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Nonlinear feedback control of bimodality in the wake of a three-dimensional bluff body

D. Ahmed* and A. S. Morgans†

  • Department of Mechanical Engineering, Imperial College London, London SW7 2AZ, United Kingdom

  • *d.ahmed18@imperial.ac.uk
  • †a.morgans@imperial.ac.uk
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Phys. Rev. Fluids 7, 084401 – Published 22 August, 2022

DOI: https://doi.org/10.1103/PhysRevFluids.7.084401

Abstract

The turbulent wake behind a square-back Ahmed body in close proximity to the ground exhibits bimodal switching. This manifests as the center of the wake switching between one of two asymmetric positions, either horizontally or vertically. Switches occur over random timescales, with the wake recovering symmetry in the long time average. The present work employs wall-resolved large eddy simulations to investigate feedback control for suppressing horizontal (lateral) wake bimodality of a square-back Ahmed body at Reynolds number, ReH∼3.3×104 based on the body height. Base-mounted pressure sensors are used to estimate the position of the wake as an input signal for the controller, while actuation targets the near-wake region via synthetic jets emanating from a gap around the perimeter of the Ahmed body base. A nonlinear feedback controller based on a Langevin model of the wake dynamics is synthesized. This successfully suppresses the wake lateral bimodal switching. However, this switching is replaced by a time-periodic streamwise motion of the large coherent structure occupying the near-wake region, leading to amplification of the higher frequency dynamical wake modes. The action of feedback control also leads to base pressure recovery and a reduction in pressure drag. Upon varying the controller parameters, a trade-off between the degree of bimodality suppression and drag reduction is observed. A maximum drag reduction of 7.4% is achieved for a semisymmetrized wake, with a fully symmetrized wake achieving 2.5% reduction. Bimodality suppression is proposed to have an indirect link to drag reduction through the effect of the wake state on the separated free shear layers and the upstream boundary layers.

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Phys. Rev. Fluids 7, 084401– Published 22 August, 2022
  • Received 9 March 2022
  • Accepted 14 July 2022
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