Characterising Flow Regimes and Transitions, Heat Transport and Energy/Enstrophy Cascades in Rapidly Rotating Thermal Convection

What is the problem?
Turbulence driven by thermal convection is a ubiquitous process that occurs in many situations in both nature and in various technologies, ranging from the atmosphere and fluid interiors of planets (including the Earth) and stars through to industrial processes such as in chemical engineering, food preparation and power generation. When convection takes place in a rotating system, convection may radically change its character, developing coherent structures that align with the axis of rotation and significantly influence the efficiency by which heat, momentum and even mass are transported within the flow. Predicting how the strength of rotation and differential heating and cooling determine the flow and its transport properties and depend on other factors such as the shape of the system are especially difficult because of the complexity of the flow and the role of nonlinear feedbacks.

Why is it important?
Predicting the properties of rotating thermal convection is well known to be important in determining the shape and intensity of flows in the atmosphere, oceans and deep interior of the Earth, for example, influencing their climate and predictability. But it is also of major importance for the design of devices such as gas turbine engines used for aircraft propulsion and power generation. Strong temperature contrasts may develop between surfaces inside various rapidly rotating components of these engines that have been found to lead to complex convection patterns that significantly affect the transfer of heat within these components. As the designers of such engines attempt to improve their fuel efficiency and performance, manufacturing tolerances e.g. between turbine blades and their shrouds are becoming more and more demanding, requiring close control of temperatures throughout the engine under all operating conditions. It is vital, therefore, to improve our understanding of, and ability to model and predict, the structure and behaviour of the turbulent convection inside these engine systems and how it responds to changing conditions.

What will this project achieve?
This project seeks to improve our understanding of rotating convection under conditions that are similar to those found (a) inside rotating cavities within components of turbine engines and (b) in highly turbulent flows encountered in the atmospheres and interiors of rapidly rotating planets. We plan to construct a laboratory experiment that can generate turbulent convective flows inside a rapidly rotating cylindrical annular tank (i.e. the cavity between two co-rotating coaxial cylinders) by heating or cooling the inner and outer cylindrical walls. The cylindrical cavity can be rotated at different speeds about a vertical axis to include conditions that are either dominated by gravity acting in the vertical direction or by centrifugal forces acting in the radial direction. The latter are most relevant to conditions inside turbine engines or the convective fluid core of the Earth or other planets, while the former emulates the conditions found in planetary atmospheres or oceans. By conducting carefully controlled experiments over a broad range of rotation rate and thermal contrasts, we aim to determine how the flow changes in character from one regime to another and to quantify the impact of these changes on properties such as heat transfer and the formation of large-scale coherent structures such as vortices and zonal jets. This would be the first time both of these regimes would have been studied in the same experimental system, allowing us to gain new insights and understanding of these flows drawn from the fields of engineering science and geophysics. We plan to compare our experimental results with numerical model simulations obtained by engineers at the Universities of Bath, Surrey and Oxford of air flows in systems similar to gas turbine engine cavities to help improve their ability to predict flow structure and behaviour.