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Stochastic modelling and feedback control of bistability in a turbulent bluff body wake

Brackston, R. D. and García de la Cruz, J. M. and Wynn, A. and Rigas, G. and Morrison, J. F. (2016) Stochastic modelling and feedback control of bistability in a turbulent bluff body wake. Journal of Fluid Mechanics, 802 . pp. 726-749. ISSN 0022-1120.

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A specific feature of three-dimensional bluff body wakes, flow bistability, is a subject of particular recent interest. This feature consists of a random flipping of the wake between two asymmetric configurations and is believed to contribute to the pressure drag of many bluff bodies. In this study we apply the modelling approach recently suggested for axisymmetric bodies by Rigas et al. (J. Fluid Mech., vol. 778, 2015, R2) to the reflectional symmetry-breaking modes of a rectilinear bluff body wake. We demonstrate the validity of the model and its Reynolds number independence through time-resolved base pressure measurements of the natural wake. Further, oscillating flaps are used to investigate the dynamics and time scales of the instability associated with the flipping process, demonstrating that they are largely independent of Reynolds number. The modelling approach is then used to design a feedback controller that uses the flaps to suppress the symmetry-breaking modes. The controller is successful, leading to a suppression of the bistability of the wake, with concomitant reductions in both lateral and streamwise forces. Importantly, the controller is found to be efficient, the actuator requiring only 24 % of the aerodynamic power saving. The controller therefore provides a key demonstration of efficient feedback control used to reduce the drag of a high-Reynolds-number three-dimensional bluff body. Furthermore, the results suggest that suppression of large-scale structures is a fundamentally efficient approach for bluff body drag reduction.

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Rigas, G.0000-0001-6692-6437
Additional Information:© 2016 The Authors. This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (, which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited. (Received 7 March 2016; revised 26 June 2016; accepted 20 July 2016; first published online 10 August 2016) R.D.B. acknowledges an Imperial College PhD Scholarship. We are also indebted to EPSRC (grant nos. EP/I005684, EP/K503733/1), the Imperial College EPSRC Impact Acceleration Account, the Climate KIC Accelerator Program and Imperial Innovations for financial support.
Funding AgencyGrant Number
Imperial College LondonUNSPECIFIED
Engineering and Physical Sciences Research Council (EPSRC)EP/I005684
Engineering and Physical Sciences Research Council (EPSRC)EP/K503733/1
Imperial College EPSRC Impact Acceleration AccountUNSPECIFIED
Climate KIC Accelerator ProgramUNSPECIFIED
Imperial InnovationsUNSPECIFIED
Subject Keywords:drag reduction, flow control, low-dimensional models
Record Number:CaltechAUTHORS:20170105-163549251
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Official Citation:Brackston, R.D., García de la Cruz, J.M., Wynn, A., Rigas, G. and Morrison, J.F. (2016) ‘Stochastic modelling and feedback control of bistability in a turbulent bluff body wake’, Journal of Fluid Mechanics, 802, pp. 726–749. doi: 10.1017/jfm.2016.495.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:73285
Deposited By: George Porter
Deposited On:06 Jan 2017 18:18
Last Modified:03 Oct 2019 16:26

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