The Formation and Evolution of Turbulent Swirling Vortex Rings Generated by Axial Swirlers

The present work investigates the formation process and early stage evolution of turbulent swirling vortex rings, by using planar Particle Image Velocimetry (PIV) and Large Eddy Simulation (LES). Vortex rings are produced in a piston-nozzle arrangement with swirl generated by 3D-printed axial swirlers in experiments. Idealised solid-body rotation is applied in LES to evaluate the effect of nozzle exit velocity profile in experiments. The Reynolds number (Re) based on the nozzle diameter D and the slug velocity U0 in the nozzle is 20,000. The swirl number S generated ranges from 0 (zero-swirl vortex ring) and 1.1, covering the two critical swirl numbers previously identified in a swirling jet. Both PIV and LES results show that the formation number F decreases linearly as S increases, with the maximum F ≈ 2.6 at S = 0 (produced by the swirler with straight vanes) and minimum F = 1.9 at S = 1.1. The corresponding maximum attainable circulation in the nozzle axis parallel plane also diminishes with increasing S. Evolution of compact rings produced by a stroke ratio L/D = 1.5 reveals that circulation decay rate is largely proportional to S. The trajectory of the vortex core in the axial direction, hence the ring axial propagation velocity, decreases as S, while that in the radial direction and the radial propagation velocity, increase with S. An empirical scaling function is proposed to scale these variables.

Influences of Rotational Speed Variations on the Flow-Induced Vibrational Performance of a Prototype Reversible Pump Turbine in Spin-No-Load Mode

During the spin-no-load mode, vibrational performance of the reversible pump turbine is an important criterion for the evaluation of the operational performances of the power station. In the present paper, the influences of rotational speed variations on the vibrational performances of the whole unit (including the top cover, the upper, and the lower brackets) are experimentally investigated with discussions of their sources and propagation characteristics. According to the whole vibrational levels and the dominant frequencies of the vibration signals obtained at the top cover, the investigated cases with different rotational speeds could be divided into three partitions with their main characteristics given as follows. In the first partition (with low rotational speeds), the vibrational level is quite limited, and its source is the pressure fluctuation generated by the swirling vortex rope in the draft tube. In the second partition (with medium rotational speeds), the vibrational level gradually increases and its source is the mechanical aspects of the impeller rotation. In the third partition (with high rotational speeds), the vibrational level is prominent with a prominent swirling vortex rope in the draft tube and intensive rotor–stator interactions in the vaneless space (VS). For the vibrations of the upper and the lower brackets, the vibrations mainly originate from the mechanical aspects of the impeller rotation and the amplitudes of the dominant frequency also increase with the increment of the rotational speed. Finally, differences between the vibrational performances of the spin-no-load mode and the generating mode are discussed.

Aerodynamic Interaction Between Incoming Vortex and Tip Leakage Flow in a Turbine Cascade

In turbines, secondary vortices and tip leakage vortices develop and interact with each other. In order to understand the flow physics of vortices interaction, the effects of incoming vortex on the downstream tip leakage flow are investigated in terms of the aerodynamic performance in a turbine cascade. Experimental, numerical and analytical methods are used. In the experiment, a swirl generator was used upstream near the casing to generate the incoming vortex, which interacted with the tip leakage vortex in the turbine cascade. The swirl generator was located at ten different pitchwise locations to simulate the quasi-steady effects. In the numerical study, a Rankine-like vortex was defined at the inlet of the computational domain to simulate the incoming swirling vortex. Incoming vortices with opposite directions were investigated. The vorticity of the positive incoming swirling vortex has a large vector in the same direction as that of the tip leakage vortex. In the case of the positive incoming swirling vortex, the vortex mixes with the tip leakage vortex to form one vortex near the tip as it transports downstream. The vortices interaction reduces the vorticity of the flow near the tip, as well as the loss by making up for the streamwise momentum within the tip leakage vortex core. In contrast, the negative incoming swirling vortex has little effects on the tip leakage vortex and the loss. As the negative incoming swirling vortex transports downstream, it is separated from the tip leakage vortex and forms two vortices. A triple-vortices-interaction kinetic analytical model and one-dimensional mixing model are proposed to explain the mechanism of vortex interaction on the aerodynamic performance.

Interaction of a Vortex Induced by a Rotating Cylinder with a Plane

In this article, we study theoretically and numerically the interaction of a vortex induced by a rotating cylinder with a perpendicular plane. We show the existence of weak solutions to the swirling vortex models by using the Hopf extension method, and by an elegant contradiction argument, respectively. We demonstrate numerically that the model could produce phenomena of swirling vortex including boundary layer pumping and two-celled vortex that are observed in potential line vortex interacting with a plane and in a tornado.