Browsing by Author "Chinyoka, Tirivanhu"

Now showing 1 - 6 of 6
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    Open Access
    Computational aeroacoustic modelling using hybrid RANS/LES methods with modified acoustic analogies
    (2017) Nyandeni, Zamashobane; Chinyoka, Tirivanhu
    This study considers a numerical approach to identifying noise mechanisms in tandem cylinders to understand aircraft landing gear as a primary contributor to airframe noise during approach and landing. Fluctuations in the flow properties induced by turbulence are computed as well as the corresponding propagations. A hybrid IDDES turbulence model is employed, to compute the boundary layer and fluctuations in the flow properties. Larsson et al. modified Curle's analogy leading to the derivation of a version of Curle's analogy that makes use of strictly time derivatives which has been proven to be less sensitive to numerical errors. Brentner and Farassat derived a formulation of the Ffowcs-Williams and Hawkings analogy for a permeable surface enclosing the acoustic sources which accounts for the quadrupole acoustic sources in the flow without the costly calculation of a volume integral. This study will consider the impact of neglecting the volume sources through a comparison of the two modified versions of Curle's and FWH analogies with the results of other CFD practitioners as well as experimental data.
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    Computational analysis of non-isothermal flow of non-Newtonian fluids
    (2015) Ireka, Ikenna Ebubechukwu; Chinyoka, Tirivanhu
    The dynamics of complex fluids under various conditions is a model problem in bio-fluidics and in process industries. We investigate a class of such fluids and flows under conditions of heat and/or mass transfer. Experiments have shown that under certain flow conditions, some complex fluids (e.g. worm-like micellar solutions and some polymeric fluids) exhibit flow instabilities such as the emergence of regions of different shear rates (shear bands) within the flow field. It has also been observed that the reacting mixture in reaction injection molding of polymeric foams undergoes self-expansion with evolution of heat due to exothermic chemical reaction. These experimental observations form the foundation of this thesis. We explore the heat and mass transfer effects in various relevant flow problems of complex fluids. In each case, we construct adequate mathematical models capable of describing the experimentally observed flow phenomena. The mathematical models are inherently intractable to analytical treatment, being nonlinear coupled systems of time dependent partial differential equations. We therefore develop computational solutions for the model problems. Depending on geometrical or mathematical complexity, finite difference or finite volume methods will be adopted. We present the results from our numerical simulations via graphical illustrations and validate them (qualitatively) against' similar' results in the literature; the quotes being necessary in keeping in mind the novelties introduced in our investigations which are otherwise absent in the existing literature. In the case where experimental data is available, we validate our numerical simulations against such experimental results.
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    Computational analysis of viscoelastic fluid dynamics with applications to heat exchangers
    (2019) Mavi, Anele; Chinyoka, Tirivanhu
    In this study, the computational analysis of a pressure driven viscoelastic fluid in a double pipe heat exchanger set-up is investigated. Non-Newtonian viscoelastic fluids in heat exchanger arrangements are encountered in various industrial applications such as power generation, refrigeration and in the food processing industry where the need for cooling and heating of liquids is required. The model problem is governed by complex, non-linear and coupled partial differential equations. These are solved using the semi-implicit finite difference method integrated with the Crank-Nicolson scheme. The pressure-velocity coupling in the momentum equations is resolved by employing the Semi-Implicit Method for Pressure Linked Equations (SIMPLE). To cope with numerical diffusion and numerical stability issues the treatment of convective terms using the upwind schemes is explored. In this work, the behaviour of viscoelastic fluids is rigorously examined by analysing the convective heat transfer from the viscoelastic core fluid of the double pipe heat exchanger to the Newtonian or viscoelastic shell fluid in the outer annulus. In addition, the effects of pressure, momentum, extra stresses, temperature, viscosity and relaxation time on the fluid temperature are investigated; both in the counter flow and parallel flow configurations. Graphical computational results are presented and discussed quantitatively and qualitatively with respect to several parameters involved in the problem.
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    Development of an efficient finite volume computational platform for the simulation of complex flows of complex fluids governed by the Rolie-Poly model
    (2019) Abuga, Jade Gesare; Chinyoka, Tirivanhu
    The flow of non-Newtonian fluids, also known as complex fluids, is an important area of fluid mechanics due to their applications in industries such as food processing, mining, chemical and plastic industries. It is therefore essential to understand the flow and general behaviour of such fluids. Examples of these include polymeric liquids (both solutions and melts), immiscible polymer blends, emulsions, suspensions of rigid and deformable particles (such as biological cells, capsules or lipid vesicles) and colloidal dispersions. Polymer melts are a category of fluids known as viscoelastic fluids and it is only recently that different forms of constitutive equations derived as simplified versions of molecular reptation-based models have been developed to describe molten polymers. The most adept mathematical models being the tube-based models which are derived from the Doi-Edwards tube-based model. These constitutive equations are able to replicate experimental data is some fairly complex flows but are not able to do so in some cases, shear-banding phenomena being one of them. Under certain flow conditions, some complex fluids have been shown to exhibit different shear bands in the flow field due to flow-induced material non-homogeneities. It is becoming increasingly clear that non-homogeneities should not be ignored in polymers or other complex fluids since flow-induced nonhomogeneities may, in some instances, be as important as the complex rheology in differentiating the flow behaviour of Newtonian and complex fluids. Thus, with the use of technology to simulate fluid flows, there has been an increase in the research towards viscoelastic models and shear banding. This has also led to an increase in the development of CFD tools to solve such flows. One of the CFD tools is the OpenFOAM CFD viscoelastic solver that has already been developed. Therefore, the development of an efficient finite volume computation platform for complex flows governed by the Rolie-Poly constitutive equation has been presented in this thesis. The test cases of the lid-driven cavity flow and the planar 4:1 contraction flow were used to validate the solver which was used with the OpenFOAM CFD package. Discrete Elastic Viscous Stress Splitting (DEVSS) technique and the Log-conformation Reformulation (LCR) methodology of Fattal and Kupferman were employed to stabilise the numerical algorithm at high Weissenberg number. For the 4:1 planar contraction flow, it is observed that the numerical results using the LCR stabilazation technique are in good agreement for a range of Weissenberg numbers whereas the DEVSS method shows good agreement for low Weissenberg numbers. The numerical results for the lid-driven cavity flow are in good agreement with the existing literature for low Weissenberg numbers for both stabilization techniques. In the course of this thesis however, capacity to deal with Rolie-Poly constitutive equation was added to the rheoTool which is a tool box for simulation viscoelastic fluid flows in OpenFOAM. A subsequent comparison of the numerical results with those from the rheoTooL solver show good agreement. Furthermore, this thesis uses a two-fluid model by coupling the stress to concentration equation to study the shear banding phenomena in Rolie-Poly fluid flow. Validation is done by comparing existing literature of shear bands using the Giesekus and Johnson-Segalman constitutive equations. The numerical results show good agreement with existing literature for the DEVSS stabilization technique.
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    Open Access
    Modelling and computational analysis of heat-transfer in multi-geometry and multi-phase flow of Newtonian and non-Newtonian fluids in pipes and channels
    (2023) Mavi, Anele; Chinyoka, Tirivanhu
    The thesis develops and computationally analyzes mathematical models for multi-geometry heat exchanger design and for multi-phase flow problems involving boiling and bubble formation. Newtonian fluids, Newtonian-Fluid-Based- Nanofluids, and non-Newtonian fluids are all considered in the thesis. The Newtonian-Fluid-Based Nanofluids (NFBN) are designed from the homogeneous mixing of a Newtonian base-fluid with solid nano-particles. Water will be used as the Newtonian base-liquid and two types of nano-particles will be considered, aluminium oxide (Al2O3) and titanium oxide (TiO2) nano particles. The non-Newtonian fluids in this thesis are modelled via the Giesekus viscoelastic constitutive equations. For the multi-geometry heat-exchanger problems, the thesis will focus attention on counterflow heat exchangers in cylindrical geometries, specifically counterflow, double-cylinder heat-exchangers. The problem statement in this direction focuses on the non-isothermal dynamics and heat-transfer characteristics for a counterflow, double-cylinder heat-exchanger design with a viscoelastic fluid flowing in the core (inner) cylinder and a Newtonian fluid flowing (in the opposite direction) in the shell (outer annulus) region. Investigations are extended to investigate the effects of using Newtonian-Fluid-Based Nanofluids (NFBN) instead of ordinary newtonian fluids in the outer annulus. The Giesekus viscoelastic constitutive model is used to model and describe the rheological behaviour of the viscoelastic core-fluid. The numerical algorithms for these problems are based on the Finite Volume Methods (FVM) implemented on the OpenFOAM software. The numerical instabilities due to the High Weissenburg Number Problem (HWNP) are resolved by employing either the Discrete Elastic Viscous Stress Splitting (DEVSS) or the Log Conformation Reformulation (LCR) techniques. The pressure-velocity coupling is resolved via the Pressure Implicit with Splitting of Operator (PISO) approach. The results illustrate that the use of NFBN as the coolant fluid leads to enhanced cooling of the hot core-fluid as compared to using an ordinary (nano-particle free) Newtonian coolant. Specifically, the results illustrate that an increase in the nano-particle volume-fraction, in the coolant shell fluid, leads to enhanced heat exchange characteristics from the hot core-fluid to the coolant shell-fluid. The multi-phase flow investigations focus on the simulation of three-phase (solid-liquid-gas) boiling flow and bubble formation problems in rectangular channels. The numerical algorithms are also based on the Finite Volume Methods (FVM) implemented on the OpenFOAM software. The numerical algorithms additionally implement both the volume-of-fluid (VOF) methods for liquid-gas interface tracking as well as the volume-fraction methods to account for the concentration of embedded solid nano-particles in the liquid phase. Water is used as the base-liquid and the solid phase is modelled via metallic nano particles, both aluminium oxide (Al2O3) and titanium oxide (TiO2) nano-particles are considered. The (aluminium oxide or titanium oxide) nano-particles are homogeneously mixed within the water base liquid. The gas phase is considered as a vapour arising from the boiling processes of the liquid-phase. In addition to the FVM and VOF numerical methodologies for the discretization of the governing equations, the pressure-velocity coupling is resolved via the PIMPLE algorithm, a combination of the Pressure Implicit with Splitting of Operator (PISO) and the Semi-Implicit Method for Pressure-Linked Equations (SIMPLE) algorithms. The simulations and results accurately capture the formation of vapour bubbles in the two-phase (particle-free) liquid-gas flow and additionally the computational algorithms are similarly demonstrated to accurately illustrate and capture simulated boiling processes. The presence of the nano-particles is again demonstrated to enhance the heat-transfer, boiling, and bubble formation processes.
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    Simulation of blood flows in a stenosed and bifurcating artery using finite volume methods and OpenFOAM
    (2022) Nagarathnam, Sunitha; Chinyoka, Tirivanhu
    Numerical simulations of the complex flows of complex (viscoelastic) fluids are investigated. The primary fluid investigated in this thesis is human blood, a complex fluid which can be modelled via viscoelastic constitutive models. The most commonly used constitutive models for viscoelastic fluids include the OldroydB, Giesekus, Johnson-Segalman, Finitely Extensible Non-Linear Elastic (FENE), Phan-Thein-Tanner (PTT) models etc. Our Numerical approach is based on the finite volume methods implemented on the OpenFOAM platform. We employ the Giesekus, Oldroyd-B, and Generalized Oldroyd-B viscoelastic constitutive models in this thesis, depending on the underlying context. Numerical validation of our results is conducted via the most used benchmark flow problems for viscoelastic fluid flow. The robust and efficient numerical methodologies are then deployed to investigate the flow characteristics, and hence illustrate various novel behavior, for blood flow in stenosed and bifurcated arteries. The present work took advantage of the availability of a reasonable set of viscoelastic constitutive model solvers within OpenFOAM, specifically the viscoelasticFluidFoam solver which we modified and developed to suit our focused needs for blood flow computations. The modified computational algorithms were successfully validated against well-known benchmark flow problems in the literature. Noting that the Giesekus viscoelastic constitutive model is a generalization of both the Oldroyd-B and Generalized Oldroyd-B models, the validation of results is carried out via the Giesekus model enabling us to develop a general-purpose code capable of simulating several viscoelastic constitutive models. The main results were otherwise presented for the Oldroyd-B and Generalized Oldroyd-B models as these are the most applicable to blood flow modelling. The results demonstrate that the velocity spurt through the stenosis is directly proportional to the constriction caused by the stenosis. The higher the blockage from the constriction, the higher the corresponding velocity spurt through the constriction. This velocity behavior, as the constriction blockage increases, correspondingly increase the wall shear stresses. High wall shear stresses significantly increase the possibility of rupture of the stenosis/blockage. This can lead to catastrophic consequences in the usual case where the stenosis is caused by tumor growth.