aCorresponding author: sommer@slf.ch
Copyright owned by the author(s) under the terms of the CC-BY 4.0 license.
A new ring-shaped wind tunnel facility to study wind-packing of snow
Christian G. Sommer1,2,a, Michael Lehning1,2 and Charles Fierz1
1WSL Institute for Snow and Avalanche Research SLF, Davos, Switzerland
2CRYOS, School of Architecture, Civil and Environmental Engineering, EPFL, Lausanne, Switzerland
Abstract. A new wind tunnel was designed and built at SLF. The facility is ring-shaped to simulate an infinite fetch. This is important for experiments where the observed processes have a slow time scale (minutes to hours). The wind tunnel was developed to study the formation of wind-packed snow. The facility is equipped with sensors to monitor environmental parameters such as wind speed, air temperature and air humidity. A SnowMicroPen and an industrial camera allow to measure properties of the snow surface. The facility is flexible and mobile having outer dimensions of 2.3x1.3x0.5 m. Airflow is created by a model aircraft propeller and the wind speed reaches values of up to 10 m/s. First experiments show that saltation is needed for wind crust formation.
1 Introduction
An open-circuit linear wind tunnel already exists at SLF and has, for example, been used to measure the saltation threshold of snow [1]. For slow processes like the formation of a wind slab/crust, the fetch of this wind tunnel is too short because drifting particles leave the facility in less than a second. A ring-shaped wind tunnel mimics an infinite fetch and is therefore better suited to study the formation of wind-packed snow. Wind-packing is the process through which a hard surface layer is formed due to wind action [2].
Ring-shaped wind tunnels have been built before. At the University of Heidelberg such facilities have been in operation since 1980 and are used to study air-sea gas transfer under different water surface conditions [3].
The primary goal of our new wind tunnel is to study the formation of wind slabs but care was taken to design a flexible and mobile facility. For example, it could also be used to study the evolution of saltating particles or for experiments in the field.
2 First prototype and experiments
To gain a basic understanding of the flow in a curved duct and how drifting snow behaves in such a situation, a prototype wind tunnel was built and used for preliminary experiments. These tests also provided valuable insight as to how such a facility is best operated.
2.1 Design
The prototype had an outer diameter of about 1.3 m. The channel was 18 cm wide and 50 cm high (Figure 1). A model-aircraft propeller created the airflow. The correct propeller was chosen based on the Cordier Diagram [4]
after estimating the pressure drop and the volume flow in the tunnel. The propeller rotated at up to 12000 rpm and was driven by an electric motor. Wind speeds of up to 12 m/s could be reached. The covers of the wind tunnel were removable and a section was made of transparent Plexiglas. The prototype was equipped with a wind speed
sensor and an instrument to measure air temperature and humidity.
Figure 1. The prototype wind tunnel deployed in the field. The outer diameter is about 1.3 m. The channel width is 18 cm. The insert shows the propeller.
2.2 Operation
The prototype wind tunnel was open at the bottom and could therefore simply be placed on the snow surface by two people. To collect fresh snow, the covers were removed and the wind tunnel was left open during a snowfall. Unfortunately, the resulting snow surface in the channel was usually not flat and appeared to have been influenced by wind. Placing the wind tunnel on the snow surface was also problematic as the rings tended to settle considerably, especially in sunny weather.
2.3 Results
Measurements of air temperature and humidity revealed considerable variations of these variables depending on whether the covers were closed or partly open.
During operation, the snow did not accumulate at the outer wall as might have been expected. However, surface features formed rapidly (Figure 2a).
339 ICSE 2016, 8th International Conference on Snow Engineering, June 14-17, 2016, Nantes, France
Large snow accumulations behind the propeller (Figure 2b) revealed the poor quality of the flow in the tunnel. Because of the centrifugal effects, a boundary layer with a logarithmic profile cannot be achieved but the flow quality should be improved nevertheless (see below).
Figure 2. (a): The snow surface in the channel showing surface features. (b): Snow accumulations on the inner wall downstream of the propeller.
3 Redesign of the wind tunnel
To increase the flow quality a honeycomb and two one- metre long straight sections were added to the tunnel (Figure 3). The straight sections also provide additional space for measurements. The covers feature four circular holes, which can be left open or closed. This allows a basic control over air temperature and humidity. When closed, both variables tend to increase. Opening the holes leads to the entrainment and mixing with surrounding air, which is generally drier and colder.
Figure 3. The redesigned wind tunnel.
The new wind tunnel features an automatic data acquisition system based on LabVIEW to measure parameters such as wind speed, air temperature and humidity at three locations as well as snow temperature and snow surface temperature. LabVIEW is also used to control an industrial camera. This camera will detect
drifting snow and allow to perform near-infrared (NIR) photography [5]. Additional measurements include the snow height (millimetre scale) and the snow hardness (using a SnowMicroPen [6]).
The wind tunnel now includes an aluminium structure that allows the tunnel to be lifted by a crane. The new facility is operated as follows: Fresh snow is collected on wooden trays outside the building, the snow-filled trays are then brought inside on a cart and the wind tunnel is lowered onto this snow surface. This ensures that the snow surface is flat and that the wind tunnel is in a stable position. The same procedure to collect snow is used in the straight wind tunnel facility [1].
4 Results
During the winter 2016, experiments were performed to test which processes are important for the formation of wind-packed snow. Based on literature, it appears that the air humidity has to be high and that the main process is fragmentation of snow particles in the saltation layer followed by sintering [2, 7]. Therefore, experiments with and without drifting snow were carried out. Thus far in our experiments, we never observed a relative humidity below about 80% even with the holes opened.
The experiments showed that no hardening occurs if there is no saltation. If there is drifting snow, the hardening is closely correlated with areas of deposited snow. Erosion without hardening and deposition appear to alternate in a random fashion in space and time. For the moment these processes can only be analysed qualitatively.
Figure 4. Relative velocity profile at the center of the channel in the main test section.
The flow quality turned out to be improved by the addition of the straight sections. The flow is still chaotic behind the propeller but can now settle sufficiently until
(a) (b)
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it reaches the test section. The honeycomb only covered the top half of the channel to limit contact with flying snow particles. Unfortunately, many particles still reached the honeycomb, so that it had to be removed due to clogging. The flow in the test section could, however, be further improved by moving the propeller to the rightmost position in Figure 3. This made the saltation intensity more uniform across the width of the channel.
Figure 4 shows a velocity profile in the main test section. As mentioned before, there is no logarithmic profile at the surface. The wind speed is almost constant over much of the channel height. The reference wind speed Vref used to normalized the profile is the wind speed measured in the usual position of the MiniAir wind speed probe.
5 Outlook
For next winter we would like to introduce a system to better control the temperatures in the wind tunnel while the air humidity is now less a concern, because there is no hardening without saltation even if the humidity is high.
At low air humidity, hardening would be even less likely.
Moreover, with saltation, the air close to the surface is almost saturated in any case.
In addition, a system to measure quantitatively erosion and deposition appears necessary. A possibility would be to use a Microsoft Kinect, which is a low-cost 3D scanner. Otherwise, the wind tunnel and its operation will remain largely the same.
References
1. A. Clifton, J. Rüedi, M. Lehning, J. Glaciol., 52, 585-596 (2006)
2. G. Seligman, Snow structure and ski fields, (1936) 3. B. Jähne, Zur Parametrisierung des Gas-austauschs
mit Hilfe von Laborexperimenten (1980) 4. O. Cordier, BWK Zeitschrift, 5, 337-340 (1954) 5. M. Matzl, M. Schneebeli, J. Glaciol., 52, 558-564
(2006)
6. M. Schneebeli, J.B. Johnson, Ann. Glaciol., 26, 107- 111 (1998)
7. V. Kotlyakov, The Snow Cover of the Antarctic and its Role in the Present Day Glaciation of the Continent (1966)
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