Séminaire ISTerre

Modal acoustic velocimetry in a gas-filled rotating spheroid

mardi 4 février 2020 - 10h00
Sylvie SU - ISTerre (soutenance de Thèse)
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Zonal flows are expected to play a major role in the heat transfer mechanism of geophysical bodies such as the Earth’s core or Jupiter’s atmosphere. However due to the turbulent regime the systems are in, and the specific force balance, dominated by rotation, it is very hard to accurately model zonal flows. This causes their formation and dynamics to still not be well understood. Experimental studies propose a complementary approach where all scales of structures coexist. However most of the existing experimental set-ups remain far from the relevant regimes found in the natural systems. We present a new laboratory experiment built to study zonal flows induced by thermal convection, called ZoRo. The originality of this new apparatus resides in the fact that the balances, forcings and geometry are closest to the geophysical bodies. In order to match those, ZoRo is a rapidly rotating spheroid (flattened sphere) filled with air. Since the working fluid is air (a gas), it is very challenging to accurately measure its velocity. To tackle to this problem, we implemented a new non-intrusive velocimetry technique based on acoustic resonances of the fluid cavity, the modal acoustic velocimetry (MAV). The quantity of interest is velocity, but other effects can influence the acoustic spectrum. In order to isolate its effect, we develop theoretical predictions that take into account the main relevant effects that influence the acoustic spectrum (cavity’s geometry, fluid’s diffusions, container elasticity, etc.). Once the velocity contribution is isolated, it is possible to retrieve the flow field using acoustic frequencies through an inverse problem. We test this method against both synthetic cases and real experimental flows measured in ZoRo. MAV is a well-suited technique for measuring global azimuthal flows such as zonal flows. It is very sensitive to variations in the flow structure, down to a few centimetres. We were able to image the internal flow velocity with non-intrusive sensors thanks to MAV. This thesis is a first landmark towards realistic zonal flows laboratory experiments. The velocimetry technique is fully developed, allowing accurate non-intrusive measurements of zonal flows.

Equipe organisatrice : Géodynamo

Amphithéâtre Killian, Maison des Géosciences, 38400 Saint Martin d'Hères