The mathematical model and numerical simulations of the solidification of a cylindrical shaped casting, which take into account the process of filling the mould cavity by liquid metal and feeding the casting through the riser during its solidification, are presented in the paper. Mutual dependence of thermal and flow phenomena were taken into account because have an essential influence on solidification process. The effect of the riser shape on the effectiveness of feeding of the solidifying casting was determined. In order to obtain the casting without shrinkage defects, an appropriate selection of riser shape was made, which is important for foundry practice. Numerical calculations of the solidification process of system consisting of the casting and the conical or cylindrical riser were carried out. The velocity fields have been obtained from the solution of momentum equations and continuity equation, while temperature fields from solving the equation of heat conductivity containing the convection term. Changes in thermo-physical parameters as a function of temperature were considered. The finite element method (FEM) was used to solve the problem.
Pouring of liquid aluminium is typically accompanied by disturbance of the free surface. During these disturbances, the free surface oxide films can be entrained in the bulk of liquid, also pockets of air can be accidentally trapped in this oxide films. The resultant scattering of porosity in castings seems nearly always to originate from the pockets of entrained air in oxide films. Latest version of ProCast software allows to identify the amount of oxides formed at the free surface and where they are most likely to end-up in casts. During a filling calculation, ProCast can calculate different indicators which allow to better quantify the filling pattern. The fluid front tracking indicator “ Free surface time exposure” has the units [cm2*s]. At each point of the free surface, the free surface area is multiplied by the time. This value is cumulated with the value of the previous timestep. In addition, this value is transported with the free surface and with the fluid flow.Experiments to validate this new functions were executed.
The cohesion and internal friction angle were characterized as quadratic functions of strain and were assumed to follow the Mohr-Coulomb criterion after the yield of peak strength. These mechanical parameters and their variations in post-peak softening stage can be exactly ascertained through the simultaneous solution based on the data points of stress-strain curves of triaxial compression tests. Taking the influence of the fault into account, the variation of strata pressure and roadway convergence with coal advancement, the temporal and spatial distribution of axial bolt load were numerically simulated by FLAC3D (Fast Lagrangian Analysis of Continua) using the ascertained post-peak mechanical parameters according to the cohesion weakening and friction strengthening model. The change mechanism of axial load of single rock bolt as abutment pressure changes was analyzed, through the comparison analysis with the results of axial bolt load by field measurements at a coal mine face. The research results show that the simulated results such as the period of main roof weighting, temporal and spatial distribution of axial bolt load are in accordance with field measurement results, so the validity of the numerical model is testified. In front of the working face, the front abutment pressure increases first and then decreases, finally tends to be stable. A corresponding correlation exists between the variation of axial bolt load and rock deformation along the bolt body. When encountered by a fault, the maximum abutment pressure, the influential range of mining disturbance and the roadway convergence between roof and floor before the working face are all increased. In the roadways along the gob, axial bolt loads on the side of the working face decrease, while the other side one increases after the collapse of the roof. As superficial surrounding rock mass is damaged, the anchoring force of rock bolts will transfer to inner rock mass for balancing the tensile load of the bolts.
The numerical investigation of the mixing process in complex geometry micromixers, as a function of various inlet conditions and various micromixer vibrations, was performed. The examined devices were two-dimensional (2D) and three-dimensional (3D) types of serpentine micromixers with two inlets. Entering fluids were perturbed with a wide range of the frequency (0 - 50 Hz) of pulsations. Additionally, mixing fluids also entered in the same or opposite phase of pulsations. The performed numerical calculations were 3D to capture the proximity of all the walls, which has a substantial influence on microchannel flow. The geometry of the 3D type serpentine micromixer corresponded to the physically existing device, characterised by excellent mixing properties but also a challenging production process (Malecha et al., 2009). It was shown that low-frequency perturbations could improve the average mixing efficiency of the 2D micromixer by only about 2% and additionally led to a disadvantageously non-uniform mixture quality in time. It was also shown that high-frequency mixing could level these fluctuations and more significantly improve the mixing quality. In the second part of the paper a faster and simplified method of evaluation of mixing quality was introduced. This method was based on calculating the length of the contact interface between mixing fluids. It was used to evaluate the 2D type serpentine micromixer performance under various types of vibrations and under a wide range of vibration frequencies.
The object of investigation was the one-strand tundish with flow control device such as gas permeable barrier (GPB). The aim of this flow control device was to activate the motion of liquid steel in the tundish longitudinal axis region. Computer simulation of the liquid steel flow and argon behaviour in isothermal turbulent motion conditions was done using the Ansys-Fluent computer program. For the validation of the hydrodynamic patterns obtained from computer simulations, a isothermal tundish glass model was used. Tundish glass model enables the recording of the visualization of fluid medium motion through the particle image velocimetry (PIV) method. Based on computer simulations, the liquid steel flow path lines in the tundish with GPB was obtained. For explain the hydrodynamic phenomena occurring in the tundish working space, the Buoyancy number has been calculated.
This paper presents a numerical investigation of fracture criterion influence on perforation of high-strength 30PM steel plates subjected to 7.6251 mm Armour Piercing (AP) projectile. An evaluation of four ductile fracture models is performed to identify the most suitable fracture criterion. Included in the paper is the Modified Johnson-Cook (MJC) constitutive model coupled separately with one of these fracture criteria: the MJC fracture model, the Cockcroft-Latham (CL), the maximum shear stress and the constant failure strain models. A 3D explicit Lagrangian algorithm that includes both elements and particles, is used in this study to automatically convert distorted elements into meshless particles during the course of the computation. Numerical simulations are examined by comparing with the experimental results. The MJC fracture model formulated in the space of the stress triaxiality and the equivalent plastic strain to fracture were found capable of predicting the realistic fracture patterns and at the same time the correct projectile residual velocities. However, this study has shown that CL one parameter fracture criterion where only one simple material test is required for calibration is found to give good results as the MJC failure criterion. The maximum shear stress fracture criterion fails to capture the shear plugging failure and material fracture properties cannot be fully characterized with the constant fracture strain.
The current numerical study focuses on the feasibility of furnishing thermal comfort in a structure, by using paraffin wax stored on a plate below the ceiling in a multi-storey building. The method is aimed to reduce energy demands at the increasing thermal loads. In summer, in daytime, walls of the building are exposed to the ambient thermal load, and heat transferred inside is absorbed by the melting wax. The study is numerical. It relates to temperature variations outside and inside, coupled with heat conduction and accumulation in walls, with radiation between the surfaces, with natural convection of air inside and melting of the wax at the ceiling. Fins spacing on the storage plate, visualization of the melting process, and its parametric investigation provide an insight into the physical phenomena. Temperature and flow fields were investigated for 3 mm and 12 mm thick layers of wax. At the specified conditions of the present study a 3 mm layer provides thermal comfort for most of the day, while a 6 mm layer may suffice for the entire day. Fluent 6.3 software was used in the computations.
In contemporary high-pressure die casting foundries, the mastery of each sequence in the production cycle is more and more important. In the paper, an example of virtual analysis of gearbox casting from Al alloy will be presented. It includes a large variety of parameters, as follows: choosing of appropriate foundry technology, calculation of computer simulation of casting process which takes into account the filling process of cold chamber and filling of cavity, model description of three phases in high-pressure die casting, flow of molten metal, solidification, formation of stress and deformations. Additionally, the optimization of cooling and heating systems will be compared with calculated volume defects, dimensions of castings and their deformations with experimentally obtained values.
The paper discusses the influence of the initial parameters on the strength parameters of S235JR steel at low stress triaxiality. The analysis was performed using the Gurson-Tvergaard-Needlem (GTN) material model, which takes into consideration the material structure. The initial material porosity was defined as the void volume fraction f₀. The fully dense material without pores was assumed and the typical and maximum values of porosity were considered for S235JR steel in order to analyse the porosity effect. The strength analysis of S235JR steel was performed basing on the force-elongation curves obtained experimentally and during numerical simulations. Taking into consideration the results obtained, the average values of the initial void volume fraction f₀ = 0.001 for S235JR steel is recommended to use in a common engineering calculations for elements operating at low stress triaxiality. In order to obtain more conservative results, the maximum values of f₀ = 0.0024 may be used.