The “S-Visu” wind tunnel is the only open loop wind tunnel in the department. Unlike closed loop wind tunnels recirculating air, air is here drawn from outside, passes through the test section and is evacuated outside thanks to a downstream centrifugal suction compressor.
This wind tunnel is thus particularly suitable for flow disturbances studies: the disturbances generated in the test section are evacuated without modifying the conditions entering the wind tunnel. The test section is 45 cm wide and 3 m long. The velocity can reach 35 m/s. Without any disturbance, the geometry, and treatment of the duct (filters, honeycomb, and fine grids) guarantee good spatial and temporal homogeneity of the flow which has a turbulence rate close to 0.3%.
Here, oscillating shutters are placed downstream the test section to generate controlled quasi-sinusoidal temporal variations of the flow velocity. The pressure losses associated with the closing of the shutters can generate a sharp decrease as strong as 50% of the streamwise velocity.
Such aerological disturbances can be recorded during a low altitude flight, particularly in an urban environment, for which the presence of buildings accentuates the intensity of apparent wind fluctuations. Thus, a micro-UAV flying at 10 m/s is likely to encounter wind gust of large spatial extension and of similar amplitude to its own forward speed. The properties and effects of atmospheric turbulence on the flight performance of these light, small-sized and low-speed aircraft are therefore very different from what is conventionally known for airliners.
The challenge is to design drones, microdrones, nanodrones robust to wind gust.
In that video, the unsteady aerodynamic response of a wing subjected to a sinusoidal variation of the streamwise velocity is studied. The wing dimensions and speeds studied are representative of a microdrone flight in an urban environment.
The desired gust properties are ensured by appropriate control of the oscillating shutters. The unsteady flow is finely characterized using a hot wire probe fixed at the inlet of the test stream. The selected gust has an average speed of 10m/s with variations of +/- 4m/s over a period of 0.8 s. The overall aerodynamic performances are measured by noting the temporal evolutions of the lift, drag and pitching moment forces for different flight angles of attack.
To analyze the origin of these performances, it is necessary to properly qualify and understand the physics of the flow around the model. Particle Image Velocimetry (PIV) provides instantaneous flow velocity fields. By appropriate post-processing, the dynamics of the flow during the wind gust can be extracted.