Thermal turbulent convection is reviewed, with emphasis on the low temperature experiments on helium gas.
In his referred paper on the problem of turbulence [l] Landau wrote "Although the turbulent motion has been extensively studied from different points of view, the very essence of the phenomenon is still lacking clearness. The problem may appear in a new light if the process of initiation of turbulence is examined thoroughly". As experimentalists, we undertook a program to study the unsteadiness of the laminar motion, starting from the onset of free convection. Following Threlfall [2], we used helium gas at low temperature as the fluid, for two main reasons. First a large span of Rayleigh numbers can be obtained by changing the gas density, going up to the liquid-gas critical point. Second, at helium temperatures, the kinematic viscosity can reach very small values, of the order of h/m. The region near the critical point is an interesting one, where the specific heat and the expansion coefficient diverge. This leads to possible huge Ra numbers, but at the same time strong non-Boussinesq effects, and critical fluctuations of the density. We will not touch here this problem. Thus, large Rayleigh (Ra) and Reynolds (Re) numbers can be reached in a closed system of reasonable size, with good thermal stability and well defined boundary conditions. Precisely, in our last cell size, 40 cm height, we can get close to Ra = 1016 and Re of the order of 10". We will describe in this paper in an informal way our main observations.
Up to now, we have used two experimental cells. The first one, a cylinder of aspect ratio one, and diameter 8.7cm, could reach 1013 in Ra number. The second one, used now, was built to reach the highest possible Ra number. Its aspect ratio is i, and its height 40 cm, and we will discuss some results obtained up to Ra = 10i4. It will be somewhat difficult to reach higher values without important non-Boussinesq effects.
As was just said, helium is used to go to the highest turbulent state. Two other interesting aspects can also be studied; The effects of droplets of liquid on the transport properties, a dramatic one, may have interesting implications