The Abdus Salam International Center for Theoretical Physics (EuTuCHe@ICTP)
| Organisation: | The Abdus Salam International Center for Theoretical Physics |
|---|---|
| Location: | Trieste, Italy |
| Website: | http://www.ictp.it |
| Contact: | Joseph Niemela |
Summary
- experimental facility to study convection in low temperature helium up to the worldwide highest level of convective turbulence (Ra 1017)
- superposition of convection and rotation (60 rpm rotation speed) in a unique set-up
- nearly ideal boundary and operating conditions
- liquid helium PIV, non-intrusive light scattering and fluorescence measurement techniques
The facility offer 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 heluim, such as particle tracking methods in super fluid flows, will constitute one of the focal points of the research at this 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 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. 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, and exceeds the capabilities of all known running DNS codes, unlike other facilities utilizing conventional fluids. The cryostat has considerable flexibility with a central shaft built-in which permits access for adding grids, etc. to the sample space.
Besides the ability to cover large ranges of Rayleigh number up to a record high value, the other attributes of this facility are 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 horizontal bounding plates compared to fluid making nearly negligible corrections due to the finite plate conduction as the dimensionless heat transfer become very large. At least one order of magnitude advantage here is noted over similar room temperature experiments using water or air. The Prandtl number is near unity and, importantly, constant over many decades of Ra, but can be made larger near critical point if desired; Boussinesq conditions can be maintained to higher Rayleigh numbers than are possible with any other existing facility. The facility is uniquely capable of attaining very high Ra in large aspect ratio cells that are important for geophysical considerations; presently, the large 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 the 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.
Present Instrumentation
Thermometry
Thermometry employing semiconductor thermometric devices has been developed in the laboratory and elsewhere. Inexpensive and readily available micron-sized thin film devices are available from a collaboration with scientists in the Ukraine. Hot wire development has historically also been a part of the laboratorys expertise, but is presently a duplication of efforts in Grenoble.
Particle Image Velocimetry (PIV)
The Facility and its coordinators have extensive experience in developing PIV for use in liquid helium both above and below the superfluid transition temperature, in collaboration with University of Oregon, University of Maryland, Yale University and the University of Birmingham, UK. This has resulted in the first ever direct visualizations of quantized vortices as reported recently in Nature magazine, and more recently superfluid vortex reconnection events.
Non-Intrusive Light Scattering in Liquid and Gaseous Helium at Low Temperatures
In conjunction with the ICTP Laser Laboratory at Elettra, presently under the coordination of J. Niemela, techniques involving the fluorescence of helium molecules as a tracer element will be developed as part of this project. This technology, relying on the excitation of helium molecules briefly to metastable states, will 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 will allow the exploitation of the large ranges of Reynolds and Rayleigh numbers thereby available, which is presently not possible with macroscopic particles. By its direct partnership with the ICTP laser laboratory, the African Laser Atomic Molecular and Optical Sciences Network (LAM) and indirect collaboration through the parent institution with the Istituto Nazionale di Fisica Nucleare (INFN), and the Fermi FEL laser laboratory the cryogenic facility affords the possibility to make considerable headway in this needed technological area.
Scientific Environment
The facility resides at the Elettra Synchrotron Laboratory in the Area Science Park, Trieste. The presence of 23 beamlines and their supporting laboratories, in addition to the neighbouring INFM (Instituto Nazionale per la Fisica della Materia), provides a stimulating research environment and access to many specialized services. By holding or hosting numerous scientific activities, including advanced schools, conferences and workshops continually throughout the year, the ICTP provides to all visitors a stimulating research environment. In additions there are Masters degree program in conjunction with neighbouring SISSA, and a Ph.D. program in environmental Fluid Mechanics jointly conducted between the ICTP, the University of Trieste, and regional governmental research agencies that provides for degree training in research directly connected to the proposed activities of the ICTP Cryogenic Facility. A superb scientific library, one of the finest and most extensive in all of Europe, is available to scientific guests at ICTP.
Outside users of the facility
The facility welcomes both theorists and experimentalists throughout Europe and the rest of the world who wish to have the unique access available at ICTP to ultra-high Rayleigh number and high Taylor number convection in either small or large aspect ratios. We are receiving letters of intent from scientists around the world to use the ICTP facilities.
From 1970 to the present, over 34 thousand European scientists have come to ICTP spending 18,750 person-months at the Center. In addition, ICTP allows connections to be made North-South and East-West between the developed and developing world. By coming to ICTP, the best researchers of the developing world not only benefit from a world class research environment, but by establishing collaborations in research while here and after returning, they themselves help strengthen the state of science in Europe.
We have had many visiting scientists, theoreticians and experimentalists, spending time either doing experimental research using our facility, or theoretical work at the ICTP main building. These include, recently, Professors Victor Steinberg of the Weizmann Institute in Israel, Peter Lucas of Manchester University, UK, Kazuo Kitahara (Japan), Jörg Schumacher (Germany), Victor Yakhot (USA), P.K. Yeung (Hong Kong/Georgia Tech.)
