Transverse velocity increments in turbulent flow using the <scp>RELIEF</scp> technique

Abstract

Non-intrusive measurements of the streamwise velocity in turbulent round 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 interrogated by laser-induced electronic fluorescence (relief). This gives access to the spatial structure of transverse velocity increments without recourse to the Taylor hypothesis. The resolution is around 25 μm, less than twice the Kolmogorov scale η for the experiments performed (with R λ ≈360–600). The technique is validated by comparison with results obtained from other techniques for longitudinal 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 departures from Kolmogorov (1941) scaling. Probability distribution functions of transverse velocity increments Δ u over separations down to 1:8η are reported for the first time. Violent events, with Δ u comparable to the r.m.s. turbulent velocity fluctuation, are found to take place with statistically significant probabilities. The shapes of the corresponding lines suggest the effect of intense slender vortex filaments.