Linear feedback control for form-drag reduction on bluff bodies with a blunt trailing edge
Author(s)
Dahan, Jeremy
Type
Thesis
Abstract
The work described in this thesis is a computational investigation applying linear feedback
control to reduce form-drag on bluff bodies with a blunt trailing edge. For such bodies, a
large portion of the aerodynamic drag is associated with an unsteady separated region or wake
downstream of the body. The development of tractable feedback strategies to control unsteady
wakes promises strong benefits, both in terms of industrial applications and for furthering our
understanding of the flow mechanisms at play.
For this purpose, large-eddy simulations are carried out where a linear feedback controller
targets an increase in the mean pressure force on the rear (base) of the body. The flows over
two distinct geometries are examined: a backward-facing step and a bluff body with a rounded
leading edge, often referred to as a D-shaped body. The control is effected by zero-net-mass-flux slot jets, responding to sensors located on the body base. Open-loop characterization provides
information on the effects of actuation and some physical insight into the relation between the
base pressure and wake dynamics. System identification is used to obtain a low-order model
of the flow's response to actuation that can be used for control.
The control strategy is based on the premise that reducing the fluctuations in the near-wake
will cause an increase in the mean base pressure, hence a reduction in form-drag. The
controllers are designed with classical frequency-domain methods, using a sensitivity transfer
function to attenuate the size of the pressure force fluctuations.
The influence of parameters such as the Reynolds number and the location and type of
actuators is studied. For all cases, low-order linear feedback controllers successfully reduce
the pressure force fluctuations and achieve sensible drag reductions. They do so with higher efficiency than the open-loop forcing considered. Uncertainties in the model and flow conditions
can be to some extent mitigated by the robustness of the controller. The results support the
conjecture linking the fluctuating and mean base pressure, although it is observed that further
work is needed before such an approach can be used for optimization.
control to reduce form-drag on bluff bodies with a blunt trailing edge. For such bodies, a
large portion of the aerodynamic drag is associated with an unsteady separated region or wake
downstream of the body. The development of tractable feedback strategies to control unsteady
wakes promises strong benefits, both in terms of industrial applications and for furthering our
understanding of the flow mechanisms at play.
For this purpose, large-eddy simulations are carried out where a linear feedback controller
targets an increase in the mean pressure force on the rear (base) of the body. The flows over
two distinct geometries are examined: a backward-facing step and a bluff body with a rounded
leading edge, often referred to as a D-shaped body. The control is effected by zero-net-mass-flux slot jets, responding to sensors located on the body base. Open-loop characterization provides
information on the effects of actuation and some physical insight into the relation between the
base pressure and wake dynamics. System identification is used to obtain a low-order model
of the flow's response to actuation that can be used for control.
The control strategy is based on the premise that reducing the fluctuations in the near-wake
will cause an increase in the mean base pressure, hence a reduction in form-drag. The
controllers are designed with classical frequency-domain methods, using a sensitivity transfer
function to attenuate the size of the pressure force fluctuations.
The influence of parameters such as the Reynolds number and the location and type of
actuators is studied. For all cases, low-order linear feedback controllers successfully reduce
the pressure force fluctuations and achieve sensible drag reductions. They do so with higher efficiency than the open-loop forcing considered. Uncertainties in the model and flow conditions
can be to some extent mitigated by the robustness of the controller. The results support the
conjecture linking the fluctuating and mean base pressure, although it is observed that further
work is needed before such an approach can be used for optimization.
Date Issued
2013-01
Date Awarded
2013-06
Copyright Statement
Attribution NoDerivatives 4.0 International Licence (CC BY-ND)
Advisor
Morgans, Aimee
Sponsor
Engineering and Physical Sciences Research Council
Publisher Department
Aeronautics
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)