The facility offers the highest Rayleigh numbers achievable anywhere in the world under strictly Boussinesq conditions, with nearly ideal boundary conditions and varying aspect ratios up to 4. State of the art micro-thermometry, in addition to development of cryogenic LIF, and the possibility to apply precision-controlled rotation is important for fundamental studies in convective turbulence, with applications in geophysics. Development of new measurement techniques for liquid helium, such as particle tracking methods in super fluid flows, will constitute one of the focal points of the research at this facility.
Facility and equipment
HRCF (High Rayleigh Number Cryogenic Facility)
The High Rayleigh Number Cryogenic Facility has as its centrepiece a thermal convection cell with maximum allowable height of 1 meter and 0.5 meter in diameter. The facility can cover a range of 12 decades of Rayleigh numbers, all in the turbulent regime, up to a world-record high of 1017 under nominally Boussinesq conditions. It is the largest such facility currently in existence anywhere in the world. The large range of Rayleigh numbers, crucially important for discerning scaling relations, is accomplished by varying the operating point of the cryogenic helium gas used as a working fluid in the facility, which allows the boundary conditions to remain constant. The maximum Rayleigh number puts the experiment at the forefront of experimental convection research, especially in terms of making additional contact with geophysical and astrophysical problems of interest. The cryostat has considerable flexibility with a central shaft which permits access for operating movable temperature probes, adding grids, etc. to the sample space accompanied by an additional top heated plate adapted and available for this purpose. For this reason the range of applications can be extended to isothermal grid flows for instance.
Another attribute of the HRCF is its nearly ideal boundary and operating conditions; namely,
- a hard, cryo-pumped vacuum that insulates the experiment from parasitic heat inputs due to conduction or convection in the surrounding space;
- truly negligible heat leakage from radiation;
- large thermal conductivity of the horizontal heated bounding plates compared to the fluid making nearly negligible any corrections due to the finite plate conduction as the dimensionless heat transfer becomes very large.
For (3) above, there is at least one order of magnitude advantage over similar room temperature experiments using water or air. The Prandtl number is near unity and can be constant over many decades of Ra. Large aspect ratio cells-- important for applications to geophysical problems—can be utilized in this facility without sacrificing high Rayleigh number. Presently, the cryostat sits on a rotating platform, capable of rotations speeds up to 60 rpm, in both directions, and with a maximum acceleration of 2x105 deg s-2 and with speed accuracy to within 1%. In non-dimensional terms, this allows access to very large Taylor numbers (Ta>1015) characteristic of atmospheric flows. With liquid in the cell, it is straightforward to maintain stably stratified conditions under strong rotation, so that turbulent zonal flows with planetary beta-effect can be studied. This work has already been proposed by Nazarenko (Warwick) and Galperin (USF) for optimization in the ICTP Facility, complementing similar turbulence research being conducted at the large Coriolis facility in Grenoble.
Instrumentation: Germanium resistance thermometers are used for averaged temperature measurements in addition to micronic Neutron-Transmutation-doped (NTD) crystals for measuring fluctuations of temperature in the bulk. Micron-sized thin film devices are also available for such fluctuation measurements. Mean pressure is measured using a series of high-precision Baratron gauges. The Facility and its collaborators have extensive experience in developing Particle Image Velocimetry (PIV) for use in liquid helium both above and below the superfluid transition temperature. In addition, in conjunction with scientists in the Elettra Synchrotron Facility, techniques involving the fluorescence of helium molecules as tracer elements are to be developed in parallel. This technology, relying on the excitation of helium molecules briefly to metastable states, would allow local measurements of velocity with a tracer particle that is more truly non-intrusive. By functioning also in the gaseous phase of helium, the technique would allow the exploitation of the large ranges of Reynolds and Rayleigh numbers available. With its direct partnership with the ICTP/Elettra laser laboratory, the African Laser Atomic Molecular and Optical Sciences Network (LAM), the Istituto Nazionale di Fisica Nucleare (INFN), and the Elettra Synchrotron Laboratory, the cryogenic facility affords the possibility to make considerable headway in this area.