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Air cushion vehicles and soil erosion
Journal o f Terramechanics, 1976, Vol. 13, No. 4, pp. 201 to 210. Pergamon Press Printed in Great Britain.
AIR
CUSHION
VEHICLES
AND
SOIL
EROSION
P. F. J. ABEELS* Summary--The use of aerostatically supported vehicles is particularly interesting in agriculture, principally because the tractive effort required to pull implements may be reduced. Then the tractor used for driving can be lighter or its outer dimensions may be more appropriate to European land ownership structures. This study verifies that the induced effects in the soil when repetitive passes of an ACV occur, do not include damage to the substrate by aeolian erosion. The main ACV characteristics influencing erosion are the speed of the air at the outlet under the skirt and the angle of incidence of the jet on the soil. The effect of the escaping air is in proportion to the square of its velocity. The angle of incidence must be chosen in relation to an equilibrium between induced erosion and operating height. The ACV with peripheral jet seems to be more appropriate when applied in agriculture. Soil resistance to erosion is a function of moisture content and of compaction. During the passage of an ACV, erosion happens in two phases. Initially, erosion is high, then the process reaches a certain level at which it remains stable. Having measured the extent of a major disadvantage of ACV's for work on agricukural land, i.e. the induced erosion, it appears that such equipment shows some promise for special conditions of use.
INTRODUCTION THE AIM o f the tests c o n d u c t e d with air cushion vehicles (ACV) is to specify the c o n d i t i o n s o f their use on agricultural soils t a k i n g into a c c o u n t the erosion process they m a y induce, p a r t i c u l a r l y in case o f repetitive passes following fixed routes. A e r o s t a t i c lift is o b t a i n e d b y b l o w i n g air into a c h a m b e r where pressure is generated because the outlets are controlled. Inflation techniques are b l o w i n g in a p l e n u m c h a m b e r o r the p e r i p h e r a l j e t (Fig. 1). O t h e r A C V systems, like diffuser, l a b y r i n t h e seal, solid wall, etc . . . . are n o t included in this study. T h e studied aspects a r e : (1) Influence o f air cushion system t y p e ; (2) Soil c o n d i t i o n s where the A C V operates, i.e. soil m o i s t u r e c o n t e n t soil c o m p a c t i o n ; (3) A C V o p e r a t i n g conditions, as: l o a d on the air cushion hovering. T h e observations c o n c e r n the characteristics a n d the quantities o f displaced particles d u r i n g o r after A C V passages. This requires erosion profile measurements. These profiles o r sections are then c o n v e r t e d into e r o d e d quanties for a unit length o f cushion a n d for a given time. *Drpartement de Grnie Rural, Facult6 des Sciences Agronomiques, Universit6 Catholique de Louvain, Belgium. 201
202
P.F.J. ABEELS
i
Peripheral ]ef
I
! i
Plenum chamber
FIG. I.
A i r cushion systems.
EXPERIMENTAL CONDITIONS The experimental prototype has dimensions and characteristics as close as possible to those of a future model available for agriculture, for example a maximum width of 2.5m. The model is in full scale so as not to be confronted immediately with scale problems. For experimental and overall dimensions reasons, the model is limited to a representative length unit of the ACV (Fig. 2), and is also convertible to different versions and adjustable in geometrical characteristics (Fig. 3). The measurements make it possible to specify all data about the air cushions obtained. INFLUENCE OF THE AEROSTATIC LIFT SYSTEM With a plenum chamber, construction of the ACV is rather simple. Air-tightness is obtained with the skirt around the vehicle. Air quantities required are high. Inflation pressure is low. The peripheral jet gets air-tightness through inflation pressure and direction (inflation angle) of the air inlet against the volume wrapped in the "cushion".
;7
FIG. 2. Experimental model.
AIR CUSHION VEHICLES AND SOIL EROSION
203
I I I
,3
I
I I
-i
FIG. 3. Differentadaptation possibilitiesfor the experimental model. Erosion is linked with ACV type and inflation angle 0, (Fig. 4). An estimation of the eroded quantities is compiled from vy and 0,.
0,, °
10
Plenum chamber 30
45
v.fl sin 0N Q'ndmaratn-t
130.78 1.02
420.50 2.30
657.51 2.73
10 152.60 0.6 to 1
Peripheral jet 30 496.12 3.4 to 3.9
45 746.76 4.2 to 5.1
Meanwhile, it should be observed that: (1) Ground clearance for a plenum chamber is only half of what is necessary for the peripheral jet; (2) A plenum chamber ACV is lighter; (3) Total pressure is slightly lower for the plenum chamber type; (4) Air flow is lower with a peripheral jet; (5) Pressure distribution for peripheral jets is such that probable erosion points can be predicted.
204
P.F.J. ABEELS
[ 45
/ /
/
/
40~
/
/
/
//
/ I / I/ / / I ,,"i I /t / /
3C o !
I I;,'"
q{ 2oI
Jd///
P l e n u m chomber
i,////I 7"/I
I0I
--------}Peripheral ]st
I
20
I 4"o 30 Sin, cm 2
t
50
I
60
FIo. 4. Erosion in relation with ACV system. In other respects, the 0, angle is defined from the vortex theory. This determines primarily the total drag for a unit length of vehicle, while the other terms* are secondary. So: Fv = p. v. z t. (1 + s i n O . ) h
(1)
h = p, v. 2 t. (1 + sin o . ) B r
(2)
or:
Consequently, the erosion increases with the term (1 + sin 0,). W h e n all other vehicle parameters are constant, a balance has to be found with h. F o r a plenum chamber ACV, t = Kh, so h --
t K
where K is a contraction factor. This decreases when 0, increases and, therefore, here also h must increase in relation with 0, for a given value of t. INFLUENCE OF SOIL CONDITIONS ON ACV INDUCED EROSION S o i l m o i s t u r e content
A n A C V manoeuvring upon a light sandy clay induces an erosion in relation to the soil moisture content. *Force per unit vehicle length due to air cushion pressure upon platform base and drag per unit vehicle length caused by the pressure of the air in the inflation inlet pipe.
A I R CUSHION VEHICLES A N D SOIL EROSION
205
/
Erosion in relation with v! and 0..
FIG. 5.
150
%
100
50
-
]
6
I
8
I
I
I0
12
~p,
FIG. 6.
I
14
16
I
18
%
Erosion in relation with soil moisture.
The following values have been determined: Soil moisture, 70 8.54 17.04
Q,,, m3 min-1 13.1 x 10-3 2.4 x 10-3
Moisture content has two favorable effects for decreasing the sensitivity of a soil to aeolian erosion. First, the specific weight increases and restrains the lifting of soil particles. Secondly, cohesion is higher when water films stay between the particles with the formation of small aggregates.
206
P.F.J.
60¸
\
5C
4C
d
\ ×
2C
o
\\Peripheral jet N% \ X\ N \ \
h0mber
t
850 FIG. 7.
\
ABEELS
I
90o
I
I
950 t000 ys, kg/drn3
\
\
I
1050
\
\
I
qt00
Erosion in relation with soil specific gravity.
So, there are no serious erosion problems when agricultural soils show high moisture contents.
Soil compaction Compaction condition is indicated by the apparent specific weight of the soil. The correlation between erosion quantities and soil specific weight is highly significant (Fig. 7). Apparent specific weight kg dm -3
Erosion Q,,,, dm 3 rain -x
1.018
2.54 0.66
0.950 1.120
5.1 1.3
0.851
The eroded quantities are clearly lower when the apparent specific weight increases. The surface o f the particles accessible to wind effects decreases. The cohesion forces generated in the upper layers o f the soil decrease and extraction of particles as well of aggregates is more difficult. After compaction, weight of the aggregates is greater and necessitates higher drawing forces. ACV OPERATING CONDITIONS
Influence of the load The increase of the load on a A C V vehicle always introduces a higher erosion under the cushion whatever the type of vehicle is used (Figs. 8 and 9). While the load increases erosion seems to pass over a m a x i m u m for the plenum chamber and through a m i n i m u m for the peripheral jet. There are two dimension parameters of the vehicle that have real and high importance for soil erosion: ground clearance (h) and air speed at the outlet under the skirt (vy) (Fig. 10).
AIR CUSHION VEHICLES AND SOIL EROSION
=q~-x_~-. . . . ×-x?-. . . . . . . .
- t~
i ¢~! ~~ I
207
---. . . .
Before
..... Afi'er --o--c---
20 k(::] 4 0 kg
FIG. 8. Erosion in relation with load (peripheral jet). These parameters change in relation with the load on the ACV. The proximity of the inflation pipes to the soil reduces the internal whirlwind in the case of peripheral jet. The solution to the mathematic general formulation, dp = p. Ve=
dR
(3)
dRe
when R e varies from Ra to R= in the thickness of the jet while it stays constant along the flow line allows it to be specified that,
R. Ye =
(4)
l'n
Re
It describes a hyperbolic form for the whirlwind when compressibility and viscosity of air are not taken into account.
I
x--x~x ~ ~ - .
x.lx j
l / *
j
~
/ ~ / / ~ .~.~" ° "
I,
Bef°re'°k' ----". . . . Af,er -20 kg -->-~---,~ 40kg
FIG. 9. Erosion in relation with the load (plenum chamber). The expression for the distribution of the pressure refers to that of the speeds, and, Pe = P .
H-1/2p.v,, ~ [ 1 k
(R~y1
\Re/ J
(5)
Moreover, the total drag per unit length of ACV is defined by the expression: F r = p. v. z t. (1 q- sin 0.) B h
(6)
208
P.F.J. ABEELS
0 ~<. *.
4~\\3 ~\I.5="A",, I.~'~ iI
fn
Opfimum
/
1
#n Constont
,'"
,oP*;/" B
0o
0.'
/ ....;"°"°Y Vo/ J "
I ~ i / w/va ' l # / / \ 's I ! 1 t II ~I I) i I i \ t \ / ~/t t tRe ~ , !h,\ h2' \ . l i / " ~ \ \, >\" " l ~ . \, ~ x " I ~'~,------~ / \\ ~, \~ .
,
, ., ~ n : Nozzle a:Atrnosphere b:Cushion e:Element j :Curved jet
F1o. 10. Thick bi-dimensionaljet model and air viscosity. and the augmentation ratio by: A = Fr/2 p, v, 2 t,
(7)
Therefore, an optimal operating height exists for an ACV with given width B, 0, and A. That optimum height corresponds to a minimum whirlwind. The increase of the load influences h and this can explain why there is a minimum for the erosion. On the other hand, for a given h, vi decreases with the load as is evident from the expressions given before and from the measured values. Peaks in the pressure under the air cushion become sharper as the load increases. This explains the aggressiveness of the jets on the soil, moreover because the increase of the pressure seems to follow a quadratic rule when related to the load. Therefore vfi has been selected to explain the eroded profile evolution. For a plenum chamber the influence of vs is clear, while operating height is related to the coefficient of contraction K. So it is connected to the value of the augmentation ratio and to the dimensions of the AC vehicle. There is a highly significant correlation between optimum yield at the sustentation and minimum erosion. So the favourable load can be determined because static pressure is interesting for the augmentation ratio while the dynamic pressure arises from an air outflow that is too fast and too aggressive. Influence o f hovering The evolution of erosion profiles when hovering is such that a certain stabilization is noted over a period of time. Steadying appears later when soils are dry initially (Fig. 11).
A I R C U S H I O N V E H I C L E S A N D SOIL E R O S I O N
I
0 fo 90
FIG. 11.
209
sec
Erosion profiles and hovering period.
Such evolution is explained when the different and successive phenomena are considered for an ACV. The rather high erosion in the beginning corresponds to the removal of the loose particles. This erosion is greater because the particles are dissociated and without cohesion. As the profile is excavated, the impact angle of the airjet on the soil is modified and reduces the erosive effect. Moreover, when soil moisture content is high, the reducing of it when hovering is rather slow. The drying effect concerns only the upper layer and only a few of the particles. Therefore, only the smallest particles are transported and there occurs a sort of clogging for the upper layer of the soil; erosion is reduced (Fig. 12).
100 --
f
./
/-/" 7 / //
06/j
/
// //
~5o
//
--
X
///~/// j ,/ / /
~/"j
x~"
Soil moisture
--- 15.8 %
17.04%
I
50
I
60
I
FIG. 12.
I
90
Time,
120
I
150
sec
Erosion in relation with hovering period.
CONCLUSIONS
The use of ACV vehicles is possible on agricultural land when some principles are respected. The air cushion of the peripheral jet type generally induces a well located erosion. With such an ACV type, ground clearance is noticeable higher than for the plenum chamber type. All ACVs must be dimensioned from a vfvalue or outlet speed of air under the skirt, and from a 0, figure or inflation angle. The speed of the air at the outlet is the more
210
P . F . J . ABEELS
injurious for the soil because erosion rises with the square of the speed. It is to be noted also that the best support conditions correspond to low air-speed. The increase of the moisture and/or of the compaction of the soil noticeably reduce the induced erosion. The load is related to the optimum values for vfand 0~. Hovering is only dangerous when moisture contents and compaction are not sufficient. Consequently, every ACV has advantages and disadvantages. It is neither an universal means of transport nor suitable for overall use. Furthermore it is a particularly convenient means when manoeuvering and transporting in unfavorable meteorological conditions. This is not only to be able to realize the operation in itself but much more to avoid all locomotion on the ground, so to protect agricultural land against heavy damage in such bad circumstances.
[1] [2] [3] [4] [5] [6] [7]
REFERENCES F. CROIX-MARIE, Le Coussin d'Air et la Manutention. Het lngenieursblad, 1970, n' 18. A. HARTING, A literature survey on the aerodynamics of air cushion vehicles. Agard Report 565 (1968). G. ABELE and H. PARROTT, Some Effects o f Air Cushion Vehicle, Operations on Deep Snow. 4th Int. Conf. I.S.T.V.S.-S.F.M., Kiruna, Sweden (1972). E. JAtrMOTTE and A. KtEORYNSKI, Dimensionnement optimal des v6hicules h coussin d'air avec jet p6riphdrique. U.L.B., N.T.S. (1965). D . M . DESOUTER, Your Book o f Hovercraft. Faber and Faber (1965). Hovercraft World. Air Age. Publications. Air Cushion Vehicles. Iliffe Publications. NOTATIONS AND DIMENSIONS FOR THE PARAMETERS
A: B: Fr: h: K: /: p~:
augmentation ratio or ACV efficiency. A C V width, L. total drag per length unit, FL 1. ground clearance, L. contraction coefficient. air cushion width at the contact skirt-soil, L. static pressure on external flow line, FL 2. Pb: static pressure at internal flow line, FL 2 p,,: static pressure into the nozzle, FL 2. Pt: total pressure, FL -2. Q,,,: quantity of soil eroded per metre skirt length, L 3 T -1. R: curve radius, L. R,,: external flow line curve radius, L. R~: internal flow line curve radius, L. Re: flow line curve radius, L. S,,,: area of the eroded section, L 2. t,,: width of the jet at the outlet o f the nozzle, L. ti: outlet width under the skirt, L. v,: air speed on the external flow line, L T 1. v,,: air speed in the nozzle, L T x. vb: air speed on the internal flow line, L T 1. ~: air speed at the outlet under the skirt, L T 1. ~: the air density into the nozzle, M L -~. 0,,: inflation angle, °.
AIR
CUSHION
VEHICLES
AND
SOIL
EROSION
P. F. J. ABEELS* Summary--The use of aerostatically supported vehicles is particularly interesting in agriculture, principally because the tractive effort required to pull implements may be reduced. Then the tractor used for driving can be lighter or its outer dimensions may be more appropriate to European land ownership structures. This study verifies that the induced effects in the soil when repetitive passes of an ACV occur, do not include damage to the substrate by aeolian erosion. The main ACV characteristics influencing erosion are the speed of the air at the outlet under the skirt and the angle of incidence of the jet on the soil. The effect of the escaping air is in proportion to the square of its velocity. The angle of incidence must be chosen in relation to an equilibrium between induced erosion and operating height. The ACV with peripheral jet seems to be more appropriate when applied in agriculture. Soil resistance to erosion is a function of moisture content and of compaction. During the passage of an ACV, erosion happens in two phases. Initially, erosion is high, then the process reaches a certain level at which it remains stable. Having measured the extent of a major disadvantage of ACV's for work on agricukural land, i.e. the induced erosion, it appears that such equipment shows some promise for special conditions of use.
INTRODUCTION THE AIM o f the tests c o n d u c t e d with air cushion vehicles (ACV) is to specify the c o n d i t i o n s o f their use on agricultural soils t a k i n g into a c c o u n t the erosion process they m a y induce, p a r t i c u l a r l y in case o f repetitive passes following fixed routes. A e r o s t a t i c lift is o b t a i n e d b y b l o w i n g air into a c h a m b e r where pressure is generated because the outlets are controlled. Inflation techniques are b l o w i n g in a p l e n u m c h a m b e r o r the p e r i p h e r a l j e t (Fig. 1). O t h e r A C V systems, like diffuser, l a b y r i n t h e seal, solid wall, etc . . . . are n o t included in this study. T h e studied aspects a r e : (1) Influence o f air cushion system t y p e ; (2) Soil c o n d i t i o n s where the A C V operates, i.e. soil m o i s t u r e c o n t e n t soil c o m p a c t i o n ; (3) A C V o p e r a t i n g conditions, as: l o a d on the air cushion hovering. T h e observations c o n c e r n the characteristics a n d the quantities o f displaced particles d u r i n g o r after A C V passages. This requires erosion profile measurements. These profiles o r sections are then c o n v e r t e d into e r o d e d quanties for a unit length o f cushion a n d for a given time. *Drpartement de Grnie Rural, Facult6 des Sciences Agronomiques, Universit6 Catholique de Louvain, Belgium. 201
202
P.F.J. ABEELS
i
Peripheral ]ef
I
! i
Plenum chamber
FIG. I.
A i r cushion systems.
EXPERIMENTAL CONDITIONS The experimental prototype has dimensions and characteristics as close as possible to those of a future model available for agriculture, for example a maximum width of 2.5m. The model is in full scale so as not to be confronted immediately with scale problems. For experimental and overall dimensions reasons, the model is limited to a representative length unit of the ACV (Fig. 2), and is also convertible to different versions and adjustable in geometrical characteristics (Fig. 3). The measurements make it possible to specify all data about the air cushions obtained. INFLUENCE OF THE AEROSTATIC LIFT SYSTEM With a plenum chamber, construction of the ACV is rather simple. Air-tightness is obtained with the skirt around the vehicle. Air quantities required are high. Inflation pressure is low. The peripheral jet gets air-tightness through inflation pressure and direction (inflation angle) of the air inlet against the volume wrapped in the "cushion".
;7
FIG. 2. Experimental model.
AIR CUSHION VEHICLES AND SOIL EROSION
203
I I I
,3
I
I I
-i
FIG. 3. Differentadaptation possibilitiesfor the experimental model. Erosion is linked with ACV type and inflation angle 0, (Fig. 4). An estimation of the eroded quantities is compiled from vy and 0,.
0,, °
10
Plenum chamber 30
45
v.fl sin 0N Q'ndmaratn-t
130.78 1.02
420.50 2.30
657.51 2.73
10 152.60 0.6 to 1
Peripheral jet 30 496.12 3.4 to 3.9
45 746.76 4.2 to 5.1
Meanwhile, it should be observed that: (1) Ground clearance for a plenum chamber is only half of what is necessary for the peripheral jet; (2) A plenum chamber ACV is lighter; (3) Total pressure is slightly lower for the plenum chamber type; (4) Air flow is lower with a peripheral jet; (5) Pressure distribution for peripheral jets is such that probable erosion points can be predicted.
204
P.F.J. ABEELS
[ 45
/ /
/
/
40~
/
/
/
//
/ I / I/ / / I ,,"i I /t / /
3C o !
I I;,'"
q{ 2oI
Jd///
P l e n u m chomber
i,////I 7"/I
I0I
--------}Peripheral ]st
I
20
I 4"o 30 Sin, cm 2
t
50
I
60
FIo. 4. Erosion in relation with ACV system. In other respects, the 0, angle is defined from the vortex theory. This determines primarily the total drag for a unit length of vehicle, while the other terms* are secondary. So: Fv = p. v. z t. (1 + s i n O . ) h
(1)
h = p, v. 2 t. (1 + sin o . ) B r
(2)
or:
Consequently, the erosion increases with the term (1 + sin 0,). W h e n all other vehicle parameters are constant, a balance has to be found with h. F o r a plenum chamber ACV, t = Kh, so h --
t K
where K is a contraction factor. This decreases when 0, increases and, therefore, here also h must increase in relation with 0, for a given value of t. INFLUENCE OF SOIL CONDITIONS ON ACV INDUCED EROSION S o i l m o i s t u r e content
A n A C V manoeuvring upon a light sandy clay induces an erosion in relation to the soil moisture content. *Force per unit vehicle length due to air cushion pressure upon platform base and drag per unit vehicle length caused by the pressure of the air in the inflation inlet pipe.
A I R CUSHION VEHICLES A N D SOIL EROSION
205
/
Erosion in relation with v! and 0..
FIG. 5.
150
%
100
50
-
]
6
I
8
I
I
I0
12
~p,
FIG. 6.
I
14
16
I
18
%
Erosion in relation with soil moisture.
The following values have been determined: Soil moisture, 70 8.54 17.04
Q,,, m3 min-1 13.1 x 10-3 2.4 x 10-3
Moisture content has two favorable effects for decreasing the sensitivity of a soil to aeolian erosion. First, the specific weight increases and restrains the lifting of soil particles. Secondly, cohesion is higher when water films stay between the particles with the formation of small aggregates.
206
P.F.J.
60¸
\
5C
4C
d
\ ×
2C
o
\\Peripheral jet N% \ X\ N \ \
h0mber
t
850 FIG. 7.
\
ABEELS
I
90o
I
I
950 t000 ys, kg/drn3
\
\
I
1050
\
\
I
qt00
Erosion in relation with soil specific gravity.
So, there are no serious erosion problems when agricultural soils show high moisture contents.
Soil compaction Compaction condition is indicated by the apparent specific weight of the soil. The correlation between erosion quantities and soil specific weight is highly significant (Fig. 7). Apparent specific weight kg dm -3
Erosion Q,,,, dm 3 rain -x
1.018
2.54 0.66
0.950 1.120
5.1 1.3
0.851
The eroded quantities are clearly lower when the apparent specific weight increases. The surface o f the particles accessible to wind effects decreases. The cohesion forces generated in the upper layers o f the soil decrease and extraction of particles as well of aggregates is more difficult. After compaction, weight of the aggregates is greater and necessitates higher drawing forces. ACV OPERATING CONDITIONS
Influence of the load The increase of the load on a A C V vehicle always introduces a higher erosion under the cushion whatever the type of vehicle is used (Figs. 8 and 9). While the load increases erosion seems to pass over a m a x i m u m for the plenum chamber and through a m i n i m u m for the peripheral jet. There are two dimension parameters of the vehicle that have real and high importance for soil erosion: ground clearance (h) and air speed at the outlet under the skirt (vy) (Fig. 10).
AIR CUSHION VEHICLES AND SOIL EROSION
=q~-x_~-. . . . ×-x?-. . . . . . . .
- t~
i ¢~! ~~ I
207
---. . . .
Before
..... Afi'er --o--c---
20 k(::] 4 0 kg
FIG. 8. Erosion in relation with load (peripheral jet). These parameters change in relation with the load on the ACV. The proximity of the inflation pipes to the soil reduces the internal whirlwind in the case of peripheral jet. The solution to the mathematic general formulation, dp = p. Ve=
dR
(3)
dRe
when R e varies from Ra to R= in the thickness of the jet while it stays constant along the flow line allows it to be specified that,
R. Ye =
(4)
l'n
Re
It describes a hyperbolic form for the whirlwind when compressibility and viscosity of air are not taken into account.
I
x--x~x ~ ~ - .
x.lx j
l / *
j
~
/ ~ / / ~ .~.~" ° "
I,
Bef°re'°k' ----". . . . Af,er -20 kg -->-~---,~ 40kg
FIG. 9. Erosion in relation with the load (plenum chamber). The expression for the distribution of the pressure refers to that of the speeds, and, Pe = P .
H-1/2p.v,, ~ [ 1 k
(R~y1
\Re/ J
(5)
Moreover, the total drag per unit length of ACV is defined by the expression: F r = p. v. z t. (1 q- sin 0.) B h
(6)
208
P.F.J. ABEELS
0 ~<. *.
4~\\3 ~\I.5="A",, I.~'~ iI
fn
Opfimum
/
1
#n Constont
,'"
,oP*;/" B
0o
0.'
/ ....;"°"°Y Vo/ J "
I ~ i / w/va ' l # / / \ 's I ! 1 t II ~I I) i I i \ t \ / ~/t t tRe ~ , !h,\ h2' \ . l i / " ~ \ \, >\" " l ~ . \, ~ x " I ~'~,------~ / \\ ~, \~ .
,
, ., ~ n : Nozzle a:Atrnosphere b:Cushion e:Element j :Curved jet
F1o. 10. Thick bi-dimensionaljet model and air viscosity. and the augmentation ratio by: A = Fr/2 p, v, 2 t,
(7)
Therefore, an optimal operating height exists for an ACV with given width B, 0, and A. That optimum height corresponds to a minimum whirlwind. The increase of the load influences h and this can explain why there is a minimum for the erosion. On the other hand, for a given h, vi decreases with the load as is evident from the expressions given before and from the measured values. Peaks in the pressure under the air cushion become sharper as the load increases. This explains the aggressiveness of the jets on the soil, moreover because the increase of the pressure seems to follow a quadratic rule when related to the load. Therefore vfi has been selected to explain the eroded profile evolution. For a plenum chamber the influence of vs is clear, while operating height is related to the coefficient of contraction K. So it is connected to the value of the augmentation ratio and to the dimensions of the AC vehicle. There is a highly significant correlation between optimum yield at the sustentation and minimum erosion. So the favourable load can be determined because static pressure is interesting for the augmentation ratio while the dynamic pressure arises from an air outflow that is too fast and too aggressive. Influence o f hovering The evolution of erosion profiles when hovering is such that a certain stabilization is noted over a period of time. Steadying appears later when soils are dry initially (Fig. 11).
A I R C U S H I O N V E H I C L E S A N D SOIL E R O S I O N
I
0 fo 90
FIG. 11.
209
sec
Erosion profiles and hovering period.
Such evolution is explained when the different and successive phenomena are considered for an ACV. The rather high erosion in the beginning corresponds to the removal of the loose particles. This erosion is greater because the particles are dissociated and without cohesion. As the profile is excavated, the impact angle of the airjet on the soil is modified and reduces the erosive effect. Moreover, when soil moisture content is high, the reducing of it when hovering is rather slow. The drying effect concerns only the upper layer and only a few of the particles. Therefore, only the smallest particles are transported and there occurs a sort of clogging for the upper layer of the soil; erosion is reduced (Fig. 12).
100 --
f
./
/-/" 7 / //
06/j
/
// //
~5o
//
--
X
///~/// j ,/ / /
~/"j
x~"
Soil moisture
--- 15.8 %
17.04%
I
50
I
60
I
FIG. 12.
I
90
Time,
120
I
150
sec
Erosion in relation with hovering period.
CONCLUSIONS
The use of ACV vehicles is possible on agricultural land when some principles are respected. The air cushion of the peripheral jet type generally induces a well located erosion. With such an ACV type, ground clearance is noticeable higher than for the plenum chamber type. All ACVs must be dimensioned from a vfvalue or outlet speed of air under the skirt, and from a 0, figure or inflation angle. The speed of the air at the outlet is the more
210
P . F . J . ABEELS
injurious for the soil because erosion rises with the square of the speed. It is to be noted also that the best support conditions correspond to low air-speed. The increase of the moisture and/or of the compaction of the soil noticeably reduce the induced erosion. The load is related to the optimum values for vfand 0~. Hovering is only dangerous when moisture contents and compaction are not sufficient. Consequently, every ACV has advantages and disadvantages. It is neither an universal means of transport nor suitable for overall use. Furthermore it is a particularly convenient means when manoeuvering and transporting in unfavorable meteorological conditions. This is not only to be able to realize the operation in itself but much more to avoid all locomotion on the ground, so to protect agricultural land against heavy damage in such bad circumstances.
[1] [2] [3] [4] [5] [6] [7]
REFERENCES F. CROIX-MARIE, Le Coussin d'Air et la Manutention. Het lngenieursblad, 1970, n' 18. A. HARTING, A literature survey on the aerodynamics of air cushion vehicles. Agard Report 565 (1968). G. ABELE and H. PARROTT, Some Effects o f Air Cushion Vehicle, Operations on Deep Snow. 4th Int. Conf. I.S.T.V.S.-S.F.M., Kiruna, Sweden (1972). E. JAtrMOTTE and A. KtEORYNSKI, Dimensionnement optimal des v6hicules h coussin d'air avec jet p6riphdrique. U.L.B., N.T.S. (1965). D . M . DESOUTER, Your Book o f Hovercraft. Faber and Faber (1965). Hovercraft World. Air Age. Publications. Air Cushion Vehicles. Iliffe Publications. NOTATIONS AND DIMENSIONS FOR THE PARAMETERS
A: B: Fr: h: K: /: p~:
augmentation ratio or ACV efficiency. A C V width, L. total drag per length unit, FL 1. ground clearance, L. contraction coefficient. air cushion width at the contact skirt-soil, L. static pressure on external flow line, FL 2. Pb: static pressure at internal flow line, FL 2 p,,: static pressure into the nozzle, FL 2. Pt: total pressure, FL -2. Q,,,: quantity of soil eroded per metre skirt length, L 3 T -1. R: curve radius, L. R,,: external flow line curve radius, L. R~: internal flow line curve radius, L. Re: flow line curve radius, L. S,,,: area of the eroded section, L 2. t,,: width of the jet at the outlet o f the nozzle, L. ti: outlet width under the skirt, L. v,: air speed on the external flow line, L T 1. v,,: air speed in the nozzle, L T x. vb: air speed on the internal flow line, L T 1. ~: air speed at the outlet under the skirt, L T 1. ~: the air density into the nozzle, M L -~. 0,,: inflation angle, °.