Density gradient experiments in cryogenic liquid helium – CryoGrad

Our approach is based on a glass cryostat, which allow full optical access. We adapt non-invasive visualization methods, such as shadowgraph and synthetic schlieren. The quantitative processing of these images is backed by model experiments at room temperature where the results can be compared to other kinds of measurements (such as PIV).
The density gradient is obtained with a temperature gradient. The high value of the thermal expansion coefficient in low temperature helium allows to obtain high density gradients, and reach buoyancy frequencies similar to natural systems, with a working fluid more than 50 times less viscous than water, hence allowing high Reynolds numbers.

Our first shadowgraph and synthetic schlieren experiments have been carried out, and allowed to ensure the sensitivity of these visualization methods in helium. They have allowed to visualize turbulent thermal plumes, and to monitor the full field density gradient in sub-cooled helium bath.

We will now investigate quantitatively the plumes and thermals, in homogeneous or stratified background, and build a small-scale Rayleigh-Bénard cell.

Belkadi et al, submitted to JFM. hal.archives-ouvertes.fr/hal-02329929

The objective of this project is to study the dynamics of density-driven flows at high Reynolds number. To that end, we will adapt the method of synthetic schlieren, well known in the laboratory for plume visualization and for internal waves experiments in water, to the case of low temperature liquid helium. It is a modern version of the classical schlieren photography, in which a background dot pattern is imaged across a fluid layer instead, allowing much easier quantitative processing of the recording. As it consists in measuring displacement, instead of intensity, the sensitivity can be made higher by adjusting the distance between the pattern and the region of interest, and by using high resolution camera. It will allow unprecedented whole-field quantitative temperature measurements in low temperature helium.

The motivation is to produce model experimental systems to study flows where buoyancy play a major role. We will use regulated heaters to control temperature gradients in the experimental cell. An unstable gradient yields turbulent thermal convection, in a range of parameters complementary to the water experiments, and in an entirely new way compared to existing cryogenic convection cells, because focused on imaging. It is very different from traditional cryogenic Rayleigh-Bénard cells, designed for fine heat transfer measurements, which have no optical access. Our intent is instead to adopt the fluid dynamicist approach that proved successful for room temperature experiments. A stable gradient allows to produce internal gravity waves, with much reduced dissipation than water, which allows unprecedented laboratory experiments much closer to oceanic flows. This was never attempted in cryogenic helium, and is made possible in our laboratory by the collaboration of experts of internal waves who have experience with laboratory experiments using salt water, and experimentalists who have had experience with low temperature hydrodynamic experiments.

The development of low temperature experiments in Lyon will strengthen our commitment to work within the national consortia on cryogenic hydrodynamics, and to reinforce the collaboration with our partners. Indeed, we have been actively participating in national projects which involve large infrastructures in Grenoble (Institut Néel and CEA Grenoble) and in CERN, and we intend to continue to do so. The experiments developed in this project are different, in nature and in scale, from what is done within these large cryogenic centers. In particular, this project will benefit from the complementarity with our room temperature experiments which we will continue to develop as well. In the long run, our low temperature experiments will provide us with tools which will increase our ability to contribute to the large-scale cryogenic turbulence projects, and therefore secure our position as long-term partners in these consortia.