Experimental investigation of three-dimensional flows induced by vortex ring interactions with oblique boundaries

Vortex rings impinging on normal surfaces have been widely studied experimentally. Though experimental studies have examined vortex ring interactions with normal boundaries and oblique boundaries at a few specific angles, an experimental study of vortex ring interaction with an inclined surface at a range of angles has not been undertaken to date. This oblique interaction introduces asymmetry in the flow, which is of interest in the study of boundary layer separation and secondary vorticity generation at boundaries. Regardless of how the primary vortex is formed, an understanding of this secondary vorticity and separation has potential application to the area of flow control, particularly for variations of synthetic jets. In addition, convective heat transfer involving impinging jets, including oblique and pulsed flows, may benefit from a greater understanding of these flow characteristics.;Finally, a few studies have suggested a correlation between coherent structures in turbulent boundary layers and vortex rings impinging on inclined surfaces. Turbulent boundary layers have been widely studied due to the applicability of turbulent flows in engineering and related fields, and coherent structures in these boundary layers are of particular interest because of their role in generating and sustaining turbulence. Once again, the limited number of studies related to vortex ring interactions with oblique boundaries have not attempted to determine the particular angles of collision that lead to similarities with coherent structures in turbulent boundary layers.;Here vortex ring interactions with oblique boundaries were studied experimentally to determine the effects of plate angle on the evolution of the primary vorticity, secondary vorticity generation, and the relationship of these features to turbulent and vortical flows near boundaries. Vortex rings were generated using a mechanical piston-cylinder vortex ring generator at jet Reynolds numbers 4000, 3000, and 2000 and stroke length to piston diameter ratios of 2.00, 1.00, and 0.75. The plate angle relative to the path of the vortex ring ranged from 3 to 60 degrees. Flow analysis was performed using planar laser induced fluorescence (PLIF), digital particle image velocimetry (DPIV), and defocusing digital particle tracking velocimetry (DDPTV).;To enhance three-dimensional analysis, a radial basis function (RBF) algorithm was developed to interpolate unstructured velocity data obtained using DDPTV onto a structured grid. To test the accuracy of this method and determine appropriate processing parameters, a numerical study was conducted using data generated from a known analytical solution, to which the interpolation was compared. The beneficial properties of RBF interpolation, namely reduced noise in the interpolated velocity field and suppression of noise amplification in gradients when compared to differencing methods, is illustrated by applying vortex identification criteria to an experimentally generated unbounded vortex ring.;Experimental results for a vortex ring impinging on an inclined plate showed the generation of secondary vorticity at the plate and its subsequent ejection into the fluid. The trajectories of the centers of circulation were plotted, and a comparison among collision angles showed increased complexity and three-dimensionality for vortex rings impinging at low angles. Higher Reynolds number vortex rings results in more rapid destabilization of the flow. The use of DDPTV enhanced the two-dimensional experimental techniques, showing an arc of secondary vorticity and secondary flow along the sides of the vortex ring as it collides with the boundary. Computation of the moments and products of kinetic energy and vorticity magnitude about the centroid of each show increasing asymmetry in the flow as it evolves.;Analysis of RBF interpolation results showed that high-resolution particle velocity data along with a sufficient number of basis functions improves the interpolation over present methods. Additionally, implementing additional constraints, especially for the velocity gradient at the boundary, improve interpolation results. Finally, the application of RBF interpolation to vortex identification for a vortex ring showed a significant reduction in noise in the derivatives of the interpolated velocity over present methods.