Near-field interaction of an inclined jet with a crossflow : LIF visualization and TR-PIV measurement

Abstract

Experiments are conducted in a water tunnel to investigate the near-field interaction of an inclined jet with a crossflow over a flat plate. The tunnel contains a jet emerging from a round pipe inclined at a 30° angle to the streamwise direction of the crossflow. The flow structures induced by the inclined jet are examined with laser-induced fluorescence (LIF) visualization and time-resolved particle image velocimetry (TR-PIV). The behaviors of the near-field flow are compared at four different jet-to-cross-flow velocity ratios (VRs): 0.25, 0.5, 0.75, and 1.0. It is found that the inclined configuration significantly weakens the interaction between the jet and the crossflow, especially at lower VRs. As such, at VR ≤0.5 (typically at VR = 0.25), the inclined jet-in-crossflow (JICF) behaves differently from a highly unsteady normal JICF at low VRs. The flow patterns are relatively simple and only weakly unsteady. A counter-rotating vortex pair (CRVP) is well observed. As VR increases up to 1.0, the inclined JICF fully detaches from the flat plate and shows the classical topology of the normal JICF at high VRs. Both CRVP and shear layer vortices are well captured in this highly unsteady flow regime. The different flow structures together with the interaction between the inclined jet and the crossflow near the jet exit are found to have a strong impact on the distribution of jet-shear layer, jet trajectory, and the jet influence on the crossflow, especially on the near-wall region. Proper orthogonal decomposition is performed on the TR-PIV results to extract the dominant fluctuating modes and reconstruct phase-averaged flow fields. It is found that the highly unsteady flow regime at VR = 1.0 is very unstable, varying between two flow patterns with different fluctuating frequencies at downstream of the jet column. The jet flow near to the exit is also found to be remarkably unsteady due to the interaction between the emerging jet and the crossflow.