Structures, Properties and Dynamics of Turbulent Vortices

Doktorsavhandling, 2015

The development of models for several phenomena that occur in turbulent single- and multi-phase flows requires improved descriptions and quantifications of turbulent vortices. In many engineering applications, the time scale of these phenomena is equal to or smaller than the lifetime of turbulent vortices; consequently, they are not adequately described by using average turbulence properties. The above mentioned phenomena are better described by the properties of single turbulent vortices, e.g. number density, size, enstrophy, energy, lifetime and vortex dynamics. In this thesis, a vortex-tracking algorithm that meets the thesis objectives was successfully developed. Using the Biot-Savart law and morphological methods, the vortex-tracking algorithm captures most of the coherent turbulent structure in individual vortices with clear separated boundaries. The novel vortex-tracking algorithm increases the total energy captured within individual vortices from 27% to 82%. The vortex-tracking algorithm works efficiently and fast. It allows for the identification of thousands of vortices individually, while different properties attributed to them can be quantified. Additionally, a new model for the number density of turbulent vortices in the entire energy spectrum was developed. This model significantly improves the prediction of the turbulent vortices number density. Moreover, it was observed that the number densities of turbulent vortices, modeled and quantified, vary at different radial locations, e.g. where the highest number density is found in the near-wall region and the lowest number density is found in the bulk of the flow. In addition to this, the average size distributions of turbulent vortices show that the sizes of vortices increase from the near-wall region to the bulk of the flow. It was concluded that the associated enstrophy and energy within turbulent vortices of the same size was log-normal distributed. The research in this thesis also examines the lifetimes of vortices. It was found that the lifetime of turbulent vortices depends on vortex size, energy and position. Also, it was concluded that the lifetime of turbulent vortices can be reasonably estimated base on their sizes and positions. Moreover, the birth frequencies of turbulent vortices were also studied.