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Wind tunnel facility
With the aim of studying snow-atmosphere interactions, Prof. Michael Lehning established a wind tunnel laboratory at SLF Davos in 2001 with the help of SNF R'Equip funding. The laboratory is located nearby Davos at 1645 m a.s.l. and is the highest in Europe. Two wind tunnels are in use in the facility, a large open-loop wind tunnel for the study of drifting snow and sand, a smaller ring-shaped tunnel for the study of the snow-crust formation. In winter, due to the availability of fresh snow on-site, this unique facility allows to conduct experiments on a natural and fresh snow cover under controlled conditions. The efforts in the development of imaging techniques, recently widely employed in wind tunnel research, combined with the possibility to test a real snow cover gives us the great opportunity to shed a new light on the physics of snow-atmosphere interactions and have the potential to bring the SLF wind tunnel laboratory to lead the exploration of the dynamics of the snow cover.
Open-loop wind tunnel
The open-loop wind tunnel is mainly employed for drifting snow experiments in winter. During summer, tests on drifting sand are also conducted, with the aim of testing new measurement devices or to compare both sand and snow saltation processes. In the past, experiments to investigate the effect of shear stress partitioning in plant canopies were conducted in this wind tunnel as well.
The wind tunnel operates in the suction mode with a flow velocity up to 20 m/s. The test section measures 14 m and has a cross section of approximately 1 by 1 m with an adjustable ceiling allowing longitudinal pressure control. To allow a rapid development of the internal boundary layer we use spires and a 4 m long roughness fetch. Depending on the purpose of the study, the setup of spires and roughness elements can be adjusted. The measurements are conducted at the downstream end of the tunnel where the boundary layer and the saltation layer are developed. Moreover tests have also been conducted on the boundary layer over flat, non-erodible, surfaces such as wooden boards or a snowpatch. The dimension of the wind tunnel, typically in the range of the environmental wind tunnels, also allows the study of the boundary layer over complex topography. The availability of snow and ice on-site further allows the investigation of the stable boundary layer development.
Figure 1: Sketch of the SLF wind tunnel setup for a drifting sand experiment
Many measurement techniques have been developed and applied in the laboratory since it was first operated, serving different purposes. At the moment the techniques that are used and maintained are the following:
Between each experiment, a Snow Penetrometer (Snow MicroPen) is also applied, when needed. Snow samples can also be collected to measure the snow water equivalent or to be scanned with the X-ray Computer Tomography available at SLF.
Additionally, air pressure, -humidity and -temperature are continuously monitored.
Figure 2: Panorama view of the open loop wind tunnel (top). Metal trays where the snow accumulates for the experiments (bottom – left). Wind tunnel view of the test section from inside (bottom – right).
Ring-shaped wind tunnel
The special circular shape of this facility allows simulating an infinitely long snow surface (infinite fetch). This is important for experiments where the observed processes have a slow time scale (minutes to hours).
Figure 3: (A) The ring-shaped wind tunnel, (B) A propeller creates the airflow.
The airflow in the wind tunnel is created by a model aircraft propeller with a diameter of 16 cm and rotation speeds of up to 12’000 rpm. Wind speeds up to 12 m/s can be reached. The channel is 20 cm wide and 50 cm high. The outer dimensions of the wind tunnel are 1.5 m wide by 2.5 m long.
The facility is equipped with an automatic data acquisition system using LabVIEW. The system monitors parameters such as the wind speed, air humidity and temperature at several locations, the snow temperature and the snow surface temperature. It is possible to control the wind speed and to some extent the air humidity and temperature. The wind tunnel also features an industrial camera to observe the snow surface. Other instruments can be installed easily because the covers of the wind tunnel can be removed.