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Velocities of Windblown Particles in Saltation: Preliminary Laboratory and Field Measurements
133
VELOCITIES
OF WINDBLOWN
PARTICLES I N
SALTATION:
PRELIMINARY LABORATORY
AND
FIELD MEASUREMENTS
85287
R. GREELEY: Dept. o f Geology, A r i z o n a S t a t e U n i v e r s i t y , Tempe, AZ S.H.
WILLIAMS: Dept. o f Geology, Arizona S t a t e U n i v e r s i t y , Tempe, AZ
J.R.
MARSHALL: NASA-Ames Research Center, M o f f e t t F i e l d , CA
g
85287
94035
gravitational acceleration
urn f r e e s t r e a m wind speed
u,
wind f r i c t i o n speed
u,.~
wind t h r e s h o l d f r i c t i o n speed
x
h o r i z o n t a l d i s t a n c e on wind t u n n e l t e s t p l a t e
E
roughness h e i g h t
1.0
INTRODUCTION The speeds o f g r a i n s c a r r i e d by t h e wind has been a t o p i c o f i n t e r e s t s i n c e
Bagnold's (1941)
p i o n e e r i n g work on
t h e physics
o f windblown p a r t i c l e s .
i n t e r e s t i n t h e s u b j e c t developed f r o m r e s e a r c h on t e r r e s t r i a l t r i a l a e o l i a n processes ( T a b l e l ) , e s p e c i a l l y sion on
Mars (Greeley e t al.,
tion i s c r i t i c a l
1982).
Our
and e x t r a t e r r e s -
i n r e g a r d t o r a t e s of
wind a b r a -
D e t e r m i n i n g p a r t i c l e speed d u r i n g s a l t a 1) t h e r a t e o f a b r a s i o n of s u r f a c e
i n t h e understanding o f :
m a t e r i a l s by i m p a c t i n g p a r t i c l e s , 2) t h e r a t e and n a t u r e o f p a r t i c l e comminution i n t h e s a l t a t i o n c l o u d , 3) t h e r e l a t i o n s h i p between s a l t a t i o n p a r t i c l e wind speed and i t s s i g n i f i c a n c e f o r sediment r e d i s t r i b u t i o n , and 4) of saltating
p a r t i c l e s i n general.
We have
i n v e s t i g a t e d p a r t i c l e speed under
simulated m a r t i a n , t e r r e s t r i a l , and Venusian e n v i r o n m e n t a l c o n d i t i o n s mine
i t s relationship t o
t h e key
parameters o f atmospheric
diameter, and wind speed (Wil'liams and Greeley, results.
I n addition,
planets which e x p e r i e n c e a e o l i a n processes:
t o deter-
density, p a r t i c l e
1983).
T h i s r e p o r t p r e s e n t s t h e methods used t o determine p a r t i c l e gives p r e l i m i n a r y
f l u x and
t h e physics
we compare
v e l o c i t i e s and
r e s u l t s among t h e
Earth, Mars, and Venus.
three
134 TABLE 1.
IMPORTANT PARAMETERS OF THE AEOLIAN ENVIRONMENTS OF EARTH, MARS, AND VENUS EARTH
VENUS
Atmospheric Composition
96%
COP
N2
S u r f a c e Pressure
3.5%
MARS
N2
77%
c02
02
21%
N2
2.7%
Ar
1.6%
H20
1%
Ar
0.9%
90 bar
95%
1 bar
7 mb
-288
-218
~
Temperature ( K e l v i n )
-730 2.2 cm/s
Hinimum t h r e s h o l d winc, r r i c t i o n speed f o r p a r t i c l e e n t r a i n m e n t ( f r o m I v e r s e n e t a l . , 1976).
\J,
1.2
250 cm/s
20.5 cm/s
APPROACH
The approach
used i n t h i s
study involved
the analysis o f
windblown par-
t i c l e s i n wind t u n n e l s u s i n g an i n s t r u m e n t t o measure d i r e c t l y t h e speed of particles,
supplemented by high-speed
second) o f g r a i n s i n f l i g h t . tained i n
a field
motion p i c t u r e s
I n addition,
experiment
(up t o
10,000
frames per
high-speed m o t i o n p i c t u r e s were ob-
t o determine
particle velocities
in
a natural
aeol ian e n v i ronment. Three atmospheric b o u n d a r y - l a y e r wind t u n n e l s were used i n t h e experiments, a l l o f which a r e a p a r t o f d i n a t e d by A r i z o n a
t h e NASA P l a n e t a r y Geology A e o l i a n
State University
(Greeley e t
al.,
Consortium coor-
1977; 1981).
l o c a t e d a t NASA's Ames Research Center, M o f f e t t F i e l d , C a l i f o r n i a : Surface ___ lating
Wind Tunnel --
(MARSWIT; Fig.
l),
m a r t i a n atmosphere c o m p o s i t i o n
t e r r e s t r i a l gravity,
a 13 m long, o p e n - c i r c u i t (Coil) and
system o p e r a t i n g a t 1 g a t a CO2 p r e s s u r e
o f 35 b a r and an
s u r f a c e o f Venus).
University,
The t h i r d
i s an open c i r c u i t , 20
b o u n d a r y - l a y e r t u n n e l (Fig. T e s t s were
run i n
aeolian a c t i v i t y i n
all
mb) at
2), a c l o s e d - c i r c u i t ambient temperature
o f 220 C ( p r o d u c i n g t h e same f l u i d d e n s i t y as t h e -90 b a r , 730 K near t h e
system simu-
surface pressures (-7
and t h e --Venus Wind Tunnel (VWT; Fig.
Two are
t h e Martian
C02 atmosphere
wind t u n n e l , l o c a t e d a t Arizona
m l o n g by 1.0 m2
State
c r o s s - s e c t i o n atmospheric
3) o p e r a t i n g a t ambient t e r r e s t r i a l c o n d i t i o n s . t h r e e wind
t u n n e l s under
optimal
t h e p l a n e t a r y environment o f concern.
s e l e c t e d f o r a n a l y s i s i n c l u d e d t h o s e most e a s i l y
c o n d i t i o n s for
The p a r t i c l e
moved by t h e wind (Fig.
sizes 4) as
135
Fig. 1. P h o t o g r a p h o f t h e M a r t i a n S u r f a c e Wind Tunnel a t NASA-Ames Research Center. The o p e n - c i r c u i t a t m o s p h e r i c b o u n d a r y - l a y e r t u n n e l i s 13 m l o n g and has a 1.1 m2 t e s t s e c t i o n l o c a t e d 5 m f r o m t h e e n t r a n c e . It i s o p e r a t e d i n a l o w pressure chamber ( b a c k g r o u n d ) i n a C O P atmosphere a t -7 mb t o s i m u l a t e m a r t i a n conditions. w e l l as common dune sand particle
bed
s i z e s (-300
covered t h e
urn) on E a r t h .
e n t i r e bed
of
the test
I n most
experiments, t h e
s e c t i o n and
there
was an
unlimited supply o f p a r t i c l e s a v a i l a b l e f o r entrainment.
2.0
WIND TUNNEL MEASUREMENTS
L a b o r a t o r y e x p e r i m e n t s have t h e a d v a n t a g e t h a t c o n d i t i o n s can
be m o n i t o r e d
and c l o s e l y c o n t r o l l e d , t h u s r e d u c i n g t h e u n c e r t a i n t i e s i n i n t e r p r e t a t i o n o f t h e results
and a l l o w i n g a w i d e r a n g e
o f c o n d i t i o n s t o be t e s t e d .
e x t r a t e r r e s t r i a l s t u d i e s , l a b o r a t o r y s i m u l a t i o n s p r o v i d e an
I n t h e case of
extremely important
data base f o r a n a l y z i n g t h e l i m i t e d r e s u l t s o b t a i n e d i n s i t u f r o m s p a c e c r a f t . P a r t i c l e s u s e d i n t h e w i n d t u n n e l e x p e r i m e n t s were o b t a i n e d f r o m commercial sources
and f o r
t h e m a r t i a n and
w e l l - s o r t e d samples. Walnut-shell
Venusian runs
Q u a r t z sands
p a r t i c l e s were
were crushed and s i e v e d t o
were used
used i n obtain the
walnut s h e l l s (-1.1 g/cm3) a l l o w s
were f u r t h e r s i e v e d i n E a r t h and
the martian
simulations;
sizes of interest.
a partial
scaling f o r the
t o obtain
Venus s i m u l a t i o n s . these p a r t i c l e s
The l o w
d e n s i t y of
low g r a v i t a t i o n a l
136
F i g . 2. View o f t h e Venus Wind Tunnel a t NASA-Ames Research C e n t e r . This closed-circuit a t m o s p h e r i c b o u n d a r y - l a y e r w i n d t u n n e l has a 1 m l o n g by (3.14)(10-L)m2 t e s t s e c t i o n and o p e r a t e s a t a m b i e n t t e m p e r a t u r e w i t h carbon d i o x i d e a t 35 b a r t o s i m u l a t e V e n u s i a n c o n d i t i o n s . field
o f Mars,
appropriate
p r e v i o u s l y (Greeley e t al., The w i n d
f o r experiments
v e l o c i t y measured i n
t h e wind
speed, urn, f o u n d above t h e b o u n d a r y l a y e r . t e r r e s t r i a l and m a r t i a n c a s e s a l l o w wind f r i c t i o n
c o n d u c t e d on
Earth
as d i s c u s s e d
1981; 1982).
speed, u,(after
t u n n e l s was
the
f r e e s t r e a m wind
Boundary-layer p r o f i l e s taken i n the
c o n v e r s i o n o f urn t o t h e
Bagnold, 1941).
c o r r e s p o n d i n g wind
F o r t h e VWT t e s t s , u,
was d e t e r -
m i n e d from urn by u s i n g t h e e q u a t i o n o f S c h l i c h t i n g ( 1 9 6 8 ) :
where x
=
d i s t a n c e down t h e t e s t p l a t e and
p a r t i c l e diameter).
Boundary l a y e r
E
=
roughness h e i g h t ( e s s e n t i a l l y the
studies are
v a r i o u s s u r f a c e roughness v a l u e s used f o r t h e VWT. p l i c a t i o n a r i s e s when sand
i s set i n motion; t h e
t r a n s f e r o f momentum f r o m t h e w i n d t o t h e r e p o r t e d here a r e approximate.
grains
c u r r e n t l y underway t o
assess
It m u s t be n o t e d t h a t a com-
u,/urn
r a t i o changes due
n motion, thus t h e
to a
u, v a l u e s
137
Fig. 3. View o f t e s t s e c t i o n o f t h e one atmosphere, o p e n - c i r c u i t w i n d t u n n e l i n t h e P l a n e t a r y Geology L a b o r a t o r y a t A r i z o n a S t a t e U n i v e r s i t y . The t u n n e l i s 19 m long and has a 1.1 m2 c r o s s s e c t i o n . Shown h e r e i s t h e e x p e r i m e n t a l s e t - u p t o f i l m windblown p a r t i c l e s i n f l i g h t a g a i n s t a c m - g r i d b a c k d r o p ( i n t e s t s e c t i o n ) using a Hycam Model I 1 m o t i o n p i c t u r e camera ( a t r i g h t ) .
2.1
PARTICLE VELOCIMETER Figure
5 i s a diagram o f
experiments.
t h e p a r t i c l e v e l o c i m e t e r used i n
The d e s i g n o f t h i s d e v i c e i s based
Forest S e r v i c e ( S c h m i d t , 1977) t o ticles.
measure t h e
the laboratory
on a system used by
v e l o c i t y o f windblown
t h e U.S. snow p a r -
The v e l o c i m e t e r u t i l i z e s a l i g h t s o u r c e w h i c h p r o j e c t s a beam a c r o s s a
s m a l l gap t o t w o p h o t o t r a n s i s t o r s .
As a w i n d b l o w n g r a i n passes t h r o u g h t h e gap,
i t i n t e r r u p t s t h e beam and c a s t s a
shadow on t h e f i r s t d e t e c t o r
w h i c h responds
p u l s e ; i t t h e n shadows t h e
second d e t e c t o r
by g e n e r a t i n g a p o s i t i v e e l e c t r i c a l which g e n e r a t e s a n e g a t i v e p u l s e .
for
These s i g n a l s
l a t e r a n a l y s e s w i t h an o s c i l l o s c o p e .
a r e r e c o r d e d on m a g n e t i c t a p e
The v e l o c i t y
i s determined from t h e
distance t r a v e l e d (known f r o m t h e geometry o f t h e windows) and f r o m t h e travel
(determined from t h e
signal displayed
on t h e o s c i l l o s c o p e ) .
signals r e p r e s e n t i n g g r a i n s e i t h e r p a s s i n g t h r o u g h t h e gap a t o b l i q u e i n clusters are Approximately
i d e n t i f i e d by
150 s i g n a l s
velocity d i s t r i b u t i o n s .
were
asymmetric o r analyzed f o r
distorted each r u n
time o f Spurious angles o r
oscilloscope tracings. to
determine p a r t i c l e
138
Fig. 4. T h r e s h o l d f r i c t i o n speed (m/s) as a f u n c t i o n o f p a r t i c l e d i a m e t e r f o r Earth, Mars, and Venus; speed r e q u i r e d f o r p a r t i c l e m o t i o n i s i n v e r s e l y proport i o n a l t o t h e atmospheric d e n s i t y ( T a b l e 1); n o t e t h a t t h e optimum g r a i n s i z e ( i d e n t i f i e d on each curve) f o r p a r t i c l e threshold i s about t h e same i n a l l three planetary environments; ( a f t e r I v e r s e n and White, 1982).
1
-
-> (I)
\
E
-
115
k
t
0 0 0l.e 8 O A W
>
z
-
0
I-
EARTH
0
0.2
L
0.10
a P A
75
-
0 I
u)
0.02 -
w 0.03 a I
I-
0
75 . 2030
10
0
50
100
1 200
5 500 1
0
P A R T I C L E D I A M E T E R ((lm)
above the
To determine t h e range o f p a r t i c l e v e l o c i t i e s a t a s i n g l e h e i g h t s u r f a c e , v e l o c i t y measurements were grouped plotted
as a h i s t o g r a m expressed as
s p e c i f i e d v e l o c i t y range. percentage o f
i n even
i n c r e m e n t s o f one
t h e percentage
f r e e s t r e a m wind speed.
This l a t t e r
function i s
o r d e r s o f magnitude.
As shown
range.
has demonstrated t h e a c c e l e r a t i o n o f
saltation Figure 6
i n Figure
path, and we c o n s i d e r t h e
experiment
t o r e f l e c t grains a l s o shows
in a
V e l o c i t i e s a r e g i v e n b o t h as an a c t u a l speed and as a useful i n nor-
m a l i z i n g d a t a f o r i n t e r p l a n e t a r y comparisons i n which w i n d speeds may White (1979)
m/s and
o f grains t r a v e l l i n g
6, p a r t i c l e v e l o c i t i e s
wide range
sampled i n
t h a t some g r a i n s
d i f f e r by
have a
i n v e l o c i t i e s measured
different parts o f
exceed t h e
wide
p a r t i c l e s along the i n our
t h e i r trajectory.
f r e e s t r e a m wind
a t t r i b u t e t h i s observation t o very high accelerations r e s u l t i n g from
speed.
We
rebound o f
s a l t a t i n g g r a i n s f r o m t h e s u r f a c e and from i n t e r g r a i n c o l l i s i o n s . Tests t h e surface. speed w i t h
were r u n t o d e t e r m i n e p a r t i c l e
As shown
i n Figure
7, t h e r e i s
height, r e f l e c t i n g , f i r s t ,
t h r o u g h t h e boundary
layer,
speed as a f u n c t i o n o f a general
t h e increase
and second,
t h e longer
h e i g h t above
increase i n p a r t i c l e
i n wind speed particle
w i t h height
trajectory
--
139
" . . ,
-
PHOTOTRANSISTORS
Fig. 5. Diagram o f t h e p a r t i c l e v e l o c i me t e r used t o measure t h e speed o f windblown g r a i n s ( f r o m Greeley e t al., 1982)
p a r t i c l e p a t h l e n g t h i n c r e a s e s w i t h h e i g h t and a f f o r d s a l o n g e r t i m e
f o r accel-
A s i m i l a r r e l a t i o n s h i p was found i n experiments s i m u l a t i n g
e r a t i o n by t h e wind.
the m a r t i a n environment (Fig. 8).
The p a r t i c l e v e l o c i m e t e r does n o t o p e r a t e i n
the Venus Wind Tunnel, and no measurements were made w i t h t h i s technique. F i g u r e 9 shows t h e e f f e c t at
a s i n g l e height.
o f wind v e l o c i t y on p a r t i c l e
Two p a r t i c l e
martian atmospheric
conditions.
f r e q u e n t l y measured
speed).
v e l o c i t i e s t h a n l a r g e r ones. wind and t h e
smaller
s i z e s (350 urn and 92 P a r t i c l e speeds
I n general,
v e l o c i t y , measured
urn) were t e s t e d under
a r e modal
smaller p a r t i c l e s
values ( t h e travel a t
most higher
We a t t r i b u t e t h i s t o a b e t t e r c o u p l i n g between t h e
p a r t i c l e s owing
t o their
larger
cross-sectional
area-
to-mass r a t i o . Figure
9 a l s o shows t h a t p a r t i c l e
markably c o n s t a n t t h r o u g h a wide may shed some l i g h t on known t h a t Kawamura,
the characteristics of particle flux.
increases i n 1951; and
v e l o c i t y a t a given height
wind
others).
remains r e -
range o f f r e e s t r e a m wind speeds. speed cause However,
increases i n it
has n o t
This r e s u l t
It has l o n g been
flux
(Bagnold, 1941;
been known
whether
the
increase i n f l u x r e s u l t e d p r i m a r i l y from h i g h e r p a r t i c l e v e l o c i t i e s o r p r i m a r i l y from i n c r e a s e s i n t h e number o f g r a i n s i n t r a n s p o r t , o r both.
F i g u r e 9 suggests
t h a t i n c r e a s e s i n f l u x may be t h e r e s u l t o f more g r a i n s i n t r a n s p o r t r a t h e r t h a n o f higher p a r t i c l e v e l o c i t i e s .
2.2
HIGH SPEED MOTION PICTURES
A Hycam Model I 1 16 mm camera (Fig.
o f windblown
g r a i n s a t f r a m i n g r a t e s up
were photographed a g a i n s t a back-drop ysis
of the f i l m .
The f i e l d
3) was used t o o b t a i n t o 10,000
frames p e r second.
w i t h a cm-scale g r i d t o
o f view had t o be
motion p i c t u r e s Grains
f a c i l i t a t e anal-
s u f f i c i e n t l y small
t o permit
i n d i v i d u a l g r a i n s t o be r e s o l v e d and y e t l a r g e enough t o c h a r a c t e r i z e t h e f l i g h t path o f t h e g r a i n s
and t o d e t e r m i n e t h e i r v e l o c i t i e s .
analyzed (-600 pm), t h e maximum u s e f u l f i e l d o f view
For t h e
largest grains
was about 6 cm h i g h
by 10
140
3 5 0 p m QUARTZ 16.1cm HEIGHT
~ ~ ' 1 0 . 2m2/ s
PARTICLE VELOCITY (m/roc) I
1
0
20
60
40
80
I
100
120
% O F FREESTREAM WIND S P E E D
Fig. 6. D i s t r i b u t i o n o f p a r t i c l e v e l o c i t i e s a t a h e i g h t o f 16.1 cm above t h e s u r f a c e f o r 350 um d i a m e t e r q u a r t z g r a i n s s u b j e c t e d t o a f r e e s t r e a m windspeed of -10.22 m/s i n a s i m u l a t i o n o f E a r t h (u, = 0.5 m/s). The v e l o c i t y d i s t r i b u t i o n i s t y p i c a l f o r b o t h wind t u n n e l and f i e l d experiments and w i l l be r e p r e s e n t e d i n subsequent f i g u r e s as a one-standard d e v i a t i o n v e l o c i t y range around a mean p a r t i c l e velocity. cm wide.
Thus f a r i n o u r i n v e s t i g a t i o n o n l y a l i m i t e d number o f runs have been
made i n which h i g h speed m o t i o n p i c t u r e s were o b t a i n e d , a l l a t E a r t h
(wind t u n -
n e l and f i e l d c o n d i t i o n s ) and Venus c o n d i t i o n s , b u t n o t m a r t i a n c o n d i t i o n s . Data were
obtained from
processed
L a f a y e t t e Analyzer o n t o a 40" by 60" GTCO 11/45 computer. i n t o the
The g r i d
image
via a
x-y d i g i t i z i n g t a b l e l i n k e d t o
a PDP
f i l m s by
p o s i t i o n , f r a m i n g r a t e , and wind
program and t h e p a r t i c l e p o s i t i o n s
f i l m sequence.
projecting the
were p l o t t e d
speed were e n t e r e d f o r each frame
in a
The reduced d a t a i n c l u d e v e l o c i t i e s i n t h e f o l l o w i n g c a t e g o r i e s :
1) particles i n r i s i n g trajectories,
2) p a r t i c l e s i n f l a t t r a j e c t o r i e s ,
t i c l e s i n f a l l i n g t r a j e c t o r i e s and 4) combined t r a j e c t o r i e s ;
3) p a r -
p a r t i c l e speeds are
g i v e n i n m/s and as a percentage o f f r e e s t r e a m wind speed (urn).
141
25 0-
E 0
20
W
0 4:
LL
a
3
15
fn W
> 0
m
EARTH CASE
10
3 5 0 ptn Q U A R T Z P A R T I C L E S
4
v
I-
r
?!
urnr 1 0 . 5 r n / s
5
W
r I
I
I
I
1
I
I
MEAN PARTICLE VELOCITY ( m / s ) 1
I
I
0 20 40 60 80 100 M E A N P A R T I C L E V E L O C I T Y (%urn)
Fig. 7. P a r t i c l e v e l o c i t i e s f o r 350 um q u a r t z g r a i n s measured a t f o u r h e i g h t s above t h e s u r f a c e i n a wind t u n n e l a t 1 b a r p r e s s u r e and a f r e e s t r e a m wind speed o f 10.5 m/s (u, = 0.5 m/s). Figure
10 shows
typical
results for
speeds i n c r e a s e w i t h h e i g h t above with the
p a r t i c l e velocimeter, although
f i l m i s limited.
E a r t h cases;
t h e surface
in
general, p a r t i c l e
similar t o the
t h e range
r e s u l t s obtained
i n the heights
analyzed by
A t t h i s t i m e , i d e n t i c a l experiments have n o t been r u n t o com-
pare r e s u l t s f r o m t h e v e l o c i m e t e r w i t h t h o s e f o r t h e m o t i o n p i c t u r e s . shows
r e s u l t s o b t a i n e d i n t h e Venus
tures.
Wind Tunnel
f r o m analyses o f
Although t h e a b s o l u t e v e l o c i t i e s a r e v e r y low
(a r e f l e c t i o n of t h e low
wind speeds on Venus), n o t e t h a t t h e p a r t i c l e s a c h i e v e n e a r l y t h e as f r e e s t r e a m
wind speed,
in
marked c o n t r a s t
t o Mars
F i g u r e 11 motion p i c -
where
same v e l o c i t y
p a r t i c l e s seldom
reach f r e e s t r e a m v e l o c i t i e s . I n a d d i t i o n , p a r t i c l e v e l o c i t i e s on r i s i n g , f l a t , and were assessed (Fig. Earth increase
12).
throughout
falling trajectories
As p r e d i c t e d by White (1979), p a r t i c l e v e l o c i t i e s the saltation
b e f o r e impact w i t h t h e s u r f a c e ,
t r a j e c t o r y , reaching
a
on
maximum j u s t
142
25
-
A
E 0
20
W
0
U
u.
U
3
15
ln W
> 0
m U
MARS CASE
10
-
c
I
a -
3 5 0 p m SHELL PARTICLES u
6 5 m/ s
5
W
I I
(
I
1
1
I
0
I
1
I
1 35
I
30 20 25 15 10 MEAN PARTICLE VELOCITY (m/s)
5
I 50
I
I
10 20 30 40 M E A N P A R T I C L E V E L O C I T Y (%Urn)
Fia. 8. P a r t i c l e v e l o c i t i e s f o r f o u r d i f f e r e n t h e i a h t s above t h e s u r f a c e for 356 w a l n u t s h e l l p a r t i c l e s s u b j e c t e d t o a f r e e s t r e a m wind speed o f 65 m/s (u, = 3.4 m/s) i n a low atmospheric p r e s s u r e (6.6 mb) t o s i m u l a t e t h e martian e n v i ronment. 3.0
FIELD STUDIES Although l a b o r a t o r y s i m u l a t i o n s e n a b l e a e o l i a n processes t o be i n v e s t i g a t e d
under c o n t r o l l e d c o n d i t i o n s , q u e s t i o n s the results
as a p p l i e d t o n a t u r a l c o n d i t i o n s .
t i o n s , a f i e l d experiment was particles.
inevitably arise
conducted t o
I n order
v a l i d i t y of
as t o t h e
t o address such ques-
obtain v e l o c i t y data
f o r saltating
The experiment was conducted 3 November 1981 a t Waddell Creek State
Beach i n C a l i f o r n i a , about 90
km s o u t h o f San Francisco.
The beach
i s one o f
t h e w i n d i e s t on t h e c o a s t and i s f a i r l y wide, e n a b l i n g a good s a l t a t i o n c l o u d t o develop across
t h e dry
p a r t o f the
beach b e f o r e
r e a c h i n g t h e area
where the
f i l m i n g t o o k place. Using t h e same i n s t r u m e n t s as ( a t a h e i g h t o f 1.0 m above Greeley e t al., taneously.
t h e surface), p a r t i c l e f l u x (using
1982), and h i g h
The p a r t i c l e
wind v e l o c i t y
employed i n t h e w i n d t u n n e l s ,
speed m o t i o n p i c t u r e s were a l l
c o l l e c t o r s , see o b t a i n e d simul-
v e l o c i m e t e r m a l f u n c t i o n e d and d a t a were
w i t h i t i n t h e f i e l d experiment.
n o t obtained
143
92vm
350um 130 120
Siia
\
E
k
0 0 A
W
10( 9c
>
n
z
3 z a
W
8C
7C
a
+ W W
6(
U
LL
5c 1
I
5
10
1
1
I
15
20
25
1
I
35
30
PARTICLE VELOCITY ( Y O D E , m / a )
Fig. 9. S t a t i s t i c a l modes o f v e l o c i t i e s as a f u n c t i o n o f f r e e s t r e a m wind speed a t a h e i g h t o f 7.1 cm above t h e s u r f a c e f o r 350 urn and 92 urn s h e l l p a r t i c l e s i n a m a r t i a n s i m u l a t i o n ; p a r t i c l e v e l o c i t y remains c o n s t a n t o v e r a wide range of freestream wind speeds. Four s u c c e s s f u l r u n s were completed, hold.
i n t h e wind t u n n e l runs. is
a l l a t wind speeds j u s t
The m o t i o n p i c t u r e s were analyzed f o l l o w i n g t h e
somewhat d i f f e r e n t
above t h r e s -
same procedures as used
F i g u r e 13 shows t h e r e s u l t s ; a l t h o u g h t h e f r o m t h o s e used
i n the
wind t u n n e l runs,
grain size the particle
v e l o c i t y d i s t r i b u t i o n s a r e w i t h i n t h e range expected.
4.0
SUMMARY AND CONCLUSIONS
Velocities o f speed,
windblown
h e i g h t above t h e
p a r t i c l e s were
ground, and
d e t e r m i n e d as
p a r t i c l e diameter f o r
functions
of wind
various conditions
s i m u l a t i n g Earth, Mars, and Venus i n e n v i r o n m e n t a l wind t u n n e l s .
S i m i l a r data,
although o f l i m i t e d range, were o b t a i n e d f r o m a f i e l d experiment
f o r comparison
w i t h t h e wind t u n n e l r e s u l t s s i m u l a t i n g t h e t e r r e s t r i a l environment.
144
-
-
EARTH CASE
E
-
ASU WIND TUNNEL
0
W
0
a Y
U
3 UJ W
> 0
m
a I-
I
-
Q W
I I
'0
I
I
I
I
I
1
5 6 7 3 4 2 MEAN PARTICLE VELOCITY (m/s)
1 I
I
I
I
I
8 I
1
I
20 30 40 50 60 MEAN PARTICLE VELOClTY
10
J
70
80
(%Urn)
F i g . 10. P a r t i c l e v e l o c i t i e s o b t a i n e d f r o m a n a l y s e s of h i g h speed m o t i o n p i c t u r e s f o r 400 q u a r t z p a r t i c l e s s u b j e c t e d t o a f r e e s t r e a m w i n d speed of 10.5 m/s (u, = 0.55 m / s ) i n t h e 1.0 b a r a t m o s p h e r i c w i n d t u n n e l a t A r i z o n a S t a t e Un iv e r s i t y
.
I n g e n e r a l , t h e r e s u l t s show
that particles
travel a t higher
speeds w i t h
i n c r e a s e d h e i g h t above t h e ground, and t h a t s m a l l e r p a r t i c l e s t r a v e l f a s t e r t h a n l a r g e r ones. not increase
However, f o r a g i v e n h e i g h t above t h e ground, p a r t i c l e speed does with higher
t h i s i s with reference t o
f r e e s t r e a m w i n d speeds.
modal v a l u e s
and
I t must
be remembered t h a t
n o t t o maximum v a l u e s ; a
few p a r t i -
c l e s do i n c r e a s e i n v e l o c i t y w i t h f r e e s t r e a m w i n d v e l o c i t i e s . Comparisons o f r e s u l t s f o r
E a r t h , Mars,
and Venus r e v e a l
some r e m a r k a b l e
differences.
As shown i n F i g u r e 14, most p a r t i c l e s a c h i e v e speeds n e a r l y equal
t o freestream
w i n d speed o n Venus, b u t
seldom a c h i e v e
Mars; E a r t h c a s e s a r e o f i n t e r m e d i a t e v a l u e s . ences
i n a t m o s p h e r i c d e n s i t y and t o
p l a n e t a r y e n v i r o n m e n t s ( T a b l e 1). V e n u s i a n atmosphere t h a n on and f o r t h e (just
t h e t h r e s h o l d w i n d speeds among
speeds), t h e
speed on the three
P a r t i c l e s a r e more e a s i l y moved i n t h e dense
Mars; c o n s e q u e n t l y , t h r e s h o l d speeds a r e
r a n g e o f w i n d speeds i n
above t h r e s h o l d
h a l f t h e wind
This i s attributed t o the d i f f e r -
w h i c h most grains
movement i s presumed
need n o t
be m o v i n g
very
v e r y low, t o occur fast to
145
-
-
3
VENUS CASE
E
VENUS WIND TUNNEL
0
Y
5 0 0 - 6 0 0 p m QUARTZ PARTICLES
W
0
ucO = 3.621111s
U
u . 2 K
__t_
3 v)
W
> 0
m
u 1
-
-
I-
-
r
-
0 W
I 0
I
I
1
,
-
I
I
1
1
I
I
I
I
I
I
I
1
60
l
I
I
l
1
1
I
3.5
2.5 3.0 MEAN PARTICLE VELOCITY (m/s)
0
--
80 90 70 MEAN PARTICLE VELOCITY (%urn)
4.0 I
100
Fig. 11. P a r t i c l e v e l o c i t i e s o b t a i n e d f r o m a n a l y s i s o f h i q h speed m o t i o n p i c t u r e s f o r 500 t o 600 pm q u a r t z p a r t i c l e s s u b j e c t e d t o a f r e e s t r e a m w i n d speed o f 3.62 m/s (u, = 0.2 m/s) a t 30 b a r s a t m o s p h e r i c p r e s s u r e i n t h e Venus Wind Tunnel. achieve
100% o f t h e w i n d speed.
C o n v e r s e l y , p a r t i c l e s on Mars must a c c e l e r a t e
very r a p i d l y t o a c h i e v e t h e speed o f t h e h i g h w i n d s r e q u i r e d f o r despite
the f a c t t h a t s a l t a t i o n path
lengths are
l o n g on Mars
most g r a i n s f a l l t o t h e s u r f a c e b e f o r e a c h i e v i n g even 50-60% o f
t h r e s h o l d , and ( W h i t e , 1979), freestream wind
speed. F u t u r e e x p e r i m e n t s w i l l i n v o l v e e x p a n s i o n o f t h e d a t a base t o a w i d e r r a n g e o f wind-tunnel
and f i e l d c o n d i t i o n s and f o r a w i d e r r a n g e o f p a r t i c l e d i a m e t e r s ,
wind speeds, and measurements f o r
v a r i o u s h e i g h t s above t h e s u r f a c e .
tion, a v e l o c i m e t e r i s under development t h a t w i l l be f i e l d - p o r t a b l e obtain
a l a r g e r number
of velocity
measurements t h a n a r e
I n addii n order t o
presently available
from h i g h - s p e e d m o t i o n p i c t u r e s . ACKNOWLEDGEMENTS A l l a s p e c t s o f t h i s work
were s u p p o r t e d
by t h e P l a n e t a r y
Geology O f f i c e ,
N a t i o n a l A e r o n a u t i c s and Space A d m i n i s t r a t i o n . We t h a n k t h e f o l l o w i n g i n d i v i d u a l s f o r t h e i r c o n t r i b u t i o n t o D.
B a l l f o r p h o t o g r a p h i c s u p p o r t , G.
Kuhl f o r d a t a r e d u c t i o n , and
Beardmore,
P.
Parkes, K.
t h i s study:
Malone,
and D.
R. Leach f o r a s s i s t a n c e i n o b t a i n i n g t h e w i n d t u n -
146 n e l d a t a and development o f t h e p a r t i c l e v e l o c i m e t e r . i n the i n i t i a l
arrangements f o r
Alan P e t e r f r e u n d a s s i s t e a
t h e f i e l d experiment.
R. Leach, M.
We thank
M a l i n , and K. Gerety f o r h e l p f u l comments on t h e manuscript.
f0
60
501
DESCENDING
0
6
4
2
8
10
PARTICLE VELOCITY (m/s) I
0
I
10
, 20
I
30
I
I
40
50
I
60
PARTICLE VELOCITY
70
80
I
I
90
100
(%Urn)
Fig. 12. P a r t i c l e v e l o c i t i e s o b t a i n e d f r o m a n a l y s i s o f high-speed m o t i o n p i c t u r e s f o r 500 t o 600 vm q u a r t z p a r t i c l e s s u b j e c t e d t o a f r e e s t r e a m wind speed of 10.5 m/s (u, = 0.5 m / s ) a t one b a r atmospheric pressure, comparing v e l o c i t i e s f o r g r a i n s on t h e r i s i n g , h o r i z o n t a l , and descending p a r t s o f t h e i r t r a j e c We p r e s e n t l y have no e x p l a n a t i o n f o r t h e bimodal v e l o c i t y d i s t r i b u t i o n tories. o f g r a i n s on t h e h o r i z o n t a l p a r t o f t h e t r a j e c t o r y .
147
-Eo
4
LL
a a
m
-
10
W
0
-
12
8
-
EARTH CASE FIELD EXPERIMENT WADDELL BEACH, CA Dp
__t_
= 300pm
6
W
> 0
m 4
W
I
2 i 4
1
0
3
2
5
4
MEAN PARTICLE VELOCITY (m/S) I
0
I
10
1
20
1
I
I
I
I
I
30
40
50
60
70
80
M E AN P A R T I C L E V E L 0 C I T Y ( % uoo)
Fig. 13. P a r t i c l e v e l o c i t i e s o b t a i n e d f r o m a n a l y s i s o f h i g h speed m o t i o n p i c t u r e s f o r n a t u r a l beach sands o f -300 um d i a m e t e r s u b j e c t e d t o a wind speed o f -6 m/s measured a t a h e i g h t o f 1 m above t h e s u r f a c e a t Waddell Creek S t a t e Beach, C a l i f o r n i a .
148
-E 0
w
30 25
0 U
;2 0 3 v)
w 15
>
0 I-
10
I
Q
i i 5 r
0
I
I
I
I
I
I
I
I
I
I
10
20
30
40
50
60
70
80
90
100
P A R T I C L E V E L O C I T Y (MODE,%u,)
F i g . 14. Comparison o f v e l o c i t i e s on E a r t h , Mars, and Venus. Two s i z e s o f p a r t i c l e s , each s u b j e c t e d t o r e l a t i v e l y l o w and h i g h w i n d speeds, were t e s t e d under E a r t h and Mars c o n d i t i o n s ; one s i z e p a r t i c l e was t e s t e d a t a l o w v e l o c i t y under Venus c o n d i t i o n s . I n g e n e r a l , g r a i n s a c h i e v e a much h i g h e r v e l o c i t y i n r e l a t i o n t o t h e w i n d speed u n d e r V e n u s i a n c o n d i t i o n s t h a n u n d e r m a r t i a n c o n d i t i o n s , and g r a i n s u n d e r t e r r e s t r i a l c o n d i t i o n s have i n t e r m e d i a t e v e l o c i t i e s . REFERENCES Bagnold, R.A., 1941. The P h y s i c s o f Blown Sand and D e s e r t Dunes: Methuen, London, 265 pp. G r e e l e y , R., R.N. Leach, S.H. W i l l i a m s , B.R. White, J.B. P o l l a c k , D.H. K r i n s l e y , J. _ Geophy. a n d J.R. M a r s h a l l , 1982. R a t e o f Wind A b r a s i o n on Mars: _ _ _ - Res., 87, pp. 10,009-10,024. G r e e l F , R., B.R. White, J.B. P o l l a c k , J.D. I v e r s e n , and R.N. Leach, 1977. Dust s t o r m s on Mars: C o n s i d e r a t i o n s and S i m u l a t i o n s : NASA T e c h n i c a l Memo. TM 78423, pp. 29. G r e e l e y , R., B.R. W h i t e , J.B. P o l l a c k , J.D. I v e r s e n , and R.N. Leach, 1981. Dust s t o r m s on Mars: C o n s i d e r a t i o n s and S i m u l a t i o n s : I n Desert Dust: Origin, C h a r a c t e r i s t i c s , and E f f e c t on Man. Geol. SOC. her.--1 T r o y Pewe, pp. l O T i 2 i 7 ' - - G r e e l e y , and J.B. P o l l a c k , 1976. Windblown d u s t on Earth, I v e r s e n , J.E., R. Mars, and Venus: J. Atmos. S c i . , 2, 2425-2429. S a l t a t i o n t h r e s h o l d on E a r t h , Mars, and I v e r s e n , J.D. and B . R T W W 1 9 8 2 . Venus: S e d i m e n t o l o g y , 29, pp. 111-119. Kawamura, R., 1951. S t u d y of sand movement by w i n d : Phy. S c i . Res. I n s t . , Tokyo Univ., 5, 95-112, ( o r i g i n a l i n Japanese; NASA t r a n s l a t i o n ) . S c h l i c h t i n g , H., 1368. Boundary-Layer Theory: M c G r a w - H i l l Book Company, New York, pp. 748. Schmidt, R.A. 1977. A System T h a t Measures B l o w i n g Snow: U.S. Dept. Agric. Res. Paper, RM -194, 80 pp. WhiteXR-79. S o i l T r a n s p o r t by Winds on Mars: J. Geophy. Res., 84, pp. 4643-4651. Williams, S.H. and R. G r e e l e y , 1983. F l u x o f w i n d b l o w n p a r t i c l e s on Venus: NASA Rept. P l a n e t a r y Geology, ( a b s t r a c t , Preliminary laboratory results: i n press).
VELOCITIES
OF WINDBLOWN
PARTICLES I N
SALTATION:
PRELIMINARY LABORATORY
AND
FIELD MEASUREMENTS
85287
R. GREELEY: Dept. o f Geology, A r i z o n a S t a t e U n i v e r s i t y , Tempe, AZ S.H.
WILLIAMS: Dept. o f Geology, Arizona S t a t e U n i v e r s i t y , Tempe, AZ
J.R.
MARSHALL: NASA-Ames Research Center, M o f f e t t F i e l d , CA
g
85287
94035
gravitational acceleration
urn f r e e s t r e a m wind speed
u,
wind f r i c t i o n speed
u,.~
wind t h r e s h o l d f r i c t i o n speed
x
h o r i z o n t a l d i s t a n c e on wind t u n n e l t e s t p l a t e
E
roughness h e i g h t
1.0
INTRODUCTION The speeds o f g r a i n s c a r r i e d by t h e wind has been a t o p i c o f i n t e r e s t s i n c e
Bagnold's (1941)
p i o n e e r i n g work on
t h e physics
o f windblown p a r t i c l e s .
i n t e r e s t i n t h e s u b j e c t developed f r o m r e s e a r c h on t e r r e s t r i a l t r i a l a e o l i a n processes ( T a b l e l ) , e s p e c i a l l y sion on
Mars (Greeley e t al.,
tion i s c r i t i c a l
1982).
Our
and e x t r a t e r r e s -
i n r e g a r d t o r a t e s of
wind a b r a -
D e t e r m i n i n g p a r t i c l e speed d u r i n g s a l t a 1) t h e r a t e o f a b r a s i o n of s u r f a c e
i n t h e understanding o f :
m a t e r i a l s by i m p a c t i n g p a r t i c l e s , 2) t h e r a t e and n a t u r e o f p a r t i c l e comminution i n t h e s a l t a t i o n c l o u d , 3) t h e r e l a t i o n s h i p between s a l t a t i o n p a r t i c l e wind speed and i t s s i g n i f i c a n c e f o r sediment r e d i s t r i b u t i o n , and 4) of saltating
p a r t i c l e s i n general.
We have
i n v e s t i g a t e d p a r t i c l e speed under
simulated m a r t i a n , t e r r e s t r i a l , and Venusian e n v i r o n m e n t a l c o n d i t i o n s mine
i t s relationship t o
t h e key
parameters o f atmospheric
diameter, and wind speed (Wil'liams and Greeley, results.
I n addition,
planets which e x p e r i e n c e a e o l i a n processes:
t o deter-
density, p a r t i c l e
1983).
T h i s r e p o r t p r e s e n t s t h e methods used t o determine p a r t i c l e gives p r e l i m i n a r y
f l u x and
t h e physics
we compare
v e l o c i t i e s and
r e s u l t s among t h e
Earth, Mars, and Venus.
three
134 TABLE 1.
IMPORTANT PARAMETERS OF THE AEOLIAN ENVIRONMENTS OF EARTH, MARS, AND VENUS EARTH
VENUS
Atmospheric Composition
96%
COP
N2
S u r f a c e Pressure
3.5%
MARS
N2
77%
c02
02
21%
N2
2.7%
Ar
1.6%
H20
1%
Ar
0.9%
90 bar
95%
1 bar
7 mb
-288
-218
~
Temperature ( K e l v i n )
-730 2.2 cm/s
Hinimum t h r e s h o l d winc, r r i c t i o n speed f o r p a r t i c l e e n t r a i n m e n t ( f r o m I v e r s e n e t a l . , 1976).
\J,
1.2
250 cm/s
20.5 cm/s
APPROACH
The approach
used i n t h i s
study involved
the analysis o f
windblown par-
t i c l e s i n wind t u n n e l s u s i n g an i n s t r u m e n t t o measure d i r e c t l y t h e speed of particles,
supplemented by high-speed
second) o f g r a i n s i n f l i g h t . tained i n
a field
motion p i c t u r e s
I n addition,
experiment
(up t o
10,000
frames per
high-speed m o t i o n p i c t u r e s were ob-
t o determine
particle velocities
in
a natural
aeol ian e n v i ronment. Three atmospheric b o u n d a r y - l a y e r wind t u n n e l s were used i n t h e experiments, a l l o f which a r e a p a r t o f d i n a t e d by A r i z o n a
t h e NASA P l a n e t a r y Geology A e o l i a n
State University
(Greeley e t
al.,
Consortium coor-
1977; 1981).
l o c a t e d a t NASA's Ames Research Center, M o f f e t t F i e l d , C a l i f o r n i a : Surface ___ lating
Wind Tunnel --
(MARSWIT; Fig.
l),
m a r t i a n atmosphere c o m p o s i t i o n
t e r r e s t r i a l gravity,
a 13 m long, o p e n - c i r c u i t (Coil) and
system o p e r a t i n g a t 1 g a t a CO2 p r e s s u r e
o f 35 b a r and an
s u r f a c e o f Venus).
University,
The t h i r d
i s an open c i r c u i t , 20
b o u n d a r y - l a y e r t u n n e l (Fig. T e s t s were
run i n
aeolian a c t i v i t y i n
all
mb) at
2), a c l o s e d - c i r c u i t ambient temperature
o f 220 C ( p r o d u c i n g t h e same f l u i d d e n s i t y as t h e -90 b a r , 730 K near t h e
system simu-
surface pressures (-7
and t h e --Venus Wind Tunnel (VWT; Fig.
Two are
t h e Martian
C02 atmosphere
wind t u n n e l , l o c a t e d a t Arizona
m l o n g by 1.0 m2
State
c r o s s - s e c t i o n atmospheric
3) o p e r a t i n g a t ambient t e r r e s t r i a l c o n d i t i o n s . t h r e e wind
t u n n e l s under
optimal
t h e p l a n e t a r y environment o f concern.
s e l e c t e d f o r a n a l y s i s i n c l u d e d t h o s e most e a s i l y
c o n d i t i o n s for
The p a r t i c l e
moved by t h e wind (Fig.
sizes 4) as
135
Fig. 1. P h o t o g r a p h o f t h e M a r t i a n S u r f a c e Wind Tunnel a t NASA-Ames Research Center. The o p e n - c i r c u i t a t m o s p h e r i c b o u n d a r y - l a y e r t u n n e l i s 13 m l o n g and has a 1.1 m2 t e s t s e c t i o n l o c a t e d 5 m f r o m t h e e n t r a n c e . It i s o p e r a t e d i n a l o w pressure chamber ( b a c k g r o u n d ) i n a C O P atmosphere a t -7 mb t o s i m u l a t e m a r t i a n conditions. w e l l as common dune sand particle
bed
s i z e s (-300
covered t h e
urn) on E a r t h .
e n t i r e bed
of
the test
I n most
experiments, t h e
s e c t i o n and
there
was an
unlimited supply o f p a r t i c l e s a v a i l a b l e f o r entrainment.
2.0
WIND TUNNEL MEASUREMENTS
L a b o r a t o r y e x p e r i m e n t s have t h e a d v a n t a g e t h a t c o n d i t i o n s can
be m o n i t o r e d
and c l o s e l y c o n t r o l l e d , t h u s r e d u c i n g t h e u n c e r t a i n t i e s i n i n t e r p r e t a t i o n o f t h e results
and a l l o w i n g a w i d e r a n g e
o f c o n d i t i o n s t o be t e s t e d .
e x t r a t e r r e s t r i a l s t u d i e s , l a b o r a t o r y s i m u l a t i o n s p r o v i d e an
I n t h e case of
extremely important
data base f o r a n a l y z i n g t h e l i m i t e d r e s u l t s o b t a i n e d i n s i t u f r o m s p a c e c r a f t . P a r t i c l e s u s e d i n t h e w i n d t u n n e l e x p e r i m e n t s were o b t a i n e d f r o m commercial sources
and f o r
t h e m a r t i a n and
w e l l - s o r t e d samples. Walnut-shell
Venusian runs
Q u a r t z sands
p a r t i c l e s were
were crushed and s i e v e d t o
were used
used i n obtain the
walnut s h e l l s (-1.1 g/cm3) a l l o w s
were f u r t h e r s i e v e d i n E a r t h and
the martian
simulations;
sizes of interest.
a partial
scaling f o r the
t o obtain
Venus s i m u l a t i o n s . these p a r t i c l e s
The l o w
d e n s i t y of
low g r a v i t a t i o n a l
136
F i g . 2. View o f t h e Venus Wind Tunnel a t NASA-Ames Research C e n t e r . This closed-circuit a t m o s p h e r i c b o u n d a r y - l a y e r w i n d t u n n e l has a 1 m l o n g by (3.14)(10-L)m2 t e s t s e c t i o n and o p e r a t e s a t a m b i e n t t e m p e r a t u r e w i t h carbon d i o x i d e a t 35 b a r t o s i m u l a t e V e n u s i a n c o n d i t i o n s . field
o f Mars,
appropriate
p r e v i o u s l y (Greeley e t al., The w i n d
f o r experiments
v e l o c i t y measured i n
t h e wind
speed, urn, f o u n d above t h e b o u n d a r y l a y e r . t e r r e s t r i a l and m a r t i a n c a s e s a l l o w wind f r i c t i o n
c o n d u c t e d on
Earth
as d i s c u s s e d
1981; 1982).
speed, u,(after
t u n n e l s was
the
f r e e s t r e a m wind
Boundary-layer p r o f i l e s taken i n the
c o n v e r s i o n o f urn t o t h e
Bagnold, 1941).
c o r r e s p o n d i n g wind
F o r t h e VWT t e s t s , u,
was d e t e r -
m i n e d from urn by u s i n g t h e e q u a t i o n o f S c h l i c h t i n g ( 1 9 6 8 ) :
where x
=
d i s t a n c e down t h e t e s t p l a t e and
p a r t i c l e diameter).
Boundary l a y e r
E
=
roughness h e i g h t ( e s s e n t i a l l y the
studies are
v a r i o u s s u r f a c e roughness v a l u e s used f o r t h e VWT. p l i c a t i o n a r i s e s when sand
i s set i n motion; t h e
t r a n s f e r o f momentum f r o m t h e w i n d t o t h e r e p o r t e d here a r e approximate.
grains
c u r r e n t l y underway t o
assess
It m u s t be n o t e d t h a t a com-
u,/urn
r a t i o changes due
n motion, thus t h e
to a
u, v a l u e s
137
Fig. 3. View o f t e s t s e c t i o n o f t h e one atmosphere, o p e n - c i r c u i t w i n d t u n n e l i n t h e P l a n e t a r y Geology L a b o r a t o r y a t A r i z o n a S t a t e U n i v e r s i t y . The t u n n e l i s 19 m long and has a 1.1 m2 c r o s s s e c t i o n . Shown h e r e i s t h e e x p e r i m e n t a l s e t - u p t o f i l m windblown p a r t i c l e s i n f l i g h t a g a i n s t a c m - g r i d b a c k d r o p ( i n t e s t s e c t i o n ) using a Hycam Model I 1 m o t i o n p i c t u r e camera ( a t r i g h t ) .
2.1
PARTICLE VELOCIMETER Figure
5 i s a diagram o f
experiments.
t h e p a r t i c l e v e l o c i m e t e r used i n
The d e s i g n o f t h i s d e v i c e i s based
Forest S e r v i c e ( S c h m i d t , 1977) t o ticles.
measure t h e
the laboratory
on a system used by
v e l o c i t y o f windblown
t h e U.S. snow p a r -
The v e l o c i m e t e r u t i l i z e s a l i g h t s o u r c e w h i c h p r o j e c t s a beam a c r o s s a
s m a l l gap t o t w o p h o t o t r a n s i s t o r s .
As a w i n d b l o w n g r a i n passes t h r o u g h t h e gap,
i t i n t e r r u p t s t h e beam and c a s t s a
shadow on t h e f i r s t d e t e c t o r
w h i c h responds
p u l s e ; i t t h e n shadows t h e
second d e t e c t o r
by g e n e r a t i n g a p o s i t i v e e l e c t r i c a l which g e n e r a t e s a n e g a t i v e p u l s e .
for
These s i g n a l s
l a t e r a n a l y s e s w i t h an o s c i l l o s c o p e .
a r e r e c o r d e d on m a g n e t i c t a p e
The v e l o c i t y
i s determined from t h e
distance t r a v e l e d (known f r o m t h e geometry o f t h e windows) and f r o m t h e travel
(determined from t h e
signal displayed
on t h e o s c i l l o s c o p e ) .
signals r e p r e s e n t i n g g r a i n s e i t h e r p a s s i n g t h r o u g h t h e gap a t o b l i q u e i n clusters are Approximately
i d e n t i f i e d by
150 s i g n a l s
velocity d i s t r i b u t i o n s .
were
asymmetric o r analyzed f o r
distorted each r u n
time o f Spurious angles o r
oscilloscope tracings. to
determine p a r t i c l e
138
Fig. 4. T h r e s h o l d f r i c t i o n speed (m/s) as a f u n c t i o n o f p a r t i c l e d i a m e t e r f o r Earth, Mars, and Venus; speed r e q u i r e d f o r p a r t i c l e m o t i o n i s i n v e r s e l y proport i o n a l t o t h e atmospheric d e n s i t y ( T a b l e 1); n o t e t h a t t h e optimum g r a i n s i z e ( i d e n t i f i e d on each curve) f o r p a r t i c l e threshold i s about t h e same i n a l l three planetary environments; ( a f t e r I v e r s e n and White, 1982).
1
-
-> (I)
\
E
-
115
k
t
0 0 0l.e 8 O A W
>
z
-
0
I-
EARTH
0
0.2
L
0.10
a P A
75
-
0 I
u)
0.02 -
w 0.03 a I
I-
0
75 . 2030
10
0
50
100
1 200
5 500 1
0
P A R T I C L E D I A M E T E R ((lm)
above the
To determine t h e range o f p a r t i c l e v e l o c i t i e s a t a s i n g l e h e i g h t s u r f a c e , v e l o c i t y measurements were grouped plotted
as a h i s t o g r a m expressed as
s p e c i f i e d v e l o c i t y range. percentage o f
i n even
i n c r e m e n t s o f one
t h e percentage
f r e e s t r e a m wind speed.
This l a t t e r
function i s
o r d e r s o f magnitude.
As shown
range.
has demonstrated t h e a c c e l e r a t i o n o f
saltation Figure 6
i n Figure
path, and we c o n s i d e r t h e
experiment
t o r e f l e c t grains a l s o shows
in a
V e l o c i t i e s a r e g i v e n b o t h as an a c t u a l speed and as a useful i n nor-
m a l i z i n g d a t a f o r i n t e r p l a n e t a r y comparisons i n which w i n d speeds may White (1979)
m/s and
o f grains t r a v e l l i n g
6, p a r t i c l e v e l o c i t i e s
wide range
sampled i n
t h a t some g r a i n s
d i f f e r by
have a
i n v e l o c i t i e s measured
different parts o f
exceed t h e
wide
p a r t i c l e s along the i n our
t h e i r trajectory.
f r e e s t r e a m wind
a t t r i b u t e t h i s observation t o very high accelerations r e s u l t i n g from
speed.
We
rebound o f
s a l t a t i n g g r a i n s f r o m t h e s u r f a c e and from i n t e r g r a i n c o l l i s i o n s . Tests t h e surface. speed w i t h
were r u n t o d e t e r m i n e p a r t i c l e
As shown
i n Figure
7, t h e r e i s
height, r e f l e c t i n g , f i r s t ,
t h r o u g h t h e boundary
layer,
speed as a f u n c t i o n o f a general
t h e increase
and second,
t h e longer
h e i g h t above
increase i n p a r t i c l e
i n wind speed particle
w i t h height
trajectory
--
139
" . . ,
-
PHOTOTRANSISTORS
Fig. 5. Diagram o f t h e p a r t i c l e v e l o c i me t e r used t o measure t h e speed o f windblown g r a i n s ( f r o m Greeley e t al., 1982)
p a r t i c l e p a t h l e n g t h i n c r e a s e s w i t h h e i g h t and a f f o r d s a l o n g e r t i m e
f o r accel-
A s i m i l a r r e l a t i o n s h i p was found i n experiments s i m u l a t i n g
e r a t i o n by t h e wind.
the m a r t i a n environment (Fig. 8).
The p a r t i c l e v e l o c i m e t e r does n o t o p e r a t e i n
the Venus Wind Tunnel, and no measurements were made w i t h t h i s technique. F i g u r e 9 shows t h e e f f e c t at
a s i n g l e height.
o f wind v e l o c i t y on p a r t i c l e
Two p a r t i c l e
martian atmospheric
conditions.
f r e q u e n t l y measured
speed).
v e l o c i t i e s t h a n l a r g e r ones. wind and t h e
smaller
s i z e s (350 urn and 92 P a r t i c l e speeds
I n general,
v e l o c i t y , measured
urn) were t e s t e d under
a r e modal
smaller p a r t i c l e s
values ( t h e travel a t
most higher
We a t t r i b u t e t h i s t o a b e t t e r c o u p l i n g between t h e
p a r t i c l e s owing
t o their
larger
cross-sectional
area-
to-mass r a t i o . Figure
9 a l s o shows t h a t p a r t i c l e
markably c o n s t a n t t h r o u g h a wide may shed some l i g h t on known t h a t Kawamura,
the characteristics of particle flux.
increases i n 1951; and
v e l o c i t y a t a given height
wind
others).
remains r e -
range o f f r e e s t r e a m wind speeds. speed cause However,
increases i n it
has n o t
This r e s u l t
It has l o n g been
flux
(Bagnold, 1941;
been known
whether
the
increase i n f l u x r e s u l t e d p r i m a r i l y from h i g h e r p a r t i c l e v e l o c i t i e s o r p r i m a r i l y from i n c r e a s e s i n t h e number o f g r a i n s i n t r a n s p o r t , o r both.
F i g u r e 9 suggests
t h a t i n c r e a s e s i n f l u x may be t h e r e s u l t o f more g r a i n s i n t r a n s p o r t r a t h e r t h a n o f higher p a r t i c l e v e l o c i t i e s .
2.2
HIGH SPEED MOTION PICTURES
A Hycam Model I 1 16 mm camera (Fig.
o f windblown
g r a i n s a t f r a m i n g r a t e s up
were photographed a g a i n s t a back-drop ysis
of the f i l m .
The f i e l d
3) was used t o o b t a i n t o 10,000
frames p e r second.
w i t h a cm-scale g r i d t o
o f view had t o be
motion p i c t u r e s Grains
f a c i l i t a t e anal-
s u f f i c i e n t l y small
t o permit
i n d i v i d u a l g r a i n s t o be r e s o l v e d and y e t l a r g e enough t o c h a r a c t e r i z e t h e f l i g h t path o f t h e g r a i n s
and t o d e t e r m i n e t h e i r v e l o c i t i e s .
analyzed (-600 pm), t h e maximum u s e f u l f i e l d o f view
For t h e
largest grains
was about 6 cm h i g h
by 10
140
3 5 0 p m QUARTZ 16.1cm HEIGHT
~ ~ ' 1 0 . 2m2/ s
PARTICLE VELOCITY (m/roc) I
1
0
20
60
40
80
I
100
120
% O F FREESTREAM WIND S P E E D
Fig. 6. D i s t r i b u t i o n o f p a r t i c l e v e l o c i t i e s a t a h e i g h t o f 16.1 cm above t h e s u r f a c e f o r 350 um d i a m e t e r q u a r t z g r a i n s s u b j e c t e d t o a f r e e s t r e a m windspeed of -10.22 m/s i n a s i m u l a t i o n o f E a r t h (u, = 0.5 m/s). The v e l o c i t y d i s t r i b u t i o n i s t y p i c a l f o r b o t h wind t u n n e l and f i e l d experiments and w i l l be r e p r e s e n t e d i n subsequent f i g u r e s as a one-standard d e v i a t i o n v e l o c i t y range around a mean p a r t i c l e velocity. cm wide.
Thus f a r i n o u r i n v e s t i g a t i o n o n l y a l i m i t e d number o f runs have been
made i n which h i g h speed m o t i o n p i c t u r e s were o b t a i n e d , a l l a t E a r t h
(wind t u n -
n e l and f i e l d c o n d i t i o n s ) and Venus c o n d i t i o n s , b u t n o t m a r t i a n c o n d i t i o n s . Data were
obtained from
processed
L a f a y e t t e Analyzer o n t o a 40" by 60" GTCO 11/45 computer. i n t o the
The g r i d
image
via a
x-y d i g i t i z i n g t a b l e l i n k e d t o
a PDP
f i l m s by
p o s i t i o n , f r a m i n g r a t e , and wind
program and t h e p a r t i c l e p o s i t i o n s
f i l m sequence.
projecting the
were p l o t t e d
speed were e n t e r e d f o r each frame
in a
The reduced d a t a i n c l u d e v e l o c i t i e s i n t h e f o l l o w i n g c a t e g o r i e s :
1) particles i n r i s i n g trajectories,
2) p a r t i c l e s i n f l a t t r a j e c t o r i e s ,
t i c l e s i n f a l l i n g t r a j e c t o r i e s and 4) combined t r a j e c t o r i e s ;
3) p a r -
p a r t i c l e speeds are
g i v e n i n m/s and as a percentage o f f r e e s t r e a m wind speed (urn).
141
25 0-
E 0
20
W
0 4:
LL
a
3
15
fn W
> 0
m
EARTH CASE
10
3 5 0 ptn Q U A R T Z P A R T I C L E S
4
v
I-
r
?!
urnr 1 0 . 5 r n / s
5
W
r I
I
I
I
1
I
I
MEAN PARTICLE VELOCITY ( m / s ) 1
I
I
0 20 40 60 80 100 M E A N P A R T I C L E V E L O C I T Y (%urn)
Fig. 7. P a r t i c l e v e l o c i t i e s f o r 350 um q u a r t z g r a i n s measured a t f o u r h e i g h t s above t h e s u r f a c e i n a wind t u n n e l a t 1 b a r p r e s s u r e and a f r e e s t r e a m wind speed o f 10.5 m/s (u, = 0.5 m/s). Figure
10 shows
typical
results for
speeds i n c r e a s e w i t h h e i g h t above with the
p a r t i c l e velocimeter, although
f i l m i s limited.
E a r t h cases;
t h e surface
in
general, p a r t i c l e
similar t o the
t h e range
r e s u l t s obtained
i n the heights
analyzed by
A t t h i s t i m e , i d e n t i c a l experiments have n o t been r u n t o com-
pare r e s u l t s f r o m t h e v e l o c i m e t e r w i t h t h o s e f o r t h e m o t i o n p i c t u r e s . shows
r e s u l t s o b t a i n e d i n t h e Venus
tures.
Wind Tunnel
f r o m analyses o f
Although t h e a b s o l u t e v e l o c i t i e s a r e v e r y low
(a r e f l e c t i o n of t h e low
wind speeds on Venus), n o t e t h a t t h e p a r t i c l e s a c h i e v e n e a r l y t h e as f r e e s t r e a m
wind speed,
in
marked c o n t r a s t
t o Mars
F i g u r e 11 motion p i c -
where
same v e l o c i t y
p a r t i c l e s seldom
reach f r e e s t r e a m v e l o c i t i e s . I n a d d i t i o n , p a r t i c l e v e l o c i t i e s on r i s i n g , f l a t , and were assessed (Fig. Earth increase
12).
throughout
falling trajectories
As p r e d i c t e d by White (1979), p a r t i c l e v e l o c i t i e s the saltation
b e f o r e impact w i t h t h e s u r f a c e ,
t r a j e c t o r y , reaching
a
on
maximum j u s t
142
25
-
A
E 0
20
W
0
U
u.
U
3
15
ln W
> 0
m U
MARS CASE
10
-
c
I
a -
3 5 0 p m SHELL PARTICLES u
6 5 m/ s
5
W
I I
(
I
1
1
I
0
I
1
I
1 35
I
30 20 25 15 10 MEAN PARTICLE VELOCITY (m/s)
5
I 50
I
I
10 20 30 40 M E A N P A R T I C L E V E L O C I T Y (%Urn)
Fia. 8. P a r t i c l e v e l o c i t i e s f o r f o u r d i f f e r e n t h e i a h t s above t h e s u r f a c e for 356 w a l n u t s h e l l p a r t i c l e s s u b j e c t e d t o a f r e e s t r e a m wind speed o f 65 m/s (u, = 3.4 m/s) i n a low atmospheric p r e s s u r e (6.6 mb) t o s i m u l a t e t h e martian e n v i ronment. 3.0
FIELD STUDIES Although l a b o r a t o r y s i m u l a t i o n s e n a b l e a e o l i a n processes t o be i n v e s t i g a t e d
under c o n t r o l l e d c o n d i t i o n s , q u e s t i o n s the results
as a p p l i e d t o n a t u r a l c o n d i t i o n s .
t i o n s , a f i e l d experiment was particles.
inevitably arise
conducted t o
I n order
v a l i d i t y of
as t o t h e
t o address such ques-
obtain v e l o c i t y data
f o r saltating
The experiment was conducted 3 November 1981 a t Waddell Creek State
Beach i n C a l i f o r n i a , about 90
km s o u t h o f San Francisco.
The beach
i s one o f
t h e w i n d i e s t on t h e c o a s t and i s f a i r l y wide, e n a b l i n g a good s a l t a t i o n c l o u d t o develop across
t h e dry
p a r t o f the
beach b e f o r e
r e a c h i n g t h e area
where the
f i l m i n g t o o k place. Using t h e same i n s t r u m e n t s as ( a t a h e i g h t o f 1.0 m above Greeley e t al., taneously.
t h e surface), p a r t i c l e f l u x (using
1982), and h i g h
The p a r t i c l e
wind v e l o c i t y
employed i n t h e w i n d t u n n e l s ,
speed m o t i o n p i c t u r e s were a l l
c o l l e c t o r s , see o b t a i n e d simul-
v e l o c i m e t e r m a l f u n c t i o n e d and d a t a were
w i t h i t i n t h e f i e l d experiment.
n o t obtained
143
92vm
350um 130 120
Siia
\
E
k
0 0 A
W
10( 9c
>
n
z
3 z a
W
8C
7C
a
+ W W
6(
U
LL
5c 1
I
5
10
1
1
I
15
20
25
1
I
35
30
PARTICLE VELOCITY ( Y O D E , m / a )
Fig. 9. S t a t i s t i c a l modes o f v e l o c i t i e s as a f u n c t i o n o f f r e e s t r e a m wind speed a t a h e i g h t o f 7.1 cm above t h e s u r f a c e f o r 350 urn and 92 urn s h e l l p a r t i c l e s i n a m a r t i a n s i m u l a t i o n ; p a r t i c l e v e l o c i t y remains c o n s t a n t o v e r a wide range of freestream wind speeds. Four s u c c e s s f u l r u n s were completed, hold.
i n t h e wind t u n n e l runs. is
a l l a t wind speeds j u s t
The m o t i o n p i c t u r e s were analyzed f o l l o w i n g t h e
somewhat d i f f e r e n t
above t h r e s -
same procedures as used
F i g u r e 13 shows t h e r e s u l t s ; a l t h o u g h t h e f r o m t h o s e used
i n the
wind t u n n e l runs,
grain size the particle
v e l o c i t y d i s t r i b u t i o n s a r e w i t h i n t h e range expected.
4.0
SUMMARY AND CONCLUSIONS
Velocities o f speed,
windblown
h e i g h t above t h e
p a r t i c l e s were
ground, and
d e t e r m i n e d as
p a r t i c l e diameter f o r
functions
of wind
various conditions
s i m u l a t i n g Earth, Mars, and Venus i n e n v i r o n m e n t a l wind t u n n e l s .
S i m i l a r data,
although o f l i m i t e d range, were o b t a i n e d f r o m a f i e l d experiment
f o r comparison
w i t h t h e wind t u n n e l r e s u l t s s i m u l a t i n g t h e t e r r e s t r i a l environment.
144
-
-
EARTH CASE
E
-
ASU WIND TUNNEL
0
W
0
a Y
U
3 UJ W
> 0
m
a I-
I
-
Q W
I I
'0
I
I
I
I
I
1
5 6 7 3 4 2 MEAN PARTICLE VELOCITY (m/s)
1 I
I
I
I
I
8 I
1
I
20 30 40 50 60 MEAN PARTICLE VELOClTY
10
J
70
80
(%Urn)
F i g . 10. P a r t i c l e v e l o c i t i e s o b t a i n e d f r o m a n a l y s e s of h i g h speed m o t i o n p i c t u r e s f o r 400 q u a r t z p a r t i c l e s s u b j e c t e d t o a f r e e s t r e a m w i n d speed of 10.5 m/s (u, = 0.55 m / s ) i n t h e 1.0 b a r a t m o s p h e r i c w i n d t u n n e l a t A r i z o n a S t a t e Un iv e r s i t y
.
I n g e n e r a l , t h e r e s u l t s show
that particles
travel a t higher
speeds w i t h
i n c r e a s e d h e i g h t above t h e ground, and t h a t s m a l l e r p a r t i c l e s t r a v e l f a s t e r t h a n l a r g e r ones. not increase
However, f o r a g i v e n h e i g h t above t h e ground, p a r t i c l e speed does with higher
t h i s i s with reference t o
f r e e s t r e a m w i n d speeds.
modal v a l u e s
and
I t must
be remembered t h a t
n o t t o maximum v a l u e s ; a
few p a r t i -
c l e s do i n c r e a s e i n v e l o c i t y w i t h f r e e s t r e a m w i n d v e l o c i t i e s . Comparisons o f r e s u l t s f o r
E a r t h , Mars,
and Venus r e v e a l
some r e m a r k a b l e
differences.
As shown i n F i g u r e 14, most p a r t i c l e s a c h i e v e speeds n e a r l y equal
t o freestream
w i n d speed o n Venus, b u t
seldom a c h i e v e
Mars; E a r t h c a s e s a r e o f i n t e r m e d i a t e v a l u e s . ences
i n a t m o s p h e r i c d e n s i t y and t o
p l a n e t a r y e n v i r o n m e n t s ( T a b l e 1). V e n u s i a n atmosphere t h a n on and f o r t h e (just
t h e t h r e s h o l d w i n d speeds among
speeds), t h e
speed on the three
P a r t i c l e s a r e more e a s i l y moved i n t h e dense
Mars; c o n s e q u e n t l y , t h r e s h o l d speeds a r e
r a n g e o f w i n d speeds i n
above t h r e s h o l d
h a l f t h e wind
This i s attributed t o the d i f f e r -
w h i c h most grains
movement i s presumed
need n o t
be m o v i n g
very
v e r y low, t o occur fast to
145
-
-
3
VENUS CASE
E
VENUS WIND TUNNEL
0
Y
5 0 0 - 6 0 0 p m QUARTZ PARTICLES
W
0
ucO = 3.621111s
U
u . 2 K
__t_
3 v)
W
> 0
m
u 1
-
-
I-
-
r
-
0 W
I 0
I
I
1
,
-
I
I
1
1
I
I
I
I
I
I
I
1
60
l
I
I
l
1
1
I
3.5
2.5 3.0 MEAN PARTICLE VELOCITY (m/s)
0
--
80 90 70 MEAN PARTICLE VELOCITY (%urn)
4.0 I
100
Fig. 11. P a r t i c l e v e l o c i t i e s o b t a i n e d f r o m a n a l y s i s o f h i q h speed m o t i o n p i c t u r e s f o r 500 t o 600 pm q u a r t z p a r t i c l e s s u b j e c t e d t o a f r e e s t r e a m w i n d speed o f 3.62 m/s (u, = 0.2 m/s) a t 30 b a r s a t m o s p h e r i c p r e s s u r e i n t h e Venus Wind Tunnel. achieve
100% o f t h e w i n d speed.
C o n v e r s e l y , p a r t i c l e s on Mars must a c c e l e r a t e
very r a p i d l y t o a c h i e v e t h e speed o f t h e h i g h w i n d s r e q u i r e d f o r despite
the f a c t t h a t s a l t a t i o n path
lengths are
l o n g on Mars
most g r a i n s f a l l t o t h e s u r f a c e b e f o r e a c h i e v i n g even 50-60% o f
t h r e s h o l d , and ( W h i t e , 1979), freestream wind
speed. F u t u r e e x p e r i m e n t s w i l l i n v o l v e e x p a n s i o n o f t h e d a t a base t o a w i d e r r a n g e o f wind-tunnel
and f i e l d c o n d i t i o n s and f o r a w i d e r r a n g e o f p a r t i c l e d i a m e t e r s ,
wind speeds, and measurements f o r
v a r i o u s h e i g h t s above t h e s u r f a c e .
tion, a v e l o c i m e t e r i s under development t h a t w i l l be f i e l d - p o r t a b l e obtain
a l a r g e r number
of velocity
measurements t h a n a r e
I n addii n order t o
presently available
from h i g h - s p e e d m o t i o n p i c t u r e s . ACKNOWLEDGEMENTS A l l a s p e c t s o f t h i s work
were s u p p o r t e d
by t h e P l a n e t a r y
Geology O f f i c e ,
N a t i o n a l A e r o n a u t i c s and Space A d m i n i s t r a t i o n . We t h a n k t h e f o l l o w i n g i n d i v i d u a l s f o r t h e i r c o n t r i b u t i o n t o D.
B a l l f o r p h o t o g r a p h i c s u p p o r t , G.
Kuhl f o r d a t a r e d u c t i o n , and
Beardmore,
P.
Parkes, K.
t h i s study:
Malone,
and D.
R. Leach f o r a s s i s t a n c e i n o b t a i n i n g t h e w i n d t u n -
146 n e l d a t a and development o f t h e p a r t i c l e v e l o c i m e t e r . i n the i n i t i a l
arrangements f o r
Alan P e t e r f r e u n d a s s i s t e a
t h e f i e l d experiment.
R. Leach, M.
We thank
M a l i n , and K. Gerety f o r h e l p f u l comments on t h e manuscript.
f0
60
501
DESCENDING
0
6
4
2
8
10
PARTICLE VELOCITY (m/s) I
0
I
10
, 20
I
30
I
I
40
50
I
60
PARTICLE VELOCITY
70
80
I
I
90
100
(%Urn)
Fig. 12. P a r t i c l e v e l o c i t i e s o b t a i n e d f r o m a n a l y s i s o f high-speed m o t i o n p i c t u r e s f o r 500 t o 600 vm q u a r t z p a r t i c l e s s u b j e c t e d t o a f r e e s t r e a m wind speed of 10.5 m/s (u, = 0.5 m / s ) a t one b a r atmospheric pressure, comparing v e l o c i t i e s f o r g r a i n s on t h e r i s i n g , h o r i z o n t a l , and descending p a r t s o f t h e i r t r a j e c We p r e s e n t l y have no e x p l a n a t i o n f o r t h e bimodal v e l o c i t y d i s t r i b u t i o n tories. o f g r a i n s on t h e h o r i z o n t a l p a r t o f t h e t r a j e c t o r y .
147
-Eo
4
LL
a a
m
-
10
W
0
-
12
8
-
EARTH CASE FIELD EXPERIMENT WADDELL BEACH, CA Dp
__t_
= 300pm
6
W
> 0
m 4
W
I
2 i 4
1
0
3
2
5
4
MEAN PARTICLE VELOCITY (m/S) I
0
I
10
1
20
1
I
I
I
I
I
30
40
50
60
70
80
M E AN P A R T I C L E V E L 0 C I T Y ( % uoo)
Fig. 13. P a r t i c l e v e l o c i t i e s o b t a i n e d f r o m a n a l y s i s o f h i g h speed m o t i o n p i c t u r e s f o r n a t u r a l beach sands o f -300 um d i a m e t e r s u b j e c t e d t o a wind speed o f -6 m/s measured a t a h e i g h t o f 1 m above t h e s u r f a c e a t Waddell Creek S t a t e Beach, C a l i f o r n i a .
148
-E 0
w
30 25
0 U
;2 0 3 v)
w 15
>
0 I-
10
I
Q
i i 5 r
0
I
I
I
I
I
I
I
I
I
I
10
20
30
40
50
60
70
80
90
100
P A R T I C L E V E L O C I T Y (MODE,%u,)
F i g . 14. Comparison o f v e l o c i t i e s on E a r t h , Mars, and Venus. Two s i z e s o f p a r t i c l e s , each s u b j e c t e d t o r e l a t i v e l y l o w and h i g h w i n d speeds, were t e s t e d under E a r t h and Mars c o n d i t i o n s ; one s i z e p a r t i c l e was t e s t e d a t a l o w v e l o c i t y under Venus c o n d i t i o n s . I n g e n e r a l , g r a i n s a c h i e v e a much h i g h e r v e l o c i t y i n r e l a t i o n t o t h e w i n d speed u n d e r V e n u s i a n c o n d i t i o n s t h a n u n d e r m a r t i a n c o n d i t i o n s , and g r a i n s u n d e r t e r r e s t r i a l c o n d i t i o n s have i n t e r m e d i a t e v e l o c i t i e s . REFERENCES Bagnold, R.A., 1941. The P h y s i c s o f Blown Sand and D e s e r t Dunes: Methuen, London, 265 pp. G r e e l e y , R., R.N. Leach, S.H. W i l l i a m s , B.R. White, J.B. P o l l a c k , D.H. K r i n s l e y , J. _ Geophy. a n d J.R. M a r s h a l l , 1982. R a t e o f Wind A b r a s i o n on Mars: _ _ _ - Res., 87, pp. 10,009-10,024. G r e e l F , R., B.R. White, J.B. P o l l a c k , J.D. I v e r s e n , and R.N. Leach, 1977. Dust s t o r m s on Mars: C o n s i d e r a t i o n s and S i m u l a t i o n s : NASA T e c h n i c a l Memo. TM 78423, pp. 29. G r e e l e y , R., B.R. W h i t e , J.B. P o l l a c k , J.D. I v e r s e n , and R.N. Leach, 1981. Dust s t o r m s on Mars: C o n s i d e r a t i o n s and S i m u l a t i o n s : I n Desert Dust: Origin, C h a r a c t e r i s t i c s , and E f f e c t on Man. Geol. SOC. her.--1 T r o y Pewe, pp. l O T i 2 i 7 ' - - G r e e l e y , and J.B. P o l l a c k , 1976. Windblown d u s t on Earth, I v e r s e n , J.E., R. Mars, and Venus: J. Atmos. S c i . , 2, 2425-2429. S a l t a t i o n t h r e s h o l d on E a r t h , Mars, and I v e r s e n , J.D. and B . R T W W 1 9 8 2 . Venus: S e d i m e n t o l o g y , 29, pp. 111-119. Kawamura, R., 1951. S t u d y of sand movement by w i n d : Phy. S c i . Res. I n s t . , Tokyo Univ., 5, 95-112, ( o r i g i n a l i n Japanese; NASA t r a n s l a t i o n ) . S c h l i c h t i n g , H., 1368. Boundary-Layer Theory: M c G r a w - H i l l Book Company, New York, pp. 748. Schmidt, R.A. 1977. A System T h a t Measures B l o w i n g Snow: U.S. Dept. Agric. Res. Paper, RM -194, 80 pp. WhiteXR-79. S o i l T r a n s p o r t by Winds on Mars: J. Geophy. Res., 84, pp. 4643-4651. Williams, S.H. and R. G r e e l e y , 1983. F l u x o f w i n d b l o w n p a r t i c l e s on Venus: NASA Rept. P l a n e t a r y Geology, ( a b s t r a c t , Preliminary laboratory results: i n press).