Flow dynamics of sweeping jet impingement upon a large convex cylinder

Abstract

Flow dynamics associated with sweeping jets impinging upon a convex cylinder are experimentally examined in a water tank using time-resolved particle image velocimetry (TR-PIV). The cylinder has a large cylinder-jet diameter ratio (D/Dh = 15) and is placed 4 Dh from the jet exit, where Dh is the hydrodynamic diameter of the jet exit. The sweeping jets with two spreading angles as a result of different Reynolds numbers are considered, and a steady impinging jet is examined for comparison. As the baseline case, the steady impinging jet can only cover a small area along the convex surface. At both Reynolds numbers, Re = 3.3 × 103 and Re = 10 × 103, the jet has a strong impact on the convex surface by direct impingement only within a small region between angular angles of θ ≈ ±8° away from the centerline. However, the sweeping jet has a spreading angle of about 30° at the lower Re = 3.3 × 103. As a result, the jet exerts a more even distribution of the impingement in a larger region along the convex surface. Near-wall vortices induced by the impingement are identified to cause early separation of the wall jet flow from the convex surface. The sweeping jet has a much larger spreading angle of about 80° at the higher Re = 10 × 103. As such, the sweeping jet impinges upon the lateral sides of the cylinder at about θ ≈ ±45°, resulting in a weak impingement around the centerline. In addition, the jet column breaks during the sweeping motion: the jet close to the exit switches to one side of the cylinder, while the remaining jet develops on the other side. Proper orthogonal decomposition (POD) analysis reveals that the remaining jet mainly contributes to the impingement at the higher Reynolds number. In general, compared with a steady jet, a sweeping jet can cover a much larger area, even half of a big convex cylinder, with strong impingement.