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.

LES.

Vortex structure

Turbulence

Vortex properties

Vortex dynamics

KB-salen, Kemivägen 4, Göteborg
Opponent: Professor Hugo Atle Jakobsen, NTNU, Trondheim, Norway.

Författare

Farideh Ghasempour

Chalmers, Kemi och kemiteknik, Kemiteknik, Kemisk reaktionsteknik

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Artikel i vetenskaplig tidskrift

Turbulence has structures known as turbulent vortices that can be considered as the muscles of the flow. Common phenomena in chemical engineering processes include coalescence, break-up, mixing and fast chemical reactions, are significantly influenced by turbulence. Usually these phenomena occur very fast, often within a few milliseconds, and modeling them is complex and requires a detailed description of turbulent vortices. One of the main mechanisms behind coalescence, break-up and mixing, is the interaction of fluid particles with single or paired vortices. The interaction timescale is smaller than the lifetime of the turbulent vortices for many engineering applications. Thus, there is no time for fluid particles to interact with more than single vortices, and, consequently, using average turbulence properties to model these phenomena is not valid. Instead, this interaction should be described by means of a detailed description of individual turbulent vortices and their turbulence properties. Such a detailed description of individual turbulent vortices is required to improve the existing models for break-up and coalescence, since these models are valid only for the inertial subrange. Moreover, a detailed description of individual turbulent vortices can be useful to develop new models for the purpose of designing new chemical engineering equipment and for predicting processes with optimal turbulence properties.

Fundament

Grundläggande vetenskaper

Ämneskategorier

Kemiteknik

Strömningsmekanik och akustik

Infrastruktur

C3SE (Chalmers Centre for Computational Science and Engineering)

ISBN

978-91-7597-130-8

Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie

KB-salen, Kemivägen 4, Göteborg

Opponent: Professor Hugo Atle Jakobsen, NTNU, Trondheim, Norway.

Mer information

Skapat

2017-10-07