Experimental investigation into the turbulence flow field of in-flight jets

In this paper, the velocity field of jets under flight condition is investigated. The aim is to provide insight into the modelling of single-point statistics (e.g. mean velocity, turbulence intensity and other central moments) and two-point statistics (e.g. cross-correlation and coherence coefficients) of subsonic jets discharged into a moving medium. An experimental campaign was performed in a recently built facility comprising an open jet wind tunnel and a jet rig. Hot-wire anemometry was used to measure the velocity fluctuations in the jet turbulence field. A moderate subsonic jet (nozzle exit Mach number equals to 0.6) was discharged into an ambient medium of speeds ranging from 0 to 100 m/s. Experimental data shows that the stretching of the jet with increasing flight-stream velocity can be calculated from the mean velocity decay downstream of the end of the jet potential core. The stretching factor is used to predict the changes in the jet turbulence statistics with the flight-stream speed. Additionally, in the region of high turbulence kinetic energy, the two-point statistics can be recovered accurately from information of the single-point statistics and the local mean velocity. Empirical models for the shear-stress, cross-correlation, and coherence functions are presented and compared to static jet data. Finally, the changes due to forward flight seen in the jet turbulence agrees well with experimental data in the near field and far field of model-scale in-flight jets. This result suggests open jet wind tunnels are capable of reproducing the full-scale problem, that is, the noise from the exhausted jet of a moving engine/nozzle.