Alban Pothérat

Separated flows

Flow separation is one the classical phenomena of fluid dynamics and it is ubiquitous. Car manufacturers often display pictures of wind tunnels where the flow around the car is visualised with smoke filaments. These closely follow the contour of the car up to the sharp angles at the back, where they inevitably separate from it. A region of low pressure develops there that "pulls" the car back and increases petrol consumption. Low pressure sucks the flow in this region, where it generates complex vortex structures that soon shed downstream. Similar effects happen at all scales, around mountains, sky-scrapers, tennis balls or air-cooled computer components.
The shape and orientation of the shed vortices determine the feedback force the flow exerts on the obstacle. But because vortices strongly mix the fluid and everything it carries, they also dictate the ability of the wake to transfer heat and chemicals. For this reason, it is common to place obstacles inside heat exchangers, or to alter the shape of ducts to control flow separation and increase the efficiency of the device. When the fluid is electrically conducting, these structures can be affected by an external magnetic field. This happens for instance in the cooling-breeding blankets of thermonuclear fusion reactors, which use a liquid metal as heat carrier, or when a probe is inserted in a liquid metal flow and generates unwanted vortices in its wake. We use shallow water models and 3D Direct Numerical Simulations to explore the fine structure of these flows, and dissect the vortex shedding mechanisms, in generic geometries such as 2D and 3D obstacles, and U-bends.