Aerodynamic analysis of transitional wings encountering high amplitude streamwise gust

We are interested here in the effect of high amplitude streamwise gusts felt by fixed rigid wings at pre-stall angles of incidence in low Reynolds numbers ($Re\approx 50 \times 10^3 - 200 \times 10^3$) flow conditions. An experimental rig has been developed to reproduce a cyclic variation of the freestream axial velocity representative of the perturbations micro-aerial vehicles can encounter in a urban environment. The temporal evolution of the aerodynamic forces (lift, drag and pitching moment) recorded during the gust show significant deviations from the quasi-static predictions that unsteady but inviscid methods are unable to capture. These deviations appear to be related to the dynamic of both the laminar separation bubble and the laminar separation without trailing edge reattachment, also known as the ``short" and ``long bubble" respectively

On the development of a turbulent boundary layer over staggered three dimensional cavities

Studies by Gowree et al. have suggested that staggered three dimensional circular cavities can potentially reduce the skin friction drag of a turbulent boundary layer developing over a flat plate. This is true up to a certain Reynolds number, after which the drag rapidly increases. The detailed mechanism behind the drag increase and the possible skin friction drag reduction was not discussed, due to the limitations in the measurement technique. In the current experimental campaign the results found by Gowree et al. have been confirmed, in addition, to more detailed insight of the turbulent boundary layer over the first rows of cavities. Through numerical simulations we aim to give a heuristic explanation of the two distinct behaviours. Here we report some quantitative evidence of the mechanism that may govern the drag increase, which is the main focus of the current study. The modification of the flow field in the presence of the cavities provides a qualitative explanation of the drag reduction benefit which is subjected for further investigation

On bio-inspired induced drag reduction techniques

The current study was aimed at identifying wing planform geometries which favours low induced drag. The geometries were defined by mathematical functions and following a subsequent parametric study, with a focus on sweep angle and taper ratio, inspired by birds wings and aquatic animals flukes or caudal fins planforms. The parametric studies were carried out using a vortex lattice method (VLM) which was validated with experimental data from literature. The main finding is a confirmation of the fact that the maximum Oswald efficiency factor, e is not exclusive to an elliptical wing planform. Despite its apparent simplicity, in fact a swept-back-tapered wing with a linear chord distribution can reach e = 1, the limit or minimum induced drag established from elliptical lift distribution. Moreover, the crescent wing geometry, such as the one displayed by the swift, can even surpass this limit

Bio-inspired vortex lift for enhanced manoeuvrability

Inspired by highly manoeuvrable species of birds like the peregrine falcon and the swift, static and dynamic computational fluid dynamics (CFD) simulations were conducted to investigate vortex lift in unsteady flows. The configuration corresponds to a 50◦ sweep delta wing with sharp leading edge at Re= 5.0×104. CFD simulations were performed using a Direct Numerical Simulation (DNS) approach with a Lattice-Boltzmann Method as well as Unsteady Reynolds Averaged Navier-Stokes (URANS) simulations. Aerodynamic forces as well as the overall structure of the leading edge vortices were compared with existing literature. The evolution of the flow structures was studied when the wing performs a pitching manoeuvre from 0◦ to 20◦ angle of attack. Close agreement between both methods was found for the static and pitching lift curves, with the URANS solver presenting substantial limitations to capture complex unsteady phenomena such as the vortex breakdown. A time lag was observed in the flow dynamics during the manoeuvre, with the vortex breakdown delayed during pitch-up resulting in an improved aerodynamics performance, but more present and intense when pitching down. A sinusoidal motion was tested with the URANS solver and compared with the linear ramp case, showing performance advantages as well as higher similarity to real manoeuvres

Boundary layer transition over a lowreynolds number rotor : effects of roughness and freestream turbulence

Two separate experiments are conducted on a three bladed NACA0012 rotor operating at low Reynolds numbers using phase-locked infrared thermography coupled with simultaneous force and torque measurements. The first, focuses on the effects of freestream turbulence on boundary layer transition over the suction side of the aerofoil of the rotor in an advancing configuration. Freestream turbulence (FST) was generated in an open section wind tunnel using grids and was characterized using Hot-Wire anemometry. In general, when the rotor was subjected to FST, an increase in thrust and efficiency was observed, which could be due to the FST suppressing flow separation or by inducing early transition. The second experiment, consisted of a parametric study on the impact of forcing boundary layer transition using roughness placed on the suction side of the aerofoil, in a hover configuration. The height of the roughness was varied from 52-220μm and was placed at all at at 10% chord, over the entire span of the blade. Force and torque measurements revealed that there could be a optimal roughness height that could lead to a performance increase

Characterisation of bursts in a turbulent boundary layer over circular cavities

Boundary layer surveys are performed to characterise the turbulent boundary layer grazing over a a surface with flush-mounted circular cavities disposed in a staggered arrangement (staggered angle of 45 deg). The momentum thickness based Reynolds number varies from Reθ = 1830 to 3380 and the cavity diameter and spacing in wall units from d+ = 130 to 250 and L+ = 587 to 1075 respectively. A decrease in the local skin friction drag with respect to a smooth baseline is evidenced as well as a thickening of the viscous sublayer. The bursts frequency profile for the perforated case is shifted away from the wall while the intensity of the bursts is reduced. The production term of the turbulent kinetic energy budget is obtained from Particle Image Velocimetry data. An upward shift and a decrease in magnitude of the peak associated with the streaks suggests that the cavities modify the near wall turbulent cycle

Laminar to turbulent transition over a rotor at low reynolds numbers

An experimental investigation on the flow topology and performance of a rotor operating at low Reynolds numbers is presented. The feasibility of laminar to turbulent transition experiments over small rotors is demonstrated. Phase-locked infrared thermography coupled with simultaneous force and torque measurements were used to study a three bladed NACA0012 rotor with a radius of 0.125 m and an angle of attack of 10 degrees. Boundary layer transition was fostered using two-dimensional (2D) and threedimensional (3D) isolated roughness elements, placed at approximately 5% and 28% of the rotor blades chord. In the smooth rotor configuration, a 3D flow topology is observed, consisting of a clear laminar region closer to the blade root and a turbulent region at the blade tip. It was found that the state of the boundary layer can significantly affect the rotor’s performance, with the forcing of laminar to turbulent transition generally resulting in a loss of performance when compared to the smooth reference rotor case

Effect of sharp edges on the unsteady flow and aerodynamic performances of a Boxfish, towards bio-inspired low-drag bluff bodies

Several studies have revealed that boxfishes, tropical fishes living in shallow waters, show exceptional aerodynamic performances despite their large volume and crosssectional area. These characteristics make the boxfish shape an interesting topic of study for drag optimisation of bluff bodies. This study focuses on the aerodynamic analysis of several mathematical shape representations of the boxfish. The effect of the edges is studied since they are responsible for the generation of vortices, which play a role in potentially manipulating the wake, resulting in an overall decrease in the drag coefficient. The effect of the sharpness or roundness of the edges on the aerodynamic performance is investigated. Aerodynamic coefficients are obtained numerically and experimentally for a range of Reynolds numbers between 3000 and 300000 and at pitch angle from −16◦ to +20◦

Towards a sustainable aerial mobility inspired by nature - Aerodynamics viewpoint

It is undoubtable that in order to reduce our current impact on the climate the greenhouse gas emissions from transportation and the energy sector should be cut down drastically. The most simplistic way will be to reduce transportation and consumption significantly, as recently seen during the nationwide lockdowns due to the Covid-19 pandemic; however this came at great economic cost which we cannot afford as a society based on our current life style. Mobility systems still rely heavily on fossil fuels and even with the rapid advances in battery technologies electric propulsion is still limited to ground mobility systems which can afford the relatively low power to mass ratio. Therefore, this brings us back to square one of our traditional design approach where efficiency and reduction of power consumption is at the heart of the design process. Hydrogen is also emerging as another cleaner alternative for fossil fuel however the power to volume ratio is slowing down its implementation, especially for aerial mobility. Hence, the reduction of power consumption becomes a priority in the design process even for these carbon free technologies. In order to revisit the need for reducing the power consumption of mobility systems here we aim to adopt a completely different approach inspired by nature, where the living can engage into complex kinematics through minimal energy intake. The concepts in nature are always looking for the best return in terms of the energy input/consumed and work-done, therefore this approach will naturally lead to a highly optimal system which consumes the least and contributes positively towards our goal of achieving a sustainable aerial mobility system. Here, techniques for aerodynamic drag reduction and manoeuvrability enhancement will be reviewed alongside the equivalent concepts that already exist in nature

Excitation of instabilies in a Blasius boundary layer by surface vibration

International audienceHere we have demonstrated that small amplitude vibration can artificially excite bothtwo dimensional (2D) and three dimensional (3D) instability modes. The 2D modes weretypical of Tollmien–Schlichting (TS) waves provided that the frequency of excitation lieswithin the unstable region of the neutral stability predicted by modal linear stabilitytheory. However, even if the frequency of the mechanically forced mode was within thestable bounds of the neutral curve the harmonics generated by the non-linear responseof the flow could develop as instability modes. Further analysis of the streamwiseand spanwise evolution of the instability modes identified from the temporal Fouriertransform confirmed the presence of 3D modes excited due to the nature of the modeshape deflection of the vibrating panel which was not uniform in the spanwise direction.The effect of spanwise non-uniformity could be increased by activating the motorsalong the spanwise direction. However, due to the forcing from a combination ofboth streamwise and spanwise motors, strong interaction with the 3D mode led to areduction in the growth rate of the TS wave in the far-field region despite higher initialperturbation generated by a larger number of motors