Large Eddy Simulation Of The Vortex Rings Interaction With Solid Boundaries

The investigation of vortex rings interaction with solid boundaries is of great im-portance in fundamentals and applications. In this dissertation, large-eddy simulation is used to study three typical flows including the vortex rings impacting on a flat wall, the vortex rings impinging on a bump, and vortex rings interacting with a circular cylinder. The results and conclusions are briefly given as follows:(1) Large-eddy simulation is used to study the normal and oblique collisions of a vortex ring with a flat wall. For the normal collision, we systematically analyze the effects of vortex core thickness on the evolution of vortical structures, the generations of loop-like and hair-pin vortices, the instabilities of primary and secondary vortex rings and the behaviors of turbulent flow. For the thin vortex ring, the secondary ring moves around the primary ring after it is generated from the wall. For the thick vortex ring, the secondary ring moves far away from the wall. This is mainly due to the stronger strength of the secondary ring for the thick ring, resulting in a larger self-induced velocity of the secondary ring compared with the induced velocity by the primary ring. In the late stage of collision, the strong vortex-vortex and vortex-wall interactions lead to the vortical structures breakdown into small scale vortices. The formations of loop-like vortices wrapping the primary and secondary vortex rings and the hair-pin vortices in the flow are investigated. The generation of the loop-like vortices is associated with the rotation and shedding of vorticity from the primary ring. Then the disconnection and deformation of the loop-like vortices are related to the generation of small-scale hair-pin vortices. Moreover, the generation of large-scale hair-pin vortices is associated with the stretching and deformation of the tertiary ring caused by the azimuthal instability. The dominant modes for the thin and thick vortex rings are identified and are consistent with the prediction of stability analysis. The instability for thick vortex ring grows more slowly than one for thin vortex ring. Turbulent flow state is examined by the-5/3decay law of perturbation kinetic energy spectra. Turbulent behaviors are analyzed based on the properties of the flow field. It is found that the turbulent kinetic energy grows rapidly during the transition process; the normal stresses contribute more than the shear stresses to the turbulence production. For the vortex ring and wall oblique collision, we mainly analyze the effect of initial angle on the evolution of vortical structures. It is found that the part of secondary vortex ring interacts with the wall leads to the incomplete secondary vortical structure when the initial angle is large.(2) Large-eddy simulation is used to study the normal and oblique collisions of a vortex ring with a bump. For the normal collision, the effects of bump height and vortex core thickness for thin and thick vortex rings on the vortical structures, vortex instability and the behaviors of turbulent flow are investigated. For the thin vortex ring, the secondary ring moves around the primary ring after it separates from the wall. Then the strong interaction happens between the second ring and the loop-like vortices which wrap both the primary and secondary rings. As the height of the bump increases, the stretching and deformation of secondary vortex ring induce a series of hair-pin vortices. After these vortices interact with the bump, they begin to break down. For the thick vortex ring, the interaction between the loop-like vortices and the secondary vortex ring becomes weak when the bump height increases, leading to the secondary ring moving far away from the wall for larger bump height. Based on the analysis of the boundary vorticity flux, it is found that the vorticity generation on the bump surface becomes weaker with the increase of the bump height. The instability of vortical structures is investigated. The thin vortex ring is more unstable than the thick ring. Further, the analysis of turbulent kinetic energy reveals the transition from laminar to turbulent state. The similar evolution manner between the strength of loop-like and hair-pin vortices and the turbulent kinetic energy indicates that the two typical vortices play an important role in flow transition from laminar to turbulent state. For the oblique collision, we investigate the effect of distance between symmetry axes of the ring and bump on the vortical structures. When the distance is larger, secondary vortex ring nearly disappears. This is mainly due to the breakdown of induced vortical structures at one side and weak vorticity induced by the primary vortex ring at the other side.(3) The interaction of a vortex ring with a circular cylinder in three dimensions is studied using large eddy simulation. We mainly investigate the complex phenom-ena and the underlying physical mechanisms after the vortex ring collides with the cylinder, such as the generation of secondary and tertiary rings; the forma-tions of hair-pin and wrapping vortices; the impact of wrapping vortices on the primary vortex ring; the force on the cylinder; the instability and breakdown of the vortical structures. Based on the evolution processes of the secondary vor-tex, three typical stages are identified. Firstly, as the vortex ring approaches the cylinder, a secondary vortex is formed and moves up from the wall; then its part moves inward to the primary vortex ring and begins to collide with the cylinder. Secondly, the strong interaction leads to the disconnection of the second ring, part of the second ring begins to wrap around the primary ring and two loop-like vortices are formed on two sides of the cylinder. Then the two loop-like vortices move oppositely due to their self-induced effect. Finally, the two vortices come into collision with each other and the breakdown of vortical structures occurs and leads to the transition of flow from laminar to turbulent regime. The total kinetic energy and enstrophy are analyzed to reveal their properties relevant to the three evolution phases. The generation of hair-pin vortices is due to the deformation of the tertiary vortex caused by the instability and the continuous stretching of the vortex induced by other vortical structures in the flow. The impact of loop-like vortices on the primary vortex can be divided into three phases:deformation of vortex, rotation of the inner core which leads to shedding of outer part of the vortex and breakdown of primary vortex into small-scale vortices.