3D acoustic Lagrangian velocimetry

摘要

We report Lagrangian measurements obtained with an acoustic Doppler velocimetry technique. From the Doppler frequency shift of acoustic waves scattered by tracer particles in a turbulent flow, we are able to measure the full three-component velocity of the particles. As a first application, we have studied velocity statistics of Lagrangian tracers in a turbulent air jet at R_λ ～ 320 and at various distances from the nozzle. The choice of an air jet is motivated by the fact that jets produce a well characterized high level turbulence and open air flows are well suited to simultaneously achieve classical hot wire Eulerian measurements. Therefore, we are also able to explicitly address the question of the differences between Eulerian and Lagrangian statistics. As Lagrangian tracers we use soap bubbles inflated with Helium which are neutrally buoyant in air and can be assimilated to fluid particles. Velocity statistics are analyzed. We show that the Lagrangian autocorrelation decays faster in time than its Eulerian counterpart.