"
.
'.Part I: 600 Inwardly Inclined Annular Jet Rovering at Zero Angle of Attack
MAY, 1962
by W. J. Scott
Enl USCH'=
HO:;~SCHOOLOELFT
VU-:GTüiG30UWKUNDE MichU cia R:.:~ :::mog 10 - DElFTASYMMETRY OF ANNULAR JET FLOW IN GROUND PROXIMITY Part I: 600 Inwardly Inclined Annular Jet Hovering at Zero Angle
of Attack
by W. J. Scott
-
.
The author wishes to th ank Dr. G. N. Patterson, Director of the Institute of Aerophysics, for the opportunity to pursue this investigation.
The suggestions and guidance of my supervisor, Dr. G. K. Korbacher, are gratefully acknowledged.
The assistance of K. Sridhar, S. Benner and P. R. Stephens with the experimental work is very much appreciated.
The work was made possible through the financial support of the Defence Research Board of Canada.
SUMMARY
I"
~. 'I r ..IIO'";:SCHOOL
Ot:
F U:'~~OUWKUNDE Mirhl ~c .~~ erweg 10 • DE1.FIFlow visualization techniques were used to investigate the flow asymmetry in a 600 inwardly inclined annular jet, hovering in close proximity to the ground at zero angle of attack. Flow patterns and pressure distributions on the ground board, nozzle base and in the cavity were obtained and are pre-sented. The observed asymmetric flow patterns are discussed. The focussed jet and a resonance phenomena were also investigated briefly.
The annular jet flow was found to be asymmetric at most heights within the ground effect at zero angle of attack. This asymmetry produces a loss in base pressure thrust as weU as an unstable pitching moment which is responsible for the inherent instability and wobbling motion of hovering ground effect machines.
1.
'
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Il. lIl. IV.V.
TABLE OF CONTENTS NOTATION INTRODUCTION. INVISCID, INCOMPRESSIBLE THEORY OF THE ANNULAR JET IN THE GROUND EFFECT
2. 1 Thin, High Aspect Ratio Annular Jet
2. 2 Thick, Low Aspect Ratio Annular Jet APPARATUS 3. 1 Air Supply 3.2 Annular Nozzle 3. 3 Ground Boards 3.4 Instrumentation 3.4. 1 Manometers
3. 4. 2 Lam pblack Plates
3.4.3 Traversing Mechanism 3.4.4 Photography
3. 4. 5 Schlieren System
3.4.6 Sound Measuring Equipment EXPERIMENT AL DEVELOPMENT
4. 1 Pressure Traverses 4.2 Lampblack Plates
4.3 Flow Patterns on Ground Board 4.4 Schlieren Pictures
4.5 Ground Board Static Pressure Distribution PRESENTATION OF RESULTS
5. 1 Pressure Traverses 5. 2 Lam pblack Flow Patterns
5.2. 1 Flow Patterns on Diametrical Plates 5.2.2 Flow Patterns on Ground Board
A. Effect of Pressure Ratio B. Effect of Aspect Ratio
5.2.3 Flow Patterns on the Nozzle Base 5. 3 Schlieren Pictures
5.4 Ground Board Statie Pressure Distributions 5.5 lnvestigation of Possible Causes of Asymmetry
Page iii 1 1 1 2 2 2 2 3 3 3 4 4 4 4 4 5 5 5 6 6 6 7 7 8 8 9 10 10 11 11 12 12
VI.
(ii)
5. 5. 1 Effect of a Very Small Angle of Attack 5. 5. 2 Effect of the Nozzle
5. 6 Additional Observations 5. 6. 1 Focussed Jet 5.6.2 Resonance
DISCUSSION OF ASYMMETRIe FLOW IN AN ANNULAR JET
6.1 Flow Model of Asymmetrie Annular Jet Flow 6.2 The General Effect of the Main Variables on the
Flow Model
6.2. 1 Aspect Ratio 6. 2.2 Pressure Ratio
6. 2. 3 Height to Diameter Ratio 6.3 Nozzle Induced Asymmetry
6. 4 Unstable Pitching Mom ent
6.5 Axially Symmetrie Regions of the Annular Jet 6.6 Effect of Compartmenting the Annular Jet Cavity 6.7 Resonance in the Annular Jet Cavity
VII. CONCL USIONS REFERENCES FIGURES Page 12 13 13 13 14 14 15 15 16 16 16 17 18 19 20 20
\. D h hl h/D J p Pa Pc t
v·
Jf
NOTATION theoretical thrust augmentation ratio nozzle aspect ratio (..)I; D ft)nozzle mean diameter
calculated. resonance frequency = sound velocity / 2h I experimental resonance frequency
height of nozzle lip above ground height of nozzle base above ground non-dimensionalheight parameter
total jet momentum with ambient exit pressure statie pressure (absolute)
atmosphere pressure base statie pressure cavity statie pressure ground statie pressure total pressure (absolute) nozzle pressure ratio
radius of thin annular jet in cross -section
exterior radius of thick annular jet in cross-section
interior radius of thick annular jet in cross-section (RI
=
Ro+
t) jet thickness at exitideal jet velocity at exit with ambient back pressure
angle of jet sheet at nozzle exit, measured from nozzle axis, (negative for inwardly inc1ined jets)
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(1) I. INTRODUCTION
This report studies the asymmetric flow set up within an inward-ly inclined annular jet curtain in close proximity to the ground. Primariinward-ly, flow visualization techniques were used in this investigation of a six inch dia-meter 600 inwardly inclined annular jet nozzle. The apparatus was first used by Garland (Ref. 3) in a study of total thrust augmentation and then by Smith (Ref. 8) in a deter.mination of base pressure augmentation. Smith noted the presence of asym~~etric flow within the annular jet curtain at h/D values smaller than 0.5. Flow pictures and pressure distributions given in some of the Princeton Symposium papers (e. g. Refs. 6 and 7) also indicate the presence of asymmetric flow.
The R,urpose of th is work is to investigate this asymmetrie flow phenomena. The shape of the asymmetrie flow pattern is sought, as well as the effect which parameters such as aspect ratio, pressure ratio and height to diameter ratio have on such asymmetric flow. The focussed annular jet and a resonance phenomena are also briefly examined and discussed.
The lampblack and kerosene technique and to alesser extent pressure probing are the main methods of investig~tion. Some use is also made of wool tufts and of a schlieren system.
Il. INVISCID, INCOMPRESSIBLE THEORY OF THE ANNULAR JET IN THE GROUND EFFECT
Some simple theory of the annular jet is presented below only in as far as it facilitates understanding of the jet curtain flow. The theory is given in-more detail by Garhtnd (Ref. 3).
2. 1 Thin, High Aspect Ratio Armular Jet
The pressure ·rise across the jet curtain (see Fig. 1) is given by Garland (Ref. 3) as.
2
Pc - Pa =
f
Vj tiR (1)and from Bernoulli
(2)
Therefore
(3 )
or the radius of the jet curtain is; - '
(
PT - Pa) R = 2t "
Pc·'- Pa
2. 2 Thick, Low Aspect Ratio Annular Jet
The pressure rise across a thick jet (see Fig. 1) is given by Garland (Ref. 3)
as:-(5)
After substituting Ro = Rl - tinto (5) and revising we have
Pc - Pa'=
~
(P T - P ) (1 -~
)1\
,a 2R1 (6)This can be solved for Rl to give the jet radius
as;-(7)
UI APPARATUS
A detailed desc'ription of the design of the annular jet test rig
used for this investigation is given in Ref. 3. ,Ho wever a brief outline
consid-ered essential for the understanding of this report will be presented below. 3. 1 Air Supply
A small gas turbine engine (Blackburn ~nd General Aircraft
Thrbomeca Palouste 500), housed in a sound proof room and remotely controlled, was used as a compressed air source for these tests. The compressed air bIed off the engine compressor could be delivered at temperatures up to 230 degrees centigrade, pressure ratios up to 3.7 and a mass flow of up to 2. 7 IbsJ sec. A large water cooler reduced the temperature of the compressed
air to below 10 degrees centigrade" depending on the test conditions. The
cooled air was delivered to theannular nozzle by means of 8 inch diameter piping as shown in Fig. 2.
3.2 Annular Nozzle
The 600 inwardly inclined annular nozzle (see Fig. 6) was
mounted horizontally on the piping as shown in Figs. 3· and 5. The diameter
of .the oi.ltet sleeve 'Was . .10 in .. and. that" of the base platewas 6 ins.
The nozzle base plate contained 28 flush static pressure taps. A centre hole, 0.8 in. in diameter was incorporated to study the effects of feeding secondary air into the flow but was sealed off for these tests and used
, < (3)
• . \0 .:I
The jet thickness could be varied continuously from 0 to 0.4 in. by rotating the threaded outer sleeve of the nozzle which could be locked in any position (Figs. 5 and 6). The useful range of aspect ratio was therefore 45 to 1000. For these tests the following aspect ratios were selected.
Aspect Ratio 67 106 151 176 263 524 3. 3 Ground Boards J et Exit Area Sj
(i~)
5.57 3.45 2.40 2.06 1. 37 0.68 Jet Thickness t(in..) .289 .180 .126 .108 .072 .036The original ground board was a 36" square metal sheet attached to a plywood backing. It was mounted vertically on a movable plat-form facing the nozzle. By means of a screw me.chanism on the platform, the ground height could be varied manually from 0 to 14.5 in. The inclination of the ground board relative to the nozzle base plate could be varied from 0 to
25 degrees.
This steel ground board was replaced by a glass plate (Fig. 3 and 4) of~" x 37~" x 37~", mounted in a woodenframework. The movable platform was modified so that there was no obstructing framework behind the centre of the ground board. Thus, lampblack and kerosene patterns produced on the ground board by the air flow could be photographed by a camera placed behind the ground board.
Later the original steel ground board was changed to fit the
modified platform. On this ground board (Fig. 5) centric to the annular nozzle, 81 static pressure taps were drilled. Thirty-three of these taps, arranged in two rows at 45 degrees to the horizontal were used in this experiment.
3.4 Instrumentation 3.4.1 Manometers
A 40 tube, 60 inch mercury manometer was used to measure the static pressure distribution on the ground board. At each test point the manometer was photographed and the pressures read from the negatives.
Four pitot probes, mounted symmetrically around the annulus upstream of the nozzle exit, were connected to a 5 tube, 90 inch mercury manometer to read the jet total pressure. This manometer was also used to
indicate pressures from a yaw probe traversing the jet sheet and air cushion. 3.4.2 Lampblack Plates
A number of th,in metal plates (see Fig. 8) were made to fit parallel to the flow in a diametric plane of symmetry between the nozzle
base and the ground board. They were used to get lampblack patterns of the
flow in the jet curtain enclosed cavity.
Three pairs of plates were made for heights (lip of nozzle to ground board) of 2", 1~", and 1". The top edge of the plate was made flush with the lip of the outer nozzle sleeve, so that there was a gap between the plate and the base varying from about O. 1" to 0.4" depending on the aspect ratio of the nozzle. A 1/8" gap was also provided at the ground board edge of the plate. These gaps were installed in an attempt to reduce interference effects of the plate on the flow. N ote that one of the plates (for h ::: 1". see Fig. 9) has no gap at the ground board edge.
Later, another pair of plates was made for h = 0.6". One of these plates had the regular gap at the base and 1/16" gap at the ground board. The other had no gaps at either edge.
3.4.3 Traversing,Mechanism
. The traversing mechanism shown in Fig. 3 and 4 was designed for making pressure traverses through the jet curtain and enclosed cavity. The mechanism has three translational movements and three angular move-ments. The traverses were done at a fixed height f~om the ground board, using one angular and one translational movement. A yaw probe of the cobra head type and astatic pressure probe were used.
3. 4. 4 Photography
A 4" x 5" camera with a wide angle lens was used to photograph the lampblack patterns on the ground board. Lighting was provided by two photoflood lights and a white cardboard reflector. The camera and photofloods we re also used to photograph the lampblack plates and the manometer board.
3.4. 5 Schlieren System
. A single mirror, reflected beam schlieren system was used to photograph the jet flow patterns. This system was more sensitive than the double mirror system because the parallel light beam passed through the jet flow twice. A BH6 mercury vapour lamp was employed as the light source and the pictures were photographed at an exposure of 1/200 seconds.
3.4.6 Sound Measuring Equipment
A wave analyser and a sound level meter were used to make frequency measurements of a resonance condition observed in the annular jet cavity.
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IV. EXPERIMENT AL DEVELOPMENT
Smith (Ref. 8) found that the base pressure di~tributions for the annular jet in ground proximity were not symmetrical. A honeycomb was placed in the 8" diameter pipe upstream of the nozzle to straighten the flow and also the connection between the nozzle and the pipe was carefully aligned . These modifications and others failed to remove the asymmetry. Smith con-cluded therefore that the asymmetry was a characteristic of the annular jet flow under certain conditions.
4. 1 Pressure Traverses
The traversing mechanism was used to do total and static pressure travers es through the annular jet curtain and its cavity within the ground effect. The traverses were done along a diametric line through the nozzle axis, parallel to the ground board and about halfway between the ground board and base. Due to the size of the traversing mechanism, the ground height had to be at least 0.33 D.
These traverses showed the presence of asymmetry in the flow and the observed pattern of flow directions indicated that the axis of the ring vortex is tilted with respect to the ground board.
A wool tuft held near the ground board on the nozzle axis demon-strated that the "direction of asymmetry" changed as a pressure probe was moved through the cavity. Traverses were therefore conducted with the di-rection of asymmetry being artificially fixed. This was accomplished by, re-stricting entrainment of atmospheric air on one side of the jet with onels hand. In doing SOl no contact was made with the jet curtain itself. The direction of
asymmetry was thus forced to point towards the hand. This did not seem to make the flow more or less asymmetrie.
4.2 Lampblack Plates
In order to find out more about the asymmetry, lampblack plates were used. Such a plate was fixed diametrically between the nozzle and the ground board, and coated with a mixture of lampblack and kerosene. Air was blown through the nozzle for a period of from 30 seconds to 5 min-utes depending on the pressure ratio. The air bleed valve was opened and closed quickly in order to minimize transient effects. The plate was then removed and photographed.
These plates verified that the ring vortex was tilted as was indicated by the pressure traverses. They also showed that there was a cross flow which _passed under the ring vortex at one side, over it at the opposite side and then was entrained into the jet curtain. This gives more meaning to the term "direction of asymmetry" which has already been used.
Since the flow is asymmetrie, a plate obviously must have an effect on the flow exeept if the direetion af asymmetry lines up with the plate. Thus the flow patterns on·the plate ean only be interpreted qualitatively. The pictures of the lampblaek plate, base and ground.board, for h/D = 3/30 (see Fig. 8) especially show that the eavity flow has been changed significantly at that low height. Thus a technique of flow visualization with less flow inter-ference was required.
4. 3 Flow Patterns on Ground Board
The steel ground board was replaeed bya glass ground board. The primary purpose of this was to observe and photograph the flow patterns on the ground board through the glass plate.
In praetiee the glass ground board was coated with a mixture of lampblaek and kerosene. Air was then blown at constant pressure ratio through the nozzle long enough to leave a good imprint on the lampblaek. The length of the run depended on the pressure ratio of the flow, but was always longer than 30 seconds. Thus the patterIf.was not a transient one.
These pietures showed th at the asymmetry persisted at all heights in varying degree except maybe at very low heights. In addition a pair of vortex ends was found on the ground board at all intermediate heights.
A few pietures were also taken of lampblaek patterns on the nozzle base. Lampblaek patterns on the base eould not be illuminated with enough contrast, so only a few were photographed. They show, however, that the pair of vortices extends from the ground to the base, at least at small ground heights. 'This indicates that the two vortiees are a feature of the cross flow.
4.4 . Schlieren Pictures
The sehlieren system showed the annular jet flow, but it was not sensitive enough to show details of the secondary (cavity) flow.
4. 5 Ground Board Statie Pressure Distributions
Statie pressure taps were installed in the original steel ground board near the end of th is investigation. Two mutually perpendicular rows of holes at 450 to the horizontal were used in these tests. The manometer board was photographed rather than read direetly because in a few cases the pressure distribution was unsteady.
The distributions obtained verified again the presenee of asymmetrie flow and provided additional details.
(7) V. PRESENTATION OF RESULTS
Practically all of the test results presented were obtained dur-ing the summer months. Unfortunately the air humidity prevented tests at pressure ratios higher than 1.2 because of condensed water in the jet. th at obscured all flow visualization attempts with lampblack.
5. 1 Pressure Traverses
Four pressure traverses were done for an annular jet of aspect ratio 151 and h/D = 10/30 (Fig. 7). These traverses were done parallel and perpendicular to the direction of flow asymmetry (cross flow) at pressure ratios of 1. 2 and 1. 8. The distance from the nozzle base was O. 90 inches.
It was necessary to artificially force the flow asymmetry to stay in the same direction throughout a traverse. Due to the humidity, water vapour condensed in the jet at PT/Pa
=
1. 8, indicating the direction of the flow asymmetry. At PT/Pa = 1. 2, the direction was found by injecting small amounts of water into the cavity through a hypodermic tube. In most cases the direction of flow asymmetry was kept within 10 degrees of the desired direction.In practice the annular jet and cavity were traversed first with a cobra head yaw probe which gave readings of total pressure and direction. This traverse was immediately followed by another one with a statie pressure probe, set at the same angle in corresponding positions as the yaw probe. The estimated maximum error in P T is 0.4" Hg and in Pc is 0.2" Hg, resulting in an error of from 10 to 20% in the jet velocity. This high error IS due to
fluctuations in jet total pres~ure. The large velocity gradient in the jet cur-tain produces an error in the measured angle of the flow, due to the finite width of the cobra .head as compared to the jet curtain thickness. The velocity vectors shown in Fig. 7 are not corrected for this error which
over-emphasizes the divergence in the annular jet sheet.
Consider the traverses for the pressure ratio of 1. 8 (Fig. 7c and d) which are more accurate due to the smaller percentage error than those at 1. 2.
The traverse perpendicular to the cross flow (=- direction of flow asymmetry) is practically symmetrical as one would expect, except for the pressure distribution at the vortex centres. From the figure it would appear that the right vortex is slightly higher than the left one. The parallel traverse (Fig. 7c) shows an asymmetrie pressure distribution with the whole annular jet curtain being shifted to the right. The centre velocity vector is inclined at 20 degrees to the nozzle axis in a direction indicating a cross flow from left to right. Examination of the traverses for both pressure ratios shows that the ri~g vortex has become elliptical (see Fig. 7c and d) with the major axis parallel to the direction of the cross flow. Figure 7c shows that the statie pressure is higher on the right side than on the left side, both at the base and along the traverse line. ThuB. .. tbe.ilow asymmetry produces an unstable pitching moment.
The annular jet curtain is still practically circular at the tra-verse height but is shifted very slightly to the right, in the direction of the cross flow .
• Ilt.~
5. 2 Lampblack Flow Patterns
5.2. 1 Flow Patterns on Diametrical Plates
The lampblack plate pictures are shown in Figs. 8, 9, and 10. For each height to diameter ratio tested, two runs were done with the lamp-black plate in the second run placed perpendicular, to its ·orientation in the first run. In most cases the plates were placed horizontally and vertically. In a few cases the natural direction of cross flow was checked beforehand, with no plate in place and then the two plates were placed parallel and perpen-dicular to this direction.
Lampblack plate patterns (Figs. 8 and 9) were obtained for as -pect ratios of 67, 106, 176 and 263 at a pressure ratio of 1. 2 and at height to diameter ratios of 10/30,.7.5/30, 5/30 and 3/30. Another series of tests (see Fig. 10) was done to illustrate the effect of pressure ratio. The height to diameter ratio in these tests was 10/30, the aspect ratios were 88.5, 151,
263, and 524 and the pressure ratios 1. 05. 1. 2 and 1. 6.
Examination of the pictures for h/D
=
3/30 (Fig. 8) shows that a plate placed in the direction of the cross flow hardly affects the flow, but a plate placed perpendicular to the cross flow changes the flow significantly. It can also be seen that a plate with no gaps between it and the base or ground board produces a p~ttern which is symmetrical about two mutually perpendicular axes. Thus the flow and therefore the pattern on the plate is likely to be more symmetrical than the flow without the inserted plate. It should be noted that this type of plate pattern is characterized by a black area in the centre from which the flow seems to be radiating. Note also from Fig. 9 that this effect decreases as the height is increased. It follows that the plate patterns become more realistic as the height increases.Examination of Fig. 9 shows that for h/D = 10/30 and 7.5/30 the pattern gets more symmetrical as the aspect ratio increases, until at AR
=
263 the pattern is quite symmetrical. It should be mentioned here that at h/D = 10/30 and 7.5/30 for AR = 263, the annular jet operates in the hysteresis region (i. e. the jet can either be focussed or unfocussed). It was found that all .plate pictures taken in this region were symmetrical. Note also th at ,in most cases the patterns at h/D = 7. 5/30 are more symmetrical than either those at higher or lower heights. The patterns at h/D = 5/30 have the same characteristics as those at 3/30.Figure 10 shows that the pressure ratio has very little effect on the flow pattern. There appears to be only a slight increase in symmetry as the pressure ratio is increased. Note that at an aspect ratio of 88.5, the cross
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flow direction reverses twice as the pressure ratio increases, thus indicating some instability at low aspect ratios. Figure lOb showing primarily pictures of focussed jets at high aspect ratios will be discussed more in section 5. 3.
5.2. 2 Flow Patterns on Ground Board
Since this flow visualization technique is the one which disturbs the flow the least, it is assumed that it gives the most reliable results. Sub-sequently, the other techniques used should be considered with caution unless their results agree with those of the flow visuaHzation technique. A few extran-eous things should be noted about the pictures (Fig. 11). At low ground heights the brush marks in the lampblack can be seen in the centre porti on of the
plctures, and should not be confused with flow lines. In some of the pictures a dark vertical Hne appears down the middle which is caused by a fold in the cardboard photographic light reflector behind the ground board. Lastly, the white rim just inside _ .. . : ... the stagnation line. circle in Fig. 11f, (i), and (1) is caused by impinging condensed water in the annular jet air.
Ground board pictures were obtained at various ground heights for pressure ratios of 1. 02, 1. 1 and 1. 2 and aspect ratios of 67 ;lQQ, 176, and
263 as shown in Fig. 11. As an example consider Fig. 11 h. At h/D
=
11/30, the cross flow appears as a "tongue" with a black tip where the flow is separat-ing from the ground board. The cross flow is directionally unsteady as can be seen by the image lines left by the tongue. Encircling the tongue is the stagna-tion line of the jet curtain.At h/D
=
10/30 (Fig. 11 h) the cross flow is steady and the stagnation line circle is largely covered over by lampblack Which theo cross flow picked up on the ground board and carried over the ring vortex back to,t,l~e ground board. The extent of the resulting black ring depends on the ,q.haracteristics of the cavity flow. At lower heights the extent of the black
region decreases.
As h/D decreases from 11/30 to 7/30 the tpngue (Fig. 11 h) decreases in length until the flow is almost symmetrical at h/D
=
7/30. At this height a pair of vortices appears at the tip of the tongue. As the height is reduced further, this pair of vortices moves· away from the centre andfarther apart. They are joined by a black line along which the flow from either side separates from the ground board. At h/D
=
1/30 the pair of vortices has disappeared or at least the velocities involved have become so small that no impression is made in the Iampblack.The black regions in the centre of the pictures are regions where the flow has either separated from the ground board or where the velocity is very low. Just inside the stagnation line the flow is radially in-ward in all pictures indicating that a ring vortex is present in all of them, although this vortex is quite narrow in some places.
Something whichthe pictures do not show directly is the unsteadi-ness of the flow at a height to diameter ratio of 4/30. lnjection of water into the ca,vity indicated th is unsteadiness for most configurations in varying degrees. If one examines the photographs for h/D = 4/30 in a number of figures. one
finds that they are not nearly as similar as picture series at other heights. The above examples illustrate the basic variation in flow pattern with changing ground height. A number of variations will also be found in some of the other figures. Following are a few of them.
In some cases (e. g. Fig. 11 c) more than one pair of vortices are attached to the ground board. When there are two pairs of vortices on the ground board it is found that the cross flow flows into the cavity from opposite quadrants (see Fig. 11c) and flows out through the other two quadrants
..
.Observation of water on the ground board at higher pressure ratios sometimes indicated as many as three pairs of vortices attached to the ground board.
Some of the figures (e. g. Fig. 11b) show patterns which are not symmetrical about a line through the centre and in the direction of the cross
flow. Others. especially Figure (11a) show patterns which are quite symmetrical about the nozzle aris. In this case there still may be some asymmetric flow, but the velocities are so small that they are not detectable.
Figures 11k and 1 show pictures for an annular jet in which the height is equal or less than the thickness of the jet. For this case. it is seen that the velocities in the cavity are significant.
Examination of the figures for aspect ratios of 67 and 106 (Figs. 11. a to f) show in each picture four pairs of disturbances through the stagnation line in the form of a cross. These are caused by the four total pressure probes in the annulus of the nozzle upstream of the exit. Some of these pictures also show disturbances at 22.5 degrees counterclockwise from those just mentioned. They are caused by the four struts still further up-stream. which support the centre body of the nozzle.
A. Effect of Pressure Ratio
From the ground board flow patterns in Fig. 11 it appears that
'pressure ratio has very little effect on the flow. In some cases the patterns at a pressure ratio of 1. 02 dtifer from those at higher pressure ratios. but this is likely due to the lower velocities present in the cavity. Gravity may affect the wet lampblack in the cavity as much as the air flow at this low pressure ratio.
B. Effect of Aspect Ratio
Aspect ratio has. a more pronounced effect on the flow than pressure ratio. For one thing the transition height increases as the aspect ratio increases. Transition height is the height where the annular jet changes to its characteristic shape when out of ground effect. An unsteadiness of the
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flow indicating the beginning of transition is shown in some of the pictures (e. g. Fig. l1c and d) at h/D = 10/30 or 11/30.
Further, the pictures indicate that the cross flow becOInes more regular or in other words the flow patterns become more symmetrical about a line through the nozzle axis. parallel to the cross flow direction, as the aspect ratio is increased. Protuberances in the nozzle interior disturb the flow more at low aspect ratios than at high ones. This may be due to the lower contraction ratios in this nozzle at low aspect ratios. This effect may have something to do also with the degree of regularity of the cross flow. 5.2.3 Flow Patterns on the NozZIe Base
Only a few pictures of lampblack patterns on the nozzle base were taken primarily because of the poor lighting.
The same procedure was followed as for the ground board pictures. Since some of the larnpblack from the base was blown onto the ground board, the ground board was photographed. cleaned off and then the base was photographed through the ground board.
Figure 12 shows the base pictures and the corresponding ground board pictures for an aspect ratio of 176 and a pressure ratio of 1. 2, at four ground heights. At higher ground heights. the pictures show that there is radial flow outwards from the centre of the base to aseparation line circling the base, a short distance from the nozzle exit annulus. This ring line appears to be the inner boundary of a kind of separationbubble, in which the flow is generally not radial.
As the height is decreased the flow no longer remains radial in the centre. The pair of vortices ending on the ground board leave a noticeable imprint on the base at h/D = 4/30. At very low heights the vortex traces on the base are about the same size as those on the ground board. Note that for h/D
=
1/30. both the base and ground board were coated with lampblack for the run.5.3 Schlieren Pictures
Some schlieren pictures of the annular jet flow are presented in Fig. 13. The knife edge of the schlieren system was perpendicular to the ground board for these pictures, such that dark areas mean density is increas-ing from left to right. Assumincreas-ing that the air temperature was constant, this means that the pressure increases rapidly across the jet curtain and then more gradually up to the nozzle axis. This is in agreement with the pressure tra-verses.
The pictures show the jet curtain to stagnate on the ground and
it appears that the intermediate aspect ratio jet strikes the ground at a steeper angle than the low aspect ratio jet. Also th~ intermediate aspect ratio jet seems to have a smaller curvature than the other.
5. 4 Ground Board Statie Pre ss ure Distributions
The statie pressure distribution on the ground board is shown in Fig. 14 for aspect ratios of 67, 106, 176 and 263, and ground heights of
1/30, 2/30, 4/30, 8/30 and 10/30 at a pressure ratio of 1. 2. In each figure the direction of the eross flow and tne pressure tap positions are shown as seen from behind the ground board.
At large heights there is considerable pressure variation (see Fig. 14a) across the ground board, with the pressure at B being mueh higher
than that at A, indicating th at at A the local jet eurtain strikes on the ground at a very shallow angle. Figures 14a and b show two distributions each for h/D = 10/30. This is beeause the flow was unsteady and the manometer board was photographed twiee. The two distributions do not necessarily represent the limits of the pressure fluctuations.
As height decreases, the pressure distributions become more
uniform. At very small heights the ring vortex hardly affects the pressure distribution on the ground board.
Note th at for AR = 67 (Fig. 14a) at h/D = 10/30, both the stagnation line circle and the ring vortex are slightly elongated and shifted in the direction of the eross flow (towards point A). At h/D :; 8/30 the stagnation line is cireular but the ring vortex is still elongated and both are shifted slightly in the direction of the cross flow. At lower heights they are both reasonably axially symmetrie. As the aspeet ratio increases there is a gradual change until at AR
=
263 both the stagnation line and the ring vortex are axially symmetrie for all heights at or below h/D = 8/30. Since thetransition height deereases with deereasing aspect ratio, this elongation and shifting appear to be eaused by the beginning of transition.
The graphs show further that at large heights, (Pg-Pa}mean is approximately proportional to the jet thickness (or I/AR), but tfiat at yery
low heights it is limited by pressure ratio.
5.5 Investigation of Possible Causes of Asymmetry
A number of things which might either cause the asymmetry or determine the direetion of the eross flow were investigated.
5. 5. 1 Effect of a Very Small Angle of Attack
Lampblaek pictures were taken of the ground board flow
pattern for an aspect ratio of 176, a pressure ratio of 1. 1 and ground heights of 2/30, 4/30 and 8/30. For each configuration the ground board was set at
a number of angles of attack, all less than one degree. The results are shown in Figs. 15a to c. The arrow under each picture points in the direction of
,(13)
These figures show that a small angle of aUack can cause the magnitude and the direction of the cross flow to change. Nevertheless the direction of the cross flow does not change so far as to line up with the angle of aUack. Note also that the effect of a small angle of attack is increased as the height is decreased.
An angle of attack of 6 minutes is equivalent to an error of 0.01
inches in height in setting the ground board parallel to the nozzle base. It is
believed that the ground board was set parallel to the nozzle base within this limit for the results presented in this paper.
5.5.2 Effect of the Nozzle
The effect of small irregularities in the nozzle was checked by turning both the outer nozzle sleeve and the complete nozzle.
The nozzle aspect ratio is changed by screwing the outer nozzle sleeve in or out. The four most frequently used aspect ratios 67, 106, 176
and. 263 correspond to turns. of the sleeve of 4, 2t 1 tand 1. Examination of
the ground board patterns for these aspect ratioa shows that the cross flow runs to the left for aspect ratios of 67 and 263 and down for aspect ratios of
106 and 176. Figure 16 shows a series of photographs for ,a pressure ratio of
1. 1 and a height of 6/30 in which the nozzle sleeve is turned by 90 degree
increments. These pictures indicate that the outer nozzle sleeve has some influence on the direction of the cross flow.
The complete nozzle was rotated 20 degrees about the axis. The two pictures in Fig. 17 indicate that the cross flow direction has also changed by 20 degrees.
No doubt, these pictures show that the nozzle itself has an effect on the direction of the cross flow, but they do not prove that the asymmetry is caused by the nozzle.
5. 6 Additional Observations
Two other phenomena were observed and partially investigated.
One concerns the focussed jet, which. is already discussed in Refs. 3 and 8;
the second one is a kind of resonance condition in the annular jet cavity. 5.6. 1 Focussed Jet
The focussed jet was investigated briefly by means of pressure
traverses, lampblack plates and schlieren .. A focussed jet of aspect ratio
524. pressure ratio 1. 8 and h/D
=
.375was
traversed at distances of .25,.5 and . 9" from the base. The result of the pressure traverses are shown in
Examination of the schlieren pictures show that there is a difference be"tween the flow with and without a lampblack plate in place.
especially at the junction of the plate and the base.
If the results of the pres.sure traverses are compared with the
lampblack pictures. a significant difference is noted in the flow pattern. Since the traverses of the annular jet cavity compare favourably with the corresponding lampblack plate pictures. it may be concluded that this effect of the plate arises only at high Reynolds numbers. This would mean that in
the annular jet lampblack plate picturesa the pattern of the jet curtain would
be suspect. but the cavity flow pattern would not. 5.6.2 Resonance
.~ For some annular jet configurations and ranges of height" a
resonance condition was found to exist in the cavity between the base and
the ground board. The range of heights expanded as the pressure ratio was
increased. At pressure ratios of 1. 1 and less there was either a narrow
height range of resonance for low aspect ratio jets or none at all.
The frequency was measured and was found to be inversely pro-portional to the distance between the base and the ground board. Calculations
showed that the wavelength corresponding to the resonance frequency was
approximately equivalent to twice the distance between the base and the ground
board. Figure 20 shows the results of frequency measurements and
calcula-tions for aspect ratios of 67 and 176 at a pressure ratio of 1. 2. The range of heights over which the measurements were taken was the range over which
the resonance existed. The ca1cul.ated frequency was based on the sound
velocity corresponding to 10oC. which was the approximate temperature of air exhausting from the nozzle. Since the experimental frequency is about 6% greater than the calculated frequency, the effective resonance tube length must have been slightly less than the base to ground board height.
The cause of the resonance appeared to be the jet curtain. Radial osciUation of the jet curtain could be seen visually for some
configura-tions by means of the schlieren system. Both the nozzle lip and the ground board were found to be not vibrating.
Resonance of thin jets could usually be stopped simply by
touch-ing the outer surface of the main jet sheet with one finger. Resonance with thick jets could be stopped only by penetrating the jet sheet with a finger. VI. DISCUSSION OF ASYMMETRIe FLOW IN AN ANNULAR JET
The preceeding experiments have pointed out many features of
annular jet flow within the ground effect. A few observations are
contradic-tory but if the criterion is used that the method which disturbs the flow the
least gives the most realistic results, then a flow model which satisfies the
(15.) 6. 1 . Flow Model of Asymmetric Annular Jet Flow
I
Consider a circular annular jet hovering within the ground effect (Fig. 21). The flow in the region between the base and the ground in general, is made up of three main parts; th~ jet curtain. the ring vortex and the cross flow.
The jet curtain is made up of the annular jet of air from the vehicle plus entrained air from the atmosphere and has. the highest velocities.
It stagnates on the ground. producing an annular region of high pressure. At low heights the jet curtain is reasonably axially symmetric. At greater heights near the transition the jet curtain starts to shift slightly and becomes elongated in the direction of the cross flow. Thus on the verge of transition. the stagna-tion ring on the ground is somewhat elliptical with its major axis parallel to and shifted with the cross flow. The jet curtain at A (see Fig. 21) now takes up a greater curvature and may not even strike the ground. while the opposite side is less curved and will " s tagnate" at a steeper angle to the ground.
The ring vortex occupies the cavity and is in contact wUh and driven by the jet curtain. At very low heights the ring vortex takes up such a narrow annular space just inside the jet curtain. that its effect is practically negligible. As the height increases it starts to tilt slightly with respect to the ground for unknown reasons and its influence on the jet curtain flow and the base pressure distribution increases. As the transition height is approached. the ring vortex shifts and becomes elliptical prior to the jet curtain. The ring vortex produces a ring of decreased pressure where it touches both the ground and the nozzle base.
The cross flow originates at B (Fig. 21) and is fed by the jet curtain. It passes from under the ring yortex. up and across the cavity and over the ring vortex at the opposite side (at A). There it is entrained by the jet curtain and carried to the outside. The region on the edge of the base (point E) is found to have a higher pressure than at the opposite side of the base (point F). This pressure difference induces a flow from point E to point F both ways around the inside surface of the jet curtain which has. a negligible effect on the jet curtain, because of its low velocity . But it does affect the
slowe~ cross flow and produces a pair of conical vortices in the cavity. which extend from the ground to the base. within the confines of the ring vortex. The cross flow diffuses the vorticity from the ground to the base. The ends of the vortices on the ground are reasonably strong for all heights below the transition height. whereas the diffused ends on the base are only significant at low heights. At very low heights the conical vortices become a pair of
cylindrical vortices.
6.2 The General Effect of the Main Variables on the Flow Model
The main variables investigated in these experiments were aspect ratio. pressure ratio and height to diameter ratio. Following. some of the effects of these variables are discussed.
6. 2. 1 Aspect Ratio
Aspect ratio is all. important parameter with respect to thrust
augm~ntation aa shown in Refs. 3 and 8. It also has a significant effect on the
flow .p~ttern.
The height at which transition takes place increases as the aspect r~tio increases. In other words high aspect ratio annular jets have a greater height range within the ground effect~ before transition to the concave or focussed jet takes place. Further. the cross flow becomes more regular {more symmetrical about a plane through the nozzle axis parallel to the direc-tion of cross flow} as the aspect ratio increases. The cross flow in low aspect ratio annular jets tends to be irregular and sometimes unsteady in direction.
At large heights (neartransition) the air flow as a whole appears to become slightly more syrnmetrical as the aàpect ratio is increased. As this feature is shown mainly by the lampblack plates~ it is not considered sufficiently proven.
Irregularities in the nozzle interior have more effect on the flow at the lower aspect ratios. This may be a characteristic of the nozzle design. as the contraction ratio is lower for lower aspect ratioa.
6. 2. 2 Pressure Ratio
Pressure ratio was found to have only a minor effect on thrust. augmentation in Refs. 3 and 8. Likewise no significant effects of pressure ratio on the cavity flow pattern were discovered in these experirnEmts. An in-crease in pressure ratio does of course increase the velocities in the flow but this does not appear to change the shape of the flow patterns.
An increase in pressure ratio does tend ta increase the unsteadi-ness in the cross flow at h/D ~ . 15. The height range of the resonance
phenomena is also increased with pressure ratio. These two effects" howeveI'. become significant only for pressure ratios equal or greater than 1. 2.
6.2.3 Height to Diameter Ratio
The height of an annular jet nozzle above the ground has a very strong effect on the thrust augmentation as weU as on the flow patterns. Some of the effects of this parameter have already been discussed in section 6. 1; others wiJl be given here.
The general changes that take "place in the internal flow patterns as the height varies from almost zero to the transition region are él:s' follows. At a height which is equal or less th,an the 'jet thickness. the jet curtain is uniform and symtnetrical. the ring vortex is practically insignificant. but random eddies are set up within the cavity. When the height becomes greater than the jet thickness. the velocities in the cavity become leas significant. At
(17)
h/D ~ O. 1 the ring vo.rtex beco.mes slightly tilted. pro.ducing a cro.ss flow with a pair o.f cylindrical vo.rtices extending fro.m the no.zzle base to. the ground. As the height increases the two. vo.rtices mo.ve downstream in the cro.ss flow and c10ser to.gether. At h/D ~ . 15 the cro.ss flo.w tends to. beco.me unsteady. More than two. vo.rtices may appear and the cro.ss flo.w may come in fro.m two sides instead o.f o.ne. As the height is, furthèr increased to. h/D =:::.25. the pair o.f vortices co.me still clo.ser to.gether and appro.ach the no.zzle axis. At this Po.int the ring vo.rtex is o.f significant size and has almo.st negligible tilt. so that the asymmetrie cross flo.w is limited to. a small regio.n aro.und the nozzle axis
(e. g. Fig. Uh). Thus the flo.w pattern is almo.st symmetrical at this height. The tro.uble is that the flo.w can easily be made asymmetrie by either slightly o.bstructing the jet curtain or by disturbing the inco.ming entrained air outside the jet curtain. A further increase in height results in the ring vo.rtex tilting with respect to the gro.und. This increases the amo.unt o.f cro.ss flo.w up to. the point where it beco.mes directio.nally unsteady and is indicative o.f the beginning o.f the transition o.f the annular jet to. its shape o.ut o.f gro.und effect.
At high gro.und heights but below trans.tiion the cavity pressure is appro.xirnately pro.po.rtio.nal ta. the jet thickness ('
Al>
'
=
J IR). whereas at very lo.w heights (he:! t) the cavity pressure also. depends on the pressure ratio. This is reasonable because the cavity pressure canno.t beco.me larger than the jet sheet stagnatio.n pressure.The effect of the ring vo.rtex increases with height. At very low heights it is insJgnificant.. At higher heights it has an appreciable effect on the pressure distribution over the base of the no.zzle and o.n the curvature of the jet curtain.
6.3 No.zzle Induced Asymmetry
If the no.zzle is respo.nsible for the asymmetrie flo.w. then the cause wilI likely be a disturbanee o.r flo.w separatio.n in so.me section of the annulus. Separatio.n may cause no.n-unifo.rmity in the velocity distribution o.r in total pressure at the nozzle exit.
Co.nsider theoretically a radial slice o.f the jet curtain with co.n-stant jet thickness and co.nco.n-stant pre.ssure difference across it. The variatio.n o.f jet radius. with to.tal pressure fo.r a thin jet is. given by Eq. 4. section 2. 1
(P T - Pa) R = 2t Pc - Pa
Thus a lo.cal reductio.n in total pressure wil! reduce the jet radius proportionally. This in turn wil! pro.duce .a greater o.utflo.w on that side o.f the annular jet, and thereby induce a cro.ss flo.w in the cavity.
The thick jet radius is given by Eq. 7
as:-It can be seen from this expression that the jet radius is more dependent on the total pressure in a thick jet. Therefore. the same disturbance or separation in the nozzie annulus will promote stronger asymmetrie effect in case of a thick jet. In fact. it was observed that the ground board flow patterns were more irregular at the Iower aspect ratios.
In order to find out if there is a total pressure variation in the nozzle. some total presBure measurements were taken. A check of the four symmetricaUy placed total pressure probes in the nozzie annulus with a
water U-tube showed no difference in pressure at the four points upstream of the nozzle exit. In addition the total pressure was measured at eight equally spaeed points around the nozzle exit, using a mercury manometer.
These pressures were found to be equal within an experimental error of less than 1%' of the gauge pressure. Therefore the asymmetry does not seem to be caused by a disturbance in the nozzie annulus. More probably. an axially sym m etric jet c urtain is inherently unstable and any minute distur -bance inside or outside the nozzle wiU make the flow asymmetrie and determine the direction of the cross flow.
The annular nozzle was machined very accurat~ly to toleranees of t 0.001 inches and polished inside. If a defect within this toleranee can produce asymmetries as observed. than an actual annular jet nozzle, built
under ordinary manufacturing conditions will certainly be subject to asymmetrie flow.
Examination of the two-dimensional annular jet flow pictures in Ref. 6 shows the presence of cross flow in the cavity. Also the base pressure distributions covering a range of jet angles, ground heights and aspect ratios for a circuIar annular jet at zero angie of attack of Ref. 7 show considerabie asymmetry. Asymmetry is therefore not something peculiar only to the UTIA annular nozzle.
6.4 Unstabie Pitc~ Moment
The base pressure distributions, presented with the pressure traverse data, show thát the cross flow produces a high pressure region on the downstream side of the base. According to Stanton-Jones (Ref. 9) and others the jet flow on the downgoing side of an annular jet splits, with the inside part passing throukh the cavity and going out the opposite (upgoing) side. If th is is the case, and it certainly seems so, then the pitching moment result-ing from the cross flow will be unstabie.
(19)
Frast (Ref. 2) states that the instability of an annular jet is due to a cross flow.
Poisson-Quinton (Ref . . 6) states that the instability is due to the
ring vort ex being displaced. Actually of course, both these things occur and produce the bighly undesirable instability.
In a number of reports the unstable pitching moment was observed
with annular jets at an angle of attack or in forward motion. Stanton-Jones (Ref. 9) and Bertelsen (Ref. 1) state that the instability at an angle of attack for a three dimensional nozzle is increased by inwardly inclining the jet curtain,
whereas Nixon (Ref. 5) states the opposite for a two-dimensional nozzle. The
present report shows that an inwardly inclined annular jet is unstable, but of
course it does not show what effect the jet angle has on stability.
Bertelsen (Ref. 1) also states that the instability increases with
jet thickness. It has not been definitely shown here that the amount of cross
flow increases with jet thicknes,s, but it has. been ahown that the cross flow is
more irregular with increasing thickness.
6.5 Axially Symmetric Regions of the Annular Jet
The ground board flow patterns show that the annular jet flow is
reasonably symmetric for h/D values less than 1/30 and also for h/D values
of approximately 7/30. Higgins and Martin (Ref. 4) found that a circular
annular jet was stabie at very low altitudes at small angles of attack. They
also found an isolated "cloud" of stability at h/D
=
O. 2. FurthermoreStanton-Jones. (Ref. 9) found that a single jet model with the jet inclined 600 inwards
was stabie for h/D less than 0.03. Therefore there should be a relation
be-tween the axial symmetry of the annular jet flow and the stability of the nozzle. Curves of thrust augmentation versus ground heights for annular
jets are usually characterized by a hump. Inspection of the base pressure aug>
mentation curves obtained by Smith (Ref. 8) shows that the hump in the curves
occurs approximately between h/D = 6/30 and 10/30. The maximum point in
the hump corresponds approximately to h/D = 7/30 where the jet flow is the
most symmetr~cal. Therefore it appears that the ring vortex is reasonably
stabie in a horizontal position at tbis height and leads to an increased augmenta-tion.
A comparison of the thlck jet theoretical augmentation curves with the experimental augmentation curves presented by Garland (Ref. 3) shows that they coincide only at the hump in the experimental curves. At other heights the experimental curves fall below the theoretical curves .
• Therefore it appears that the asymmetry or tiIting of the ring vortex leads to
6.6 Effect of Compartmenting the Annular Jet Cavity
Be.sides giving a picture of the flow, the lampblack plates show the effect of partitioning the flow. At low heights a plate caused the flow to become symmetrical about two mutually perpendicular planes. This was
more evident for a plate with no gaps at the base or ground board than one with gaps. At higher heights for higher aspect ratiosthe flow,was more symmetrical with a plate in place than without. Therefore. one diametric partition may already be quite beneficial. especially for high aspect ratio annular jets. 6. 7 Resonance in the Annular Jet CavitY
Resonance due to radial vibration of the jet curtain may be a problem for high pressure ratio GEM's as it greatly increases the noise level. The frequency of the resonance will be m uch lower for a full sized GEM thari - for a six inch diameter nozzle because of the greater height. Also. no
re-sonance was found at very low heights and low pressure ratios. Therefore for most full sizeq GEM's there should be negligible or no resonance.
If this phenomena did occur it can likely be stopped by putting a damper in the jet curtain. Control flaps, in or against the jet curtain, if mounted suitably shou1d damp the vibration as weil as control the jet curtain orientation.
VII. CONCLUSIONS
An axially symmetrie annular jet flow within the ground effect is unstable for most ground heights. In general, the flow is stabIe in an asymmetrie state. A minute ang1e of attack or a small disturbance in the nozz1e will determine the direction of the cross flow. This asymmetry pro -duces an unstable pitching moment on the nozzle, leading to the well-known wobbling motion of GEM's. The flow asymmetry also results in a 10ss of base pressure thrust. and therefore thrust augmentation.
The flow symmetry and therefore the stability and thrust augmenta-Hon of the annular jet may be improved by emp10ying one diametric partition in the cavity. This is most effective for high aspect ratio jets. Lowaspect ratio jets win require more than one partition.
Since the ring vortex p1ays such an important part in the phenomena of flow asymmetry, the bad effects of asymmetry might we1'.L be reduced by decreasing the strength of the ring vortex. This cou1d be done by instailing a low speed secondary jet curtain immediately a10ng the inside edge of the main jet curtain. Of course the tota1 presf)ure of this jet would still have to be slightly higher than cavity pressure. This possib1e so~ution
wou1d be most effective for a high pressure ratio primary jet. For a 10w pressure ratio jet. it is equivalent to using a thick jet.
More experimenta1 work has to be done on the effect of ang1e of attack on the asymmetry of the cavity flow. The effect of jet curtain inclina-tion shou1d also be investigated further.
1. Bertelsen. W. R. 2. Frost. J. C. M. Earl. T.D. 3. Garland. D. B. 4. Higgins. H.C. Martin. L. W. 5. Nixon. W. B. Sweeney. T. E. 6. Poisson-Quinton. Ph. 7. Sacks. D.G. 8. Smith. R. E. 9. Stanton-Jones. R. REFERENCES
Experience with Several Man-Carrying
Ground Effect Machines. Princeton University Symposium Oct .• 1959.
Flow Phenomena of the Focussed Annular Jet. Princeton University Symposium Oct .•
1959.
Studies of Ground Effect on an Inwardly Inclined Annular Jet. Part I: Apparatus and Method of Testing; Effects of Aspect Ratio and Pressure Ratio. UTIA T. N. No. 37. August 1960.
Effects of Surface Geometry and Vehicle Motion on Forces Produced by a Ground Pressure Element. Princeton University Symposium. October 1959.
A Review of the Princeton Ground Effect Program. Princeton University Symposium October. 1959.
Two-Dimensional Studies of a Ground Effect Platform. Princeton University Symposium October. 1959.
Ground Cushion Flow Visualization Studies. Princeton University Symposium October. 1959.
Studies of Ground Effect on a 600 Inwardly Inclined Annular Jet. Part II: Base Plate Thrust Augmentation and Pressure Distri-bution;- Effects of Aspect Ratio. Pressure Ratio and Height Above Ground; Jet Stability. UTIA TN No. 47. May. 1961.
The Development of the Saunders -Roe Hover-craft. SR-NI. Publication No. TP. 414.
VlI""G UIG OUWKUNDE Miehi I ,. ,;tqltrw g 10 • DE
D/2
t«R
<t:
I
°/2
h
FIG. 1 THEORETICAL REPRESENTATION OF THE THIN AND THICK ANNULAR JET
COOLER
THIN WALL PIPE
EXPANS/ON JOINT
ANNULAR
NOZ2LE
GROUND
::<DIFFUSER
FLEXl8LE COUPLING
PALOUSTE
GAS
TURBINE
ENGIN
FIG. 2 DIAGRAMMATIC LAYOUT OF COMPRESSED AIR FACILITY
AND ANNULAR JET TEST RIG
FIG. 4 TRAVERSING MECHANISM WITH YAW PROBE IN PLACE
FIG. 5 ANNULAR JET NOZZLE AND STEEL GROUND BOARD WITH STATIe PRESSURE HOLES
c:::::::JJ
FIG.6
Nozzle End
Sleeve
Piping
HALF SECTION THROUGH ANNULAR NOZZLE ASSEMBLY SHOWING
FIG. 8(a) SIMULTANEOUS LAMPBLACK FLOW PICTURES ON BASE AND GROUND BOARD WITH NO PLA TE PRESENT FOR A NOZZLE OF AR = 176, PT/Pa = 1.2 AND h/D = 3/30
left side right side
FIG. 8(b) SIMULTANEOUS LAMPBLACK FLOW PICTURES ON GROUND
BOARD AND VERTICAL NOZZLE AXIS PLANE FOR A NOZZLE
"
gaps
FIG. 8(c) SIMULTANEOUS LAMPBLACK FLOW PICTURES ON BASE, GROUND BOARD, AND HORI-ZONTAL NOZZLE AXIS PLANE FOR A NOZZLE OF
h/D
10/30
7.5/30
5/30
top bottom left right
VERTICAL HORIWNTAL
FIG. 9(a) LAMPBLACK PICTURES OF THE FLOW IN A NOZZLE AXIS PLANE FOR A NOZZLE OF AR
=
6710/30
7.5/30
5/30
PARALLEL TO CROSS FLOW NORMAL TO CROSS FLOW
FIG. 9(b) LAMPBLACK PICTURES OF THE FLOW IN A NOZZLE AXIS PLANE FOR A NOZZLE OF AR
=
106h/D
10/30
PARALLEL TO CROSS FLOW NORMAL TO CROSS FLOW
7.5/30
5/30
top bottom left right
VERTICAL HORIZONTAL
FIG. 9(c) LAMPBLACK PICTURES OF THE FLOW IN A NOZZLE AXIS PLANE FOR A NOZZLE OF AR = 176
10/30
7.5/30
5/30
top bottom
VERTICAL PLANE left HORIZONTAL PLANE right
FIG. 9(d) LAMPBLACK PICTURES OF THE FLOW IN A NOZZLE AXIS PLANE FOR A NOZZLE OF AR
=
263PT/Pa
1. 05
1.2
1.6
AR
=
88.5 AR=
151FIG. 10(b) PT/Pa 1. 05 1.2 1.6 AR
= 263
AR=
524EFFECT OF PRESSURE RATIO ON NOZZLE AXIS PLANE FLOW PATTERN AT h/D = 10/30
:::: " ;;:i r-. ij r~ ):1: i''''' ~ ;::: ~ :':-"' 0 '" J
r:-a
r.l
""e""",,-:0:2
°2
0 ' z o oor-~m o "T't rT"1 ....r--."
h/D
=
10/30 h/D=
9/30 h/D = 8/30 h/D = 7/30h/D
=
5/30 h/D = 3/30 h/D = 2/30 h/D=
1/30 FIG. 11 (a) LAMPBLACK PICTURES OF THE FLOW ON THE GROUND BOARD FOR A NOZZLE OFh/D = 4/30 h/D= 3/30
h/D = 2/30 h/D= 1/30
FIG. l1(b) LAMPBLACK PICTURES OF THE FLOW ON THE GROUND BOARD FOR A NOZZLE OF AR = 67 AND
h/D = 10/30 h/D
=
8/30 h/D = 6/30h/D
=
4/30 h/D=
3/30 h/D=
2/30 h/D=
1/30FIG. 11 (c) LAMPBLACK PICTURES OF THE FLOW ON THE GROUND BOARD FOR A NOZZLE OF
h/D= 6/30 h/D = 5/30 h/D
=
4/30h/D = 3/30 h/D = 2/30 h/D = 1/30
FIG. 11(d) LAMPBLACK PICTURES OF THE FLOW ON THE GROUND BOARD FOR A NOZZLE OF AR
=
106; PT/Pa=
1. 02h/D = 11/30 h/D = 10/30 h/D = 8/30
h/D= 6/30 h/D= 5/30 h/D = 4/30
h/D= 3/30 h/D = 2/30 h/D = 1/30
FIG. 11( e) LAMPBLACK PICTURES OF THE FLOW ON THE GROUND BOARD FOR A NOZZLE OF AR = 106; PT/Pa = 1. 1
h/D = 6/30 h/D= 5/30 h/D = 4/30
h/D = 3/30 h/D = 2/30 h/D = 1/30
FIG. 11(f) LAMPBLACK PICTURES OF THE FLOW ON THE GROUND BOARD FOR A NOZZLE OF AR
=
106; PT/Pa=
1. 2h/D= 10/30 h/D= 8/30 h/D= 6/30 h/D = 4/30 h/D = 3/30 h/D = 2/30
lil
,. -~ '.'*,
\' , '-'><.,;,!(.:'
'.1. " , ' , : " ,. 11.( .'!~;l ' ! : . ~ .,,"\,~', "'- ... \' ,. lri~ J ,,,'..2:~' ... , ... ... , .~)
, " .1, . '. ''''. , d~t~"
' l . l
''];; .
.:r,. ,,,:j
~--:-:,~
':
t~"
; k-~,:_~.'
~ir1\..
';'-""'~~".,~'
..
"~:~,
~, ";;:;"'jt ,'~ '" ""1~;/!~~:"~
\' ... -, -,;t;'-,.-, \ Ir " t l , " 'Y" ,,!,!\ 4.,., ' , " . , ~ ;#49, '. - " h/D = 5/30 h/D = 1/30FIG. 11(g) LAMPBLACK PICTURES OF THE FLOW ON THE GROUND BOARD FOR A NOZZLE OF AR = 176
h/D = 4/30 h/D = 3/30 h/D= 2/30 h/D = 1/30
FIG. 11(h) LAMPBLACK PICTURES OF THE FLOW ON THE GROUND BOARD FOR A NOZZLE OF AR = 176
h/D= 11/30 h/D = 10/30 h/D = 9/30 h/D= 8/30
h/ D
=
7/30 h/D= 6/30 h/D = 4/30 h/D = 2/30FIG. 11(i) LAMPBLACK PICTURES OF THE FLOW ON THE GROUND BOARD FOR A NOZZLE OF AR = 176
AND PT/Pa = 1. 2
--h/D = 4/30 h/D'= 3/30 h/D = 2/30
h/D= 1/30
FIG. 11{j) LAMPBLACK PICTURES OF THE FLOW ON THE GROUND BOARD FOR A NOZZLE OF AR = 263
AND PT,/Pa = 1. 02 J
h/D=9/30 h/D = 8/30 h/D=7/30
h/D= 5/30 h/D = 3/30 h/D=2/30
h/D = 1/60 h/D = 1/120 h/D = 1/120
FIG. l1(k) LAMPBLACK PICTURES OF THE FLOW ON THE GROUND BOARD FOR A NOZZLE OF AR
=
263; PT/Pa=
1. 1h/D= 6/30
h/D=5/30 h/D=4/30
h/D = 3/30
h/D=2/3Q h/D
=
1/60h/D= 1/120
FIG. 11(1) LAMPBLACK PICTURES OF THE FLOW ON THE GROUND BOARD FOR
h/D= 8/30 h/D = 5/30 h/D= 4/30 GROUND BOARD -~
•
"
•
t'l ~;j ~~ f. ,~ ':>.. / I:, "'i' :;.i). . , ..*~.,;,
~;:.:;.. . ,~, . ,/;~,c':.
J
~~ ,;~ '.';;/ ji' / tip.'
h/D= 1/30 COATED UNCOATED 0lIl( . . ~ ~FIG. 12 SIMULTANEOUS LAMPBLACK PICTURES OF THE FLOW ON THE BASE AND THE GROUND BOARD
AR = 67 PT/Pa
=
1. 2h/D
=
8/30 h/D = 4/30 h/D= 2/30AR
