ON THE EFFECTS OF VELOCITY PROFILES ON THE TOPOLOGICAL STRUCTURE OF A JET IN CROSS FLOW

The effect of jet exit velocity profile on the topological structure of a round jet in a cross flow is investigated experimentally using the laser induced fluorescence technique. The study is focused on two velocity profiles, namely top-hat and parabolic profiles. The jet Reynolds number considered ranges from 350 to 1750, with the corresponding velocity ratios varying from 1 to 5. The results show that the top-hat jets are more unstable and likely to shed shear layer vortices than parabolic jets. Interestingly, it is found that the parabolic jets penetrate higher into the flow, but the largescale structures appear to be less coherent than those of the top-hat jets. These findings suggest that the characteristics of a jet in a cross flow is not only a function of the Reynolds number and the velocity ratio, but also a function of the shear layer thickness and hence velocity profile. INTRODUCTION The study of jets in cross flows (JICF) has immense relevance to engineering applications such as film cooling for turbine and combustors, fuel injection for burners, thrust reversers for propulsive systems as well as in the development of S/VTOL aircrafts. Its relevance in areas involving the dispersion of effluents in waterways and of pollutants in the atmosphere via chimneys and smoke stacks has long been recognized by the research community. Early research on JICF was confined mainly to determining the mean paths of the deflected jets (Jordinson, 1956 and Margason, 1968). Later, the focus was shifted to the measurement of the axial velocity decay and turbulence intensity along the jet axis by Keffer & Baines (1963), Pratte & Baines (1967) and Andreopoulos (1982 and 1985). Their results show that fluid entrainment and hence the mixing process, is substantially more intensive for a JICF than for a Figure 1 : Schematics of a jet in cross flow phenomenon as illustrated by Kelso et al (1996). free jet. This resulted in a surge of research focusing on better mixing of fluids. Recent studies see a rise in the use of computational simulations to predict both the fluid entrainment and the deflected jet trajectory as well as to verify earlier experimental work (Yuan & Street, 1996 and 1998). The numerical results of Yuan & Street looked promising and hold great potential for further investigations. A more detailed review of the subject over the last fifty years can be found in Margason (1993), Fric & Roshko (1994) and Kelso et al (1996). Earlier experimental work on JCIF has revealed a complex system of interacting vortical structures resulting from the interaction between a transverse round jet and a cross flow boundary layer (Figure 1). The main flow features can be summarised as follows: Flat wall Horseshoe vortex system Shear layer Counter-rotating vortex pair