Drag reduction on a blunt body by self-adaption of rear flexibly hinged flaps

We study the aerodynamics of a blunt-based body with rear flexibly-hinged rigid flaps, subject to a turbulent flow of Reynolds number Re = 12000, under aligned and cross flow conditions with yaw angle β = 0◦ and β = 4◦. To that aim, different values of the equivalent torsional stiffness are considered, to cover the range of reduced velocity U∗ = (0, 3.48] in water tank experiments. The effect of the angular deflection of plates on the drag and near wake flow is analyzed, experimentally and numerically. The results show that, in the range of U∗ herein considered, the plates undergo an inwards quasi-static, self-adaptive deflection, which is symmetric for yaw angles β = 0◦ and asymmetric for β = 4◦. In particular, the plates feature small mean deformation angles for values of U∗ < 1, whereas a sharp and monotonic increase of such deflection occurs for U∗ > 1, i.e. for lower values of the hinge’s stiffness, with an asymptotic trend towards the larger values of U∗. A critical value of reduced velocity of U∗ ≃ 0.96 is obtained as the instability threshold above which plates depart from their initial equilibrium position. The progressive streamlining of the trailing edge translates into significant reductions of the associated mean drag coefficients. Thus, reductions close to 19% with respect to reference static plates configurations are obtained for the most flexible case of U∗ = 3.48 for both β = 0◦ and β = 4◦. A close inspection of the near wake reveals that the inwards progressive mean displacement of the plates yields a reduction in the recirculation bubble size. A symmetric evolution of the recirculating bubble is observed for β = 0◦, whereas the bubble becomes asymmetric for β = 4◦, with a larger leeward clockwise vortex. In both cases, the drag coefficient is shown to vary linearly with the global aspect ratio of the recirculating bubble. The analysis of the numerical results shows that the reduced extension of the recirculating bubble significantly alters the formation length and intensity of the eddies size and associated pressure. It is observed that despite the local pressure decrease in the vortices shed from the trailing edges, the plates self adaption reduces their size and prevents the eddies from entering the cavity, thus, creating a dead flow region with a consequent pressure increase at the body base.Junta de Andalucia FEDER-UJA 1262764Universidad de JaenEuropean CommissionSpanish MCIN/AEI PDC2021-121288-I00European Union Next Generation EU/PRT

Experimental analysis of the effect of local base blowing on three-dimensional wake modes

Wake modes of a three-dimensional blunt-based body near a wall are investigated at a Reynolds number . The targeted modes are the static symmetry-breaking mode and two antisymmetric periodic modes. The static mode orientation is aligned with the horizontal major -axis of the base and randomly switches between a positive and a negative state leading to long-time bistable dynamics of the turbulent wake. The modifications of these modes are studied when continuous blowing is applied at different locations through four slits along the base edges (denoted L for left, R for right, T for top and B for bottom) in either four single asymmetric configurations or two double symmetric configurations (denoted LR and TB). Two regimes, referred to as mass and momentum, are clearly identifiable for all configurations. The mass regime, which is fairly insensitive to blowing momentum and location, is characterized by the growth of the recirculating bubble as the total injected flow rate is increased, and is associated with a base drag reduction and interpreted as resulting from the equilibrium between mass fluxes feeding and emptying the recirculating region. A simple budget model is shown to be in agreement with entrainment velocities measured for isolated turbulent mixing layers. The strength of the static mode is reduced up to 20 % when the bubble length is maximum, whereas no change in the periodic mode frequencies is found. On the other hand, the momentum regime is characterized by the deflating of the recirculating bubble, leading to base drag increase, and it is interpreted by the free shear layer forcing, which increases the entrainment velocity, thus emptying the recirculating bubble. In this regime the static mode orientation is imposed by the blowing symmetry. Lateral L and R (respectively top/bottom T and B) blowing configurations select or states in the horizontal (respectively vertical) direction, while bistable dynamics persists for the symmetric LR and TB configurations. The shape of periodic modes follows the changes in wake static orientation. The transition between the two regimes is governed by both the total injected flow rate and the location of the injection

Unveiling the competitive role of global modes in the pattern formation of rotating sphere flows

The wake flow past a streamwise rotating sphere is a canonical model of numerous applications, such as particle-driven flows, sport aerodynamics and freely rising or falling bodies, where the changes in particles' paths are related to the destabilization of complex flow regimes and associated force distributions. Herein, we examine the spatio-temporal pattern formation, previously investigated by Lorite-Diez &amp; Jimenez-Gonzalez (J. Fluid Mech., vol. 896, 2020, A18) and Pier (J. Fluids Struct., vol. 41, 2013, pp. 43-50), from a dynamical system perspective. A systematic study of the mode competition between rotating waves, which arise from the linearly unstable modes of the steady-state, exhibits their connection to previously observed helical patterns present within the wake. The organizing centre of the dynamics turns out to be a triple Hopf bifurcation associated with three non-axisymmetric, oscillating modes with respective azimuthal wavenumbers m = -1, -1 and -2. The unfolding of the normal form unveils the nonlinear interaction between the rotating waves to engender more complex states. It reveals that for low values of the rotation rate, the flow field exhibits a similar transition to the flow past the static sphere, but accompanied by a rapid variation of the frequencies of the flow with respect to the rotation. The transition from the single helix pattern to the double helix structure within the wake displays several regions with hysteric behaviour. Eventually, the interaction between single and double helix structures within the wake lead towards temporal chaos, which here is attributed to the Ruelle-Takens-Newhouse route. The onset of chaos is detected by the identification of an invariant state of the normal form constituted by three incommensurate frequencies. The evolution of the chaotic attractor is determined using of time-stepping simulations, which were also performed to confirm the existence of bi-stability and to assess the fidelity of the computations performed with the normal form