Multiphysics CFD modelling of incompressible flows at Low and Moderate Reynolds Numbers

In this Ph.D. thesis, a novel high-resolution Godunov-type numerical procedure has been developed for solving the unsteady, incompressible Navier-Stokes equations for constant and variable density flows. The proposed FSAC-PP approach encompasses both artificial compressibility (AC) and fractional step (FS) pressure-projection (PP) methods of Chorin [3, 4] in a unified solution concept. To take advantage of different computational strategies, the FS and AC methods have been coupled (FSAC formulation), and further a PP step has been employed at each pseudo-time step. To provide time-accurate solutions, the dual-time stepping procedure is utilized. Taking the advantage of the hyperbolic nature of the inviscid part of the AC formulation, high-resolution characteristics-based (CB) Godunov-type scheme is employed to discretize the non-linear advective fluxes. Highorder of accuracy is achieved by using from first- up to ninth-order interpolation schemes. Time integration is obtained from a fourth-order Runge-Kutta scheme. A non-linear fullmultigrid, full-approximation storage (FMG-FAS) acceleration technique has been further extended to the FSAC-PP solution method to increase the efficiency and decrease the computational cost of the developed method and simulations. Cont/d

Validation of a magneto- and ferro-hydrodynamic model for non-isothermal flows in conjunction with Newtonian and non-Newtonian fluids

This work focuses on the validation of a magnetohydrodynamic (MHD) and ferrohydrodynamic (FHD) model for non-isothermal flows in conjunction with Newtonian and non- Newtonian fluids. The importance of this research field is to gain insight into the interaction of non-linear viscous behaviour of blood flow in the presence of MHD and FHD effects, because its biomedical application such as magneto resonance imaging (MRI) is in the centre of research interest. For incompressible flows coupled with MHD and FHD models, the Lorentz force and a Joule heating term appear due to the MHD effects and the magnetization and magnetocaloric terms appear due to the FHD effects in the non-linear momentum and temperature equations, respectively. Tzirtzilakis and Loukopoulos [1] investigated the effects of MHD and FHD for incompressible non-isothermal flows in conjunction with Newtonian fluids in a small rectangular channel. Their model excluded the non-linear viscous behaviour of blood flows considering blood as a Newtonian biofluid. Tzirakis et al. [2, 3] modelled the effects of MHD and FHD for incompressible isothermal flows in a circular duct and through a stenosis in conjunction with both Newtonian and non-Newtonian fluids, although their approach neglects the non-isothermal magnetocaloric FHD effects. Due to the fact that there is a lack of experimental data available for non-isothermal and non-Newtonian blood flows in the presence of MHD and FHD effects, therefore the objective of this study is to establish adequate validation test cases in order to assess the reliability of the implemented non-isothermal and non-Newtonian MHD-FHD models. The non-isothermal Hartmann flow has been chosen as a benchmark physical problem to study velocity and temperature distributions for Newtonian fluids and non-Newtonian blood flows in a planar microfluidic channel. In addition to this, the numerical behaviour of an incompressible and non-isothermal non-Newtonian blood flow has been investigated from computational aspects when a dipole-like rotational magnetic field generated by infinite conducting wires. The numerical results are compared to available computational data taken from literature

Numerical investigation on various heat exchanger performances to determine an optimum configuration for charge air cooler, oil and water radiators in F1 sidepods

The present work focuses on a three-dimensional CFD approach to predict the performance of various heat exchangers in conjunction with non-isothermal transitional flows for motorsport applications. The objective of this study is to determine the heat transfer, pressure drop and inhomogeneous flow behaviour for distinct heat exchangers to identify an optimum configuration for the charge air cooler, water and oil radiators placed in the sidepods of a formula one (F1) car. Therefore, a comprehensive analysis of various heat exchanger configurations has been carried out in this work. In order to assess the reliability of the obtained results, a mesh sensitivity study along with additional parametric investigations have been performed to provide numerical parameters predicting accurately (a) the heat transfer rate at the fluid-solid interface and (b) the sporadic separation. As a result of the performed validation procedure in this study, the aerodynamic- and thermal boundary layer development along with the convective characteristics of the air flow have been captured accurately near to the heated surface. The characterization of a heat exchanger core and a core configuration in a closed domain is also possible with this procedure. The presented three-dimensional CFD approach could overcome the difficulties of macroscopic heat exchanger and porous media methods for F1 applications, because it can be used to predict the heat transfer and pressure drop related to the mass flow rate correlation curves. The contribution of fins to the total heat transfer rate has been predicted theoretically, and application benchmark test cases have been presented to analyze five different heat exchanger configurations in accordance with the 2014 formula one technical regulations. The numerical data extracted directly from three-dimensional CFD simulations can be used in the sidepod design process of the external cooling system of F1 engines

A unified fractional-step, artificial compressibility and pressure-projection formulation for solving the incompressible Navier-Stokes equations

This paper introduces a unified concept and algorithm for the fractionalstep (FS), artificial compressibility (AC) and pressure-projection (PP) methods for solving the incompressible Navier-Stokes equations. The proposed FSAC-PP approach falls into the group of pseudo-time splitting high-resolution methods incorporating the characteristics-based (CB) Godunov-type treatment of convective terms with PP methods. Due to the fact that the CB Godunov-type methods are applicable directly to the hyperbolic AC formulation and not to the elliptical FS-PP (split) methods, thus the straightforward coupling of CB Godunov-type schemes with PP methods is not possible. Therefore, the proposed FSAC-PP approach unifies the fully-explicit AC and semi-implicit FS-PP methods of Chorin including a PP step in the dual-time stepping procedure to a) overcome the numerical stiffness of the classical AC approach at (very) low and moderate Reynolds numbers, b) incorporate the accuracy and convergence properties of CB Godunov-type schemes with PP methods, and c) further improve the stability and efficiency of the AC method for steady and unsteady flow problems. The FSAC-PP method has also been coupled with a non-linear, full-multigrid and full approximation storage (FMG-FAS) technique to further increase the efficiency of the solution. For validating the proposed FSAC-PP method, computational examples are presented for benchmark problems. The overall results show that the unified FSAC-PP approach is an efficient algorithm for solving incompressible flow problems

Multiphase Eulerian simulations of a sedimentation process in a solid-fluid particle-laden flow

In this paper, modelling details have been investigated for a multiphase settling process in a two-dimensional particle-laden flow. Unsteady simulations have been performed by using an Eulerian-Eulerian multiphase approach. A preliminary mesh sensitivity study showed that the numerical results might become oscillatory when the grid spacing is comparable with the solid particle diameter, which indicates that excessive mesh refinement is undesirable. In these multiphase flows, the interaction between the fluid and solid phases is modelled relying on purely heuristic arguments, which is a major source of uncertainties. Therefore fluid-solid exchange and drag coefficient models have been compared and assessed in terms of their accuracy. Since the ANSYS-FLUENT commercial software package provides only a few of these approaches, the majority of the models have been implemented through User-Defined Functions (UDFs) in C programming language. The results showed that the choice of an exchange model has considerable impact on the solution and the best agreement has been achieved by employing the formulation proposed by Schiller and Naumann [8]. However, only minor differences have been experienced between the distinct drag models for this specific problem due to their similar behaviour over the investigated settling Reynolds number range

Discontinuous Galerkin finite element investigation on the fully-compressible Navier–Stokes equations for microscale shock-channels

Microfluidics is a multidisciplinary area founding applications in several fields such as the aerospace industry. Microelectromechanical systems (MEMS) are mainly adopted for flow control, micropower generation and for life support and environmental control for space applications. Microflows are modeled relying on both a continuum and molecular approach. In this paper, the compressible Navier–Stokes (CNS) equations have been adopted to solve a two-dimensional unsteady flow for a viscous micro shock-channel problem. In microflows context, as for the most gas dynamics applications, the CNS equations are usually discretized in space using finite volume method (FVM). In the present paper, the PDEs are discretized with the nodal discontinuous Galerkin finite element method (DG–FEM) in order to understand how the method performs at microscale level for compressible flows. Validation is performed through a benchmark test problem for microscale applications. The error norms, order of accuracy and computational cost are investigated in a grid refinement study, showing a good agreement and increasing accuracy with reference data as the mesh is refined. The effects of different explicit Runge–Kutta schemes and of different time step sizes have also been studied. We found that the choice of the temporal scheme does not really affect the accuracy of the numerical results

Energetikai és környezetvédelmi rendszerek kísérleti és számítástechnikai modellezésének és a vonatkozó szerkezetek és folyamatok optimálásának összekapcsolása = Connection between experimental and numerical modelling of power engineering and environmental protection system and the optimisation of related processes and structures

A pályázat keretében elkészült több áramlási és hőtani berendezésben lejátszódó energetikai folyamat numerikus modellezése és jelentős részüknél az üzemük, illetve a kialakításuk optimálása. Ezek: 1. Ipari üzemcsarnokok szellőzésének és a szennyeződés épületen kívüli tovaterjedésének modellezése, a szellőzés optimálása 2. Spirális hőcserélők optimális kialakításának elemzése a. Síkbeli spirális hőcserélő modellezése b. Villamos fűtésű spirális hőcserélő modellezése c. Lapos lángú égővel fűtött spirális hőcserélő modellezése és üzemének optimálása 3. Hőkezelő kemencék gázcirkulációjának modellezése és optimálása a. Kísérleti kemence hőtechnikai modellezése b. Villamos fűtésű alagútkemence modellezése c. Hőkezelő csőkemence modellezése és a kemence, illetve a tüzelés optimálása 4. Elszívóhálózatok modellezése, kialakításuk és üzemük optimálása 5. Villamosan fűtött fémszál körüli áramlás modellezése Az optimálások a Dynamic_Q eljárással és genetikus algoritmus felhasználásával készültek. A számítási eredmények validálására irodalmi adatokat, saját üzemi és laboratóriumi méréseinket használtuk fel. A kutatás végeredményeként kialakult egy olyan eljárás, amelynek segítségével a legkülönbözőbb áramlási- és hőtani folyamat FLUENT programrendszerben történő numerikus modellezése és a matematikai optimálás összekapcsolható. A módszert kiterjesztettük saját kódú numerikus eljárás optimálási eljárásba való beépítésére is. | Within this OTKA research project several tasks were carried out. Analysis and numerical modelling of flow and thermal process in equipments were done, and in some cases their shape or the operation efficiency were optimised too. These works were the following: 1. Numerical modelling and optimisation of natural ventilation of industrial workshops, 2. Numerical modelling of spread and dispersion of air-pollution outside a workshop, 3. Analysis of spiral heat-exchanger for optimal shape (a 2D spiral heat-exchanger, a 3D spiral heat-exchanger with electric heat, and one heated with a flat-flamed burner). 4. Modelling and optimising the gas circulation of heat-treating furnaces (an experimental furnace, an electrically heated tunnel furnace, a heat-treating pipe-furnace). 5. Modelling air-sucking networks, optimization of their shape and operation. 6. Flow modelling around an electrically heated wire. The optimisation tasks were carried out by using the Dynamic-Q and the Genetic Algorithm separately. For validating the computations own experimental (laboratory) and operational (on site) measurements were used as well as data of technical literature. As the result of the research a new method have been developed, by which the numerical simulation (modelling) of any flow and thermal process with the FLUENT software package can be coupled with mathematical optimisation. The method was extended also for coupling the optimization algorithm with home-made numerical code

On a novel approximate solution to the inhomogeneous Euler–Bernoulli equation with an application to aeroelastics

This paper focuses on the development of an explicit finite difference numerical method for approximating the solution of the inhomogeneous fourth-order Euler–Bernoulli beam bending equation with velocity-dependent damping and second moment of area, mass and elastic modulus distribution varying with distance along the beam. We verify the method by comparing its predictions with an exact analytical solution of the homogeneous equation, we use the generalised Richardson extrapolation to show that the method is grid convergent and we extend the application of the Lax–Richtmyer stability criteria to higher-order schemes to ensure that it is numerically stable. Finally, we present three sets of computational experiments. The first set simulates the behaviour of the un-loaded beam and is validated against the analytic solution. The second set simulates the time-dependent dynamic behaviour of a damped beam of varying stiffness and mass distributions under arbitrary externally applied loading in an aeroelastic analysis setting by approximating the inhomogeneous equation using the finite difference method derived here. We compare the third set of simulations of the steady-state deflection with the results of static beam bending experiments conducted at Cranfield University. Overall, we developed an accurate, stable and convergent numerical framework for solving the inhomogeneous Euler–Bernoulli equation over a wide range of boundary conditions. Aircraft manufacturers are starting to consider configurations with increased wing aspect ratios and reduced structural weight which lead to more slender and flexible designs. Aeroelastic analysis now plays a central role in the design process. Efficient computational tools for the prediction of the deformation of wings under external loads are in demand and this has motivated the work carried out in this paper

Equilibrium molecular dynamics modeling of diffussion and adsorption of fluids in armchair single walled carbon-nanotube

The aim of this paper is to study adsorption and diffusion of gases and liquids especially Argon and Carbon-dioxide in single walled carbonnanotube at room temeperature using equilibrium molecular dynamic simulation. The simulation domain is developed by the large atomic/molecular massively parallel simulator (LAMMPS). The domain consists of a simulation box of volume 100 x 100 x 100 A having periodic boundary conditions at the x. y and z direction.The adsorption and diffusion of different chiral- ity of carbonnanotubes are reported. The Molecular Dynamics Simulation (MD) result shows that single walled carbonnanotube have affinity to attract carbon dioxide to itself than argon, with argon acting as a catalyst for adsorption of more C02 confirming a high adsorption at higher loading. The highest adsorption and diffussion inside the Single-walled carbon-naotube (SWCNT) was determined at certain loading and temperature. The SWCNT is as-sumed to be rigid due to the fact that, flexibility is insignificant and can increase computational time. This study will bring about a better understanding of storage and filtering of gases in SWCNTs and so leading its usefullness in applications such as gasification for jet engines, Co2 removal in the international space station, desalination for water systems, air purification, longer space batteries and enhanced oil recovery

A generalised and low-dissipative multi-directional characteristics-based scheme with inclusion of the local Riemann problem investigating incompressible flows without free-surfaces,

In the present study, we develop a generalised Godunov-type multi-directional characteristics-based (MCB) scheme which is applicable to any hyperbolic system for modelling incompressible flows. We further extend the MCB scheme to include the solution of the local Riemann problem which leads to a hybrid mathematical treatment of the system of equations. We employ the proposed scheme to hyperbolic-type incompressible flow solvers and apply it to the Artificial Compressibility (AC) and Fractional-Step, Artificial Compressibility with Pressure Projection (FSAC-PP) method. In this work, we show that the MCB scheme may improve the accuracy and convergence properties over the classical single-directional characteristics-based (SCB) and non-characteristic treatments. The inclusion of a Riemann solver in conjunction with the MCB scheme is capable of reducing the number of iterations up to a factor of 4.7 times compared to a solution when a Riemann solver is not included. Furthermore, we found that both the AC and FSAC-PP method showed similar levels of accuracy while the FSAC-PP method converged up to 5.8 times faster than the AC method for steady state flows. Independent of the characteristics- and Riemann solver-based treatment of all primitive variables, we found that the FSAC-PP method is 7–200 times faster than the AC method per pseudo-time step for unsteady flows. We investigate low- and high-Reynolds number problems for well-established validation benchmark test cases focusing on a flow inside of a lid driven cavity, evolution of the Taylor–Green vortex and forced separated flow over a backward-facing step. In addition to this, comparisons between a central difference scheme with artificial dissipation and a low-dissipative interpolation scheme have been performed. The results show that the latter approach may not provide enough numerical dissipation to develop the flow at high-Reynolds numbers. We found that the inclusion of a Riemann solver is able to overcome this shortcoming. Overall, the proposed generalised Godunov-type MCB scheme provides an accurate numerical treatment with improved convergence properties for hyperbolic-type incompressible flow solvers