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
Rλ≈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
Δu over separations down to 1:8η are reported for the first time.
Violent events, with Δu 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.