Reducing the pressure drag of a D-shaped bluff body using linear feedback control

Longa, L. Dalla; Morgans, Aimee S.; Dahan, Jeremy A.

doi:10.1007/s00162-017-0420-6

Cited by 19 publications

(18 citation statements)

References 36 publications

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“…A wall-adapting local eddy-viscosity sub-grid-scale model is employed to represent the smaller turbulent scales, this model accounting for the vanishing behaviour of the eddy viscosity near the walls. This solver has been extensively tested and validated by Temmerman (2004) and more recently applied to a range of bluff body flows (Dahan, Morgans & Lardeau 2012;Dalla Longa, Morgans & Dahan 2017;Evstafyeva et al 2017).…”

Section: Simulations Set-up and Validationmentioning

confidence: 99%

Simulations of the bi-modal wake past three-dimensional blunt bluff bodies

2019

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The bi-modal behaviour of the turbulent flow past three-dimensional blunt bluff bodies is simulated using wall-resolved large eddy simulations. Bi-modality (also called bi-stability) is a phenomenon that occurs in the wakes of three-dimensional bluff bodies. It manifests as a random displacement of the wake between preferred off-centre locations. Two bluff bodies are considered in this work: a conventional square-back Ahmed body representative of road cars, and a simplified lorry, which is taller than it is wide, with its aspect ratio corresponding to a 15 % European lorry scale model. To our knowledge, this is the first time that the asymmetric bi-modal switching behaviour of the wake, observed experimentally, has been captured in simulations. The resulting unsteady flow fields are then analysed, revealing instantaneous topological changes in the wake experiencing bi-modal switching. The best-resolved case, the simplified lorry geometry, is then studied in greater detail using modal decomposition to gain insights into the energy content and the dominant frequencies of the wake flow structures associated with the asymmetric states. High-frequency snapshots of the switching sequence allow us to propose that large hairpin vortices are responsible for the triggering of the switching.

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Section: Simulations Set-up and Validationmentioning

confidence: 99%

Simulations of the bi-modal wake past three-dimensional blunt bluff bodies

2019

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“…It is envisaged that in suppressing these fluctuations, a mean base pressure recovery will be achieved, giving a form-drag reduction. This strategy has been successfully applied to 3D blunt bluff body geometries (Dahan et al 2012;Dalla Longa, Morgans & Dahan 2017).…”

Section: Devinant and Kourta 2015mentioning

confidence: 99%

Simulation and feedback control of the Ahmed body flow exhibiting symmetry breaking behaviour

2017

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The present work considers the low-Reynolds-number wake flow behind a squareback Ahmed body, in close proximity to a ground. At low Reynolds numbers such wakes are known to undergo a series of bifurcations to a state that breaks reflectional symmetry. The symmetry breaking of the wake also persists at turbulent high Reynolds numbers, where it manifests as bi-modal behaviour with random switching between the asymmetric states. Thus far, it has only been possible to study the low-Reynolds-number sequence of bifurcations experimentally and mathematically. The present work presents the first numerical simulations capturing the sequence of symmetry breaking bifurcations that occur. A study of how the wake topology changes throughout suggests that interaction between the closer top/bottom pair of parallel shear layers can only dominate once there is sufficient underbody flow. When this occurs, the two main vortex structures in the wake switch from being horizontally to vertically aligned. A linear feedback control strategy, designed to attenuate base pressure force fluctuations, is then implemented. This causes an accompanying reduction in drag and re-symmetrisation of the wake. Analysis using the dynamic mode decomposition confirms that the wake shedding mode is re-symmetrised. This work motivates future attempts to capture wake symmetry breaking and bi-modality in numerical simulations, and application of a promising feedback control strategy at higher, turbulent Reynolds numbers.

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“…(Bagheri et al (2009); Semeraro et al (2011); Barbagallo et al (2009); Rowley & Dawson (2017) for a comprehensive review). Alternatively, if system identification is used to derive a reduced-order model (ROM) directly from inputoutput data (Cattafesta et al 1999;Kegerise et al 2002;Illingworth et al 2011;Hervé et al 2012;Dahan et al 2012;Dalla Longa et al 2017), the resulting models will typically be of manageable dimension. Suitable ROMs for closed-loop control may also be obtained by gradually increasing the spatial discretization from a coarse mesh (Jones et al 2015).…”

Section: Introductionmentioning

confidence: 99%

Linear iterative method for closed-loop control of quasiperiodic flows

et al. 2019

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This work proposes a feedback-loop strategy to suppress intrinsic oscillations of resonating flows in the fully nonlinear regime. The frequency response of the flow is obtained from the resolvent operator about the mean flow, extending the framework initially introduced by McKeon & Sharma (J. Fluid Mech., vol. 658, 2010, pp. 336–382) to study receptivity mechanisms in turbulent flows. Using this linear time-invariant model of the nonlinear flow, modern control methods such as structured ${\mathcal{H}}_{\infty }$-synthesis can be used to design a controller. The approach is successful in damping self-sustained oscillations associated with specific eigenmodes of the mean-flow spectrum. Despite excellent performance, the linear controller is however unable to completely suppress flow oscillations, and the controlled flow is effectively attracted towards a new dynamical equilibrium. This new attractor is characterized by a different mean flow, which can in turn be used to design a second controller. The method can then be iterated on subsequent mean flows, until the coupled system eventually converges to the base flow. An intuitive parallel can be drawn with Newton’s iteration: at each step, a linearized model of the flow response to a perturbation of the input is sought, and a new linear controller is designed, aiming at further reducing the fluctuations. The method is illustrated on the well-known case of two-dimensional incompressible open-cavity flow at Reynolds number $Re=7500$, where the fully developed flow is initially quasiperiodic (2-torus state). The base flow is reached after five iterations. The present work demonstrates that nonlinear control problems may be solved without resorting to nonlinear reduced-order models. It also shows that physically relevant linear models can be systematically derived for nonlinear flows, without resorting to black-box identification from input–output data; the key ingredient being frequency-domain models based on the linearized Navier–Stokes equations about the mean flow. Applicability to amplifier flows and turbulent dynamics has, however, yet to be investigated.

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Reducing the pressure drag of a D-shaped bluff body using linear feedback control

Cited by 19 publications

References 36 publications

Simulations of the bi-modal wake past three-dimensional blunt bluff bodies

Simulations of the bi-modal wake past three-dimensional blunt bluff bodies

Simulation and feedback control of the Ahmed body flow exhibiting symmetry breaking behaviour

Linear iterative method for closed-loop control of quasiperiodic flows

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