B l o w i n g

S n o w

Drift-FluxModellingandNumerical

SimulationofSnow-Accumulation

S undsbø

Per-A rne

Key Words: Snow Drift, Snow Fence, CFDabout 10 m/s,measured at a reference height of 10 m.He
estimated that 90% of the total mass flux was within the
ABSTRACT^{saltation layer.Thisis probably true formost snow drift}
This paper presentsa methodfor two-dimensional numeri-^{situationswitha similarvelocityandsnowquality,but}
calsimulationsofsnowdrift.Thenumericalmodelis^{average snow particle sizes may vary due to climate and}
based ontwo-phase,gas-gastheorywhere snow and air^{storage conditions.Creep may be the prevailing drift fac-}
mainly are transported as dilute suspension. Transport by^{torforsnow consisting of large grains that are aged fora}
creep and saltation isconsidered by imposing transport^{few days without being exposed for drifting winds (Tabler,}
above the snow surface according to experimental knowl-^{1988b).In these situation snow deposition forms in wave}
edge. Snow is accumulated where the calculated friction^{patterns similar to sand waves that often can be observed}
velocity is below the thresholdfor particle movements and^{on a sandy beach. Otherdrift occurrences are dominated}
the new snow surface willaffect the wind velocityfield^{by driftofparticles small enough to gointosuspension.}
similar to real conditions. The numerical model is imple-^{Thesesmallerparticlesareeasilycarriedaway bylarge}
mented in a general transient computer code based on fi-^{eddies extendingup to hundredsof meters above the snow}
nite-volume technique. Effects on the packing due to stor-^{surface. Since saltation is a phenomenon limited by short}
age conditions, such as rainfalland temperature variations^{trajectory heights it islikely toassume that transport by}
arenot considered.Simulation of the developmentof snow^{suspension is significant around tall constructions orin}
accumulations around a typical porous snow fence is per-^{high mountain terrain.}
formed and the result seems to be in good agreementwith
experimental data.^{The flow problemconsidered}
Snow drift is considered as a two-phase flow problem that
INTRODUCTION^{is modelled by gas-gas technique and with the useof some}
The mechanismof snow drift has beenthe subject for many^{empirical rules.Transport of the snow phase is governed}
investigators during the years (Schmidt, 1980). Most stud-^{by the airflowand a small relative velocitybetween the}
ies havebeen carried out byperforming experiments in^{two phases allows drift and settling with respect to the air}
the field (Tabler, 1988b) or in wind tunnels(Iversen, 1979).^{phase. It is assumed that the amount of snow in transport}
These methods arestill important to strengthenour knowl-^{issmallcompared tothetotalmixtureofsnowand air.}
edge regarding the physics of snow drift but of less value^{Effects fromdrifting snow particles on the mean airflow}
as design tools forthe common engineer.^{are therefore neglected. This one-way coupling where the}
Initiation of particle movements along the ground oc-^{airphase controls the snow transport,is debatable,espe-}
curswherethesurfaceshearstressreachesacertain^{cially forthe ratherimportant modelling of saltation.On}
criticalvalue.Afurtherincreaseabove thisthreshold^{the other hand, the depositedsnow is considered to affect}
shear stress will extend the vertical distribution of snow^{mean velocitycalculations.Deposited snow istreated as}
particles and consequently increase the snow transport.^{a solid surface as long as the surface shear stress is below}
Snow willbe transporteddownwinduntilthe surface^{the limitforerosion.Thus,the one-waycoupling situa-}
shearstressis reduced beneath the threshold value for^{tionbetweenairbornesnowandwindisnowreversed}
relocationofdepositedparticles.Thewindcannot^{since the deposited snow is the controlling factor.}
longermaintain a vertical distribution of particles and^{Snow transport in the saltation layerisdominated by}
depositions are formed.^{heavy particle transport andthere exists no rigorous theory}
The surface shear stress is related to the vertical wind^{for this type of modelling. Consequently, empirical knowl-}
profile bythe friction velocity(u=[!]t_o / r ),whichisde-^{edgeisappliedtomodeltranspo}
giv^{rtbybothcreepand}
termined byfield measurements.Aen type of snow^{saltation.Transportofsnowin suspensionbythemean}
withacertainqualityandparticledistributionseems^{flow isconsidered,but the contribution of turbulent dif-}
tohave a characteristic threshold value forfriction ve-^{fusion on suspended snow is neglected.}
locityorsurface shearstress forwhich transport prop-^{In general,the flow situation considered in this paper}
ertiesare defined.^{is two dimensional,mechanical snow drift.Temperature}
Snow drift may occur as creep, saltation or suspension.^{variations,phase changes and storage conditionsare ne-}
In most situations where snow drift analysis is requested,^{glected even if they might have a substantial effect on the}
all the three transport modes are present. Creep, saltation^{overall result. There are some questions arising about how}
and suspension refer to the way particles move by rolling^{to apply such statistically expected climate factors in nu-}
on thesurface,jumping nearthe surface and being sus-^{merical modelling.}
pended inthe windand broughtfurtherawayfromthe
surface.Investigationofblowingsnowperformedby
Kobayashi (1972) concludes withthat saltation is thedomi-
nantmechanismforsnowtransportatwindspeedsof

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