Simulation and feedback control of simplified vehicle flows
File(s)
Author(s)
Evstafyeva, Olga
Type
Thesis
Abstract
The present doctoral work investigates wake flow behind a square-back Ahmed body at laminar and turbulent Reynolds numbers. At low Reynolds numbers the Ahmed body wake undergoes a series of bifurcations towards symmetry breaking state. Using Large Eddy Simulations, which resolve the boundary layer down to the wall, the sequence of symmetry breaking bifurcations is captured numerically for the first time. Large Eddy Simulations are then used to investigate the reflectional symmetry breaking behaviour in the turbulent regime, where it manifests as slow and random bi-modal switching between the asymmetric states. The bi-modal switch from one state to the other is captured for the first time numerically, allowing to study the effects of wake displacement on the instantaneous wake structures.
A linear feedback control strategy, designed to attenuate base pressure force fluctuations is tested on the Ahmed body flow. At low Reynolds numbers the strategy is successful; a moderate base pressure recovery and a concomitant drag reduction are achieved and the wake is re-symmetrised. A similar control approach is also benchmarked at higher Reynolds numbers, where the flow exhibits turbulent separation. The preliminary results suggest that the controller is unlikely to reduce drag and other actuation/sensing arrangements need to be investigated. This work motivates future attempts to develop a linear feedback controller based on the strategy described herein, but with better selected actuation and sensing. Further work is also required to better understand the physical mechanism behind bi-modal switching in the wake.
A linear feedback control strategy, designed to attenuate base pressure force fluctuations is tested on the Ahmed body flow. At low Reynolds numbers the strategy is successful; a moderate base pressure recovery and a concomitant drag reduction are achieved and the wake is re-symmetrised. A similar control approach is also benchmarked at higher Reynolds numbers, where the flow exhibits turbulent separation. The preliminary results suggest that the controller is unlikely to reduce drag and other actuation/sensing arrangements need to be investigated. This work motivates future attempts to develop a linear feedback controller based on the strategy described herein, but with better selected actuation and sensing. Further work is also required to better understand the physical mechanism behind bi-modal switching in the wake.
Version
Open Access
Date Issued
2018-01
Date Awarded
2018-07
Copyright Statement
Attribution NoDerivatives 4.0 International Licence (CC BY-ND)
Advisor
Morgans, Aimee
Sponsor
Engineering and Physical Sciences Research Council
Grant Number
1743454
Publisher Department
Mechanical Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)