The interaction region associated with twin jets and a normal crossflow

An experimental investigation of the interaction between twin jets and a normal crossflow using real-time, quantitative, video image analysis of smoke-seeded jets to yield information concerning overall jet growth rates, the widths of the interface regions, intermittencies, p.d.f.s, and spectra of the interface fluctuations is presented. The cases covered are side-by-side and in-line jets, with a nozzle spacing of 5 nozzle exit diameters and a jet velocity/crossflow velocity ratio of 8, together with the single jet cases. Additional data, in the form of velocity vector distributions from five-hole pressure probe measurements, were obtained for comparison with the intermittency profiles in planes across the jets for the case of side-by-side jets with a nozzle spacing of 5 diameters and a ratio of 6. Finally, a parametric study of the effect on the overall jet penetration of velocity ratio (from 4 to 10), nozzle spacing (from 1 to 5 diameters) and nozzle orientation to the crossflow (from in-line to side-by-side, but with normal injection angle) was carried out using a new digital image processing system. The velocity data for the side-by-side case shows that the deflected jets are dominated by one single vortex pair such that the inner vortex of each jet pair are not evident. The video analysis of the three main configurations, side-by-side, in-line and single jet, shows that in all cases the width of the mixing region at any downstream location is similar in magnitude to the jet half-width (as defined by the mean interface location). The interface widths are similar for the side-by-side and in-line jets but these are greater than the single jet for the intermittency distributions across the interface regions which will enable correlation between the different jet configurations. The parametric study shows that, for a given jet ratio, the in-line jets (0 deg angle) penetrate further than those set at other angles to the crossflow and that changes of angle between 30 deg and 60 deg have less effect upon the overall penetration. For the in-line case it is the second jet, issuing into the lower static pressure region downstream of the first jet, that penetrates furthest into the crossflow.