Simulation and feedback control of the flow past the Ahmed body
Creators: name: Ahmed, Dania
Publisher: name: Imperial College London
Year: 2023

description: This research investigates the turbulent flow past a blunt bluff body. The square-back Ahmed body is considered a canonical bluff body, representing a simplified road vehicle. Wall resolving Large Eddy Simulations (LES) were used to investigate the dynamics of the unforced flow and accordingly inform control strategies for drag reduction. The turbulent wake behind a square-back Ahmed body in close proximity to the ground exhibits bi-modal switching. This manifests as the centre 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 investigates numerically feedback control for suppressing horizontal (lateral) wake bi-modality of a square-back Ahmed body at Re_H ∼ 3.3 × 10^4 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 synthesised. This successfully suppresses the wake lateral bi-modal switching. However, this switching is replaced by a time-periodic streamwise motion of the large coherent structure occupying the near-wake region. Further, the controller amplifies the higher frequency dynamical wake modes. The action of feedback control also leads to base pressure recovery and a reduction in pressure drag. A trade-off between the degree of bi-modality suppression and drag reduction is observed upon varying the controller parameters. A maximum drag reduction of 7.4% is achieved for a semi-symmetrised wake, with a fully symmetrised wake achieving a 2.5% reduction. Wake bi-modality is found to be sensitive to different parameters, including the free stream turbulent intensity, the underbody flow and the dynamics of the upstream boundary layers developed along the longitudinal surfaces. Boundary layers separate and reattach close to the body’s fore-end before reaching the base. Following this recently-established link by Hesse and Morgans (2021), the second part of this research investigates the effect of suppressing boundary layer separations on wake bi-modality. Hairpin vortices, formed at the reattachment points, grow along the surfaces before breaking down upstream of the base. The resultant smaller vortices from the top and side surfaces interact as they are convected downstream, which is suggested to be a trigger of the wake bi-modal switching. Suppressing boundary layer separations interrupts this interaction, which is found to have a damping effect on the fluctuations just upstream of the base. Steady suction was applied on the longitudinal surfaces of the body to suppress boundary layer separations. The results showed that horizontal bi-modality is completely suppressed by suppressing the separation of the boundary layers on the surfaces normal to the switching direction without affecting the vertical wake position. Different configurations for suppressing boundary layer separations affect the momentum and the turbulent kinetic energy of underbody flow. The wake can fully be symmetrised by reducing the momentum of the underbody flow with a reflected vertical symmetric position. The results of these cases open doors for using feed-forward controllers with actuation significantly upstream the base separation to reduce the drag rather than forcing the wake directly, which involves some trade-offs between different dynamics in the wake.