A computational model for defect prediction in shape castings based on the interaction of free surface flow, heat transfer, and solidification phenomena

High-integrity castings require sophisticated design and manufacturing procedures to ensure they are essentially macrodefect free. Unfortunately, an important class of such defects—macroporosity, misruns, and pipe shrinkage—are all functions of the interactions of free surface flow, heat transfer, and solidication in complex geometries. Because these defects arise as an interaction of the preceding continuum phenomena, genuinely predictive models of these defects must represent these interactions explicitly. This work describes an attempt to model the formation of macrodefects explicitly as a function of the interacting continuum phenomena in arbitrarily complex three-dimensional geometries. The computational approach exploits a compatible set of finite volume procedures extended to unstructured meshes. The implementation of the model is described together with its testing and a measure of validation. The model demonstrates the potential to predict reliably shrinkage macroporosity, misruns, and pipe shrinkage directly as a result of interactions among free-surface fluid flow, heat transfer, and solidification.

Finite volume methods applied to the computational modelling of welding phenomena

This paper presents the computational modelling of welding phenomena within a versatile numerical framework. The framework embraces models from both the fields of computational fluid dynamics (CFD) and computational solid mechanics (CSM). With regard to the CFD modelling of the weld pool fluid dynamics, heat transfer and phase change, cell-centred finite volume (FV) methods are employed. Additionally, novel vertex-based FV methods are employed with regard to the elasto-plastic deformation associated with the CSM. The FV methods are included within an integrated modelling framework, PHYSICA, which can be readily applied to unstructured meshes. The modelling techniques are validated against a variety of reference solutions.

Modelling the fluid dynamics and coupled phenomena in arc weld pools

A 3D model of melt pool created by a moving arc type heat sources has been developed. The model solves the equations of turbulent fluid flow, heat transfer and electromagnetic field to demonstrate the flow behaviour phase-change in the pool. The coupled effects of buoyancy, capillary (Marangoni) and electromagnetic (Lorentz) forces are included within an unstructured finite volume mesh environment. The movement of the welding arc along the workpiece is accomplished via a moving co-ordinator system. Additionally a method enabling movement of the weld pool surface by fluid convection is presented whereby the mesh in the liquid region is allowed to move through a free surface. The surface grid lines move to restore equilibrium at the end of each computational time step and interior grid points then adjust following the solution of a Laplace equation.

Investigation into the performance of turbulence models for fluid flow and heat transfer phenomena in electronic applications

Computational Fluid Dynamics (CFD) is gradually becoming a powerful and almost essential tool for the design, development and optimization of engineering applications. However the mathematical modelling of the erratic turbulent motion remains the key issue when tackling such flow phenomena. The reliability of CFD analysis depends heavily on the turbulence model employed together with the wall functions implemented. In order to resolve the abrupt changes in the turbulent energy and other parameters situated at near wall regions a particularly fine mesh is necessary which inevitably increases the computer storage and run-time requirements. Turbulence modelling can be considered to be one of the three key elements in CFD. Precise mathematical theories have evolved for the other two key elements, grid generation and algorithm development. The principal objective of turbulence modelling is to enhance computational procedures of efficient accuracy to reproduce the main structures of three dimensional fluid flows. The flow within an electronic system can be characterized as being in a transitional state due to the low velocities and relatively small dimensions encountered. This paper presents simulated CFD results for an investigation into the predictive capability of turbulence models when considering both fluid flow and heat transfer phenomena. Also a new two-layer hybrid kε / kl turbulence model for electronic application areas will be presented which holds the advantages of being cheap in terms of the computational mesh required and is also economical with regards to run-time.

Investigation into the performance of turbulence models for fluid flow and heat transfer phenomena in electronic applications

Heat is extracted away from an electronic package by convection, conduction, and/or radiation. The amount of heat extracted by forced convection using air is highly dependent on the characteristics of the airflow around the package which includes its velocity and direction. Turbulence in the air is also important and is required to be modeled accurately in thermal design codes that use computational fluid dynamics (CFD). During air cooling the flow can be classified as laminar, transitional, or turbulent. In electronics systems, the flow around the packages is usually in the transition region, which lies between laminar and turbulent flow. This requires a low-Reynolds number numerical model to fully capture the impact of turbulence on the fluid flow calculations. This paper provides comparisons between a number of turbulence models with experimental data. These models included the distance from the nearest wall and the local velocity (LVEL), Wolfshtein, Norris and Reynolds, k-ε, k-ω, shear-stress transport (SST), and kε/kl models. Results show that in terms of the fluid flow calculations most of the models capture the difficult wake recirculation region behind the package reasonably well, although for packages whose heights cause a high degree of recirculation behind the package the SST model appears to struggle. The paper also demonstrates the sensitivity of the models to changes in the mesh density; this study is aimed specifically at thermal design engineers as mesh independent simulations are rarely conducted in an industrial environment.

Numerical modelling of electrodeposition phenomena

Electrodeposition is a widely used technique for the fabrication of high aspect ratio microstructure components. In recent years much research has been focused within this area with an aim to understanding the physics behind the filling of high-aspect ratio vias and trenches on PCB's and in particular how they can be made without the formation of voids in the deposited material. This paper describes some of the fundamental work towards the advancement of numerical models that can predict the electrodeposition process and addresses: i) A novel technique for interface motion based on a variation of a donor-acceptor technique ii) A methodology for the investigation of stress profiles in deposits iii) The implementation of acoustic forces to generate replenishing electrolytic flow circulation in recessed features.

The effects of cadence and power output upon physiological and biomechanical responses to incremental arm-crank ergometry

The aim of this study was to examine the effects of cadence and power output on physiological and biomechanical responses to incremental arm-crank ergometry (ACE). Ten male subjects (mean +/- SD age, 30.4 +/-5.4 y; height, 1.78 +/-0.07 m; mass, 86.1 +/-14.2 kg) undertook 3 incremental ACE protocols to determine peak oxygen uptake (VO2 peak; mean of 3 tests: 3.07 +/- 0.17 L.min-1) at randomly assigned cadences of 50, 70, or 90 r.min-1. Heart rate and expired air were continually monitored. Central (RPE-C) and local (RPE-L) ratings of perceived exertion were recorded at volitional exhaustion. Joint angles and trunk rotation were analysed during each exercise stage. During submaximal power outputs of 50, 70, and 90 W, oxygen consumption (VO2) was lowest for 50 r.min-1 and highest for 90 r.min-1 (p < 0.01). VO2 peak was lowest during 50 r.min-1 (2.79 +/-0.45 L.min-1; p < 0.05) when compared with both 70 r.min-1 and 90 r.min-1 (3.16 +/-0.58, 3.24 +/-0.49 L.min-1, respectively; p > 0.05). The difference between RPE-L and RPE-C at volitional exhaustion was greatest during 50 r.min-1 (2.9 +/- 1.6) when compared with 90 r.min-1 (0.9 +/- 1.9, p < 0.05). At VO2 peak, shoulder range of motion (ROM) and trunk rotation were greater for 50 and 70 r.min-1 when compared with 90 r.min-1 (p < 0.05). During submaximal power outputs, shoulder angle and trunk rotation were greatest at 50 r.min-1 when compared with 90 r.min-1 (p < 0.05). VO2 was inversely related to both trunk rotation and shoulder ROM during submaximal power outputs. The results of this study suggest that the greater forces required at lower cadences to produce a given power output resulted in greater joint angles and range of shoulder and trunk movement. Greater isometric contractions for torso stabilization and increased cost of breathing possibly from respiratory-locomotor coupling may have contributed increased oxygen consumption at higher cadences.