Göttingen Turbulence Facility
| Organisation: | Max-Planck-Institute for Dynamics and Self-Organization |
|---|---|
| Location: | Göttingen, Germany |
| Website: | http://www.ds.mpg.de |
| Contact: | Eberhard Bodenschatz |
Investigations of the fundamental properties of turbulence require flows with high Reynolds numbers under well-controlled conditions. The flow properties need to be resolvable by modern measurement technology from the largest to the smallest spatial and temporal scales. On Earth the highest turbulence levels (Reynolds numbers~107) are found in the atmospheric boundary layer. Even the most violent flows on Earth, such as plinian volcanic eruptions, have similar turbulence levels. The observations of natural flows are difficult, as the conditions are rarely stationary and the scales of the flow are very large and make detailed measurements utmost difficult. For example, when considering the turbulent motion of clouds in the atmospheric boundary layer, the largest scales of the flow are typically 100m, while the smallest scales are fractions of millimeters. Another complication is that clouds are carried by a mean wind. With current measurement technology it is very difficult, if not impossible, to resolve all scales of the dynamics of turbulent clouds. In addition, the turbulence generation mechanisms in nature are multifold and it is difficult to investigate how the turbulence depends on its generation mechanisms. In the foreseeable future computational fluid dynamics can substitute experiments only for moderate Reynolds numbers at idealized flow conditions. Therefore, experimental facilities generating turbulent flow fields at high Reynolds numbers are essential for the research. The properties of turbulence can either be measured from the spatial (Eulerian) perspective or from the perspective of particles carried by the flow, the so-called Lagrangian perspective. While Eulerian measurements, have traditionally been conducted in wind tunnels with hot wire anemomentry, only recently it has become possible to conduct Lagrangian measurements with high accuracy at high Reynolds number thanks to the advance in imaging technology. High Reynolds-numbers at manageable temporal and spatial scales under well-controlled conditions can be realized by employing the principle of physical self-similarity. The turbulence Reynolds number scales as Re=pvL/n, where p is the density of the fluid, v is the fluctuating velocity, L is the energy injection scale and n is the molecular dynamical viscosity. One-way to achieve high Reynolds numbers is to use cryogenic He gas. In this case very high Reynolds numbers can be achieved, however, with very small spatial and temporal scales that currently cannot be fully resolved by measurement technology. In addition, many methods well tested at room temperature are difficult to use at cryogenic temperatures. An alternative way is to increase the density by pressurizing the gas. This increases the Reynolds numbers, as the molecular dynamical viscosity is approximately independent of pressure. In addition, by using a heavy gas, like sulfur hexafluoride (SF6) it is possible to reach high Reynolds numbers already at moderate pressures of only 10-20 bar. In Göttingen we have decided to go the second path.
We have installed two facilities that use pressurized SF6 gas at up to 19 bar. The institute installed a gas handling and liquefaction system that handles and stores 12 tons of SF6. The gas is used in two facilities - the Göttingen Turbulence Tunnel and the Göttingen U-Boot. While the first is a wind tunnel with an extra long measurements section to allow particle tracking in the decaying turbulence behind a passive or active grid, the later is used for the investigations in Lagrangian mixers and of turbulent thermal convection. In addition to serving the in-house experimentalists, the GTF provides visitors with the possibility to study phenomena in turbulent flows under very well controlled conditions at high Reynolds and Rayleigh numbers.
The Building
Both facilities are housed in a newly constructed building that has been optimized for vibration isolation and high temperature stability. It has been equipped with control systems allowing the safe use of the pressurized gases and of lasers. High bandwidth fiber optics links the facilities to a 80 processor data analysis cluster. A 36m² class 1000 clean room is available for micro fabrication of sensors, like the hotwire probes needed for the Eulerian measurements in the Turbulence Tunnel.
The U-Boot
This is a general-purpose pressure vessel. It has been designed to house different experiments. Similarly to the turbulence tunnel, all equipment can be used inside the vessel for measurements from heat transport and PIV to 3D-Langrangian Particle Tracking (LPT). Experimental inserts available include a von Kármán type mixer with Rλ~3,500 and a turbulent cylindrical Rayleigh-Benard experiment of 1.1m diameter and 2.2m height that will reach Rayleigh numbers as large as Ra~1015. The Turbulence Tunnel: Reynolds numbers of up to Re~107 are possible in this recirculation tunnel when filled with SF6 at 15bar. The tunnel is upright and consists of two measurement sections with a cross-sectional area of 1.9m² and lengths of 9 m and 7 m, respectively. Passive or active grids that are mounted at the entrance of each measurement section generate the turbulence. Two sleds, driven by linear motors, are installed that allow measurement devices (e.g. cameras and optics) to be moved with the mean velocity (up to 5 m/s) of the circulating gas. The tunnel is pressure and temperature controlled, and has optical and electrical access. The circulating gas can be filtered to <1µm in order to provide a clean gas. Equipment can be installed either on the sleds or all along the measurement sections. Measurement equipments to be used in the tunnel are high-speed cameras, LDV/PDA system, and hot wires.
Specifications
Tunnel
length 18 m, height 6 m, inner diameter 1.8 m, pressure 1 mbar - 15 bar, temperature: 20-35°C, mech. power 210kW, cooling power 280kW, kin. visc. SF6 (15bar): 1.5x10-7m²/s. Expected properties: <u>max =5m/s, urms,max=1m/s, Lmax=0.45m, Rλ.max ~104, εmax =1.4 W/kg, η>8 μm, τη>0.4msec.
U-Boot
length 5.3 m, max. height 4 m, outer diameter 2.5 m, straight cylinder length 4 m, dome 1.5 m high and 1.2 m in diameter, pressure 1 mbar-20 bar, temperature: 20-35ºC, cooling power <50 kW. Expected properties: Ramax = 7x1014 at high Ra only weakly non Boussinesq.
Properties Rayleigh-Benard cell: Ramax = 7x1014 at high Ra only weakly non Boussinesq; Lagrangian stirrer: <u>max=0m/s, urms,max=1m/s, Lmax=0.1m, Rλ,max=4500, εmax =5.5W/kg, η>5μm, τη>0.2msec.
Gas Handling System
4 tanks, 2.8m³ each, height 4 m, diameter 1 m, oper. pressure 1 mbar-15 bar, evacuation of tunnel air (1 bar → 1 mbar )2.5h, pressurizing SF6 1 mbar→ 15 bar: 8 h, depressurizing SF6 15 bar → 1 mbar: 22 h, filling with air (1 mbar → 15 bar): 48 h
Measurement Systems
3D-LPT (36kHz), TOMO-PIV, LDV, PDPA, hotwires, lasers, optics, data analysis and storage cluster.
