J. Lefevre

Laser Velocimeter for Wind Tunnel Measurements

Laser velocimetry is an optical technique used to make local measurements of mean velocity and turbulence in fluid flows without disturbing them. Laser velocimetry principles are briefly recalled; the optical and mechanical elements making up a velocimeter are described. Their purpose is to create two laser beams of equal intensity which are focused and crossed in a probe volume ¿ where a fringe pattern is formed; the light scattered by submicron particles passing through ¿ is collected by an optics rigidly connected to the emitting optics and focused on a photomultiplier. Among the various signal processing techniques, counters are now widely used, in connection with minicomputers. The different means used to determine the velocity sign and to measure simultaneously two components of the velocity are described; the simultaneous measurement of the three velocity components is more a matter of technology and cost than a theoretical problem. The modular operational velocimeter developed at ONERA is described; as examples of application, some typical results obtained in different aerodynamic flows are reported.

Turbulence induced by an acoustic field. Application to acoustic agglomeration

The efficiency of sonic coagulation and precipitation of particles in an acoustic field is strongly reinforced, above the acoustic intensity corresponding to the enhancement of acoustic turbulence. The aim of this work is to evaluate the turbulence properties of the experimental system in order to calculate the acoustic agglomeration kernels and precipitation rates. The knowledge of the aerodynamic behavior of the fluid allows verification of the presence of turbulence in the system. Turbulence spectra have been measured in order to provide for the evolution of turbulence; the largest structures degenerate into small eddies that are the sites of sonic coagulation. Finally, we evaluate the energy dissipated through heat in the system, which is a parameter representative of turbulence.