Velocity turbulence properties in the near-field region of axisymmetric variable density jets

This study concerns turbulent jets with density variations. The local statistical properties, obtained for the velocity field and the mean mass fraction, are needed both for better understanding of the density effects and for modeling of such flows. Special attention is paid to the near-field region (x/D-j less than or equal to 20), where there is a lack of experimental results. A vertical axisymmetric turbulent jet emitted from a fully developed pipe flow, weakly confined in an air coflow, is investigated. Apart from the constant density case (air/air), two situations with variable density are treated with a density ratio varying from 0.14 [He/air] to 1.5 [CO2/air], with the momentum flux M(j) maintained constant. Even though there is no exact similarity for jets when density varies, some pseudosimilarity laws can be established, and they are experimentally well confirmed. It is also found that the values of the correlations [u(2)v], [uv(2)], and [uw(2)] are negative in the central region (r/L(u) <0.5), and this is valid for the three gases considered here. On the other hand, the use of the results for calculating balances of some of the flow conservation equations (i.e., continuity, momentum, and kinetic energy) allows us to confirm some measurements and particularly to estimate some terms that are not, and cannot be, measured at present in variable density flows, such as the dissipation rate of turbulent kinetic energy. (C) 1996 American Institute of Physics.