TURBULENCE MEASUREMENTS IN AXISYMMETRICAL JETS OF AIR AND HELIUM .1. AIR-JET

A turbulent round jet of air discharging into quiescent air was studied experimentally. Some x-wire hot-wire probes mounted on a moving shuttle were used to eliminate rectification errors due to flow reversals in the intermittent region of the jet. Moments of velocity fluctuations up to fourth order were measured to characterize turbulent transport in the jet and to evaluate current models for triple moments that occur in the Reynolds stress equations. Fourth moments were verv well described in terms of second moments by the quasi-Gaussian approximation across the entire jet including the intermittent region. Profiles of third moments were found to be significantly different from earlier measurements: [uv2], [uw2] and [u2v] are found to be negative near the axis of the jet. The Basic triple moment model that included turbulent production and models for the dissipation and the return-to-isotropy part of the pressure correlations was found to be unsatisfactory. When mean-strain production and a model for rapid pressure correlations were also included, predictions were satisfactory in the fully turbulent region. The consistency of the measurements with the equations of motion was assessed: momentum flux across the jet was found to be within +/- 5% of the nozzle input and the integral of radial diffusive flux of turbulent kinetic energy across the jet calculated from the measured third moments was found to be close to zero.