Experimental investigation of the effects of background system rotation on the flow dynamics of circular vortex rings and non-circular vortex loops

点击翻译按钮获取中文摘要

For his PhD Mr Marcel Salmon will be conducting an experimental research project in the area of rotating fluid flows. In particular, the project will investigate aspects of the dynamics and the stability of vortex rings subject to background system rotation. Vortices are often referred to as the 'sinews and muscles' of fluid motion. Their dynamics affect the vast majority of all processes in nature and technology that involve liquids and gases in motion. Examples include, for instance, liquid-mixing processes in industrial production processes, vortices encountered in flow around cars or aeroplanes, weather phenomena or ocean circulation of global scale. Background system rotation induces Coriolis forces. These forces are well known to fundamentally alter the dynamics of fluid flows in comparison to the corresponding, non-rotating flow equivalent. Coriolis forces result in many, often counter-intuitive, phenomena absent in non-rotating flows. In the published literature there only exists a very limited number of results on the effects that background system rotation has on the dynamics and the stability of vortex rings in a rotating fluid. From our previous research, we already know that Coriolis forces act destabilizing on vortex rings and that they induce several secondary flow structures. What remains to be established is what exactly causes the destabilization and to investigate how the secondary flow structures depend on the independent system parameters. That is, how the flow depends on the rotational velocity of the system and the generation parameters of the vortex rings. For Mr Salmon's study vortex rings will be generated inside a large water-filled tank (diameter 1m, height 2m) mounted on our unique large-scale rotating-turntable facility. The rings form when water is ejected from a fully computer-controlled vortex-generator, a nozzle-piston arrangement. Mr Salmon will use our rig to conduct and analyse in-depth Particle-Image-Velocimetry (PIV) measurements. PIV is the leading, laser-based technology to perform velocity measurements in fluid flows. The vortex rings will be ejected from the circular vortex generator nozzle which is submerged in the water inside the tank. The nozzle is mounted on the axis of rotation and the vortex rings will be ejected such that they propagate downwards within the tank and along the rotational axis. Marcel will study the flow field within and surrounding the travelling vortex rings. The data will be compared to results for rings in non-rotating fluids. These comparisons will involve data from Marcel's own measurements for non-rotating flow and literature data for rings in non-rotating systems. Benchmark data for vortex rings in non-rotating systems are abundant in the relevant scientific and engineering journals. Depending on the progress of the research the project also aims to begin investigating aspects of the dynamics of non-circular vortex loops, in rotating and non-rotating flow. There exists essentially no research on the dynamics of such non-circular vortex loops. That is so, because until recently, it was very difficult to manufacture complex, non-circular generator nozzles which, for instance, enable one to superpose wavy undulations on the ejected vortex structure. However, the recent advances in additive manufacturing now enable producing such nozzle structures easily. Therefore, producing such nozzles to perform first experiments with them represents a highly attractive new research direction that has the potential to uncover hitherto entirely unknown, fundamental, new flow phenomena.