László Könözsy

Columnar to Equiaxed Transition During Ingot Casting Using Ternary Alloy Composition

The present paper deals with the formation of macrosegregation in a benchmark ingot using (Fe-C-Cr) ternary alloy composition. The numerical investigation of complex multiphase phenomena is a difficult study, because the thermophysical properties depend strongly on the temperature, concentration, pressure and chemical composition as well. For the numerical modeling of solidification and melting processes different phases (e.g. liquid, equiaxed crystals and columnar dendrite trunks) have been considered. The mass, momentum, energy conservation and species conservation equations for each phase have been solved. The Eulerian-Eulerian model equations have been implemented in the commercial Finite Volume Method based FLUENT-ANSYS v6.3 CFD software using User-Defined Functions (UDF). The mass transfer has been modelled by diffusion controlled crystal growth by applying an advanced tip tracking algorithm for columnar solidification. The modeling of the grain density transport has been improved. The derivatives of the mass fraction quantities for each component appear in the nucleation rate term. It means that we obtain a new term of the right hand side of the grain density transport equation for using ternary alloy composition. This paper focuses on both the process and simulation parameters and their influence on the macrosegregation formation. The results show that the macrosegregation pattern does not change significantly above a well-chosen number of grid cells, and the computational time could be decreased, when the time step size has been increased.

Numerical investigation of grid influence on formation of macrosegregation

Abstract The investigation of grid influence on numerical prediction of the formation of macrosegregation is an important issue in the point of view of numerical modelling. The estimation of numerical accuracy for the simulation of complex multiphase phenomena is a difficult modelling process, since the thermophysical properties depend on the temperature and concentration as well. The numerical stability and accuracy of the modelling also depend on the chosen time step and grid size. This paper focuses on the grid influence and modelling questions on macrosegregation in a benchmark ingot using Fe-0·34 wt-%C steel. The FLUENT-ANSYS v6·3 commercial software does not have built-in multiphase solidification and melting module for simulating columnar to equiaxed transition. Therefore, a multiphase model was implemented using User-Defined Functions. The number of grid cells has been increased from 180 to 4300 to define an optimal grid size, to prove the reliability of the model implementation based on solution accuracy. The results show, the macrosegregation pattern does not change significantly above a well-chosen number of grid cells.

Numerical and Experimental Investigation of NH4Cl Solidification

This paper deals with the validation of a volume averaged multiphase solidification model based on a benchmark experiment using NH4Cl-H2O as model alloy and Particle Image Velocimetry (PIV) as optical measurement method. For the numerical modelling of the solidification, different phases (e.g. liquid, equiaxed grains and columnar dendrite trunks) have been considered. The mass, momentum, energy conservation and species transport equations for each phase have been solved. The Eulerian-Eulerian model equations have been implemented in the commercial Finite Volume Method based software FLUENT-ANSYS v6.3 using User-Defined Functions (UDF). The mass transfer has been modelled by diffusion controlled crystal growth. The simulation of the NH4Cl-H2O solidification has been numerically investigated as a two-dimensional unsteady process in the cross-section of a 100 x 80 x 10 (mm3) experimental benchmark. Since during the experiment both columnar and equiaxed growth of NH4Cl have been observed, both phenomena have been considered in the simulation. The predicted distribution of the solidification front and the measured thickness of the mushy zone have been compared with the measurements.

Experimental and numerical investigations of NH4Cl solidification in a mould Part 2: numerical results

Abstract This paper deals with the validation of a multiphase solidification model based on a benchmark experiment presented in Part 1. For the numerical modelling of NH4Cl–H2O solidification, the three different phases liquid, columnar dendrite trunks and equiaxed grains have been considered. The mass, momentum, energy conservation and species transport equations for each phase have been solved. The multiphase Eulerian-Eulerian model equations have been implemented in the Finite Volume Method based commercial software FLUENT-ANSYS using User-Defined Functions (UDF). The simulation of the NH4Cl–H2O solidification has been numerically investigated as a twodimensional unsteady process representing a cross-section of a 100 × 80 × 10 mm experimental benchmark. During the experiment both columnar and equiaxed growth of NH4Cl were observed, therefore both phenomena were considered in the simulation. The predicted distribution of the solidification front has been compared with the measurements.

Airbox Design, Analysis and Improvement for a High Performance Road Racing Sidecar

It has been well documented that the performance of an engine can be adjusted through altering the design of the air intake system. A full four stroke Otto cycle is an unsteady cycle with the continuous charging and discharging of the intake system. The characteristics of airflow within the inlet system can be described as a function of frequency. This frequency can be altered to aid in increasing the volumetric efficiency (VE) of the engine at a desired point/s. This paper reviews the air intake system used by Dave Molyneux Racing (DMR) for their sidecar racing in the Isle of Man Tourist Trophy (TT). An iterative design process found that the airbox design needed the intake pipe to be as straight as possible and facing the free stream flow. Turning vanes were used in the intake pipe and airbox to aid in ensuring uniform pressure across the four bell mouths of the intake system.

On Approximate Riemann Solvers within the Concept of the Unified Fractional-Step, Artificial Compressibility and Pressure Projection Method

This study focuses on the investigation of various Godunov-type treatments of the convective fluxes of Navier–Stokes equations by employing a Fractional-Step, Artificial Compressibility and Pressure-Projection (FSAC-PP) formulation. The FSAC-PP approach unifies Chorin’s fully-explicit Artificial Compressibility (AC) and semi-implicit Fractional-Step Pressure-Projection (FS-PP) methods for solving the incompressible flow problems. In this work, we study various Riemann solvers by using the FSAC-PP formulation with respect to convergence behaviour and numerical solution accuracies. Furthermore, two benchmark test problems have been investigated as laminar flows in a channel between two parallel flat plates and in a lid-driven cavity. Simulations have been performed at different moderate Reynolds numbers (Re = 100, Re = 400, and Re = 1000) in which cases reference data available in the literature. Numerical solutions have been considered for Rusanov, HLL and HLLC Riemann solvers compared to the case when the Riemann problem is excluded from the numerical procedure. The MUSCL scheme has been employed with a third-order spatial approximation, and fifthand ninthorder WENO interpolation schemes have also been considered. The computational results have been shown to be very accurate compared to previous studies, and when any Riemann solver is excluded from the numerical procedure.

Performance Evaluation of a Two-Dimensional Lattice Boltzmann Solver Using CUDA and PGAS UPC Based Parallelisation

The Unified Parallel C (UPC) language from the Partitioned Global Address Space (PGAS) family unifies the advantages of shared and local memory spaces and offers a relatively straightforward code parallelisation with the Central Processing Unit (CPU). In contrast, the Computer Unified Device Architecture (CUDA) development kit gives a tool to make use of the Graphics Processing Unit (GPU). We provide a detailed comparison between these novel techniques through the parallelisation of a two-dimensional lattice Boltzmann method based fluid flow solver. Our comparison between the CUDA and UPC parallelisation takes into account the required conceptual effort, the performance gain, and the limitations of the approaches from the application oriented developers’ point of view. We demonstrated that UPC led to competitive efficiency with the local memory implementation. However, the performance of the shared memory code fell behind our expectations, and we concluded that the investigated UPC compilers could not efficiently treat the shared memory space. The CUDA implementation proved to be more complex compared to the UPC approach mainly because of the complicated memory structure of the graphics card which also makes GPUs suitable for the parallelisation of the lattice Boltzmann method.

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 [2].