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Home > Large Equipments > Low turbulence wind tunnel

Low turbulence wind tunnel

The Lespinard wind tunnel, named after its founder, is one of the large facilities of LEGI lab. It is used to study turbulent flows (single and two phase), and is used by PhD students, permanent staff and many students from Grenoble Alpes University for experimental projects on the study of turbulence.

The wind tunnel is a closed circuit (see figure). Its overall dimensions are 5 meters high, 16 meters long and 2.5 meters wide. The test section is 4 meters long and has a square section (0.75x0.75 m²). The operation of two motors rotating at the same speed with opposite directions allows both to avoid turbulence on a large scale (laminar case) and the insertion of a grid (turbulent case).

Speeds can reach up to 45 m/s in the test section. The stability of the flow is assured to 0.2%. A plenum equipped with fine grids, followed by a convergent section reduces the residual turbulent intensity to a very low level (<0.1%, for a completely empty test section). The wind tunnel, originally built to investigate the laminar-turbulent transition, is used by several teams for various studies, including:

  • The diffusion of a passive/active scalar in grid turbulence.
  • The measure of vorticity on developed turbulence via acoustic techniques.
  • Preferential concentration of inertial particles on a turbulent flow.
  • The settling velocity of inertial particles on a turbulent flow.
  • Different aspects of fundamental turbulence: grid turbulence, turbulent wakes, boundary layers, etc.

Instruments, sensors and the 3D traverse system

Velocity can be measured through a pitot tube installed in the convergent section using a micromanometer. A hot-wire anemometry system (single and/or multiple wire) can also be used. Several imaging techniques are also possible: high-speed imaging of water droplets on a turbulent flow, Particle Imaging Velocimetry (PIV) and Phase Doppler Interferometry (PDI), among others. The test section of the wind tunnel is protected by a leather cabin that allows us to use the laser safely.

A 3D system displacing the probes within the whole section (hot wire, sonic or cup anemometers...) is installed in the test section. It has recently been calibrated to display the actual position of the probes on all three axes.

The active grid

The active grid is installed upstream, at the beginning of the test section. It is composed by 16 axes (8 horizontal and 8 vertical, see image). Each one of them is connected to a motor and can be controlled independently. A software (developed internally using LabVIEW) allows to control the velocity and direction of each motor. The active grid therefore allows to generate bespoke turbulence (homogeneous isotropic turbulence, boundary layers, shear profiles...).

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The wind tunnel has been used for research and teaching projects since it was built in 1965. It has recently been refurbished (change of the motors in 2013, replacement of the test section in 2015 for a transparent one, addition of a remote-control mode to drive both engines at the same time with an external power supply in 2017). Future developments for this large facility include: speed PID control by using the analog output of the micromanometer, control of the three-dimensional displacement system by a LabVIEW software.