Summary

• worldwide largest experiment (7.0 m x 6.3 m) to study highly turbulent convection in air with unrivalled spatial and temporal resolution
• full measurement access into a very well controlled space of 250 m³ with a regular and easily changeable geometry
• cutting edge flow measurement techniques: LDV, PIV, PTV, ultra-small temperature probes
• roughly isotropic turbulence with zero mean velocity in the centre unlike to all the wind-tunnel-like facilities

The "Barrel of Ilmenau (BOI)" presents a large-scale Rayleigh-Bènard experiment built up to study highly turbulent convection in its pure form but recently also used e.g. to test novel measurement techniques or to verify indoor flow computations.

The fundamental study of convective flows is a very important issue in these days quite relevant to very ongoing questions related to the earth climate, the weather or the properties of the geomagnetic field as well as to a large number of technical applications. Despite of the giant progress in the computational technique it is still quite hard or sometimes even impossible to predict these very complex flows and therefore simplified model experiments are an exclusive way to study their properties. One of the best-known and in the last hundred years intensively explored models is the Rayleigh-Bènard (RB) experiment an adiabatic box which is heated from below and cooled from above (see figure). The BOI presents such an apparatus in which a turbulent air flow can be investigated up to Rayleigh numbers of Ra=10¹² (Ra=β*g*ΔT*H³/(ν*κ)). The experimental facility consists of a virtually adiabatic cylinder filled with air and shielded at the sidewall by an active heating system. An electrical heated plate at the bottom as well as a free hanging cooling plate at the top triggers the convective motion of the air between them. Both plates with a diameter of 7 meters are carefully designed to maintain a very constant and homogeneous temperature with a deviation below 1 K. A further unique feature of the apparatus is that the distance between the both plates can be varied continuously between 0.05 meters and 6.30 meters by lifting the cooling plate. While experiments using the maximum height are interesting to depict flows with highest complexity and the broadest variation of turbulent structures, the geometry at moderate heights is closer to the geometry of typical geophysical flows. Compared to the similar low-temperature helium facility at ICTP able of achieving higher Rayleigh numbers (up to Ra=10¹⁷) with rotation, in our facility the access for the measurement of velocity or temperature is very easy and these quantities can be measured with an unrivalled spatial and temporal resolution. The BOI also complements the GTF and the CICLoPE experiment, being basically large wind tunnels, since it enables the user to study highly turbulent flows at very small velocities and superimposed by a heat flux.

In order to investigate the flow field inside the cell a broad variety of modern flow measurement technique is available: a 3d laser Doppler anemometer (LDA), a 3d particle tracking velocimeter (PTV, under development), a 2d particle image velocimeter (PIV), a 3d hot-wire system and a multi-channel system for temperature measurements with temperature probes of a size of 125 µm. This technique will be improved and complemented by novel hot-wires, ultra-small velocity and temperature sensors and optimized algorithms for PTV. All the experimental data shall be published in a standardized form in the joint database of the EuHIT infrastructure.