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Title: Simulation, modelling and feedback control of the flow around a simplified square-back road vehicle
Author: Dalla Longa, Laurent
ISNI:       0000 0004 7658 7338
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2018
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The present work investigates the use of wall-resolved Large Eddy Simulations (LES) to compute the flow around simpli fied square-back road vehicles. The objective being to simulate numerically the unsteady flow past such blunt bluff bodies and to assess the use of a linear feedback control technique for drag reduction. Wall-resolved large-eddy simulations are employed as a test-bed, but the control strategy could be transferrable to moving vehicle experiments as both the sensing (base pressure sensors) and the actuation (synthetic jets) are body-mounted. The control strategy attempts to reduce the pressure drag by acting on the wake rather than manipulating the flow separation location which is fi xed for square-back road vehicles. This technique is known as direct wake control. The control technique exploits the link between wake flow fluctuations and mean drag reduction. The flow response to the synthetic jets is characterised using system identi cation, and controller design is via shaping the frequency reponse to achieve wake fluctuation attenuation. The first bluff body studied is an in finite spanwise blunt bluff body called D-shaped body. It exhibits two interacting shear layers and a large recirculation area. The designed controller successfully attenuates base pressure fluctuations, increasing the time-averaged pressure on the body base by 38%. The second bluff body is a three-dimensional simplifi ed lorry in presence of a fixed floor. The wake flow is composed of four detached shear layers, which form the envelope of a large low-pressure recirculation area. For this case, unforced simulations captured numerically, for the first time, the wake bi-modality. Bi-modality manifests as a random displacement (switching) of the wake between preferred off-center locations. The unsteady flow fi eld is studied in great details using modal decomposition. High-frequency snapshots of the switching sequence allow us to propose an explanation for the triggering of the bi-modal switching. Finally, various feedback control confi gurations are assessed. Base pressure fluctuations are signifi cantly reduced as targeted but it appears challenging to obtain a clear mean base pressure recovery. The best confi guration yields to a mean base pressure increase of 2.1%.
Supervisor: Morgans, Aimee Sponsor: Imperial College London ; Engineering and Physical Sciences Research Council ; Renault-Nissan (Firm)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral