Studies of ground effect on an inwardly inclined annular jet. Part 1. Apparatus and method of testing; effects of aspect ratio and pressure ratio

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TECHNISCHE

HOGESCHOOL

DELFT

VLiEGTUIGBOUWKUNDE Michiel de Ituyterweg 10 • DELFT

l'

feb. 1961

STUDIES OF GROUND EFFECT ON AN INWARDLY INCLINED ANNULAR JET

Part 1: Apparatus and Method of Testing; Effects of Aspect Ratio and Pressure Ratio

by

D.B. Garland

Part 1: Apparatus and Method of Testing; Effects of Aspect Ratio · And Pressure Ratio

by

D. B. Garland

The author wishes to express his thanks to Dr. G. N. Patterson for the opportunity to pursue this' investigation and to Avro Aircraft Limited for leave of absence. very gene rous financial assistance and construction of apparatus.

The suggestions and supervision of Dr. G. K. Korbacher are gratefully acknowledged.

Thanks are due to A. B. Bailey for valued assistance and to R. E. Smith for the extensive help with calibration and testing.

The work was made possible through the financial support of the Defence Research Board of Canada

..

A systematic but not exhaustive series of tests was carried out with a 60° inwardly inclined annular nozzle over a wide range of nozzle aspect ratios (67 ~ AR ~ 524), pressure ratios (up to 3.0), with and without the presence of simulated ground. Total thrust force was mea-sured and converted to augm entation ratio, using the ideal thrust of an equivalent circular nozzle in free-air as a basis. Nozzle angle of attack was he~d at zero degrees. Base pressure distributions and nozzle mass flows were also measured but will be included in Part n ,to be published later.

Agreement between test results and available theory is only moderate. A very sudden jet focussing phenomenon was observed at high nozzle aspect ratios (AR

>

250) as weIl as a very pronounced hysteresis effect in the focussing-unfocussing region.

1. 1I. lIL IV.

V.

VI. T ABLE OF CONTENTS NOTATION INTRODUCTION

NON-VISCOUS THEORY OF THE ANNULAR JET CLOSE TO THE GROUND

2. 1 Thin, High Aspect Ratio Annular Jet 2.2 Thick, Low Aspect Ratio Annular Jet TEST FACILITIES 3. 1 Design Considerations 3. 2 Description of Apparatus EXPERIMENTAL PROCEDURE 4. 1 Calibration 4. 2 Method of Testing 4. 3 Accuracy of Measurements RESULTS AND DISCUSSION

5. 1 Presentation of Results 5. 2 Discussion of Results

5.2.1 Annular, Unfocussed Jet With Ground Effect

5.2.2 Jet Focussing

5. 2.3 Behaviour in Free-Air

5. 2. 4 The Concave or the Focussed Jet CONCL USIONS REFERENCES FIGURES ii 1 1 1 3 8 8 9 10 10 11 12 12 12 13 13 14 14 15 16 17

D F j J m N p NOTATION

Experimental thrust augrn entation ratio Theoretical thrust augmentation ratio

7t'D Nozzle aspect ratio _t Nozzle rnean diameter Nozzle base diameter

Thrust of an ideal circular nozzle in free-air Pressure thrust on nozzle base

Mornentum thrust of the annular jet

Pressure thrust of the annular jet (when Pe:J: Pa)

Total thrust of annular nozzle Height of nozzle base above ground Non-dimensional height parameter

Jet rnornentum per uni~ length of circumference

Tatal jet rnornentum with ambient exit or back pressure Jet rnass flow

Number of turns of the ground-board lead-screw (h/Do =N / 30) Static pressure (absolute units)

Atmospheric pressure Base static pressure Jet exit static pressure

Tatal pressure (absolute .. untts) J et exit total pressure

Nozzle pressure ratio

R S· J t t/h

v

_J

·

Radius of annular jet in cross -section J et exit area

Jet thickness at exit

Non-dimensional jet thickness parameter

Jet velocity at nozzle exit

Ideal jet velocity at exit with ambient back pressure

Angle of nozzle exit, measured from the nozzle axis,

negative for inward-pointing jets.

1. INTRODUCTION

As far as the author is aware the earliest work on ground effect with an annular jet was carried out by J. C. M. Frost in 1953 and is briefly mentioned in Ref. 1. Since that date the phenomenon has been intensively studied in several European countries as well as in the U. S. A. and Canada.

The first unc1assified report published by NACA (Ref. 2) seemed to indicate a simpie, orderly pattern of results and when com-pared with equally simple non-viscous theory (Ref. 3) the agreement was excellent. Further work by N. A. S. A. (Ref. 4) and others (Ref. 5) and (Ref. 6) however, revealed peculiar behaviour with higher aspect ratio

jets, i. e., thin jets surrounding large diameter bases.

Obviously, viscous effects we re beginning to play a noticeable part in what is essentiallY a viscous regime. In order to approach an

understanding of these effects a study of past literature on plane and cir-cular"jet mixing phenomena became necessary before application to annular jet problems was made (Ref. 7). A method using potential flow theory to predict jet mixing effects was also tried (Ref. 8). The unknown effect of large curvature on the entrainm ent characteristics of plane and annular jets is probably a very important parameter and is now being studied at UTIA and elsewhere.

, 0 This report deals with the systematic testing of a 9 = - 60

inwardly inclined annular jet over a wide range of jet exit areas, pressure ' ,_.

ratios and heights above ground. Thrust augmentation near the ground

was studied and the effect of jet focussing investigated. The term "focussed

jet" is one used to describe the flow from an annulus which, by virture of

the Coanda effect, attaches itself to the nozzle base.

Il. NON-VISCOUS THEORY OF THE ANNULAR JET, CLOSE TO THE GROUND

This section of the report is included for the sake of com-pleteness and ease of reference.

2. 1 Thin, High'" Aspect Ratio Annular Jet

The assumptions are made that the flow is incompressible and non-viscous (i. e., no mixing takes place between the jet and the surrounding air), that the jet thickness is small compared with height above ground and that the total jet momentum remains constant along the jet path. For,the.:case of the two-dimensiona1. 'annular' jet, the momentum per unit 'circumference' also remains a constant at any section downstream of the nozzle but for the axisymmetric annular jet with which we are con-cerned, due to the increase in jet circumference near the ground as the jet

spreads out, the momentIolm per fooi dec;:reases. The appro~imate theory for the axisymmetric annular jet ignores this effect. Comparison with the "exactf theory of Ref. 3 shows th at this is justifiabl-e especially if the jet nozzle is inwardly.inc1ined. The approx.i,mate theory proceeds as follows.

Considering. an element of the curved jet (.:Fig. 1). for equilibrium of pressure and centrifugal forces

R

~9

tfp

:t fjVj 2

_~R

69

or

~p

:0: D·V·2

1!L

rJ J R

If

t is the (constant) thickness of the jet then

,.

But ain&e and 2 'p = DL -p =LJ.V. . ~D a '~J J

LJ.v

2 t • -..;.J _ _ I J j 21f ro • j R

=

h

'1, -

Pa • _ _ J _ _ 2 1f' ro h Hence the thrust force due to basepressure is

2

Fb zo 11' ro J

=

J

The thrust due to jet momentum alone is . Fj = Jcos 9₀

therefore the tot al thrust is

t R FT

=

J (cos 8₀

+

1 - sin 8₀) . 4 h/Do

Ut

(2) (3) (4) (5) (6) (7) (8) (9)

The theoretica! augmentation ratio. based on the ideal thrust J, is then

= cos 8₀

+

1 - sin 8₀ 4h!Do .'

(10),

2.2 Thick, Low Aspect Ratio Annular Jet

For a 'thick' jet the effect of non-ambient back pressure becomes important, causing a reduction in mass flow and jet momentum in genera!. Several authors (for example, Ref. 9) have demonstrated the effect of jet thickness, at least to a first order. The expression for theoretical augmentation ratio developed in Ref. 9 is written here in a

form such that comparison with the thin jet equation is immediately possible. In addition, two slight refinements of the theory were evaluated. The first was to more accurately express the geometrical nozzle exit area and the second was to include the thrust term due to the non-am bient exit pressure acting on the exit area. For the particular configuration tested these two modifications were found to cancel each other very closely.

As for the thin jet theory, it is again assumed that the flow is incompressible and non-viscous, that Bernoulli's equation holds through-out the jet and, for simplicity, that there is a single centre of curvature for all particles in the jet (Fig. 2). Again, as in the approximate, axisym-metric, thin-jet theory the variation of jet momentum per foot along the jet path is ignored. But it should be mentioned that in thè~thièk::jet theory of Ref. 13 this effect is included.

Following from equations (1) and (2) we can write, for any point in the jet,

and from Bernoulli th at

Hence dp

=

dR R 2 dR R

Integrating between the limits of Rl to Ro where the corresponding pressures are Pb and Pa we get

(::)

or

(11)

(12)

Therefore we can express the increment in base pressure

.1

p as P - P _{b a T .} = (P - P ) _a J (14) Now (15) But _h Rl

=

( I - sin '₉₀₎ and J J approximate1y (see Eq. (4» Hence

The base thrust Fb

=

(Pb - Pa) _{is therefore}

F b

=

~

[

(I - sin 90

J

h/Do

J

= J [( 1 -

~in

90

J

4h/Do

Î

1 - '2 - 2 ( I - sin 90 ) 2 ] h/Do t

(I-Sin90~2J

Do 4h/Do

Î

(17)

The second term represents the 10ss in base pressure thrust due to the reduction in jet curtain strength (mornentum).

.

'

(5)

The non-ambient exit pressure also causes a reduction in jet exit velocity. One can express the new jet momentum approximately as

dR (18)

Integration of equation (12) wil! give

(

l!0~2

P t , ""' P = _{(PT , -} P ) - -

-J e J a R for any radius R.

Hence the velocity V

(19)

i. e.

The jet momentum can therefore be expressed as

Fj

=

1fDo

1

Rl (ljV/

(~o)2

dR

R

o

The modified jet reaction thrust is therefore

F, cos 9

=

J ( 1 - t ) cos 9

J 0 - 0

Rl

The expression for the total thrust is then

.

'

sineeR

o

= R 1 - t (20) - sin 90

J

cos 9 0 4 h/Do

Î

(21 )

J [ f"\ 1 - sin 9₀

=

cos 00

+

4h/Do - 4 cos 9₀

(...!...)(

1 - sin 90 ) Do 4h/Do _ 2 ( _ t ) ( 1 - sin 90 " 2

J

. Do 4h/Do

Î

The thick jet augmentation ratio is therefore

A

th

=

cos 90 + '1 - sin 90 -

4f...!...)f1 -

sin 90 ) cos 9

~Do

4h/Do 0 4h/D o - 2 2 - sin 9_{0 )} 4h/Do

This expres'sion is plotted in Figs. 3 and 4.

A first refinement, referred to earlier, is to write the nozzle exit area as

S·

=

J

rather than the approximate expression

Equation (15) therefore becOInes

Pb - Pa =

2

-(;1) ]

and equation (17) becomes

F _{b -} -(1+ J t Do

[ (

1 -

sin

9

0 \ _

2

(D t )

(1 -

sin

9

0 \

2]

4h/Do

J

0 .4h/Do /

A second refinement is to include in the nozzle thrust a term due to the non-ambient exit or back pressure

i.e., _{F p}

=

1

(Pe-Pa)

s·

_J dS (22) (23) (24) (25) (26) (27 )

Using Bernoulli' s equation to get 2 2 Pe - Pa =

-!

f

j (V j - V e ) equation (27) becomes 2 2 (V. - V ) dR J e

where D

=

Do

+

2 (Rl - R) cos 9₀ (see Fig. 2)

and the nozzle momentum thrust is

Hence

Putting Rl

=

F

+

F

=

j P

h _{Eq. (31) reduces ultimately to}

=~D02

{2(D

t

o)

[1

+

~

(-1

:o:i:----I~o

]

-

t~S

(cos

(28) (29) (30) (31 )

- 2 cos 9₀

-D:

r

log e (1 -

{{~-Sin

9

_{0 )}

The augmentation ratio is then which becom es - cos 9₀ t/Do Fb

+

_(Fj

+

F p) cos 9₀ J - sin 9_{0 _)} 4h/Do

J

_ _ t

)2

10g

(_1 )]

Do 1 - ~ (l-sin 90 ) (33 ) (34)

Several computations of this expression gave results a1.most

identical to those of Eq. (23), which has been used throughout this report

as the basis of comparison with experimental findings.

lIl. TEST F ACILITIES

3. 1 Design Considerations

From its conception, the compressed-air facility at UTIA

(Figs. 5 and

9)

was designed to minimize pressure losses in the system in

order to make full use of the capabilities (pressure ratio 3. 5) of the

Turbomeca Palouste 500 Gas Turbine supplying the air.

Two general problems face all those who try to design

apparatus to measure the thrust of a fluid jet:

1) Because the means of conveying the fluid (in th is case, air)

to the nozzle usually has to be a pipe of considerable strength and stiffness,

it will therefore carry a significant proportion of any thrust force experienced by the nozzle. Hence any attempt to connect the nozzle to a weighing or

force measuring device has to take account of the thrust force absorbed by the delivery pipe.

Various methods have been tried to overcome this problem. Ref. 2 shows a system of rotatable couplings in the pipe and this, pre-sumably, is a practical solution for relatively small diameter pipes where the friction forces or moments in the coupling are smal!. Apart from the objectionable disturbance to the pipe flow due to the right-angle bends at the couplings, the large diameter pipe (8" dia. ) in the UTIA installation would have m eant proportionately larger frictional forces in the thrust

m easuring system.

Air bearings have been used elsewhere in the place of

flexible or rotating couplings. These have negligible friction (although

sometimes misalignment forces) but as there is a loss of air through them the flow measuring section must therefore be mounted downstream of the air bearings. This usually implies a considerably longer and more cumber-some pipe system.

A third method, adopted at UTIA, was to reduce the magnitude of the thrust force absorbed in the supply pipe system by arranging its

stiffness to be much less than that of arestraining strut and strain gauge dynamometer. For such a system there is an optimum stiffness of the beam or ring, carrying the strain gauges. A beam which is too stiff will not experience large surface stresses (required for accurate operation of strain gauges) and a beam not stiff enough will, in this two com ponent redundant structure, allow a large proportion of the nozzle thrust to escape into the supply pipe.

2) The second problem area concerns the accurate

measure-ment of nozzle mass flow, required when calculating jet momeasure-mentum. At

UTIA, the use of standard orifice plate meters had to be ruled out because of the associated high pressure loss and length of pipe required up stream

and down stream of the orifice. For the latter reason venturi meters were

also not feasible. Flow meters, being expensive and of limited range, were also ruled out and the only recourse left was to use a pitot static tube and traverse a given pipe cross section as accurately as possible, using the integrated velocity distribution to obtain the mass flow through the pipe.

In practice, the flow at this section chosen was still rather turbulent and

this reduced the accuracy of the mass flow determination somewhat . .

3. 2 Description of Apparatus

Compressed-air was supplied by a small gas turbine engine (Blackburn and General Aircraft Turbomeca Palouste 500), the air being

bIed off the com pressor at temperatures up to 2300C and pressures up to 55

psia. The engine was mounted on a simple frame and housed in a small room designed with walls and doors having a high noise attenuation (Noise

level inside was 136 db - outside 92 db at 35, 000 rpm). Intake air was

brought in from outside the building and the exhaust was ducted out through a muffler. The engine and air delivery were remotely controlled.

The hot compressed-air was diffused to an 8" diameter pipe and passed through a cooler supplied with mains water. lmmediately downstream of the cooler was a length of thin wall pipe of low-stiffness, followed by the mass flow measuring section, which consisted of a tra-versible pitot-static tube (connected to an inclined tube manometer )and a thermometer.

The final pipe section (a settling length) was supported from a simple steel frame with very thin steel straps (Fig. 7). Downstream of the rigidly mounted cooler, therefore, the piping system was free to

deflect perfectly elastically under load, except for the restraint offered by a sensitive ring-type dynamometer, its output being fed to an SR-4 strain indicator.

Attached to the extremity of the last pipe section by a rotatable flange was the annular nozzle, shown in Figs. 7 and 8. It

embodied an outer sleeve, running on a screw thread, to which was attached the outer wal! of the annular nozzle. For the range of nozzle exit angles envisaged, changing nozzle exit area simply entailed rotating the outer sleeve. The nozzle exit angle was variable (i. e., inner and outer pieces were made in detachable pairs for 9₀

=

-300, -450 and -600 )

but for this report only the -600 _{nozzle was tested.}

Stat ic pressure over the nozzle base plate was measured at 29 points. Jet total pressure was indicated by four pitot tubes equally spaced around the annulus just upstream of the nozzle exit. All pressure leads were taken out to mercury manometers through the centre body and its support struts.

For future research, a secondary flow duct exhausting at the centre of the base plate was an integral part of the model, but for the present series of tests it was blanked-off (to preclude any possibility of air leaking past the butterfly valve. ).

The annular jet exhausted towards a vertically mounted ground board, the position of which could be varied over a range of 20 inches ( 0 L

hl

d ~ 3) by means of a lead screw (Fig. 9). Variability of

ground board angle (i. e., angle of attack) was incorporated into the rig but only zero angle of attack was used for this series of tests.

IV. EXPERIMENT AL PROCEDURE

4. 1 Calibration

The basic load (thrust) calibration was carried out using a ring-type dynamometer "A" (see Fig. 10) in compression between the annular nozzle base plate and the ground board, the load being applied by rotating the lead screw. Dynamometer "A" had been previously calibrated

on a Tinius-Olsen testing machine. lts output was recorded on an SR-4 strain indicator. The output of the rig dynamometer "B", also pre-calibrated, was read on a second SR-4 indicator. During loading and unloading both indicators were read simultaneously. Evaluation of the results showed that 65% of the thrust applied at the nozzle was being taken by the ring dynamometer, the rest being 'lost' in the supply pipe.

To make certain that the supply pipe, when pressurized would have the same stiffness as when at ambient pressure, several calibrations were made with the nozzle exit area closed down to zero and various pressures up to 70 inches of mercury in the pipe. There was no change of stiffness but expansion of the pipe under pressure caused a zero shift in the dynamometer reading.

Af ter running air through the system for a few minutes the pipe temperature would reach equilibrium in approaching the temperature of the air passing through it. The resultant thermal contraction also caused a zero shift in dynam om eter reading. During subsequent testing, therefore, it was necessary to take frequent zero readings; fortunately, a simple operation since it merely required closing the air delivery valve momentarily.

Results of the mass flow calibration by pitot-static traverse of the measuring section wil! be included in Part II, to be published later. 4.2 Method of Testing

It was found that for a given setting of the air delivery valve

and nozzle exit area, movement of the ground board produced a change

in total pressure at the jet exit. Therefore, the test method used was to

maintain a constant ground-board position and jet exit area and vary the jet exit momentum by suitably adjusting engine speed and air delivery valve position.

The 29 nozzle base pressures, indicated on a multi-tube mercury manometer, were photographed at each of the more than 350 test points. Results of the analysis and integration of the base pressures for the determination of base thrust wil! be included in Part IT.

Hysteresis effects on the focussing and unfocussing of the jet were investigated by a series of tests using the moving ground board

method. Over the relatively smal! range (. 15 L.. h/D ~ .35) of ground

heights, the slight variation in jet total pressure did not seriously affect the range of hysteresis.

The measurements which had to be recorded for each of the 350 test points were:

1) Ground board position

2) Jet exit area

3) Jet exit total pressure

4) Total temperature at measuring section

5) Reference dynamic head at measuring section

6) SR-4 strain indicator reading of rig

(dynamometer "B" output)

7) 29 Base (static) pressures (photographed)

4. 3 Accuracy of Measurements

Measurement of nozzle pressure ratio is believed accurate

to better than ±

10/0.

Total measured thrust force was probably accurate

to

!

2 lb. The maximum force measured was about 500 lb. and the

minimum was approximately zero.

v.

RESULTS AND DISCUSSION

5. 1 Presentation of Results

The total thrust or lift force is presented as an augmentation

ratio versus ground proximity parameter, nozzle pressure ratio and nozzle

aspect ratio. Of the several methods in use to define augmentation ratio,

one of the simplest and most useful is that based on the ideal thrust of a

convergent, circular nozzle with exit area equal to that of the annular nozzle being tested. This augmentation ratio can be defined as:

)Act

.

(-SJ-

.

_F_P-T-j- )

Ideal

This definition is used in preference to FT / J (as found in the theory)

because the use of pressure rat_{ios greater than critical (PT . /Pa}

">

1. 89)

implies nozzle exit pressure greater than ambient. This aug\nentation ratio was evaluated for each of the 350 test points. The function

F ~SjPTj) !DEAL is, of course, dependent only on the nozzle pressure ratio

PT./Pa. It is included, for reference, as Fig. 11.

J

The definition of augmentation ratio used in this report is by no means a perfect one. From a practical view point, a definition in terms of thrust per air horse power expended in the jet seems more desirable, but th en a knowledge of exit velocity distribution is required.

Further discus sion on this topic wiU be found in Refs. 9 to 13, and in

5. 2 Discussion of Results

5.2.1 Annular, Unfocussed Jet with Ground Effect

The augmentation ratio for nozzles of all aspect ratios tested very close to the ground (see Figs. 12a and b) shows a tendency to increase

up to PT·/Pa ~ 2.2 and then to remain almost constant upto PT./Pa = 3

at least {or AR

<

263. However, for the higher aspect ratios (52:f) closest

to the ground (h/Do

=

1/30, see Fig. 12a) the maximum augmentation

(approximately 7.7 for AR

=

524) decreases at pressure ratios greater

than 2.3. At larger distances from the ground (h/Do

=

4/30), the

aug-mentation ratio seems to become almost independent of pressure ratio and

AR (see Fig. 12c).

For small values of h/Do (i. e., 1/30 and 2/30) the effect of

nozzle aspect ratio is very pronounced for all pressure ratios (see Fig. 13, a to e). Augmentation ratio increases rapidly with increase in aspect ratio

especially in the region up to AR ~ 300. Above this value the rate of

increase diminishes. As height above ground is increased the effect of

aspect ratio becomes smaller until at h/Do

=

8/30 and above there is no

appreciabie aspect ratio effect (see Fig. 13b) for the unfocussed (annular)

jet. It is expected, however, that further testing in the low aspect ratio

region (AR <::. 200) may show some variation with aspect ratio if the jet is

focussed.

Comparison with Fig. 4 shows that experimental trends due to aspect ratio and height above ground effects of the annular jet are accounted for reasonably weil. A further comparison with Fig. 3, made in Figs. 14 a to f, shows that agreement is best for the highest values of h/Do at which the annular jet curtain configuration could be maintained. Very close to

the ground (h/Do

=

1/30) the best augmentation ratio for the PT/Pa

=

3.0

jet, achieved with the nozzle of aspect ratio of 524 (see Fig. 14a) is only

53% of the theoretical "thin" jet value and 65% of the theoretical "thick"

jet value. It should be noted also that the maximum possible augmentation

occurs at h/Do

=

0 when the full jet total pressure. will act over the who ie

base area.

According to a recent report (Ref. 13), the theoretical thick

jet analysis is applicable only for values of hIt

>

3 (or h/Do"> 37f' / AR)

This is illustrated in Fig. 3 by the pecl:l.li.ar behaviour of the theoretical

curves at low hIt values, i. e., low aspect ratio jets very close to the

ground. As the exact, inviscid analysis of Ref. 13 uses a different

definition of augmentation ratio, its results, therefore, cannot be used for a direct comparison with the experimental findings of this report.

The effect of plotting augmentation ratio versus hIt instead of h/Do can be seen by comparing Figs. 15 and 16. The more orderly separation of the curves in the latter figure is noticeable and presumably

It is believed that the major cause of the discrepancy between experimental and theoretical values of augmentation ratio is the detrimental presence of the toroidal-shaped vortex enclosed by the annular jet curtain. This vortex, the existence of which has been known for some time (e. g. Ref. 5), was demonstrated in these experiments by its effect

on the base pressure distribution (see Fig. 17). It is made up of entrained

air (Fig. 18) and its strength is governed, presumably, by such factors as jet exit velocity, velocity gradient through the jet and radius of curvature of the jet.

5.2.2 Jet Focussing

Very apparent at the two highest aspect ratios (AR = 524 and

263), is the phenomenon of jet 'focussing' (see Figs. 14a, 14b and Figs.

15a to 15c). Above a certain height above ground the annular jet curtain would suddenly attach itself to the nozzle base plate and form a central,

circular or tree-trunk jet, (see Fig. 19). From the change in base

pressure distribution (see Figs. 17 and 20), it is apparent that this strong

attachment is a manifestation of the Coanda effect. Within the ground cushion, jet focussing reduces the aug.mentation ratio by strongly reducing the high positive base pressures enclosed within the annular jet curtain.

Associated with the focussing phenomenon is a pronounced hysteresis effect (Fig. 21). Because the focussed jet is a very stable entity it was possible to reduce the height above ground appreciably below the h/Do at which focussing occures and still preserve the tree-trunk pattern. A further reduction in h/Do eventually caused a very sudden

"de-focussing" at a particular height which was repeatable to approximately

±

0.5% of h/Do. Within the region of hysteresis the annular jet could ,

artificially be made to focus by deflecting the jet curtain (e. g. by hand) towards it centre line. When focussed the jet behaved very much like a

circular jet exhausting through a plane surface parallel and close to

the ground (see Ref. 14) in that a considerable loss of lift occurred. At aspect ratios less than 260 (see Figs. 14c and d) jet focussing did not occur s.uddenly and for the lowest aspect ratios tested

(see Figs. 14e and f) did not seem to occur at all. Fig. 15a, especially,

shows a poorly defined pattern of fall-off of augmentation,ratio with h/Do.

Base pressure distributions in these regions were of ten very unsteady and some times stronglyassymmetrical.

5.2.3 Behaviour in Free-Air

Partly as a result of the low force levels being measured and partly due to an unaccountable but stable jet assymmetry, the force data at large distances above ground (essentially 'free-air') were rather erratic. When combined with observations of the base pressure distri-bution however some insight into the flow mechanism was obtained.

In free air the flow pattern is apparently decided by the

nozzle aspect ratio only. The two highest aspect ratio nozzles (AR

=

524

and 263, Figs. 14a and b), for example, exhibited the focussed jet con-figuration and gave thrust forces approaching the ideal for equivalent

circular nozzles. Generally, the higher pressure ratios gave slightly higher augmentation ratios. The two lowest aspect ratios tested showed a slight but entirely positive base pressure distribution. This is believed due to a flow pattern of the type shown in Fig. 22, which can be considered to be one of a family of possible flow patterns (see Fig. 23) the general

shape and influence of which on base pressure seems to be determined by the

jet exit angle, 9_{0 ,}

In Reference 2 and elsewhere, when 9₀

=

00 the term

11 tu lip 11 (see Fig. 23) is used to describe the shape of the annular jet in

free air. As is now well known, a negative base pressure is set-up by

this form of the annular jet. It would appear that at some angle of

inclination, 90' of the annular nozzle the tulip or convex flow pattern is converted either into the conical or the concave pattern and a positive base

pressure is finally established. For the angle tested here, (9

=

-600₎

however, the magnitude of the base pressure thrust is still not sufficient to overcome the cosine loss of the jet exit momentum and the experimental augmentation ratio, Aex' therefore not only amounts to less than one but also to less than that for high aspect ratio focussed jets.

A transit through the flow pattern such as that of Fig. 23 would explain why there are no abrupt changes in the augmentation curves of the low aspect ratio nozzles.

5.2.4 The Concave or the Focussed Jet

High aspect ratio annular jets produce, almost independ-ently of pressure ratio, the highest augmentation ratios which, however,

abruptly . drop to values below unity (see Figs. 14a, b, 15 and 16) as

soon as the height above ground exceeds about 0.3 Do or 50 t. This drop and the observed large hysteresis loop could prove to be serious draw-backs to the practical usefulness of focussed jets.

At lower AR's annular jets (see Figs. 14c to f, 15 and 16), showaless abrupt or even gradual decrease of Aex with increasing

ground distance and the hysteresis loop disappears. It is suspected that

these characteristics are associated with the concave annular jet con-figuration (see Fig. 23). However, further testing is required to

establish this assumption. A special traversing gear for velocity, static pressure and flow angle measurements across the annular jet sheet or within the jet curtain cavity is contemplated. In addition, flow

VI. CONCLUSIONS

This study, one of the first systematic series of tests of a

e

~ 600 _{inwardly inc1ined annular nozZIe, has shown the need for a}

theoretical evaluatiQn of the influence of the vortex flow set up within the annular jet curtain. Until sorne allowance can be made for the reduction in effective jet momentum of the jet due to the action of these vortices and the associated entrainment, no good agreement of 'thin' or 'thick' jet theory with experimental results can be expected. What agreement has been reached in certain regions of h/Do and nozzle aspect ratio seems to be fortuitous.

Undoubtedly, the sudden focussing and unfocussing action of the inc1ined jet observed in these tests, together with the large

hysteresis effect, would present difficulties to the operation of practical ground effect machines of high aspect ratios at moderate ground heights

where this phenomenon was noticed, unless it can be controlled artificially.

On the other hand, the possibility of flying an annular focussed jet machine in free-air with no loss of thrust (due to adverse base pressure lift) is apparent, provided a suitably high pressure ratio and nozzle aspect ratio is used. However, such a machine would be less efficient in terms of thrust available per horsepower required.

1. Frost, J.C.M. Earl, T.D. 2. Von Glahn, U. 3. Chaplin, H. R. 4. Davenport, E. E. Kuhn, R. E. Sherman, 1 R. 5. Poisson-Quinton, Ph. 6. Tinajero, A. A. 7. Boehler, G. D. 8. Chaplin, H. R. 9. Pinnes, R. W. 10. Rethorst, S. Royce, W. W. REFERENCES

Flow phenomena of the Focussed Annular Jet. Princeton University Symposium October 1959.

Exploratory study of Ground Proxirnity Effects on Thrust of Annular and

Circular Nozzles. NACA TN 3982 April, 1957.

Theory of the Annular Nozzle in Proximity to the Ground. David Taylor Model Basin, Aero Report 923, July, 1957.

Static Force Tests of Several Annular Jet Configurations in Proxirnity to Srnooth and Irregular Ground. Langley Research Centre, NASA TN D-168, Nov. 1959.

Etude en Courant Plan sur Ie Principe des Plates-Formes a Effet de Sol. O. N. E. R. A. La Recherche Aeronautique, No. 71

July-Aug. 1959.

Comparison of Experimental to Theoretical Design Parameters of a 6 Inch Annular Jet Model with a Jet Angle of -450 _{Hovering in}

Proximity to the Ground and Experimental Results for Forward Flight.

David Taylor Model Basin, Aero Report 954 May 1959.

Aerodynam ic Theory of the Annular Jet. Part 1, Aerophysics Company, Washington, D. C., December, 1958.

Effect of Jet Mixing on the Annular Jet. David Taylor Model Basin, Aero Report,

953. February 1959.

A Power Plant ManIs Look at the Ground Effect Machine. NAVAER Research

Division, Report No. DR-1958, April 1959.

The Annular Jet and Thrust Augmentation. Vehic1e Research Corporation, Dec. 1958.

11. Rethorst, S. Royce, W. W. 12. Matthews, G. B. Wosser, J. L. 13. Strand, T. 14. Spreeman, K. P. Sherman, 1. R.

Lifting Systems for V. T. O. L. Vehicles Vehicle Research Corporation

1.A.S. Paper No. 59-123, June, 1959. Ground Proximity: A Critical Review. Office of Naval Research.

1. A. S. Paper No. 59-121, June 1959.

Exact, Inviscid, Incompressible Flow Theory of Statie Peripheral Jets in Proximity to the Ground.

Convair (San Diego)

Aerodynamics ZA-305, December 1959 Effects of Ground Proximity on the

Thrust of a Sim ple Downward Directed Jet Beneath a Flat Surface.

+

FIG.! THEORETICAL REPRESENTATION OF THIN ANNULAR JE.T

(EQUIVALENT TO TWO-DIMENSIONAL JET IF t IS ASSUMED

h

Theoretical - Thick Jet

FIG.3 THEORETICAL THRUST AUGMENTATION RATIO VS.

FIG.4 THEORETICAL THRUST AUGMENTATION RATIO VS.

FLOW MEASURIN c;. EX PANSION :JOINT ANNULAR ADDITIONA L PIThT-STAT1.C-. TUBE

i-

SETTL,N" LEN "TH

.1

,

DYNAMOMETER IFFU5ER. FLEXIBLE GrROUND BoARD \ PALDUSIE G-AS-TUR.BINE ENGINE

FIG.6 DIAGRAMMATIC LAYOUT OF COMPRESSED AIR F ACILITY

AND ANN ULAR JET TEST RIG

BASE PLATE CENTRE !-IOlE

I

1 1 ) \ \t--~ 1 1 CJl c:::.=g

DYNAMOMETER

B

_{GJ~.OUND ~OA.RD}

--...

~

0

-

- _I---- T

ANNULAR NOZ=LE

7

FIG.I0 DIAGRAM ILLUSTRATING METHOD OF LOAD (THRUST) CALIBRATION ... ~ ~ ~

_F

Ir / i\. --;

h.

!-Hl

~

..Hl

lI'

",

I

-,

t+

-Critical Pressure Ratio

t--I

Nozzle Press ure Ratio

f-+ I

_

\t}f~t:,

:t -_ ·,-FF~:j:lft:I:tH

JBif:p:t

F

'-Itdi n-.t# .

Height Above Ground, h/Do = 1/30

AR

=

524

67

. -r- .

r -:- r -r Nozzle Pressure Ratio

r-FIG. 12(a) EFFECT OF NOZZLE PRESSURE RATIO ON

A UG MENT ATION RATIO

263 176 151 132 PT-/p _{J a} : _{_ .} +1

-,

lt· H+ I-+~ '1:1: ' : -I:!=i+ .. ~ :. ··H+ t· ti

1·

1±~ .rW -"1t~~ - . :I

-!Ut

:-i:ti 'rl-

'f+J:

.

Cl!

it.d:!:· .

lt

l::

l4.

-i±

21±

H

Ili

~

..

lP:

til

I~: .

..:p:l~Iti!·':""fl-rl·l·t1-'· -+ ï -- h· ·r4-l..L-it .IX .

. f+I"

. ,tl IW +"~i " t ·(Imi.. I± i-i+f ~ ' I:ï: ' I± i±-V '-f·n

-,

-

'1;

':

rJ:Hi~ :'r:.;:t:'~", !ittJ' . ~ ~ I,1f. ~i

I

t.

.·:- .'

-

.'

rm

:I:rr -

': ,t -l!: ' : _., H " 1:1- -', " ' I:!:t-ll _:!). r't· - "Y !-r11l;'~-j-~"-'::llI l·I{~rt· '!l~' hl~

t·

r'

f{'

..

u : :~ i ' ! CP' jJ' ,; :~j.j '~H- " -. -J. Tft .,

8:

..

~r'

:

~It.~~:t·, i:t: _. ,"J_ .. 1+r ,H ,~~ - +1. - r+ ntntt, W: . 1+ + , , -H

.

..

J:

F.f: I' . '1 f.. ;:l. .. ". ,.... , ,:.,, :' ,,':['1':1'" -

t

" ~" I--l. 1· ... 11 - .-, H· .... ',""1-1- ...J . .,. -' .. +

FIG . 12(b) EFFECT OF NOZZLE PRESSURE RATIO ON

A UG MEN TA TION RATIO

AR = 524 263 176 151

~'ffitt tt::

r

+i-'- --tl:! --.;:i-n 1 ~ :!±i--·EJ:r,t,,' ,

'Tf:::

,i·r :. ~ l+ -: ,T

~.

_~

,

ti ~ -1+

Height Above Ground, h/Do = 4/30

Symbol

-, _{NOzzle Pressure Ratio}

-I

IT

FIG . 12(c) EFFECT OF NOZZLE PRESSURE RATIO ON

• L' + -f ,-. Annular .L Focussed r , r Nozzle -,

FIG. 13(a) EFFECT OF NOZZLE PRESSURE RATIO ON

A UG MENT A TION RA TIO

..

1/30 I 2/30

1-1

1

:_ 3/30 1-1+ CT 4/30 : ~ ,r, !V):

f+-H+

OOffiftl1f-i-

F

~

• 6/30 10/30 T r , - .-:=J:H:l=rH-H,-,-, H tttlt-

,H:

1 Ir,! Aspect Ratio

rttrrn

. . . :rjfl~ t-!:H LtiH -HH-ë!' . tt:t++iJ_-

_.

êtJ:J';;f'

,

H

~'Ifll, ~:_ij:-+j rrH-' t ,. -" ~::+ I ...l,i.' ~;··--IJ+

10/30 12/30

Nozzle Aspect Ratio

FIG.13(b) EFFECT OF NOZZLE PRESSURE RATIO ON A UGMENT A TION RATIO

h/Do

=

1/30 2/30 3/30 4/30 6/30 8/30 10/30

1+,. -H-I+f- -" '-;+1- I-H1++ ,-.' -1+ ' <+: -j' !-tIJ _{ftq +-H---}

r,d_

_'" -t - r

I

tL::-r.*

1+ !tIr.- ,

EEt1$!$1

-Ht 10/30 12/30

Nozzle Aspect Ratio

-j

FIG. 13(c) EFFECT OF NOZZLE PRESSURE RATIO ON AUGMENTATION RATIO 2/30 3/30 : 4/30 6/30 8/30 E • -j

T

10/30

12/30

Nozzle Aspect Ratio

FIG. 13(d) EFFECT OF NOZZLE P:RESSURE RATIO ON

AUGMEN TATION RATIO

1/30 2/30 3/30 4/30 6/30 8/30 Focussed

Hi+tlt++-t _. Eir4r 'H FIG. 13(e) 10/30 . 12/30

Nozzle Aspect Ratio

EFFECT OF NOZZLE PRESSURE RATIO ON A UG MENT ATION RATIO

2/30 3/30 4/30 6/30 8/30 Focussed

Theoretical - Thin Jet Theoretical - Thick Jet

Annular

Focussed

Theoretical - Thin Jet Theoretical - Thick Jet

Symbol

Annular (Unfocussed)

Focussed

t

L'

Theoretical - Thin Jet

Theoretical - Thick Jet

Annular

Focussed ~

Annular

++

-+-Theoretical - Thin Jet

Theoretical - Thick Jet

Concave 'G

Annular

Theoretical - Thin Jet

Theoretical - Thick Jet

PT./P Symbol

J a

Concave

Theoretical - Thin Jet

Theoretical - Thick Jet

,

-Nozzle Aspect Ratio Symbol

Annular

Concave

Focussed

FIG. 15(a) EFFECT OF GROUND PROXIMITY AND ASPECT RATIO ON AUGMEN TA TION RATIO

Nozzle Aspect Ratio Symbol

Annular

Concave

Focussed

FIG. 15(b) EFFECT OF GROUND PROXIMITY AND ASPECT RATIO

Nozzle Aspect Ratio Syrnbol

Unfocussed

Concave ~

Focussed

FIG.15(c) EFFECT OF GROUND PROXIMITY AND ASPECT RATIO

+t-,-+~~-, '0 _;+-l-:+, _{I..o..,..- "tUt} '-+ .' ~ :r~B ~~~~~~~~ j:;

Nozzle Pressure Ratio 1.4

H • ft"':~- ... , ... ·.LI~ ~ .!-" .. +t!-I_,-:...1l _+,+, .!:t: ~

Nozzle Aspect Ratio Symbol

Jl::E2:lri-+-4 ~I:±'

lli ~....J,...;..l,;;"";'~j 1+ Annular ~-rF

.,

-+ nu '.:ti ,I~o '.f j:: Concave ~ _'+' Focussed ±±:! LHili '++ +H~

~...I...9±:I±l30l±! t-,I:{

FIG. 16(a) EFFECT OF JET THICKNESS PARAMETER AND

ASPECT RATIO ON AUGMENTATION RATIO

'-+! ld l"

11

I

p

J1%

~-I±:t +1-'-8-1~ _tlir:!:: .4.0-0 l:H

lEW

_11~ '1 _;~

mrt

tT,.

0; , ''-

'

-

Im*

~~-+t :Et_., ffi

g:--

"."

i~~~ ;~ _1.~:tt:r!C . .:;::::-_::1: H

~

~-+"--I"""I!I '"' ~ .. ";i:J:J TT ' - - ' - I . _ _

.---1.,1--Noz'Zle Aspect Ratio

Annular

Focussed

FIG. 16(b) EFFECT OF JET TmCKNESS PARAMETER AND ASPECT RATIO ON AUGMENTATION RATIO

Concave ~

Nozzle Pressure Ratio = 3.0

Annular 1 • .',. tJ:j. 1-11-1 ···- I":='-~- -- . . . . .. --~ '---...

Nozzle Aspect Ratio /mSymbol

t:±I±±: I~

~F'~

Focussed

I..i=

FIG. 16(c) EFFECT OF JET THICKNESS PARAMETER AND ASPECT RATIO ON AUGMENTATION RATIO

.. 13t1m~~~++dl~~

I

W11'1·j1jti_llt

tH

I

+,'

I

.

_{j _!l} HIt G,tI·itth--H . . ,I

m

~ h-~

-

,

~ --+-~ UiliIb'! -;

o

I~

FIG.17 FIG.18

I

I

I ,I i ! Vortex \ ~

~r-.r-f'Î)

NO'ZZLE ~SE DIAMETER

~I

TYPICAL BASE PRESSURE DISTRIBUTION WITH UNFOCUSSED ANNULAR JET

5TAGNAT/ON STREAMlINE

VORTE't

Separated Flow Region

~~

,

~~

' -

~

~_.-/

\

{

I

)

I

'

\~

--I ,

Lb) FocV.sSED OR TREE. - TRIJNK JET

o

- ve

FIG.20

SE,PARATED Fl-DW RE Go/ON /'

NOZ'Z...LE BA ~E DIAMETE~

TYPICAL BASE PRESSURE DISTRIBUTION WITH FOCUSSED JET

Nozzle Aspect Ratio = 524

Nozzle Pressure Ratio = 1.4

Annular (Unfocussed)

Focussed

1 H ~

r ' . -f .

-ti-.

Nozzle Aspect Ratio = 524

Nozzle Pressure Ratio = 2.2

Annular (Unfocussed)

Focussed

-r ~ I

Nozzle Pressure Ratio = 3.0

Annular (Unfocussed)

Focussed

Symbol

Annular or Unfocussed

Max. PTj/Pa

Focussed

"

~

~

/

\ \

. I

I

\

\

/

/

\

\.

(

I

I

I

\

FIG.22 FLOW PATTERN WITH LOW ASPECT RATIO

Convex or Tulip

FIG. 23

Ground Effect Cases

I

+t=:

b

~

l

_

~

_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ ..1..-_ _ .. _ _ _ _ _ Concave

(Low Aspect Ratios)

Conical Focussed

(High Aspect Ra tios)

FLOW PATTERNS FOR CYLINDRICAL (Go = 0°) AND CONICAL (Go = -600)