DOUBLE ROW VORTICAL STRUCTURES IN THE NEAR-FIELD REGION OF A PLANE WALL-JET

The qualitative and quantitative behaviour of double row vortical stru ctures in the near held region of a plane wall jet are studied experim entally by flow visualization and hot-wire measurements. Ensemble aver aging is employed to investigate the interaction of vortices with the wall. In the flow visualization study, a double row vortical structure , which includes a primary vortex formed in the outer layer region and a secondary vortex induced in the inner layer region, and the vortex liftoff phenomenon are clearly observed during the development of the wall jet. The phase averaged results of the velocity measurements show that the instability leading to induction of the secondary vortex is stimulated by the primary vortex. In the early stage of wall jet trans ition, the inflection point of the inner layer velocity profile moves transversely from the wall surface to the inner layer region due to pa ssage of the well-organized primary vortex in the outer layer region. The inner layer instability is thus induced and the instability wave r olls up to form the secondary vortex. Furthermore, the secondary vorte x will convect downstream faster than the primary vortex, and this dif ference in convective speed will lead to the subsequent phenomenon of vortex lift-off from the wall surface.