Thesis--M. Phil., University of Hong Kong.

The tumbling tip vortex effect of a reduced-scale, 1 m diameter, four-bladed rotor during hover is studied using vortex methods, combined with a center of mass analysis approach. Measurements of all three components of the velocity field are acquired using a stereo PIV system synchronized to capture up to 500 degrees of vortex age, with 10 degree wake age offsets, during hover conditions. The nominal operating condition of the rotor is at a rotational rate of 1520RPM, corresponding to ReC = 248,000 with a chord length of 58.5mm. The rotor is operated with a pitch of 7.2± 0.5 degrees and a CT/sigma of 0.045. The far wake vortex tumbling phenomenon is captured and described. It is shown that tip vortices from two blades tumble through approximately 90 degrees of rotation before they coalesce. It is also seen that the constituent parent vortices do not combine to create a stronger daughter vortex as was previously thought to happen. Instead, the merged vortex has a lower large-radius circulation than either of its parent vortices. An accurate characterization and prediction of the trajectory of the far wake vortex tumbling can enhance the ability to predict and alleviate the resuspension of particles during brownout as well as provide a database for far wake validation of CFD codes. / text

published_or_final_version / Mechanical Engineering / Master / Master of Philosophy

Abstract In this thesis, the structure and the dynamics of vortices are studied from the standpoint of the hydrodynamical theory of superfluids. In the hydrodynamical theory a superfluid is described by a continuous order parameter field. In the case of superfluid helium-4 this field is a complex-valued function of position and time. However, in superfluid helium-3 the order parameter is a complex-valued 3 × 3 matrix. The first part of this work consists of studies on structures that appear in the order parameter field, when a vessel filled with superfluid helium-3 in the A phase (3He-A) is rotated in an external magnetic field. Among the most common of these structures are the so-called continuous vortices. They exist in several different forms. In addition to vortices, other possible structures include the vortex sheet of 3He-A that was discovered at the Low Temperature Laboratory of Helsinki University of Technology (currently Aalto University) in late 1993. In this thesis, these structures were studied by finding stationary vortex configurations that minimize the free energy of the superfluid. An algorithm for minimizing the free energy was implemented by writing a computer program. This program was then used to study the structure of a few vortex types, inferred to be the most probable ones. In addition, regular lattices formed by these vortices, including the vortex sheet, were studied. A phase diagram for vortex lattices was constructed by comparing the free energy of various lattice structures as a function of rotational velocity and external magnetic field. The study of vortex structures also lead to a discovery of a new type of vortex in 3He-A, later named the LV3 vortex. In the second part of the work, the dynamics of vortices was studied using a filament model of vortex motion, which also has its theoretical justification in the hydrodynamical model of superfluids, but where the detailed structure of the vortex core is not relevant. The specific problem under consideration here was the motion of a quantized vortex in a rotating elongated cylinder filled with superfluid, and how the motion of the vortex depends on temperature and the rotational velocity of the vessel. The study of vortex motion was simplified using scaling laws. A new type of scaling law was discovered, which both simplified the specific problem under study, and made the results more general. In summary, the research in this thesis touched upon two somewhat complementary areas, i.e. the structure of continuous vortices in 3He-A and the dynamics of thin vortex lines, which is more applicable to superfluid 4He or to the B phase of superfluid helium-3 (3He-B). However, these areas complement each other in advancing the general scientific understanding about the properties of superfluids.

The objective of this research was to investigate parameters affecting vortex ring formation and propagation and their application to mixing of fluids. To this end both empirical and numerical simulation experiments were conducted. The empirical experiments involved observations and measurement of the volume, displacement and velocity of vortex rings generated from a 5 cm diameter tube. The results revealed that there is an optimal range of generation injection velocity for various mixing requirements. The numerical simulations were done using a commercial package, FLUENT. Both tube type and plate orifice type vortex ring generators were investigated. Also the affects of a central shaft and various projections on the control of the motion of a vortex ring. All models considered a polar model cylindrical tank with a diameter to height ratio of 3:10. The average injection velocity was in the range of 0.7 m/s to 3 m/s. When simulating the tube type generator various injection velocity profiles and value were investigated, which resulted in a fitted correlations of nondimension displacement versus non-dimension time as a function of infection profile. In order to control the forward motion of vortex rings some obstructions were considered. It was found that the trajectory and energy of a vortex ring can be controlled with shape and geometries of baffles. In the simulations for the orifice plate type generator, a moving mesh technique was used. As expected a pair of vortex rings were produced per half cycle of the plate oscillation, but they did not travel as fast as expected. Recommendations have been made to improve the simulation accuracy. It has also been found that the FLUENT package will not properly simulate turbulent vortex rings. However, this may be because a vortex ring is not truly homogeneously turbulent. The use of a laminar model appears to give quite good agreement with empirical data for tube type vortex ring generator. The results of this research are expected to be useful for the optimization of the design of vortex ring mixing systems. / Thesis / Master of Engineering (ME)

No description available.

Buoyancy-induced, columnar vortices (dust devils) that are driven by thermal instabilities of ground-heated, stratified air in areas with sufficient insolation convert the potential energy of low-grade heat in the near-surface air layers into a vortex flow with significant kinetic energy. A variant of the naturally-occurring vortex is deliberately triggered and anchored within an azimuthal array of vertical, stator-like flow vanes that form an open-top enclosure and impart tangential momentum to the radially entrained air. The induced flow within the enclosure may be ultimately exploited for power generation by coupling the vortex to a vertical-axis turbine. The fundamental mechanisms associated with the formation, evolution, and dynamics of an anchored, buoyancy-driven columnar vortex that is formed within such an enclosure over a heated ground plane are investigated in laboratory experiments. Specific emphasis is placed on the dependence of the vortex cellular structure and vorticity production and sustainment mechanisms on the thermal resources and the magnitude and direction of the entrained flow that is regulated by the flow vanes. Manipulation of vorticity concentrations and advection are exploited in order to modify and optimize the available mechanical energy within the induced flow field and, therefore, the extractable power. Finally, anchored vortices are formed in the natural environment within a scaled field prototype of the flow enclosure using only insolation as the source of buoyancy. These field tests demonstrated formation and sustainment of energetic columnar vortices that enable potential thermomechanical link for tapping the gravitational potential energy of the unstable air layers for power generation.

This research concentrates on the swirl motion, and in particular the flow structure which develops under its action, in swirl burner/furnace systems. Although the Reynolds numbers for such systems are usually large and well into the turbulent regime, periodic oscillations and associated instabilities are still prevalent. The predominant coherent structure is the so-called precessing vortex core (PVC) which is a three-dimensional, time-dependent phenomenon. It is helical in shape, twisted against the flow, and precesses around the geometric centre of the system, in the sense of the flow. The aim of this work was to numerically model this instability in a 2MW industrial-size system, under isothermal conditions. A fully three-dimensional, time-dependent model was developed using the CFD (Computational Fluid Dynamics) software FLUENT. This study first presents an overview of publications on vortex breakdown, a similar phenomenon observed initially on delta wings, in order to highlight its significant features. A summary was also made of various recent studies, experimental and theoretical, carried out at Cardiff University, in the same equipment as used in the present work. This review allows a better understanding of the phenomenon and constitutes a basis for further validation of the mathematical model. Numerous flow pattern characteristics have- been predicted, which agree qualitatively with different published studies, such as crescent shaped regions of maximum axial and tangential velocities, off-centred reverse flow zone, and spiralling vortex core. Quantitatively, the agreement is good, in terms of range of velocities and frequency. However, the predicted flow pattern could. not be maintained in time and tended back to axisymmetry, possibly due to numerical diffusion. Grid refinement could not, however, be envisaged due to the practical limits of the available machines. Nevertheless, these results are encouraging and prove that mathematical modelling of these complex flows is a realistic objective.

No description available.

published_or_final_version / Physics / Doctoral / Doctor of Philosophy