Wed March 27th 2013
Seminar Double diffusive convection - Application in the Geosciences
Ulrich Hansen


Double diffusive convection takes place if the density of the fluid is influenced by at least two components with different molecular diffusivities. Convection in the oceans is a classical example, as salt and heat determine the density of the sea water. Interesting double diffusive phenomena occur, if one of the components acts as driving, the other as restoring force. A fluid with a stably stratified salt profile, however being unstably stratified with respect to temperature, may serve as an example. Besides two Rayleigh numbers, measuring the strength of the driving (restoring) force,, the ratio of molecular diffusivities is of key importance. Often this is expressed as the Lewis number, being the ratio of thermal to compositional diffusivies. Such scenarios, in which the fast diffusing component (i.e. temperature) acts as a driving force, while the slowly diffusing component (composition) has a restoring influence, are called the diffusive regime. The finger regime describes the opposite case, i.e. driven by the slowly diffusing component and retarded by the fast diffusing component. In the seminar I will address some fundamental aspects of double diffusive convection, especially the case of subcritical flow. At a sufficient high Lewis number, strongly subcritical flow can exist, once excited by a finite amplitude perturbation. Another important aspect of double diffusive convection is the self-organization into layered flow structures from non-layered starting conditions. This phenomenon is important for many natural systems, ranging from layer evolution in the oceans to the layered structure of virtually all planets. I will discuss the evolution of layers in planetary bodies as a result of double diffusive convection. On a smaller scale, structures in magmatic systems (for example Kilauea, Hawaii) can potentially be explained by the interplay of compositionally and thermally driven forces. Double diffusive convection in the finger regime can create fine-scale
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The 10th Complex Motion in Fluids 2021
Max Planck Gesellschaft
Centre for Scientific Computing