Browsing by Author "Frank, Michael"
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Item Open Access Effects of surface roughness on shear viscosity(American Physical Society, 2017-03-13) Papanikolaou, Michail; Frank, Michael; Drikakis, DimitrisThis 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.Item Open Access Heat transfer across a fractal surface(American Institute of Physics (AIP), 2019-10-02) Frank, Michael; Papanikolaou, Michail; Drikakis, Dimitris; Salonitis, KonstantinosThe 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.Item 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, MichaelDespite 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.Item Open Access Molecular studies of confined liquids and nanofluids for passive thermal management(2015-03) Frank, Michael; Drikakis, Dimitris; Asproulis, N.; Murray, Angus;The constant technological advances in integrated circuits and electronic systems experienced over the last few years have resulted in large temperature gradients. These can damage electronic devices. Current cooling methods are unable to cope with highly demanding applications such as military systems. Furthermore, for applications in which failure is not an option, a lack of sufficient thermal management can be a limiting factor in the design and addition of functionality. The aim of this research project is to provide possible solutions to the overheating of electronics. Following an in depth review of the state-of-the-art in cooling technologies, we have identified nanofluidics and nanofluids as promising candidates for thermal management. However, systems characterised by such small dimensions are governed by surface phenomena. Sometimes, continuum computational methods such as Computational Fluid Dynamics (CFD) are inadequate in providing a detailed description of such effects. Instead, molecular methods, such as Molecular Dynamics (MD), study systems at a higher resolution and can potentially provide a more accurate understanding of such systems. This thesis uses MD to understand how the thermodynamic properties of liquids and nanofluids are modified by spatial restrictions. An important finding is that heat is transferred differently in confined and unconfined liquids. Following this realisation, an analysis of the system parameters is carried out to understand how to optimise the heat conductance of such systems. We also consider confined nanofluids. Different materials are modelled and compared with respect to their possible practical use as thermal management agents. The thermodynamic behaviour discovered has not been described elsewhere and has potentially high practical importance. Although in its infancy, we believe that it can eventually provide a framework for the design of efficient cooling devices.Item Open Access Nanoflow over a fractal surface(American Institute of Physics (AIP), 2016-08-01) Papanikolaou, Michail; Frank, Michael; Drikakis, DimitrisThis paper investigates the effects of surface roughness on nanoflows using molecular dynamics simulations. A fractal model is employed to model wall roughness, and simulations are performed for liquid argon confined by two solid walls. It is shown that the surface roughness reduces the velocity in the proximity of the walls with the reduction being accentuated when increasing the roughness depth and wettability of the solid wall. It also makes the flow three-dimensional and anisotropic. In flows over idealized smooth surfaces, the liquid forms parallel, well-spaced layers, with a significant gap between the first layer and the solid wall. Rough walls distort the orderly distribution of fluid layers resulting in an incoherent formation of irregularly shaped fluid structures around and within the wall cavities.