Abstract
The pressure drag of blunt bluff bodies is highly relevant in many practical applications, including to the aerodynamic drag of road vehicles. This paper presents theory revealing that a mean drag reduction can be achieved by manipulating wake flow fluctuations. A linear feedback control strategy then exploits this idea, targeting attenuation of the spatially integrated base (back face) pressure fluctuations. Large-eddy simulations of the flow over a D-shaped blunt bluff body are used as a test-bed for this control strategy. The flow response to synthetic jet actuation is characterised using system identification, and controller design is via shaping of the frequency response to achieve fluctuation attenuation. The designed controller successfully attenuates integrated base pressure fluctuations, increasing the time-averaged pressure on the body base by 38%. The effect on the flow field is to push the roll-up of vortices further downstream and increase the extent of the recirculation bubble. This control approach uses only body-mounted sensing/actuation and input–output model identification, meaning that it could be applied experimentally.
Highlights
Fluid flows around blunt bluff body shapes occur in a wide range of engineering applications
This approach has previously been successful for backward-facing step (BFS) flows [15]; the present study reveals some theory underpinning the link between mean drag and fluctuations, and extends application to bluff bodies exhibiting interacting shear layers
The present work has presented a concept for reducing the aerodynamic drag of bluff body flows, based on sensing and manipulating only fluctuating flow quantities
Summary
Fluid flows around blunt bluff body shapes occur in a wide range of engineering applications. For blunt bluff bodies, such as the D-shaped body or square cylinder, above a critical Reynolds number an absolute wake instability [8,20] generates alternating shedding of vortices at characteristic frequencies, known as a von Kármán vortex street Numerical simulations of such flows have been as a test-bed for feedback control strategies, for example using Galerkin projection of the Navier–Stokes equations onto a lowdimensional subspace [21] or using on adjoint-based methods [22]. The aim is to use linear feedback control to achieve an increase in mean pressure on the back face or base of the geometry, so as to achieve a pressure drag reduction This approach has previously been successful for BFS flows [15]; the present study reveals some theory underpinning the link between mean drag and fluctuations, and extends application to bluff bodies exhibiting interacting shear layers.
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