Document
N89- 15022
DEVELOPMENT AND TESTING O F A UNIQUE CAROUSEL W I N D TUNFiEL TO EXPERIMENTALLY DETEZrlINE THE EFFECT O F GRAVITY AND THE INTZRPARTICLE FORCE ON THE PHYSICS O F WIND-BLOWN PARTICLES R.N. Leacha, R . Greeleya,B. White=,J. Xversen3.
I n t h e study of planetary aeolian processes t h e e f f e c t of g r a v i t y i s n o t r e a d i l y modeled. G r a v i t y appears i n t h e equations of p a r t i c l e mction along w i t h t h e i n t e r - p a r t i c l e f o r c e s b u t t h e two are not separable. A wind tunnel t h a t permits multi-phase f l o w experiments w i t h wind blown p a r t i c l e s a t v a r i a b l e g r a v i t y has been b u i l t and experiments have been conducted a t reduced g r a v i t y . The equations of p a r t i c l e motion i n i t a t i o n ( s a l t a t i o n threshold) w i t h v a r i a b l e g r a v i t y were experimentally v e r i f i e d and t h e i n t e r p a r t i c l e f o r c e was separated.
Wind tunnel s suf f er from several shortcomings i n aeol i an experiments, p r i m a r i l y due t o l i m i t a t i o n s i n s i r e . The Reyr;olds Number t h a t most s t r o n g l y a + f e c t s s a l t a t i o n threshold is b a s 4 a n t h e distance from t h e tunnel entry, and f o r most experiments a long distance i s required t o obtain a s u f f e c i e n t l y l a r g e Reynolds Number t o o b t a i n t h e corresponding f u l l y developed t u r b u l e n t boundary 1ayer.This presents a problem, e s p e c i a l l y when t h e equipment i s t o be f l i g h t or space borne.
A uniquely designed Carousel Wind Tunnel allows f o r t h e long f l o w distance i n a small sized tunnel since t h e t e s t s e c t i n n i s a continous loop ahd develops t h e required t u r b u l e f i t bol;ndar)r layer. The Carousel Wind Tunnel consists o f two concentric drums w i t h t h e t e s t s e c t i o n being t h e e n t i r e space between t h a drum-,.
D i f f e r e n t i a l r o t a t i o n of t h e drums causes an a i r f l o w between t h e drums which e n t r a i n s p a r t i c l e s placed there. Rotation of t h e outer drum produces a pseudo g r a v i t y f o r c e holding t h e p a r t i c l e s t o t h e surface i n t h e same manner t h a t g r a v i t y does. The f o r c e i s p s u e d o i n t h a t t h e p a r t i c l e s f e e l t h e f o r c e o n l y w h i l e i n contact w i t h t h e surface. The tunnel i s t o be used i n a micro g r a v i t y environment such as on t h e space s t a t i o n or i n t h e s h u t t l e .
A prototype model of t h e tunnel where o n l y t h e i a n e r drum r ~ t a t e s has been b u i l t and t e s t e d i n t h e K C 135 "Weightless Wmder I V " zero g a i r c r a f t operated by NASA Johnson Space Center.
Thus f o r these t e s t s t h e g r a v i t y l e v e l was changed by t h e external environment r a t h e r than by t h e r o t a t i o n o f t h e outer driim. Reduced o r z e r o g i s obtained when the a i r c r a f t , a f t e r obtaining a s u i t a b l e excess airspeed, climbs a t a 45O angle and then enters a parabolic or n e a r l y parabolic t r a j e c t o r y which produces t h e reduced o r z e r o g f o r up t o 30 seconds, Figure 1.
The a i r c r a f t i s able t o f l y 40 o r more such t r a j e c t o r i e s i n a s i n g l e f l i g h t .
The wind tunnel, Figure 2, i s b u i l t of c l e a r polycarbonate p l a s t i c and t h e inner drum i s made t o s p i n b y means of a v a r i a b l e speed f r a c t i o n a l horsepower e l e c t r i c motor connected through a b e l t drive. The outer drum i s 60 c m i n diameter and 30 cm wide.
The inner drum i s 40 c m i n diameter and i s s i r e d t o provide a close f i t along t h e s i d e walls. There i s a removable p a n e l i n t h e lower s i d e w a l l f o r i n s e r t i n g and removing aeolian t e s t material.
A-5 3 The d r u m speed i s monitored by an AC voltmeter d r i v e n by a inductance pick-up which is energized by a magnet attached t o t h e motor shaft. T h i s is c o r r e l a t e d w i t h t h e actual drum r p m as determined w i t h a photo-tachometer. A g r a v i t y meter u t i l i z i n g a s e n s i t i v e accelerometer d i s p l a y s t h e g r a v i t y l e v e l . The rpm, g r a v i t y l e v e l and p a r t i c l e motion are recorded by video camera during t h e t e s t s f o r l a t e r analysis. The tunnel i s mounted on a stand b o l t e d t o t h e f l o o r of t h e KC 135.
The experiments were done i n t h e f o l l o w i n g manner: a small q u a n t i t y of aeolian material was placed i n t h e t e s t s e c t i o n and t h e inner drum was spun a t a speed below t h a t which would cause any p a r t i c l e movement. A s t h e a i r c r a f t entered i t s marieuver t h e g r a v i t y l e v e l a t which s a l t a t i o n threshold occured was recorded along w i t h t h e drum r o t a t i o n speed. The a i r c r a f t d i d both t e r a g and low g manuevers ranging from 0.05 t O 0.5 9. Ey varying t h e drum r o t a t i o n speed f o r subsequent manuevers a matrix o f data p o i n t s were obtained. Often t h e drum speed was e i t h e r t o o h i g h so t h a t a speed much above threshold was obtained o r too low sa t h a t no p a r t i c l e movement took place. The drum s p e e d could not be changed r a p i d l y enough t o t o a d j u s t t h e speed during a maneuver, however as t h e f l i g h t s progressed experience allowed a S e t t e r choice of i n i t i a l drum r o t a t i o n speed, obtaining values clctser t o s a l t a t i o n threshold. The video tape was analysied a f t e r t h e f l i g h t so t h a t data obtained even on those maneuvers t h a t exceeded threshold could b e used by n o t i n g t h e momentary g level at which p a r t i c l e movement began. The t e s t data were p l o t t e d and a reference l i n e drawn through t h e minimum v e l o c i t y where s a l t a t i o n occufed, Figures 3 % 4.
Data were obtained f o r two s i z e s of material. Closely graded ground walnut s h e l l s w i t h median diameters o f 700 and 1288 microns were used i n t h e two experiments conducted. Walnut s h e l l w e r e used instead of sand f o r several reasons: ( 1 ) they are n o t as abrasive as sand and do n o t scratch t h e wind tunnel as sand o r other material does; (2) t h e r e i s a great amount o f of data on t h e s a l t a t i o n p r o p e r t i e s o f walnut s h e l l from previous experiments i n t h e MARSWIT f a c i l i t y 4 and (3) walnut s h e l l d o n o t become as h i g h l y charged by e l e c t r o s t a t i c s as other material, perhaps due t o t h e i r moisture content (about EX by weight).
These data were c o r r e l a t e d w i t h t h e f r i c t i o n threshold v e l o c i t y a t s a l t a t i o n threshold by c a l i b r a t i n g t h e Carousel Wind Tunnel w i t h a s e r i e s of p a r t i c l e s of known f r i c t i o n thresholds as obtained i n conventional aeolian wind tunnels, thus g i v i n g a curve of u* verses drum r o t a t i o n a l speed, Figure 5.
The f l i g h t data were corrected f o r t h e nominal a i r c r a S t cabin pressure of 12.25 p s i and t h e datum p o i n t s c l o s e s t t o t h e reference l i n e a r e presented along w i t h t h e t h e o r e t i c a l curve Gbtained from t h e equation A-54 The data correlates w e l l w i t h t h e g r a v i t y t e r m f o r v a l u e s o f g l e a s t h a n 1, F i g u r e s 6 t 7. An a t t e m p t w a s also made to a b t a i n v a l u e s of s a l t a t i o n t h r e s h o l d from 1-0 g t o 1.9 g d u r i n g t h e aircraft p u l l u p and p u l l o u t maneuvers. T h i s w a s d o n e by s p e e d i n g up t h e drum d u r i n g t h e maneuver u n t i l s a l t a t i o n occured.
T h e data d i d n o t p r o v e s a t i s f a c t o r y d u e t o t h e slow acceleration of t h e drum mentioned earlier and also d u e to t h e f a c t t h a t there is a l a g t i m e between t h e t i m e t h a t t h e drum r e a c h e s a r o t a t i o n s p e e d and t h e t i m e t h a t t h e air f l o w r e a c h e s a c o n s t a n t v a l u e .
T h e a b o v e e q u a t i o n c a n b e w r i t t e n i n t h e f o r m : If p l o t s are made of r h o u..*= vs. rho,qD, a n d t h e i n t e r p a r t i c l e force ( I , ) is z e r o t h e s e should g o through t h e o r i g i n . T h e s e are p r e s e n t e d i n F i g u r e s 8 and 9. I t a p p e a r s t h a t t h e c u r v e s i n t e r c e p t t h e y - a x i s a t a s m a l l p o s i t i v e v a l u e .
i n d i c a t i n g t h a t t h e i n t e r p a r t i c l e f o r c e h a c been i d e n t i f i e d ar;d s e p a r a t e d from t h e g r a v i t y force f o r t h e s e t w o testc.
F u t u r e work i n c l u d e s f u r t h e r e x p e r i m e n t s w i t h w a l n u t s h e l l i n t h e KC 135 w i t h s h a r p l y g r a d e d p a r t i c l e s o$ w i d e l y v a r y i n g median s i z e s i n c l u d i n g v e r y s m a l l p a r t i c l e s t o see how i n t e r p a r t i c l e force v a r i e s w i t h p a r t i c l e s i z e , and also e x p e r i m e n t s w i t h o t h e r a e o l i a n material.
A r i z o n a State U n i v e r s i t y , Tempe, Az 85257, = U n i v e r s i t y of C a l i f o r n i a , D a v i s Ca. 95615,
= Iowa State U n i v e r s i t y , Ames, Id. 50010
4Geophysical R e s e a r c h L e t t e r s , V a l . 3 no. 8 , pp 417-420 Gree?ey.
et.al.
A-55
FLIGHT PATH OF KC135 AIRCRAFT
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FIGURE 1 A-5 6 RPM SQUARED VS. GRAVITY RPM SQUARED VS. QRAVITY 1080 MICRON WALNUT SHELL 700 MICRON WALNUT SllELL EXPERIMENTAL EXPERIMENTAL FLIGHT DATA FLIGHT D A T A e l.o - 0 PARTICLE MOVEMENT 0 PARTICLE MOVEMENT I 1 0 NOMOVEMENT o NO MOVEMEN r .Q
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F L I G H T D A T A FLIGHT DATA INTERPARTICLE FORCE I I I .20 0 .w .08 .I2 .16 g PP DP F I G L P E E FlOURE 9 A-5 8