Science Pictures

PIV Measurement of the Flow Downstream of a Helicopter Rotor

copyright © ONERA 1996-2006
PIV (Particle Image Velocimetry) Measurement of the Flow Downstream of a Helicopter Rotor.
The vorticity field (rotation rate) of the flow is traced. Blue and red structures correspond to the wake and vortices produced by the successive movements of the blades. The two big structures in the top right and top left corners of the picture represent the tip vortices resulting from the blade lift.
(A. Le Pape - DAAP, J.-C. Monnier - DAAP et J. Gatard - DSFM)

The animation above shows how vortices evolve with rotor rotation.
(.avi - 550 ko)

The main cause for helicopters' noise pollution is the interactions of the numerous vortices generated by rotor rotation with the blades - interactions which produce for example the characteristic noise of a landing helicopter. Moreover, vortices emitted by the rotor interact with the fuselage, making the helicopter less stable and more difficult to handle. Notice that the fuselage also limits the performances of the rotor by resisting to the evacuation of its wake.

Rotor-fuselage interactions are the most difficult to compute, because they require a full modeling of the entire helicopter. An experimental approach is often necessary, to get a global understanding of the problem and to provide a database to validate the calculation codes.

A model of the Dauphin Helicopter was tested in Onera's F1 wind-tunnel for this purpose.

F1 Windtunnel:
Model and Visualization Devices

To study the velocity field around the helicopter, a nonintrusive optical technique called PIV (Particle Image Velocimetry) is used. The flow field is seeded with very small particles (here, particles of olive oil) and briefly illuminated with a laser sheet. A high-definition CDD camera captures two successive images that are separated by a small increment in time. Comparison of the two images permits to measure the displacement of the particles in the illuminated laser sheet and to compute the flow velocity in this plane.

The very detailed data obtained by the PIV unit of the DAAP department are precious for aerodynmaics studies.

Last Update: November 15, 2004 - © ONERA 2009 - Terms of use