An investigation into the dynamics of gravitational and surface-tension driven flows in discretely heated systems

Student thesis: Doctoral Thesis


Periodically distributed wall-mounted hot blocks located at the bottom of a layer of liquid with a free top interface are representative of a variety of technological applications in several fields (including, but not limited, to mechanical engineering, materials science, and the energy sector; related examples being power plants, solar energy collectors, nuclear reactors, energy storage systems and furnaces or crucibles). All these systems tend to create patterns in their surrounding fluid that are reminiscent of the classical modes of Rayleigh Bénard or Marangoni–Bénard convection. In the present thesis, these subjects are investigated giving much emphasis to understanding how ensemble properties arise from the interplay of localized effects, i.e., different sources of heat. In particular, the problem is addressed from both numerical and experimental points of view. First numerical simulations are used to get relevant information about the dynamics of discretely heated systems for the case of opaque fluids (liquid metals) for which experiments are not possible, then the case of a transparent oil is considered. Through the used numerical framework, the emerging planforms are identified, and the statistics of the associated heat transport mechanisms are put in relation with the spatially averaged behavior of the underlying thermal currents. It is shown that in some cases, all these features can be directly mapped into the topography at the bottom of the layer. In other circumstances, these systems contain their own capacity for transformation, i.e., intrinsic evolutionary mechanisms are enabled, by which complex steady or unsteady patterns are produced. It is shown that self organization driven by purely surface-tension or mixed buoyancy–Marangoni effects can result in ‘quantized states’, i.e., aesthetically appealing solutions that do not depend on the multiplicity of wall-mounted elements. The problem is also investigated experimentally considering an oil with temperature-dependent properties. By means of a concerted approach based on the application of a thermographic visualization technique, multiple temperature measurements at different points and a posteriori computer-based reconstruction of the spatial distribution of wavelength, it is shown that tuning of the physical topography at the bottom and the difference of temperature between the liquid and the external (gaseous) environment can still be used to enable internal feedback control over the spontaneous flow behavior. For a fixed geometry and temperature difference, variations in the emerging pattern can also be produced by changing the thickness of the liquid layer, which indirectly provides evidence for the additional degree of freedom represented by the relative importance of buoyancy and Marangoni effects. As a final variant, the case where the fluid container is inclined to the horizontal direction is also considered and it is shown that this fluid-dynamic system is prone to develop a rich set of patterns, which include spatially localized (compact) cells, longitudinal wavy rolls, various defects produced by other instabilities and finger-like structures resulting from an interesting roll pinching mechanism.
Date of Award26 Apr 2023
Original languageEnglish
Awarding Institution
  • University Of Strathclyde
SponsorsUniversity of Strathclyde
SupervisorMarcello Lappa (Supervisor) & Monica Oliveira (Supervisor)

Cite this