The flow over a topography or a step is a fundamental problem in fluid dynamics with relevance to many fields and circumstances. In the present analysis direct numerical simulation (DNS) is initially used to examine the properties of the thermofluiddynamic field in two-dimensional channels with a heated obstruction located on the bottom. The involved dynamics include forced flow driven by injection of cold fluid and the buoyancy convection of thermal origin, which naturally emerges in these channels as a result of the prevailing temperature gradients.
The sensitivity of these systems to thermal buoyancy for each considered rate of fluid injection (measured through the related Richardson number, Ri) is explored by varying parametrically the corresponding Rayleigh number (Ra) over a large interval of orders of magnitude (up to the onset of chaos). Different orientations of the step with respect to the forced flow are assumed (Forward-Facing and Backward-Facing Steps) and two alternate paradigms are considered for the bottom of the considered channel, namely an adiabatic or kept-at-constant temperature (hot) boundary.
Through this conceptual framework and using a peculiar analysis hierarchy where selected effects are intentionally switched on or off depending on the targeted regime, a kaleidoscope of situations is revealed in the (Ri, Ra) space, which differ in terms of flow patterning behaviour, thermal plume generation phenomena, intensity of heat exchange at the walls and bifurcation scenario. Comparison of forward facing and backward facing step configurations indicates that, besides the differences, these two systems display interesting analogies.
These are further explored by removing the constraint of twodimensionality and allowing the flow to develop along the spanwise direction. To reduce the scale of the three-dimensional problem to a level where it is affordable, however, this study is developed in the framework of a large eddy simulation (LES) approach. The results of the three-dimensional simulations are used to clarify some still poorly known aspects, i.e., the dynamics in proximity to the point where the abrupt change in the channel cross-sectional area occurs and the effect of problem dimensionality on the flow behaviour at different length scales.
|Date of Award||18 Mar 2022|
- University Of Strathclyde
|Supervisor||Marcello Lappa (Supervisor) & Marco Fossati (Supervisor)|