherence between a measurable wall-quantity, being the wall-shear stress fluctuations, and the streamwise and wall-normal velocity fluctuations in a turbulent boundary layer. Synchronized spatial and temporal velocity data from two direct numerical simulations and a fine large eddy simulation at and 2000 are employed. This study shows that the spectral energy of the streamwise velocity fluctuations that is stochastically incoherent with wall signals is independent of Reynolds number in the near wall region (up to the viscous-scaled wall-normal height ). Consequently, the streamwise energy-fraction that is stochastically wall-coherent grows with Reynolds number due to the increasing range of energetic large scales. This thus implies that a wall-based control system has the ability to manipulate a larger portion of the total turbulence energy at off-wall locations, at higher Reynolds numbers, while the efficacy of predicting/targeting the small scales of the near wall cycle remains indifferent with varying Reynolds number. Coherence values of 0.55 and 0.4 were found between the streamwise and wall-normal velocity fluctuations at the near wall peak in the energy spectrogram, respectively, and the streamwise fluctuating friction velocity. These coherence values, which are considerably lower than 1 (maximum possible coherence) suggest that a closed-loop drag reduction scheme targeting near wall cycle streaks alone (based on sensed friction velocity fluctuations) will be of limited success in practice.

Item Type:	Journal article
Publication Title:	International Journal of Heat and Fluid Flow
Creators:	Samie, M., Baars, W.J., Rouhi, A., Schlatter, P., Örlü, R., Marusic, I. and Hutchins, N.
Publisher:	Elsevier
Date:	December 2020
Volume:	86
ISSN:	0142-727X
Identifiers:	Number Type 10.1016/j.ijheatfluidflow.2020.108683 DOI S0142727X20304434 Publisher Item Identifier 1378969 Other
Divisions:	Schools > School of Science and Technology
Record created by:	Linda Sullivan
Date Added:	28 Oct 2020 10:44
Last Modified:	01 Sep 2022 03:00
URI:	https://irep.ntu.ac.uk/id/eprint/41417

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