Nicolas Mazellier

We report a comprehensive study of the wake of a porous disc, the design of which has been modified to incorporate a swirling motion at an inexpensive cost. The swirl intensity is passively controlled by varying the internal disc geometry, i.e. the pitch angle of the blades. A swirl number is introduced to characterise the competition between the linear (drag) and the azimuthal (swirl) momenta on the wake recovery. Assuming that swirl dominates the near wake and non-equilibrium turbulence theory applies, new scaling laws of the mean wake properties are derived. To assess these theoretical predictions, an in-depth analysis of the aerodynamics of these original porous discs has been conducted experimentally. It is found that, at the early stage of wake recovery, the swirling motion induces a low-pressure core, which controls the mean velocity deficit properties and the onset of self-similarity. The measurements collected in the swirling wake of the porous discs support the new scaling laws proposed in this work. Finally, it is shown that, as far as swirl is injected in the wake, the characteristics of the mean velocity deficit profiles match very well those of both laboratory-scale and real-scale wind-turbine data extracted from the literature. Overall, our results emphasise that, by setting the initial conditions of the wake recovery, swirl is a key ingredient to be taken into account in order to faithfully replicate the mean wake of wind turbines.

This paper describes the evolution of the boundary layer around a cylinder and its contribution to the drag force in supersonic rarefied flows. Experiments are

Superhydrophobic (SH) coatings have been demonstrated to reduce drag in various applications, though they have yet to be widely used or adopted due to practical issues. Various studies have demonstrated the prospect of reducing drag via SH surfaces both in turbulent and laminar flow regimes. However, the beneficial wall-slip effect produced may disappear depending on the surface geometry and flow conditions. The main mechanisms considered behind the decrease in performance are Marangoni-induced stresses and air/liquid interface deformation. In the present study, another mechanism is proposed to explain the loss of performances of SH surfaces in laminar flow regimes. Here we consider the flow of air inside the plastron and the associated momentum loss induced by roughness elements with different geometric characteristics. The effects of air motion within the plastron are coupled to the outer fluid with a homogenized boundary-condition approach. To this end, numerical simulations at the scale of the roughness elements were conducted as a function of the porosity and the tortuosity of the domain to determine the slip velocity at the air/liquid interface. The homogenized boundary condition is then implemented in a theoretical model for the outer flow to compute drag on SH spheres at low Re numbers. Experiments of laminar SH falling spheres indicate that high values of the tortuosity and low values of the porosity lead to a loss of performances when considering drag reduction. As anticipated, a three-dimensional printed sphere with low tortuosity and similar porosity demonstrated near-optimal drag reductions. A comparative study between the predicted values and experiments shows that the homogenized model is able to accurately predict the drag on SH surfaces for values of the porosity and tortuosity estimated from microscopy images of the SH textured surface.

Machine learning control is applied in real-time to an airfoil equipped with variable-velocity jets and pressure sensors in a closed-loop wind tunnel. The objective of the control is to improve the lift-to-drag ratio using an array of variable velocity jets located at the leading edge of the wing model. Pressure sensors are located along the chord, at mid-span, and are used in a feedback control strategy where the symbolic control laws are optimized using a linear genetic programming control (LGPC) algorithm. For angles of attack at the onset of stall, we show that the best control law is able to outperform the best open-loop control strategy in the case of a single-input multiple-output control loop. Results are reported for Reynolds numbers at both half a million and one million. In particular, the lift-to-drag ratio is improved by 4% compared to the best open-loop strategy, which corresponds to a relative increase by a factor 3. In the case of leading-edge separation conditions, LGPC builds a control law performing similarly to the best open-loop strategy while minimizing the actuation power. This study suggests that in order to improve the control authority, the LGPC strategy is sufficiently mature. This control methodology highlights that in order to improve the aerodynamic performances, novel fluidic actuators providing access to the orientation of the jets could enable breakthroughs for high-Reynolds-number experimental demonstrators.

Super Hydrophobic (SH) coatings are widely used to mitigate drag in various applications. Numerous studies have demonstrated that the beneficial wall-slip effect produced by these materials disappears in laminar flow regimes. The main mechanisms considered to be behind the decrease in performance are Marangoni-induced stresses and air/liquid interface deformation. In the present study, a new mechanism is proposed to explain the loss of performances of SH-surfaces in laminar flow regimes. Here we consider the flow of air inside the plastron and the associated momentum loses induced by roughness elements with different geometric characteristics. The effects of air motion within the plastron is coupled to the outer fluid with a homogenised boundary condition approach. To this end, numerical simulations at the scale of the roughness element were conducted as a function of the porosity and the tortuosity of the domain to determine the slip velocity at the air-liquid interface. The homogenised boundary condition is then implemented in a theoretical model for the outer flow to compute drag on SH-spheres at low Re numbers. Experiments of laminar SH falling spheres indicate that high values of the tortuosity and low values of porosity lead to a loss of performances when considering drag reduction. As anticipated, a 3D printed sphere with low tortuosity and similar porosity demonstrated near-optimal drag reductions. A comparative study between the predicted values and experiments shows that the homogenised model is able to accurately predict the drag on SH surfaces for values of the porosity and tortuosity estimated from microscopy images of the SH textured surface.