Study on three-dimensional flow structures of a sweeping jet using time-resolved stereo particle image velocimetry

Abstract

The three-dimensional flow structures that were induced by a sweeping jet emitted from a sweeping jet actuator to an open space were examined experimentally using stereo and two-dimensional PIV (particle image velocimetry) measurements. A new method was proposed to obtain phase-averaged flow field based on proper orthogonal decomposition, which is more robust to local flow fluctuations than existing methods. The jet Reynolds number (Re) was fixed to 8000 and the jet exit had a square shape with a hydraulic diameter (Dh) of 10 mm. The three-dimensional time-averaged flow fields of the sweeping jet were first reconstructed using multiple two-dimensional flow fields along various planes. In the time-averaged flow field, the jet flow distribution generally showed a V-shape pattern, which consisted of two inclined jet columns with an angle of approximately 90°. As the jet traveled away from the jet exit, the shape of jet column also changed from an elliptical shape to a circular shape because of the confinement effect of the jet exit. Stereo-PIV measurements were then conducted at two cross-sectional planes with distances of 3Dh and 8Dh away from the jet exit. In the plane 3Dh from the jet exit, one pair of counter-rotating vortices formed during the switching period when the jet swept from one side to the other. This is because the jet switching motion can enhance the jet shear layer, as in the case of a jet in crossflow. On the other hand, on the plane 8Dh away from the jet exit, the counter-rotating vortices dissipated quickly owing to the strong turbulence and weak switching motion.