CERES :: Browsing by Author "Papanikolaou, Michail"

Browsing by Author "Papanikolaou, Michail"

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Open Access
Alternation of the dynamic coefficients of short journal bearings due to wear
(Emerald, 2015-05-31) Papanikolaou, Michail; Farmakopoulos, Michael G.; Papadopoulos, Chris A.
Purpose – Wear in journal bearings occurs when the operating conditions (high load, high temperature,low angular velocity or low viscosity), downgrade the ability of the bearing to carry load. The wear depth increases because the rotor comes in contact with the bearing surface. Wear in journal bearings affects their characteristics because of its influence on the thickness of the fluid film. This influence can be detected in the dynamic behavior of the rotor and especially in the dynamic stiffness and damping coefficients. The paper aims to discuss these issues. Design/methodology/approach – In this paper, the effect of wear on the rotor dynamic stiffness and damping coefficients (K and C) of a short journal bearing is investigated. K and C in this work are estimated by using two methods a semi-analytical method and finite element (FE) analysis implemented in the ANSYS software. Findings – The main goal of this research is to make the identification of wear in journal bearings feasible by observing the alternation of their dynamic coefficients. Both of the methods implemented are proven to be useful, while FE analysis can provide more accurate results. Originality/value – This paper is original and has not been published elsewhere.
Open Access
A computational framework towards energy efficient casting processes
(Springer, 2018-12-01) Papanikolaou, Michail; Pagone, Emanuele; Salonitis, Konstantinos; Jolly, Mark R.; Makatsoris, Makatsoris
Casting is one of the most widely used, challenging and energy intensive manufacturing processes. Due to the complex engineering problems associated with casting, foundry engineers are mainly concerned with the quality of the final casting component. Consequently, energy efficiency is often disregarded and huge amounts of energy are wasted in favor of high quality casting parts. In this paper, a novel computational framework for the constrained minimization of the pouring temperature is presented and applied on the Constrained Rapid Induction Melting Single Shot Up-Casting (CRIMSON) process. Minimizing the value of the pouring temperature can lead to significant energy savings during the melting and holding processes as well as to higher yield rate due to the resulting reduction of the solidification time. Moreover, a multi-objective optimization component has been integrated into our scheme to assist decision makers with estimating the trade-off between process parameters.
Open Access
Contact stiffness effects on nanoscale high-speed grinding: A molecular dynamics approach
(Elsevier, 2019-07-04) Papanikolaou, Michail
One of the most important grinding parameters is the real depth of cut which is always lower than its programmed value. This is because in reality abrasive grains of the grinding wheel are not fixed but attached to a bonding material which is deformed during the process. In this study we investigate the effect of the contact stiffness between a single abrasive grain and the workpiece on the depth of cut and the grinding process characteristics via three-dimensional Molecular Dynamics (MD) simulations. Contact stiffness has been modelled by attaching a single trapezoid abrasive grain to a spring in the normal grinding direction. MD experiments have been repeated due to the stochastic nature of the grinding process in favour of statistical accuracy. Various grinding speeds have been considered while the case of a rough abrasive-workpiece interface has been investigated as well using fractal models. Our results indicate that the trajectory followed by the abrasive grain is not a straight line, as in the case of a rigid abrasive, but a curved one, asymptotically converging towards the equilibrium point which corresponds to the selected value of the spring stiffness. This behaviour alongside the grinding velocity and rough abrasive-workpiece interface have been found to affect the grinding forces, friction coefficient, morphology of the ground surface and subsurface temperature. The present MD model has also been proven to be capable of capturing the thermal softening phenomenon at the abrasive-workpiece interface.
Open Access
Design optimisation of the feeding system of a novel counter-gravity casting process
(MDPI, 2018-10-11) Papanikolaou, Michail; Pagone, Emanuele; Georgarakis, Konstantinos; Rogers, Keith; Jolly, Mark R.; Salonitis, Konstantinos
The appropriate design of feeders in a rigging system is critical for ensuring efficient compensation for solidification shrinkage, thus eliminating (shrinkage-related) porosity and contributing to the production of superior quality castings. In this study, a multi-objective optimisation framework combined with Computational Fluid Dynamics (CFD) simulations has been introduced to investigate the effect of the feeders’ geometry on shrinkage porosity aiming to optimise casting quality and yield for a novel counter-gravity casting process (CRIMSON). The weighted sum technique was employed to convert this multi-objective optimisation problem to a single objective one. Moreover, an evolutionary multi-objective optimisation algorithm (NSGA-II) has been applied to estimate the trade-off between the objective functions and support decision makers on selecting the optimum solution based on the desired properties of the final casting product and the process characteristics. This study is one of the first attempts to combine CFD simulations with multi-objective optimisation techniques in counter-gravity casting. The obtained results indicate the benefits of applying multi-objective optimisation techniques to casting processes
Open Access
Effects of crystallinity on residual stresses via molecular dynamics simulations
(American Physical Society, 2022-10-05) Papanikolaou, Michail; Rodriguez-Hernandez, Francisco; Jolly, Mark R.; Salonitis, Konstantinos
Mechanical properties of materials are highly dependent on microstructure. One characteristic example is tensile stresses at the grain boundaries, which is one of the most critical factors in crack nucleation. Although experimental techniques have significantly evolved during the past decades with respect to obtaining highresolution snapshots of the microstructure with methods such as scanning electron microscopy, the quantitative estimation of continuum quantities, such as localized stresses, still remains a very challenging task. The molecular dynamics simulation method has been proven to be a quite effective simulation tool for providing insights in such challenges due to its high spatial and temporal resolution. In this study, molecular dynamics simulations have been performed to obtain a spatial resolution of the residual stresses in solidified aluminum. A best-effort realistic microstructure was obtained by starting from a pure aluminum block which was initially melted and subsequently quenched under various cooling rates, and finally relaxed. The obtained results suggest that residual stresses are higher in absolute terms at the vicinity of grain boundaries than at the grain interiors, and higher crystallinity has been found to be correlated to lower residual stresses. Moreover, it has been shown both qualitatively and quantitatively that grain boundaries undergo tensile loading, in contrast to the grain interiors which are compressed; this result comes to support the conclusions of quite recent experimental investigations, showing that the residual stress is tensile at the grain boundaries and gradually transits into compressive in the grain interiors, and highlights the potential of molecular dynamics simulation to capture nanoscale physical phenomena.
Open Access
Effects of surface roughness on shear viscosity
(American Physical Society, 2017-03-13) Papanikolaou, Michail; Frank, Michael; Drikakis, Dimitris
This paper investigates the effect of surface roughness on fluid viscosity using molecular dynamics simulations. The three-dimensional model consists of liquid argon flowing between two solid walls whose surface roughness was modeled using fractal theory. In tandem with previously published experimental work, our results show that, while the viscosity in smooth channels remains constant across the channel width, in the presence of surface roughness it increases close to the walls. The increase of the boundary viscosity is further accentuated by an increase in the depth of surface roughness. We attribute this behavior to the increased momentum transfer at the boundary, a result of the irregular distribution of fluid particles near rough surfaces. Furthermore, although the viscosity in smooth channels has previously been shown to be independent of the strength of the solid-liquid interaction, here we show that in the presence of surface roughness, the boundary viscosity increases with the solid's wettability. The paper concludes with an analytical description of the viscosity as a function of the distance from the channel walls, the walls’ surface roughness, and the solid's wetting properties. The relation can potentially be used to adjust the fluid dynamics equations for a more accurate description of microfluidic systems.
Open Access
Energy resilient foundries: the “Small is beautiful” projects
(Springer, 2022-02-05) Jolly, Mark R.; Salonitis, Konstantinos; Pagone, Emanuele; Papanikolaou, Michail; Saxena, Prateek
Applying the concept of “small is beautiful” into a conservative relatively low technology manufacturing sector where the “economies of scale” argument has been used to build ever more so-called efficient process lines is a major challenge. The energy efficiency of the casting process has only been investigated in a limited fashion. The two “Small is Beautiful” projects aimed to introduce a new concept into foundries with regards to the use of their resources. The new philosophy, “small is beautiful”, starts by encouraging the use of high-quality feedstock, only melting what is required and only when it is required. Recycling of internal scrap is not necessarily acceptable but an aim for higher yields is. Applying counter gravity casting methods to improve yield and give enhanced quality is encouraged as is the recovery of low-grade heat from solidification. The present paper discusses the research undertaken and the key findings from the two projects.
Open Access
An experimental and simulation screening of X-65 steel weldment corrosion in high flow rate conditions
(Elsevier, 2024-04-01) Nofrizal, Nofrizal; Wulandari, Meyliana; Impey, Susan; Georgarakis, Konstantinos; Papanikolaou, Michail; Raja, Pandian Bothi
Over many decades, the oil and gas industry has encountered significant challenges due to weldment corrosion. The issue of internal-pipeline local corrosion at the welded joint region has garnered significant concern, especially due to the combined impact of high shear stress and electrochemical corrosion. This combination can lead to pipeline rupture with relative ease. Hence, a new approach to screen the flow corrosion of X-65 steel via electrochemical methods, predicting fluid shear stress and velocity using computational fluid dynamic (CFD) simulation is positively tested and presented here. For that, the X-65 steel specimens were cut/designed as the inner, centre, and outer electrodes of the target to analyse the Weld Metal (WM), Heat Affected Zone (HAZ), and Parent Metal (PM). Electrochemical screening was carried out simultaneously at a flow of 10 m/s using a brine solution saturated with CO2. The PM and HAZ will corrode less than the WM, in some cases at 30–23% of the rate of the WM. Thus in an environment of uninhibited brine saturated with CO2 at 10 m/s, preferential weld corrosion (PWC) is expected to occur. In addition, the surface morphology screening (scanning electron microscope with energy dispersive x-ray analysis, X-Ray diffraction, focus ion beam, raman spectroscopy) was employed to monitor the corrosion damage on the metal surface and also to support the electrochemical measurements (linear polarization resistance, galvanic measurement, and electrochemical impedance spectroscopy).
Open Access
Fractal roughness effects on nanoscale grinding
(Elsevier, 2018-10-20) Papanikolaou, Michail; Salonitis, Konstantinos
Three-dimensional Molecular Dynamics simulations have been performed to investigate the effects of fractal roughness on nanoscale grinding. The first part of this investigation focuses on the effects of the workpiece rough top surface on the grinding process characteristics, with special emphasis placed on the friction coefficient, the grinding forces and the subsurface temperature. The second part focuses on the alternation of the aforementioned parameters due to the rough abrasive contact surface. Rough surface profiles have been generated using the multivariate Weierstrass-Mandelbrot function while the abrasive has been modelled as a trapezoid. The irregular surface topography has been controlled by tuning the value of the fractal dimension Ds . The aforementioned experiments have been repeated for various values of the cutting depth. Our results indicate that the grinding process parameters are mainly dependent on the cutting depth as well as the abrasive lower surface profile, which also defines the interface contact area between the workpiece and the abrasive, rather than the topography of the workpiece top surface.
Open Access
Grain size effects on nanocutting behaviour modelling based on molecular dynamics simulations
(Elsevier, 2020-11-16) Papanikolaou, Michail; Salonitis, Konstantinos
Grain size is one of the most critical factors affecting the mechanical and thermal properties of metallic materials. In this study the effect of the grain size of a workpiece made of pure Aluminium on the nanocutting process has been investigated via means of Molecular Dynamics simulations. The polycrystalline workpiece has been generated starting from a Face-Centred Cubic block of Aluminium atoms which was melted and subsequently quenched under various cooling rates in order to control the average grain size. The case of a monocrystalline workpiece has been considered as well. The generated workpieces were ground by a diamond abrasive. Simulations have been repeated in order to eliminate any statistical errors. The obtained results suggest that the average grain size of the workpiece significantly influences almost every aspect related to the nanocutting process. More specifically, it has been found that the cutting forces increase and the friction coefficient decreases with the grain size. Very small grain sizes lead to lower thermal conductivity and consequently high temperature at the cutting region. Finally, it has been shown that the high residual stresses at the grain boundaries can be relieved as the tool passes on top of the workpiece; this phenomenon resembles heat treatment. In summary, the nanocutting behaviour of polycrystalline materials depends on the average grain size and significantly differs from the case of monocrystalline materials. This should be taken into account in future numerical models of nanocutting processes
Open Access
Heat transfer across a fractal surface
(American Institute of Physics (AIP), 2019-10-02) Frank, Michael; Papanikolaou, Michail; Drikakis, Dimitris; Salonitis, Konstantinos
The effects of surface irregularities and imperfections on the thermal resistance at a solid-liquid interface have been investigated using molecular dynamics. The molecular model comprises liquid argon confined between silver walls. The surface roughness was designed using fractal theory, introducing stochastic patterns of multiple scales that resemble realistic surface geometries. In agreement with most previous studies, we find that increasing the strength of the solid-liquid interactions monotonically reduces the thermal resistance across smooth interfaces. Yet, the behavior of the thermal resistance across rough surfaces is more complex. Following the initially anticipated decrease, the thermal resistance starts to increase once the strength of solid-liquid interaction increases past a threshold. We attribute the above behavior to two competing phenomena, namely, the area of the solid-liquid interface and the introduction of vibrational anharmonicities and localization of phonons resulting from the surface roughness. Finally, we demonstrate that, for the same fractal dimension and depth of surface roughness, different surfaces practically have the same thermal resistance, solid-liquid radial distribution function, and liquid density profiles. We conclude that the above fractal parameters are useful in deriving reduced models for properties related to the surface geometry.
Open Access
Investigation of the subsurface temperature effects on nanocutting processes via molecular dynamics simulations
(MDPI, 2020-09-10) Papanikolaou, Michail; Hernandez, Francisco Rodriguez; Salonitis, Konstantinos
In this investigation, three-dimensional molecular dynamics simulations have been performed in order to investigate the effects of the workpiece subsurface temperature on various nanocutting process parameters including cutting forces, friction coefficient, as well as the distribution of temperature and equivalent Von Mises stress at the subsurface. The simulation domain consists of a tool with a negative rake angle made of diamond and a workpiece made of copper. The grinding speed was considered equal to 100 m/s, while the depth of cut was set to 2 nm. The obtained results suggest that the subsurface temperature significantly affects all of the aforementioned nanocutting process parameters. More specifically, it has been numerically validated that, for high subsurface temperature values, thermal softening becomes dominant and this results in the reduction of the cutting forces. Finally, the dependency of local properties of the workpiece material, such as thermal conductivity and residual stresses on the subsurface temperature has been captured using numerical simulations for the first time to the authors’ best knowledge
Open Access
Large-scale molecular dynamics simulations of homogeneous nucleation of pure aluminium
(MDPI, 2019-11-12) Papanikolaou, Michail; Salonitis, Konstantinos; Jolly, Mark R.; Frank, Michael
Despite the continuous and remarkable development of experimental techniques for the investigation of microstructures and the growth of nuclei during the solidification of metals, there are still unknown territories around this topic. The solidification in nanoscale can be effectively investigated by means of molecular dynamics (MD) simulations which can provide a deep insight into the mechanisms of the formation of nuclei and the induced crystal structures. In this study, MD simulations were performed to investigate the solidification of pure Aluminium and the effects of the cooling rate on the final properties of the solidified material. A large number of Aluminium atoms were used in order to investigate the grain growth over time and the formation of stacking faults during solidification. The number of face-centred cubic (FCC), hexagonal close-packed (HCP) and body-centred cubic (BCC) was recorded during the evolution of the process to illustrate the nanoscale mechanisms initiating solidification. The current investigation also focuses on the exothermic nature of the solidification process which has been effectively captured by means of MD simulations using 3 dimensional representations of the kinetic energy across the simulation domain.
Open Access
Life cycle and energy assessment of automotive components manufacturing: The dilemma between aluminium and cast iron
(MDPI, 2019-07-03) Salonitis, Konstantinos; Jolly, Mark R.; Pagone, Emanuele; Papanikolaou, Michail
Considering the manufacturing of automotive components, there exists a dilemma around the substitution of traditional cast iron (CI) with lighter metals. Currently, aluminum alloys, being lighter compared to traditional materials, are considered as a more environmentally friendly solution. However, the energy required for the extraction of the primary materials and manufacturing of components is usually not taken into account in this debate. In this study, an extensive literature review was performed to estimate the overall energy required for the manufacturing of an engine cylinder block using (a) cast iron and (b) aluminum alloys. Moreover, data from over 100 automotive companies, ranging from mining companies to consultancy firms, were collected in order to support the soundness of this investigation. The environmental impact of the manufacturing of engine blocks made of these materials is presented with respect to the energy burden; the “cradle-to-grave approach” was implemented to take into account the energy input of each stage of the component life cycle starting from the resource extraction and reaching to the end-of-life processing stage. Our results indicate that, although aluminum components contribute toward reduced fuel consumption during their use phase, the vehicle distance needed to be covered in order to compensate for the up-front energy consumption related to the primary material production and manufacturing phases is very high. Thus, the substitution of traditional materials with lightweight ones in the automotive industry should be very thoughtfully evaluated
Open Access
Metal casting energy efficient metrics for material selection of automotive parts
(Springer, 2018-12-01) Pagone, Emanuele; Papanikolaou, Michail; Salonitis, Konstantinos; Jolly, Mark R.
The automotive sector is one of the main end-use markets for metal casting worldwide. The strong competitive pressure typical of this industry have been influenced in the recent years by sustainability as a new factor promoted by legislation, increased societal awareness of relevant instances and resource scarcity. Energy efficiency, although only a part of sustainability, is important for the metal casting practice because of its nature of large consumer of energy per unit product. Therefore, the effective use of appropriate energy efficient metrics in foundries is of great interest. In this work, a set of indicators developed by the authors (and derived by traditional metrics) to analyse the energy performance of foundries will be used to compare high pressure die casting processes producing car transfer cases with different suitable materials. On the basis of this analysis, it will be shown that the most energy efficient material can be identified whereas the traditional metrics cannot detect such opportunity.
Open Access
Minimising defect formation in sand casting of sheet lead: a DoE approach
(MDPI, 2020-02-13) Prabhakar, Arun; Papanikolaou, Michail; Salonitis, Konstantinos; Jolly, Mark R.
Sand casting of lead sheet is a traditional manufacturing process used up to the present due to the special features of sand cast sheet such as their attractive sheen. Similarly to any casting process, sand casting of lead sheet suffers from the presence of surface defects. In this study, a surface defect type, hereby referred to as ‘grooves’, has been investigated. The focus has been laid on the identification of the main factors affecting defect formation in this process. Based on a set of screening experiments performed using Scanning Electron Microscopy (SEM) as well as the existing literature, a number of factors affecting the formation of such defects was identified and their corresponding significance was estimated using the Analysis of Variance (ANOVA) technique. The obtained results suggest that the most significant factor affecting defect formation in sand casting of lead sheet is the composition of the moulding mixture. Defect formation was also proven to be dependent on the sand grain fineness, the quality of the melt and some of the interactions between the aforementioned process parameters. Finally, an optimal set of process parameters leading to the minimisation of surface defects was identified.
Open Access
Molecular dynamics simulations of confined liquids in nanochannels with rough walls
(Cranfield University, 2017-02) Papanikolaou, Michail; Drikakis, Dimitris
During the past few decades Micro-Electromechanical systems (MEMS) have been increasingly used in various engineering domains ranging from electronics to biological sciences as nowadays they can be massively produced in numerous shapes and with various compositions. Additionally, the development of the manufacturing techniques has allowed MEMS to be easily integrated into devices and expand their applications as sensors and actuators. The future of MEMS seems to be more than promising; however the small scales involved in this type of devices give rise to phenomena that cannot be treated by continuum simulations such as Computational Fluid Dynamics (CFD) or Computational Structural Dynamics (CSD). On the contrary, Molecular Dynamics (MD) Simulations are considered to be an effective approach in investigating the flow behaviour and the rheological properties of liquids in the nanoscale. The aim of this PhD project is to establish and implement Molecular Dynamics Models for the investigation of nano-scale liquid flows and the fluid properties in nanochannels with rough walls. This thesis uses MD to investigate the effect of nano-scale roughness on the slip length, the fluid viscosity and the Kapitza resistance. Rough nanochannel walls have been modelled with the help of the multivariate Weierstrass - Mandelbrot (W-M) function which has been used in the past to describe fractally rough surfaces being common in nature. A number of different approaches have been used to extract the aforementioned thermodynamic and flow properties including Equilibrium Molecular Dynamics (EMD) and Non-Equilibrium Molecular Dynamics (NEMD) Simulations. The outcomes of this research suggest that surface roughness can greatly affect the flow behaviour of highly confined liquids as well as their thermodynamic behaviour. Therefore they could potentially be used as a first step for the selection of the surface treatment and finishing techniques of MEMS devices according to the desired fluid behaviour.
Open Access
Molecular dynamics simulations of the evolution of residual stresses during rapid solidification of aluminium
(Springer, 2021-02-24) Papanikolaou, Michail; Salonitis, Konstantinos; Jolly, Mark R.
It is well known that residual stresses are quite common in castings and they emerge due to uneven cooling conditions. Nowadays, the development of atomistic modelling techniques has allowed for the in-depth investigation of the solidification process mechanics as well as the distribution of residual stresses in the simulation domain. In this study we have performed three-dimensional molecular dynamics simulations to investigate the evolution of residual stresses during homogeneous nucleation in pure aluminium as well as their distribution over the simulation domain. A simulation box containing 1 million aluminium atoms placed on the sites of a face centred cubic (FCC) lattice has been melted and subsequently quenched under various cooling rates. The potential energy as well as the formation of grains has been monitored during quenching stages. Moreover, the present analysis is expanded to the distribution of the grain size and the number of grains as a function of the cooling rate. Finally, the obtained results suggest that the cooling rate significantly affects the distribution as well as the final magnitude of residual stresses in the solidified structure
Open Access
Molecular dynamics simulations of the solidification of pure aluminium
(Springer, 2020-01-28) Papanikolaou, Michail; Salonitis, Konstantinos; Jolly, Mark R.
Despite the continuous and remarkable development of experimental techniques for the investigation of microstructures and the growth of nuclei during the solidification of metals, there are still unknown territories around the topic of nucleation during solidification. Such nanoscale phenomena can be effectively observed by means of Molecular Dynamics (MD) simulations which can provide a deep insight into the formation of nuclei and the induced crystal structures. In this study, MD simulations have been performed to investigate the solidification of Aluminium melt and the effects of process parameters such as the cooling rate and hydrostatic pressure on the final properties of the solidified material. A large number of Aluminium atoms have been used in order to investigate the grain growth over time solidification. The population of the Face Centred Cubic (FCC) and amorphous (or non-crystalline) phases has been recorded during the evolution of the process to illustrate the nanoscale mechanisms during solidification. Finally, the exothermic nature of the solidification process has been effectively captured by measuring the temperature of the Al atoms during grain formation.
Open Access
Multi-criteria decision-making for the life cycle of sustainable high pressure die casting products
(Inderscience, 2020-04-23) Pagone, Emanuele; Papanikolaou, Michail; Salonitis, Konstantinos; Jolly, Mark R.
Although a significant body of literature has been devoted to establish metrics capable of measuring the performance of manufacturing systems (including foundries) and their influence on decision-making, there is a scarcity of comprehensive and organic studies on performance indicators encompassing sustainability. The objective of this investigation is the selection of the most suitable material for the manufacture of an automotive component using the High Pressure Die Casting (HPDC) process. The performance of three different alloys (Aluminium-A380, Magnesium-AZ91D and Zinc-ZA8) was evaluated based on four different classes of metrics, namely: (a) cost, (b) time, (c) quality and (d) sustainability. The metrics selected refer to the overall product life cycle and process characteristics and have been normalised by mass to extend the applicability of the selection method to parts produced with similar process and design specifications but different mass. The deterministic TOPSIS method has been adopted to weigh and combine the different metrics used and drive the decision making process. According to the results, although the zinc alloy appears to be the most expensive option, it should be favoured over the two alternatives due to its significantly superior performance with respect to the quality and sustainability criteria. The current investigation demonstrates that the implementation of the TOPSIS method in combination with the added sustainability dimension influences the decision making process and challenges well-established decision making trends in the automotive industry during the past few decades.