Laurence Pietri

Simultaneous measurements of temperature and velocity fluctuations in a slightly heated jet combining a cold wire and Laser Doppler Anemometry

Simultaneous measurements of two velocity components and temperature are performed combining Laser Doppler Anemometry (LDA) and cold wire thermometry. LDA is a common technique suitable for velocity measurements in turbulent jets where strong turbulence intensities and reverse flows may exist, but temperature measurements in association with LDA are difficult because the fine wire response is altered by the seeding deposit, so that the wire must be regularly cleaned. Results reported herein concern velocity–temperature correlations, as well as velocity and temperature marginal probability density functions and temperature (or velocity) probability density functions conditioned by the sign of the velocity (or temperature) fluctuation. The evolution of these various quantities is analysed in order to better understand the mixing properties in the near-field of a turbulent jet where the initial conditions still have a strong influence. It is shown that, while the velocity field tends to relax rather quickly (within a few nozzle diameters from the exit) to almost gaussian statistics, the temperature properties are still significantly skewed towards the hot jet exit temperature until x/Dj about 7–8. On the contrary, the signature of the cold ambient temperature vanishes rather quickly.

Estimation of velocity spectra in variable density jets using laser–Doppler anemometry

The present paper is concerned with turbulent jet flows in which the density varies due to exchanges of mass. Such flows have gained some attention over the last few years but, to our knowledge, this is the first time results are reported for the influence of density variations on velocity spectra. Our measurements are obtained by laser–Doppler anemometry and part of the paper is also devoted to testing several procedures for estimating spectral densities from the randomly sampled data. Comparisons between turbulent helium and air jets confirm that the lighter gas jet develops at stations nearer from the exit nozzle and that the turbulent Reynolds number Rλ remains the essential parameter to describe the distribution of turbulent scales in strong variable density jets.