Experimental data obtained in various turbulent flows are analysed by means of orthogonal wavelet transforms. Several configurations are analysed: homogeneous grid turbulence at low and very low Reλ, and fully developed jet turbulence at moderate and high Reλ. It is shown by means of the wavelet decomposition in combination with the form of scaling named extended self-similarity that some statistical properties of fully developed turbulence may be extended to low-Reλ flows. Indeed, universal properties related to intermittency are observed down to Reλ≃10. Furthermore, the use of a new conditional averaging technique of velocity signals, based on the wavelet transform, permits the identification of the time signatures of coherent structures which may or may not be responsible for intermittency depending on the scale of the structure itself. It is shown that in grid turbulence, intermittency at the smallest scales is related to structures with small characteristic size and with a shape that may be related to the passage of vortex tubes. In jet turbulence, the longitudinal velocity component reveals that intermittency may be induced by structures with a size of the order of the integral length. This effect is interpreted as the signature of the characteristic jet mixing layer structures. The structures identified on the transverse velocity component of the jet case turn out on the other hand not to be affected by the mixing layer and the corresponding shape is again correlated with the signature of vortex tubes.
The statistical properties of the velocity fluctuation components at moderate Re are studied in homogeneous and non-homogeneous turbulence by an experimental technique. Measurements are performed by means of a hot-wire anemometer with X-probe downstream of a screen for several positions, x/M ϭ9 up to 109, where M is the screen mesh size, with Re ranging from 37 to 82. The homogeneity of the flow is analyzed by means of velocity measurements in different transverse positions and a direct evaluation of the local isotropy of the flow is performed by means of velocity spectra. The scaling properties of the statistical moments of the structure functions up to the order of six, are investigated in the extreme positions by means of the extended self-similarity ͑ESS͒ method and the intermittency exponents are detected for both homogeneous and non-homogeneous flow conditions. A comparison of the longitudinal and transverse intermittency exponents as functions of the position is then performed and discussed in addition to the analysis of the transition from the anomalous to a regular scaling for small spatial separation.
Flow visualizations and phase-averaged particle image velocimetry (PIV) measurements of a jet in crossflow configuration at very low Reynolds numbers (Rej ≃ 100) are performed in a water tunnel for jet-to-cross-stream velocity ratios R ranging from 1.5 to 4.5. The PIV vector fields and flow visualizations, carried out by injecting methylene blue dye and by the laser induced fluorescence (LIF) technique, are analysed to characterize the effect of R on the formation and evolution of large-scale vortices. It is shown that two distinct flow regimes are established depending on R, with R ≃ 3 being a transitional value. At low R, the longitudinal vorticity dynamics is dominated by the so-called wake-like structures which are shown to be strictly connected to the streamwise counter-rotating vortices (CRVP) which drive the destabilization of the jet flow. On the other hand, at large R, vortices with positive and negative vorticity are coupled together. The establishment of these different behaviours is interpreted physically as an effect of the jet Reynolds number which plays an essential role on the destabilization mechanisms which lead to the formation of the jet shear-layer structures. In any case, the onset of instability is driven by mechanisms which are different from those characteristic of free jets.
Experimental data obtained in the near region of a turbulent jet flow (from x/D=0 up to x/D=19, where D is the jet diameter and x is the axial coordinate) are processed through wavelet decomposition. A wavelet based technique for coherent structures identification is applied to the longitudinal and transverse velocity data series obtained by X probe hot wire anemometry measurements. This methodology permits the evolution with x of the time signature of coherent structures to be analyzed. The statistics of the local turbulent energy magnitude is also discussed and the temporal dynamics is studied by the analysis of the time of appearance of the most energetic structures evolving with x/D.
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