OCOlivier Cadot

Olivier Cadot

University of Liverpool
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Olivier Cadot has a total of 131 co-authors. Here are the top co-authors by number of shared publications:

Latest Publications

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    Asymmetry, Bistability, and Bifurcations in the Wake of Simplified Vehicles

    by Olivier Cadot, Vladimir Parezanović

    This review presents research from the past 15 years that has been motivated by the discovery of the bistability phenomenon in the turbulent wake dynamics of bluff bodies with rectangular bases. Works involving theory, experiment, numerical simulation, and modeling have been published on a broad scope encompassing the original laminar bifurcation; its evolution in the turbulent wake regime; the impact on the body aerodynamics; and the sensitivity of the wake asymmetry to geometry, freestream conditions, and flow control. The review aims to provide a unifying context for the bistability phenomenon and evaluate its contribution to the aerodynamic performance of bluff bodies relevant to a wide majority of land vehicles and some naval superstructures.

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    Momentum decomposition of the pressure field

    by Taihang Zhu, Chao Xia, Jiabin Pang, Olivier Cadot, Jonathan F. Morrison

    The generative mechanism of the pressure field is fundamental in turbulence research and flow control, especially when subject to turbulent separation at high Reynolds numbers. To this end, we propose a physics-based framework—momentum decomposition of the pressure field—which uses the momentum equations to decompose the spatial pressure gradient into quantifiable momentum budgets. Each momentum budget corresponds to a distinct physical mechanism, enabling a rigorous decomposition and quantitative evaluation of the inherent mechanisms governing pressure generation. Incorporating results of a direct numerical simulation of laminar cylinder flow, this method is first demonstrated, after which the three-dimensional large eddy simulation results of a complex turbulent bluff body wake at Re D = 1.88 × 10 5 are analyzed with a comparison of flow control. In the near wake, the bulk of low pressure originates from the streamwise convection of the centripetal flow outside the wake, which manifests as a streamline curvature effect from the term u ¯ ∂ v ¯ / ∂ x , while the pressure variation inside the wake receives main contributions from Reynolds stresses. Under flow control, due to the streamline curvature modification near the base edge, the pulsed jet produces favorable pressure gradients from the mean flow term u ¯ ∂ v ¯ / ∂ x contributing to the bulk of base-pressure recovery. Overall, the proposed framework provides a fundamental flow diagnostic tool and a rigorous foundation for designing and analyzing flow control strategies, since pressure control can be rationally linked to the momentum budgets manipulation, generally applicable to flow regimes governed by the Navier-Stokes equations.

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    Self-adaptive elastic flaps with bending and torsion for 3D blunt body drag reduction

    by J.M. Camacho-Sánchez, M. Lorite-Díez, Y. Fan, J.I. Jiménez-González, O. Cadot

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    A review on the asymmetry, steady instability and bistability in the wake of industrial ground vehicles

    by Olivier Cadot

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    Recirculating flow structures of a square-back Ahmed body at a variable attitude

    by Yajun Fan, Chao Xia, Guglielmo Minelli, Simone Sebben, Olivier Cadot

    The recirculating flow at the rear of a flat-base three-dimensional body with ground proximity is investigated for different body attitudes defined by the pitch angle varying in the range $-1.5^\circ \lt \alpha \lt +2.6^\circ$ and the yaw angle in the range $0^\circ \lt \beta \lt +12^\circ$ . Experiments measuring the three components of the mean velocity field in two perpendicular planes intersecting the recirculation area as well as the base pressure distribution are conducted for 50 different attitudes. They provide a clear correlation between the orientation of the spatially averaged reversed flow and the gradient at the centre of the base pressure distribution. Both vectors are found to be in the same so-called w- plane, that is perpendicular to the base of which the azimuthal position changes with the body attitude due to either the flow orientation at the base separation or sometimes to a ground separation for large nose-up pitch. Numerical simulations of the same geometry realised for 10 attitudes show satisfactory agreement with the force coefficients measured in the experiment. Base flow variations induced by attitude changes are also well captured, particularly that of the w- plane. The full three-dimensional simulation data are used to show that the inner structure of the separation bubble is always a tilted recirculation torus, where the tilt orientation is given by the base pressure gradient. At the bubble closure, a pair of longitudinal vortices symmetrically located on both sides of the w- plane are permanently observed with circulations consistent with the circulation of the dividing streamline separation in the w- plane.

    Cited by 4
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    Equilibrium of fluid fluxes in the wake of a three-dimensional flat-back bluff body

    by T.I. Khan, L. Pastur, O. Cadot, V. Parezanović

    The turbulent wake behind a flat-back Ahmed body is investigated using stacked stereoscopic particle image velocimetry. The wake is disturbed by a steady jet from the centre of the base and the effects are quantified for key blowing rates. The unactuated wake exhibits bistable dynamics in the horizontal plane that are completely subdued for the optimal blowing case, yielding a base drag reduction of 9 %. The three-dimensional mean wake is reconstructed and used to evaluate the wake mass fluxes whose equilibrium determines the recirculation length. The results for the unactuated wake show that up to 80 % of replenishment fluid flux entering the recirculation bubble from the free-stream flow is provided through the low-pressure side of the base, where the symmetry-breaking shear layer roll-up occurs near the base. For the optimal blowing configuration, where the wake becomes symmetric, the flux of wake replenishment is severely reduced. This flow configuration results in elongated shear layers on all sides, which terminate the bubble with a roll-up of reduced intensity at a further downstream location. The dominant cause of bubble growth and the accompanying drag reduction is attributed to the momentum of the base blowing, and the new regime is referred to as the ‘favourable momentum regime’. Similar trends are observed when the model is at $5^{\circ }$ yaw where a reduction of drag and yaw-induced asymmetry are obtained. Proper orthogonal decomposition of the wake reveals the coherent structures related to the bistable flow and the symmetric wake under optimal blowing coefficient.

    Cited by 10
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    Experimental and computational study of aerodynamic loads on the NATO Generic Destroyer

    by N. Fernandez, I. Owen, O. Cadot, White

    Cited by 7
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    Experimental study on the effect of adaptive flaps on the aerodynamics of an Ahmed body

    by J. M. Camacho-Sánchez, M. Lorite-Díez, J. I. Jiménez-González, O. Cadot, C. Martínez-Bazán

    We perform an experimental study on the turbulent flow around a square-back Ahmed body of height, h, at varying Re=u∞h/ν∈[0.5,1]×105, where u∞ is the free-stream velocity and ν is the incoming flow kinematic viscosity, under different yaw angles β=(0∘,−5∘,−10∘), to analyze the use of rear flexibly hinged parallel plates as a control strategy to reduce the drag in a self-adaptive manner under changing flow conditions. The model implements rear parallel rigid flaps of depth d=0.5h, which are mounted with torsional joints through embedded flexible foils of calibrated thickness. This holding system restricts the motion of the plates to a rotary displacement, θ. The fluid-structure dynamics is characterized by the reduced velocity, defined as U*=u∞/fnh, where fn is the natural frequency of rotary oscillations of the hinged plates, measured in free-decay tests of flaps. In fact, we have explored the range of reduced velocity, U*=[0,65], varying u∞ and consequently Re. We perform force and pressure measurements to quantify the variations of the drag and the base pressure coefficients while laser displacement sensors are used to obtain the angular flaps motion. Results show that the hinged plates decrease the drag coefficient of the original body by nearly 4.4% for flow conditions aligned with the body axis. Under cross-flow conditions, their efficiency is even larger, attaining relative reductions drag of nearly 9.1% at β=−10∘ (13.5% in comparison with a body with fixed rigid plates of the same depth). Such variations are shown to be associated with a passive reconfiguration process of rear flaps. Additionally, hinged flaps are shown to interact with the reflectional-symmetry-breaking (RSB) modes, typically present in the wake of three-dimensional bodies. At aligned conditions, the interaction with the RSB modes is characterized by two regimes, in such a way that the hinged flaps manage to partially stabilize the RSB modes, and consequently to inhibit the bistable behavior at low values of U* (in a similar manner to rigid flaps), while at high values of U*, they respond dynamically to switches between the opposite wake deflections of the RSB modes, deviating themselves accordingly.

    Cited by 11
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    Suppression of the wake steady asymmetry of an Ahmed body by central base bleed

    by Tauha Irfan Khan, Vladimir Parezanović, Luc Pastur, Olivier Cadot

    Base blowing is applied through a small, centrally located square aperture at the base of a flat-backed Ahmed body. In addition to the expected drag reduction effect at low momentum injection (i.e., base bleeding effect), a major result is the complete suppression of the steady asymmetry of the wake. Both the maximum drag reduction and the minimum magnitude of the base pressure gradient are achieved for the same optimal blowing coefficient of the actuator. Independent force measurements corroborate the suppression of the wake asymmetry. Different scales of base blowing reveal a similar maximum drag reduction and asymmetry suppression, where the optimal blowing coefficient is found to scale with the bleed-to-base area ratio as (Sj/S)1/2.

    Cited by 17
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    Wake transitions and steady -instability of an Ahmed body in varying flow conditions

    by Yajun Fan, Vladimir Parezanović, Olivier Cadot

    The paper investigates experimentally the flow past a flat-back, taller than wide Ahmed body having rectangular base aspect ratio $H/W=1.11$ in the context of ground vehicle aerodynamics. Parametric studies at Reynolds number $2.1\times 10^5$ explore the sensitivity of the aerodynamic force and body pressure distribution with respect to varying flow conditions defined from variable ground clearance $C$ (taken at mid-distance from the front and rear axles of the body), pitch angle $\alpha$ , and yaw angle $\beta$ (equivalent to a crosswind). Two-dimensional parametric maps in the parametric spaces $(\beta,C)$ and $(\beta,\alpha )$ are obtained for parameter ranges covering real road vehicle driving conditions. The study of the base pressure gradient reveals non-trivial and sharp transitions identified as significant changes of near wake orientation in both parametric spaces. All unsteady transitions correspond to fluctuation crises of the vertical gradient component only. These transitions are summarized in phase diagram representations. Both phase diagrams in $(\beta,C)$ and $(\beta,\alpha )$ parametric spaces can be unified at large yaw in the $(\beta,C_r)$ space, where the rear clearance $C_r$ separating the lower edge of the base from the ground is a simple function of the pitch $\alpha$ and the clearance $C$ . The impacts of the wake transitions are clearly identified in the base drag, drag, lift and side force coefficients. The contribution of the steady near wake $z$ -instability (Grandemangeet al.,Phys. Fluids, vol. 25, 2013a, pp. 95–103) is assessed by repeating the sensitivity analysis with a rear cavity. As reported previously, the rear cavity suppresses the steady instability by symmetrizing the wake. A domain for the existence of the instability is finally proposed in the attitudes map $(\beta,\alpha )$ defined from regions where the mean lateral force coefficients are significantly decreased by the presence of the rear cavity. In addition, it is found that the steady instability forces the wake to be less unsteady for all attitudes that do not correspond to unsteady transitions.

    Cited by 31
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