High Reynolds number turbulent boundary layers in low temperature classical or superfluid helium – ECOUTURB

The technical objective of the project is to use smooth disks or disks with a controlled roughness, and operate them in cryogenic liquid helium above and below the superfluid transition (the mass fraction of the superfluid phase is a function of the temperature).
The flow will be characterized both by global measurements (torque, wall pressure, mean vorticity which are spatially integrated), and local or almost-local measurements with miniature sensors inside the bulk of the flow (miniature anemometers, thermometers, dynamic acoustic scattering and second sound attenuation). The torque measurements are the most sensitive to the viscous sublayer and will allow us to probe the smallest scales (boundary layer width), where the normal and superfluid flows are thought to differ. The system will therefore be highly flexible.
The scientific objectives that we wish to address are:
• The behavior of a turbulent boundary layer at atmospheric Reynolds numbers and the way this type of variable boundary layer evolves, in presence of a turbulent bulk, into a turbulent boundary layer. This is the equivalent, but in closed flow, of the drag crisis. The extrapolation to these Reynolds numbers from the current laboratory experiments is not trivial. We have the only laboratory experiment capable of reaching those Reynolds numbers;
• The acoustic scattering inside a boundary layer at very high Reynolds numbers;
• For both points above, the alteration of the boundary layer properties when part of the fluid becomes superfluid;

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Our project relates to the study of Von Kármán (VK) flows (between counter-rotating disks) with smooth or rough disks using low temperature helium as the working fluid, either in classical or superfluid phases.
The high-power cooling station in INAC-SBT (0.4 MW electrical power supply providing 400 W cooling power at 1.8 K) and the SHREK infrastructure allows to operate such a flow with large disks (close to 80 cm in diameter), much larger than other available cryogenic systems, and is presently the only facility in the
world capable of sustaining stationary turbulence at Reynolds numbers up to 10^8 with normal and superfluid liquid helium. When the driving disks have no blades the flow will be dragged into motion by the effect of shear, in contrast with more usual cases in which it is pushed (or pulled) by blades. In the former situation, momentum is transferred to the fluid directly from the boundary layers and turbulence in the bulk can be qualified as “shear induced”.
In this configuration, we can study the bulk turbulence, the boundary layers and their interaction. One way is to perturb the boundary layers and measure changes in their properties. To that end, disks with tailored roughness will be used to monitor the shear forcing mechanism via the introduction of a new fixed length scale.
Using these configurations, we can address several physical important questions about boundary layers, high Reynolds number turbulence: on the one hand, with a classical fluid (normal helium), we can reach atmospheric Reynolds numbers, and study its dynamics; on the other hand, the drag (the torque on
the disks) is a global (i.e. space integrated) quantity which is sensitive to very small scales (the viscous sublayer in the normal case), much smaller than the resolution of available local probes. At this scale, we expect a difference between normal and superfluid flows, which paradoxically does not appear at larger
scales. If the typical roughness size is chosen close to the inter-vortex spacing, there will be new quantum hydrodynamical effects, never explored so far.
throughout these studies, we plan to perform both global and local measurements.
The former consists in torque measurements, parietal pressure and acoustic scattering, as well as large scale Pitot tube, rugged anemometers and large He II second sound probes.
The latter is more challenging and will require further developments of original sensors, such as new micron-size hot-wires suited to the low temperatures and
second sound tweezers to gain access to the local density of quantum vortex lines, and also new types of local velocity sensors based on cleanroom technologies, such as cantilever anemometers.