| |
By Per Olowson, ÅF-Energi&Miljö AB, Göteborg, Sweden
Courtesy of the Swedish Touring Club (STF)
View the pdf of this article
The CFD model of the mountain station
illustrates the surrounding hilly terrain
in the north of Sweden
Drifting snow can cause significant problems
in open terrain. Snowdrifts usually develop
behind the crests of mountains or hills,
in surface depressions, or around obstacles such
as buildings, fences, or trees. In exposed, windy
locations, the amount of snow that accumulates
due to snowdrifts can be hundreds of times greater
than the snow that falls directly on the ground.
Strong winds and low visibility due to drifting snow
are often reasons for closing mountain roads. Around
buildings, snowdrifts can cause serious problems
with accessibility and the blockage of emergency
exits. Snowdrifts also increase the wear and tear
on buildings, as well as the maintenance costs.
The snow deposited around a building in a windy
place is usually more a result of the wind transporting
and redistributing the snowflakes than of
free falling snow. The wind patterns around a
building therefore play a strong role in determining
where and how much snow will accumulate.
The Swedish Touring Club (STF) has a number
of mountain stations that are open for the
public throughout the year. One of the stations,
Sylarnas mountain station, is situated in the north
of Sweden, 1000 meters above sea level, close
to the Syl massif. Since the station is positioned
in open terrain in a valley, it is exposed to hard
winds and snowdrifts. In the early 1980s, a new
station was built after a fire destroyed the old station.
When designing the new building, wind loads
and snowdrift problems were taken into consideration
and wind tunnel tests were made of
the construction. Despite the tests performed, however,
the problems with snowdrifts around the
building have been so severe that STF now is planning
to re-build the station.
At the time when the new station was built,
only experimental methods were available to assess
the snowdrift potential. Today, increased computing
power and advanced numerical methods
have made it possible to use CFD as a powerful
yet cost-effective tool to solve the complex two-phase
problem that snowdrift represents. STF therefore
asked ÅF to evaluate different retrofit
proposals with the aid of CFD.
Based on drawings, topological data, and wind
statistics, a computational model was created. In
order to validate the methods used, the existing
building was simulated first and comparisons were
made with observations of the snowdrift around
the building. Agreement with these observations
was good, so simulations of new building
designs were made.
The air flow and transport of snow particles
were simulated using the mixture model in
FLUENT. By applying an apparent viscosity to the
snow phase, the particles were slowed down in
regions with high snow concentration and/or low
shear stress. When the snow phase is slowed, the
density of the blowing snow increases, and this
in turn slows the wind speed. Since accumulated
snow is a result of a history of varying wind
directions and speeds, a number of representative
wind conditions were included in transient
simulations performed for each geometry.
The calculations showed that it is better to make
the building higher than lower. It is also important
to avoid steps in the construction, such as
exterior stairwells. The roof angle was also found
to have a significant influence on the amount of
drifted snow. With the aid of the CFD study, valuable
information regarding the snowdrift at the
mountain station was achieved. Moreover, the
increased knowledge about the nature of snowdrifts
will very likely lead to major cost reductions
for STF in the future.
Strong winds cause recirculating flow and subsequent
snow drifts near the house
A surface of constant snow volume fraction is used to
illustrate the drifts that form around the building
Early in the transient simulation, snow drifts begin to
form in expected locations in the vicinity of the house
|
|
|
