The understanding of fundamental relationships in turbulent convection flows is relevant for a number of highly topical issues related to climate change, weather forecasting and the generation of the Earth's magnetic field, but also for a variety of technical flow processes. Despite the great progress in computer technology, it is still very difficult or often even impossible to describe or predict these very complex flow processes. Simple model experiments are therefore an effective means of studying the properties of these flows. One of the model experiments that has been intensively studied over the past 100 years is the Rayleigh-Bénard experiment. It consists of an adiabatic test cell in which a fluid is heated from below and cooled from above (see figure). The "Ilmenau barrel" represents such an experiment, in which turbulent air flows up to Rayleigh numbers Ra=10^12 (Ra=β*g*ΔT*H3/(ν*κ) can be investigated. The experiment consists of a cylindrical cell containing air, the side wall of which is nearly adiabatic due to an active compensation heating system. An electric heating plate at the bottom and a cooling plate freely suspended above it drive the thermal convection. Both plates with a diameter of 7 m are designed in such a way that a uniform temperature with a maximum spatial deviation of 1 K and a maximum temporal variation of 0.02 K is established on their surface. Another unique feature of this experiment is that the distance between the heating and cooling plates can be varied continuously between 0.05 m and 6.30 m. This allows the temperature to be adjusted to the desired value. While the highest Rayleigh numbers can be achieved in experiments with maximum plate spacing, geometries with smaller plate spacing are more similar to typical geophysical flows. Compared to similar experimental setups, where significantly higher Rayleigh numbers up to Ra=10^17 can be achieved by using cryogenic helium, the metrological approach in our experiment is much simpler and velocity and temperature can be measured with much higher spatial and temporal resolution.
The "Ilmenau barrel" represents a large-scale Rayleigh-Bénard experiment in which highly turbulent convection flows can be studied experimentally. It is also used, for example, to verify the simulation of room air flows or to test new measurement methods.
World's largest Rayleigh-Bénard experiment (7.0 m x 6.3 m) for the study of highly turbulent convection flows in air
allows unrestricted measurement of the velocity and temperature field with unprecedented spatial and temporal resolution
state-of-the-art flow measurement technology, such as 3D laser Doppler velocimetry, 3D particle tracking velocimetry, 2D particle image velocimetry, multi-channel temperature measurement system with microthermistors
- nearly homogeneous and isotropic turbulence in the inner core of the Rayleigh-Bénard experiment, free of artificial mechanical excitation
Heat transfer through flowing media plays an important role in nature and technology. Convective heat flows are usually turbulent and three-dimensional and thus very difficult to characterize. Therefore, the time and location dependent velocity and temperature fields are investigated in model experiments, e.g. in Rayleigh-Bénard cells with the working fluids air, SF6, water and liquid metals. The flow and temperature fluctuations, which are decisive for the turbulent heat flow, are determined with non-contact methods such as laser Doppler anemometry (LDA), particle image velocimetry (PIV), ultrasonic Doppler velocimetry (UDV) and laser-induced fluorescence (LIF) as well as with thermographic liquid crystals (TLC). The results of the model investigations are used, among other things, for the validation of numerical simulations (CFD).