Measurements and treatment of LDA signals, comparison with hot-wire signals

Abstract

 An experimental investigation was carried out in the wind tunnel F2 of the ONERA Fauga centre for the measurement of the characteristics of a turbulent wake behind a wing. As these measurements require the calculation of time and space correlations, two different types of acquisition means are used : a pair of crossed hot wires and a one-dimensional laser-Doppler anemometer. The non-intrusive characteristics of laser anemometry allow the measurement of fluid velocity upstream of a hot wire probe without disrupting the flow. It is well known that LDA generates individual realisations of randomly sampled velocity data because the random arrival of seeding particles in the measurement volume is nonperiodic. A detailed study of this random sampling quantifies the deviations from the theoretical lows, shows the limiting factors of this sampling, and gives a characterisation of the particles arrival law. The simultaneous acquisition of the two velocity signals at very close points allows a good comparison between the signals. A statistical analysis of the two signals enables us to precisely measure the error value of the velocity estimation made by the anemometer.

The spectrum analysis of the laser signal coupled with one of the hot wire signals requires resampling the signal at constant steps. Two different methods of interpolation are analysed: the sample and hold interpolation method and the linear interpolation method. The influence of these interpolation methods on the spectrum of LDA signals is studied. Different estimators are then calculated to evaluate the convection velocities and the coherence length of the turbulence.