TRANSVERSE VELOCITY INCREMENTS IN TURBULENT-FLOW USING THE RELIEF TECHNIQUE

Non-intrusive measurements of the streamwise velocity in turbulent rou nd jets in air are performed by recording short-time displacements and distorsions of very thin tagging lines written spanwise into the flow . The lines are written by Raman-exciting oxygen molecules and are int errogated by laser-induced electronic fluorescence (RELIEF). This give s access to the spatial structure of transverse velocity increments wi thout recourse to the Taylor hypothesis. The resolution is around 25 m u m, less than twice the Kolmogorov scale eta for the experiments perf ormed (with R-lambda approximate to 360-600). The technique is validat ed by comparison with results obtained from other techniques for longi tudinal or transverse structure functions up to order 8. The agreement is consistent with the estimated errors - a few percent on exponents determined by extended-self-similarity - and indicates significant dep artures from Kolmogorov (1941) scaling. Probability distribution funct ions of transverse velocity increments du over separations down to 1.8 eta are reported for the first time. Violent events, with du comparab le to the r.m.s. turbulent velocity fluctuation, are found to take pla ce with statistically significant probabilities. The shapes of the cor responding lines suggest the effect of intense slender vortex filament s.