Simultaneous measurements of temperature and velocity fluctuations in a slightly heated jet combining a cold wire and laser doppler anemometry
Résumé
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.