Fluid Mechanics

Projects

Feature Project: Transition to Turbulence in Complex Fluids

Pumping fluids through pipes is an important part of many industrial processes. Turbulence during pumping greatly reduces its efficiency. In theory avoiding turbulence is achievable as a disturbance over a certain minimum size is needed to trigger it. Knowledge of this minimum size tells us how smooth the pipe needs to be in order to avoid turbulence. However, it has recently been shown with classical fluids, such as oil and water, that this minimum size is very low indeed – so low that in practice it may be entirely impossible to avoid turbulence for anything above moderate flow rates. 

A potential solution would be to change the characteristics of the observed turbulence. Adding small quantities of polymers to a fluid can change its fundamental properties. In this way it is possible to create a shear thinning fluid – one which becomes less viscous as it is forced through a pipe. Although for classical fluids reducing the viscosity actually increases the chances of turbulence, shear thinning fluids exhibit two types of turbulence. Along with the problematic turbulence observed for water or oil, there is also an intermediate ‘weak turbulence’ that has a much smaller impact on pumping efficiency. This proposal seeks to explore and understand the ‘weak state’ and to find conditions for holding off turbulence to improve pumping efficiency.

• Characterising the Earth’s liquid core

  Characterising the Earth’s liquid core

  Much of the mystery surrounding the Earth's dynamics (its magnetic field, plate tectonics) lies in the nature of the convective patterns within its liquid core, and in particular in the region called the “Tangent Cylinder (TC).

  This Leverhulme Trust-sponsored project , led by Prof. Alban Potherat, aims at characterising the state of the liquid core of the Earth- a 2200km thick region between the solid iron inner core and a thick rocky mantle. Heat and solute elements released from here drive thermal and compositional convection, which is in turn shaped by both the planet's fast rotation and its magnetic field. These complex phenomena directly determines the global dynamics of the planet: how fast it cools, how much energy it dissipates, how much energy drives the movement of the large pieces of the Earth's crust and upper part of the mantle, referred to as plate tectonics, and how much its rotation slows down. It also sustains the Earth’s magnetic field through the ‘dynamo’ process.

  Because the core lies beyond reach, the very nature of flow within it is unclear so progress towards a physical and computational interpretation is impeded by insufficient insight into the state of convection in the core. A number of forces, sometimes competing, are also involved: extremely large buoyancy forces that drive convection, opposing damping forces, Coriolis forces due to the Earth’s rotation and electromagnetic force.

  This project combines the most up-to-date numerical techniques from nonlinear hydrodynamic stability theory with geophysical insight and understanding. By initially focussing on the Tangent Cylinder reduces the complexity of the geometry to more manageable levels with two decisive advantages: first, the magnetic field there is predominantly axial and similar to that of solenoidal magnets, enabling us to visualise the nonlinear states found theoretically in an experimental model of the TC obtained by rotating a heated cylinder filled with a weakly conducting but transparent electrolyte inside very high magnetic fields. Second, convection is around 10 times less critical inside the TC than outside it, thus more amenable to numerical and experimental analysis.

  The main goals of the project are to:

  1. Create a complete stability diagram for the states of magneto-convection in a rotating cylinder;
  2. Experimentally observe nonlinear magneto-convective states predicted theoretically for the first time;
  3. Among these states, to identify and characterise those consistent with flow patterns inferred from satellite measurements of the Earth magnetic field (flow intensity, topology, heat transferred).