With this project, we propose a numerical investigation into a variety of dynamics and effects produced by the interaction of an advancing solidification front with thermally-driven convection in the melt. Related technological applications abound in the fields of thermal, mechanical, nuclear and chemical engineering. The project will concentrate on different types of substances (opaque metals and transparent materials), different forms of convection (thermogravitational, thermocapillary or thermovibrational flows) and geometrical configurations.
PROJECT DETAILS [full details of the project]:
Over recent years, the progressive refinement of manufacturing techniques in the main field of crystal growth has enabled the production of advanced materials with “controlled” microscopic structures suitable for a variety of advanced applications. Despite such improvements, however, relevant industrial production methods are still adversely affected by undesired fluid-dynamic instabilities that develop when the material is in a liquid state. Indeed, the “purity” and “perfection” of the resulting crystalline structures, which are in general crucial factors for the exploitation of such materials and the success of related applications, depend strongly on the (fluid-dynamic) phenomena that occur when the transition from an initial liquid state (melt) to the final solid phase takes place.
Control of convective instabilities occurring when the considered material is in a liquid state, in general, is regarded as an essential topic from a “product quality” perspective. Although a plethora of studies have been appearing over recent years motivated, completely or in part, by the need to elaborate new means to mitigate such instabilities (and hence to produce single crystals of higher quality), unfortunately, most of these studies were focusing on idealized geometries with limited translational applicability to effective production methods.
With this project, we propose an investigation into a variety of dynamics and effects produced by the presence of flows of different natures during the solidification of a material from an initial liquid state.
Different materials will be considered, which range from typical metals, semiconductors or superconductors  to the transparent oxide materials used for opto-electronic and other applications (these oxides are widely used as transparent “metallic” electrodes or solar cells and flat panels including liquid crystal displays (LCDs) and organic light emitting diodes (OLEDs), ).
Different types of thermal convection and their interplay with the presence of a solidification front (advancing through the considered physical domain) will be considered, including (but not limited to) thermogravitational, thermocapillary and thermovibrational flows.
The considered geometrical configurations will be those traditionally used to model classical techniques for the production of these materials (e.g., the Bridgman, Floating-Zone and Czochralski methods).
The research will involve the application of relevant numerical techniques. The student will be trained to use OpenFoam and other numerical codes available at the Department of Mechanical and Aerospace Engineering. Specific numerical examples with growing complexity will be consideredto provide the student with an increased understanding of the main cause-and-effect relationships driving fluid flow and determining its properties.
 M. Lappa, R. Savino, (2002), “3D analysis of crystal/melt interface shape and Marangoni flow instability in solidifying liquid bridges”, Journal of Computational Physics, 180 (2): 751-774.
 M. Lappa (2018), On the Formation and Propagation of Hydrothermal waves in Solidifying Liquid Layers, Computers &Fluids, 172, 741-760.
From a theoretical point of view, training will be provided with regard to 1) the general background (importance of this kind of research and potential practical applications), 2) governing parameters, 3) gravitational phenomena in fluids, 4) Marangoni thermal effects, 5) Vibrational effects, 6) Numerical methods for the solution of the fluid flow equations 7) Mathematical Models for solidification and related techniques (e.g., the so-called enthalpy method).
Subject areas - Manufacturing, Materials Science, Mechanical Engineering, Semiconductors