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

An experimental investigation was carried out in the wind tunnel FZ of the ONERA Fauga centre for the measurement of the characteristics of a turbulen t 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 anemomete r. The non-intrusive characteristics of laser anemometry allow the measurem ent of fluid velocity upstream of a hot wire probe without disrupting the f low. It is well known that LDA generates individual realisations of randoml y sampled velocity data because the random arrival of seeding particles in the measurement volume is nonperiodic, A detailed study of this random samp ling quantifies the deviations from the theoretical lows, shows the limitin g factors of this sampling, and gives a characterisation of the particles a rrival law. The simultaneous acquisition of the two velocity signals at ver y 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 met hods of interpolation are analysed: the sample and hold interpolation metho d and the linear interpolation method. The influence of these interpolation methods on the spectrum of LDA signals is studied. Different estimators ar e then calculated to evaluate the convection velocities and the coherence l ength of the turbulence.