Computational Parametric Study Research Articles

Cross-shear implementation in sliding-distance-coupled finite element analysis of wear in metal-on-polyethylene total joint arthroplasty: Intervertebral total disc replacement as an illustrative application

Computational simulations of wear of orthopaedic total joint replacement implants have proven to valuably complement laboratory physical simulators, for pre-clinical estimation of abrasive/adhesive wear propensity. This class of numerical formulations has primarily involved implementation of the Archard/Lancaster relationship, with local wear computed as the product of (finite element) contact stress, sliding speed, and a bearing-couple-dependent wear factor. The present study introduces an augmentation, whereby the influence of interface cross-shearing motion transverse to the prevailing molecular orientation of the polyethylene articular surface is taken into account in assigning the instantaneous local wear factor. The formulation augment is implemented within a widely utilized commercial finite element software environment (ABAQUS). Using a contemporary metal-on-polyethylene total disc replacement (ProDisc-L) as an illustrative implant, physically validated computational results are presented to document the role of cross-shearing effects in alternative laboratory consensus testing protocols. Going forward, this formulation permits systematically accounting for cross-shear effects in parametric computational wear studies of metal-on-polyethylene joint replacements, heretofore a substantial limitation of such analyses.

Effects of structural and material length scales on stress-induced martensite macro-domain patterns in tube configurations

This paper studies the effects of structural and material length scales on the equilibrium domain patterns in thin-walled long tube configurations during stress-induced elastic phase transition under displacement-controlled quasi-static isothermal stretching. A nonconvex and nonlocal continuum model is developed and implemented into a finite element code to simulate the domain formation and evolution during the phase transition. The morphology and evolution of the macro-domains in different tube geometries are investigated by both analytical (energy analysis) and numerical (nonlocal finite element) methods. Energy minimization is used as the principle to explain the experimentally observed macroscopic domain patterns in a NiTi polycrystal tube. It is found that the domain pattern, as the minimizer of the system energy, is governed by the relative values of the material length scale g, tube-wall thickness h and tube radius R through two nondimensional factors: h/ R and g/ R. Physically, h/ R and g/ R serve as the weighting factors of bending energy and domain-wall energy over the membrane energy in the minimization of the total energy of the tube system. Theoretical predictions of the effects of these length scales on the domain pattern are quantified and confirmed by the computational parametric study. They all agree qualitatively well with the available experimental observations.

Micron-Level Actuators for Thermal Management of Microelectronic Devices

The failure rate of electronic equipment depends on the operating temperature. Although demand for more effective cooling of electronic devices has increased in the last decades because of the microminiaturization in device sizes accompanied by higher power dissipation levels, there is still a challenge for engineers to attain improved reliability of thermal management for intermediate and low-heat-flux systems. In the present study, an innovative alternative method is proposed and a computational parametric study has been conducted. A single microchip is placed in a two-dimensional channel. Different synthetic jet configurations are designed as actuators in order to investigate their effectiveness for thermal management. The effect is that the actuator enhances mixing by imparting momentum to the channel flow, thus manipulating the temperature field in a positive manner. The best control is achieved when the actuator is placed midway on the chip length and increasing the throat height. Also, using nozzle-like throat geometry increases the heat transfer rate from the microchip surface. Doubling the number of the actuators, optimally placing them, and phasing their membrane oscillations all improve the cooling.

1D Transient Model for Frost Heave in Polymer Electrolyte Fuel Cells: II. Parametric Study

A computational model based on a frost-heave mechanism has been developed to simulate the physical damage process for polymer electrolyte fuel cells (PEFCs) during shutdown to a frozen state. In previous work by the authors, an analytical model was developed and two modes of physical damage were proposed: diffusion media (DM) punch through caused by ice-lens formation under the channel at the interface of DM and catalyst layer (CL) and catalyst delamination caused by supercooled unfrozen water flow from the electrolyte to the interface of CL and electrolyte, where it is rapidly frozen. In this study, a computational parametric study was conducted to assess the controlling parameters in the physical damage process, which were found to include drainage rate of DM, CL, and electrolyte, initial water content in the electrolyte, virgin material cracks or initial interfacial gaps between the DM and the CL, the irreducible water content of PEFC components, and heat-transfer rate and directional gradient within the membrane electrode assembly. The ice-lens thickness at the interface of DM and CL under the channel is not severe from a single cycle but can grow over many cycles, causing damage.

Preliminary Study on Biomechanics of Vertebroplasty

Algorithm development for the automatic finite element modeling of patient vertebra. To present a technique for automatic generation of patient specific computational fluid dynamics (CFD) models for intraosseous PMMA cement flow simulation. The secondary objective is to demonstrate the possibility of using resultant PMMA cement distribution for post-PVP stress-strain analyses. There are no noninvasive methods for the visualization of PMMA cement flow. In addition, optimum volume and distribution of PMMA cement are still not known. Computational models that allow patient specific intraosseous PMMA cement flow visualization as well as postvertebroplasty mechanical evaluation would be advantageous. We developed an algorithm coded into a custom platform that inputs patient CT datasets. Hounsfield unit values were used to assign permeability values as well as modulus to the finite element model before analyses. Several user inputs are required, and these reflect the decisions made by physicians that practice vertebroplasty. As a case study, we isolated a single L1 vertebra from patient CT dataset and used our platform for model generation. Simulated vertebroplasty was performed for different PMMA cement volumes and at different placements to study the effects of varying distribution. Increased needle injection pressure was observed as the volume of PMMA increases and as the distribution of PMMA is in close proximity to the cortical walls. Stiffness of augmented vertebral body also increases with increased volume of PMMA administered. Varying distributions, for the same volume, of PMMA cement did not alter stiffness drastically. Our custom platform and technique for modeling vertebral bodies may contribute significantly to the science of vertebroplasty. Intraosseous PMMA cement flow can be visualized before vertebroplasty, and needle position altered for optimization. Also, parametric computational studies on the postvertebroplasty biomechanical effects of vertebroplasty are now enhanced with such a modeling capability.

A computational model of a PEM fuel cell with finite vapor absorption rate

The paper presents a new computational model of non-steady operation of a PEM fuel cell. The model is based on the macroscopic hydrodynamic approach and assumptions of low humidity operation and one-dimensionality of transport processes. Its novelty and advantage in comparison with similar existing models is that it takes into account the finite-time equilibration between vapor and membrane-phase liquid water within the catalyst layers. The phenomenon is described using an additional parameter with the physical meaning of the typical reciprocal time of the equilibration. A computational parametric study is conducted to identify the effect of the finite-time equilibration on steady-state and transient operation of a PEM fuel cell.

Toward a Model for Local Drug Delivery in Abdominal Aortic Aneurysms

The formation of an abdominal aortic aneurysm (AAA) may eventually result in rupture, an event associated with a 50% mortality rate. This work represents a first step toward improving current stress estimation techniques and local transport simulations in AAA. Toward this aim, a computational parametric study was performed on an axisymmetric cylindrical FEM of a 5 cm AAA with a 1.5 cm thick intraluminal thrombus (ILT). Both the AAA wall and ILT were modeled as porohyperelastic PHE materials using estimated values of AAA wall and ILT permeability. While no values for AAA wall permeability could be found in the literature, the value of ILT permeability was taken from a previous investigation by Adolph et al.(7) Peak stresses, fluid velocities, and local pore pressure values within the ILT and wall were recorded and analyzed as a function of the cardiac cycle. While peak wall stress values for the PHE models did not largely differ from corresponding solid finite element simulations (186.2 N/cm(2) vs. 186.5 N/cm(2)), the stress in the abluminal region of the ILT increased by 17.4% (7.7 N/cm(2) vs. 6.5 N/cm(2)). Pore pressure values were relatively constant through the ILT while there were significant pore pressure gradients present in the AAA wall. The magnitude of fluid velocities varied in magnitude and direction throughout the cardiac cycle with large fluctuations occurring on the luminal surface. The combination of the patient-specific PHE AAA FEMs with mass transport simulations will result in spatially and time-varying concentration distributions within AAA, which may benefit future pharmaceutical treatments of AAA.

Computational Model of Physical Damage during Freeze/Thaw of PEFCs

The modes of degradation that can occur during freeze/thaw of a PEFC have become a recent topic of interest. Direct evidence of physical damage to fuel cell components have been observed by some groups, but is not yet fully understood. A computational model based on a frost heave mechanism has been developed to simulate the physical damage process and help achieve design solutions. In a previous study by the authors, two modes of physical damage have been proposed: 1) diffusion media punch through caused by ice lens formation under the channel at the interface of diffusion media and catalyst layer; 2) catalyst delamination caused by sub-zero unfrozen water diffusion from the electrolyte to the interface of catalyst layer and electrolyte, where it is rapidly frozen. The results of an ongoing computational parametric study are detailed in this paper. Controlling parameters in the physical damage process have been determined to be 1) drainage rate of diffusion media and catalyst layer, 2) drainage rate of the electrolyte, 3) heat transfer rate within the MEA, and 4) the irreducible water content of PEFC components.

Hydrogen isotope separation by permeation through palladium membranes

An inverse isotope effect is observed for both the solubility and the diffusivity of hydrogen in palladium, i.e., the lightest isotope has the highest solubility and lowest diffusivity. For permeation of hydrogen isotopes through palladium alloy membranes when diffusion is the rate determining step tritium is the least permeating isotope. Since the absolute values of hydrogen, deuterium and tritium permeabilities in palladium differ considerably, the effect could be employed as a basis for technical isotope separation. Using an experimentally verified mathematical model, a parametric computational study was performed to show the net isotope effects in permeate and bleed flows when feeding a technical permeator with various hydrogen isotope mixtures under different feed and permeate pressures. The technical feasibility of hydrogen isotope permeation as a method for separation is discussed with regard to the necessary process control for either a single permeator or a cascade.

Time-dependent behaviour of reacting concrete I: Mechanism and theory

It is observed that mature concrete exhibits time-dependent deformation under sustained load on exposure to adverse chemical environment. The mechanical behaviour of such reacting elastic solids like concrete can be explained by using Dissolution-Precipitation Mechanism according to which the creep is caused by the gradual transformation of stressed reactant solid phase/s into stress-free solute before reaction. Using this mechanism, constitutive equations for the reacting isotropic linear elastic solids under arbitrary state of stress have been proposed in this Part I of the paper. The various aspects of long-term mechanical response of stressed reacting hardened concrete like creep, recovery, relaxation, mobility, dissipation, divergence, etc., have been studied. A principle of superposition has also been derived. The mechanism proposed and theory constructed here have been evaluated and computational parametric study and practical relevance of the work done have been presented in Part II of the paper.

A computational parametric study on the permeability of intra-luminal thrombus and aortic wall within abdominal aortic aneurysms

A computational parametric study on the permeability of intra-luminal thrombus and aortic wall within abdominal aortic aneurysms

Parametric Study of Heat Release Preceding a Blunt Body in Hypersonic Flow

A computational parametric study is presented illustrating the effects of upstream heat release on drag and heating rate in hypersonic e ow over an axisymmetric blunt body. A chemical nonequilibrium viscous e ow is consideredwith seven species;Park’ stwo-temperaturemodelisalso used to takeaccountofthermally nonequilibrium phenomena. Three parameters that control the heat release are introduced, and their effects on the e ow structure, specie cally with respect to drag and heating rate, are quantie ed. Results show that heat release upstream of the body can reduce not only the aerodynamic drag but also the aerodynamic heating rate at the body surface. Computedresultsachieved areductionof23%in drag and 74%in heating rate,relativetobaselinevaluesintheabsence of heat release. The reduction mechanism is discussed, and dynamic pressure is shown to play an important role in the drag reduction mechanism.

Directed Energy Air Spike による空力加熱率減少の数値シミュレーション

This paper presents a computational parametric study showing how heat release affects aerodynamic characteristics, drag and heating rate, of a hypersonic flow over an axisymmetric blunt body. A chemical non-equilibrium viscous flow is assumed together with seven chemical species and finite-rate chemical reactions. Results show that heat release in the upstream of the body can reduce not only the aerodynamic drag but also the aerodynamic heating rate on the body. Three parameters which control heat release are introduced and their effects on the flow structure, especially on the drag and the heating rate, are presented. It is shown that the drag is reduced to 23% of the baseline value (the drag of the flow without heat release) and the aerodynamic heating rate to 76%.

Novel liquid blanket configurations and their hydrodynamic analyses for innovative confinement concepts

Hydrodynamics analyses as a part of the APEX (advanced power extraction) study demonstrates the potential application of applying swirling thick liquid walls to innovative confinement concepts such as field reversed configuration (FRC), spherical torus (ST) and heavy-ion fusion (HIF). This paper addresses the design and the hydrodynamic aspects of fusion relevant swirling flow including 3-D velocity distribution, variations of the flow height in axial and azimuthal directions and hydrodynamic flow stability. Numerical hydrodynamic analyses using a 3-D code with Flibe as the working fluid, shows that a thick liquid first-wall/blanket (>0.6 m) can be maintained in a circular vacuum chamber of 2 m radius by injecting the liquid layer from one side through a swirl flow generating inlet with axial (7 m/s) and azimuthal (10 m/s) velocity components. Parametric computational study indicated that the liquid layer thickness in axial and azimuthal directions is strongly dependent on the inlet axial, azimuthal velocity values and gravitational acceleration. It also shows that a uniform liquid layer thickness can be maintained for axial and azimuthal inlet velocities of 11 and 13 m/s in a cylindrical chamber with a 2 m radius and 12 m length. The swirling liquid wall idea is applied successfully to ST and HIF configurations. A 2D linear stability analysis using potential flow theory (Reynolds number is ∼10 6 for liquid wall thickness of ∼0.5 m) of the swirling flow in the azimuthal flow direction suggested that mean flow is stable when the surface tension, gravitational acceleration, and the centrifugal force effects are considered.

Solder joint reliability of plastic ball grid array with solder bumped flip chip

A computational parametric study on the solder joint reliability of a plastic ball grid array (PBGA) with solder bumped flip chip (FC) is presented. The basic configuration of the PBGA is 27mm package‐size and 1.27mm ball‐pitch. There were three kinds of ball population: four‐row perimeter grid array with/without thermal balls, and full grid array. A total number of 24 cases, involving various chip sizes, chip thicknesses and substrate thicknesses, were studied. The diagonal cross‐section of the PBGA‐printed circuit board (PCB) assembly was modeled by plane‐strain elements and was subjected to uniform thermal loading. Through mismatch of coefficient of thermal expansion (CTE), and lack of structural compliance, the solder joints were stressed to produce inelastic deformation. The accumulated effective plastic strain was evaluated as an index for the reliability of solder joints. The present study revealed the effects of aforementioned design parameters on the solder joint reliability of FC‐PBGA assemblies. Some peculiar phenomena were identified.

Blunt-Body Wave Drag Reduction Using Focused Energy Deposition

A parametric computational study of energy deposition upstream of generic two-dimensional and axisymmetric blunt bodies at Mach numbers of 6.5 and 10 is performed utilizing a full Navier-Stokes computational fluid dynamics code. The energy deposition modifies the upstream shock structure and results in large wave drag reduction and very high power effectiveness. Specifically, drag is reduced to values as low as 30% of baseline drag (no energy deposited into flow) and power effectiveness ratios (ratio of thrust power saved to power deposited into the flow) of up to 33 are obtained. The fluid dynamic and thermodynamic bases of the observed drag reduction are examined

Use of Computer Simulation to Aid the Understanding of Microstructural Changes Observed in Heat-Treated AlN Ceramics.

A parametric computational study, using the Monte Carlo method, was performed to examine the effect of γs1/γss value and Monte Carlo step (MCS) on the key microstructural characteristics of aluminum nitride (AlN)/yttrium aluminate system. The γsl/γss values studied in the simulation ranged from 0.1 to 1.0 and covered the non-wetting and wetting situations for this liquid phase sintered material. Grain morphology and contiguity of the resultant microstructures were determined. Validation experiments were performed on sintered and heat-treated AlN ceramics to study the effect of time, temperature, and grain boundary phase level on the microstructural development. In addition to contiguity, dihedral angle and grain boundary phase chemistry were also measured. The mimicking of experimental and computational results suggested that the remarkable microstructural changes observed during heat treatment at 1800°C might be the result of variations in the γs1/γss value with temperature. There was also good agreement between the simulated microstructures and the AlN ceramics with different liquid phase fractions. Dihedral angle measurements of the ceramic materials confirmed the appropriateness of the γs1/γss values used in the simulation.

Mathematical modelling of macro- and micro-infiltration in resin transfer moulding (RTM)

Abstract A mathematical model is proposed to describe the macro- and micro-infiltration through reinforcements of bimodal porosity distribution in resin transfer moulding. The model is based on Darey's law incorporating mechanical, capillary and vacuum pressures and covers three modes of infiltration. Permeabilities and capillary pressures are calculated at macro- and micro-levels. A numerical analysis is presented where at each numerical location the flowrate is split between three possible types of flow on the basis of mass balance and a combination of permeability magnitude and local flow potential. Parametric computational studies are carried out to study the flow of a model Newtonian fluid through woven cloths where the following parameters are varied: fibre volume fraction, fibre tow diameter and injection pressure. Predicted variables include micro- and macro-infiltration times and apparent global permeability. The latter was found to depend on the injection pressure, in the regime of relatively low injection pressures, where the variations also depended on the fabric architecture and fibre volume fraction.

Computer Simulation of Solder Bridging Phenomena

Solder bridging is investigated under the assumption that liquid solder bridges are equilibrium capillary surfaces and that the principal factor that determines whether a bridge will freeze to form a permanent short is its configurational stability. A computational parametric bridge stability study is conducted to determine the response of bridging to the system volume, the distance between pads, the contact angle between the liquid metal and resist surface and the relevant physicochemical properties of the liquid metal.

Study on Independent Effects of Diesel Engine Operating Conditions on Nitric Oxide Formation and Emissions Through Schematical Combustion Simulation

A mathematical model has been used to evaluate the effect of diesel engine operating conditions on nitric oxide formation and emissions. Although there are several formation pathways during diesel combustion, only the supposed most important one was considered in the present work: the thermal mechanism. The effect of the engine operating conditions was simulated by means of schematical variations of local temperature and fuel–air ratio of the mixture. The evolution of these local variables is described by three characteristic time intervals, which define the timing of the following items: the temperature and pressure peaks; the dilution of burnt products with surrounding air; and the decay of temperature and pressure due to expansion. In addition, the temperature peak was related to the adiabatic flame temperature, which was previously calculated for each mixture condition. A computational parametric study was carried out by varying each of the local mixture parameters and characteristic times, either separately or in combinations. The trends obtained show a qualitative agreement with those obtained by other authors from experimental tests on different diesel engines. The analysis of these trends contributes to a better understanding of the sources of NO diesel emissions and could provide guidelines for cleaner diesel engine design.