Structure and Dynamics of Bounded Vortex Flows With Different Nozzle Heights

A bounded vortex flow is generated by a nozzle with a central suction outlet surrounded by inlet jets with a 15 deg inclination in the azimuthal direction. The jets impinge on a flat surface called the impingement surface. The circulation introduced by azimuthal tilting of the inlet jets is concentrated at the flow centerline by the suction outlet to form a wall-normal vortex, with axis nominally orthogonal to the impingement surface. An experimental study was conducted in water to examine the structure and dynamics of bounded vortex flows with balanced inlet and outlet flow rates for different values of the separation distance h between the nozzle face and the impingement surface. The experiments used a combination of laser-induced fluorescence (LIF) to visualize the vortex and jet flow structure and particle-image velocimetry (PIV) for quantitative velocity measurements along a planar slice of the flow. Different liquid flow rates were examined for each separation distance. The results show that a stationary wall-normal vortex is formed at small separation distances, such as when the ratio of h to the inlet jet radial position R is set to $h/R=0.67$⁠. When the separation distance is increased such that $h/R=1.3$⁠, the intake vortex first becomes asymmetric, drifting to the one side of the flow, and then bifurcates into a vortex pair that rotates in a V-state around the flow centroid. At large separation distances (e.g., $h/R=6.7$⁠), the intake vortex adopts a spiral structure that is surrounded by the inlet jets, with upward-flowing exterior fluid at the center of the spiral vortex structure. The arms of this spiral are advected downward with time by the inlet jet flow until they reach the impingement surface. Knowledge of this flow structure at different separation distances is necessary in order to design systems that utilize this flow field for enhancement of particle removal rate or heat/mass transfer from a surface.