13.ijaest vol no 5 issue no 1 a parametric study on ambient pressure effects on super circulation...

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A parametric study on ambient pressure effects on

super circulation over a simple ramp

Valeriu Drãgan―POLITEHNICA‖ University Bucharest, Faculty of Aerospace Engineering Str. Gheorghe Polizu, nr. 1, sector 1, 011061,

Bucharest, Romania

E-mail: [email protected]

Abstract — This paper describes a study of the lift effect

obtained by super circulation, an aerodynamic effect discovered

by Henri Coandã that relies heavily on the Coandã effect.

A parametric two dimensional CFD study has been carried out

with two goals in mind, the primary goal was to see the impact of

the ambient pressure on the super circulation effect and also a

secondary goal to investigate the super circulation processes

themselves. Early parametric studies have been performed by

various authors however the parameterizations provided in the

available literature is applicable only to some particular aircraft

configurations. The value of this study is that it provides a bare

geometric parameterization that can be used in a wider variety of

applications from aircraft lift and actuators to fluidic actuators

and machinery. The tests showed no dependency between the

ambient pressure and the super circulation effect which

encourage us to state that an aeronautical application – that must

operate both at high and low altitudes- is feasible. Further study

has shown that the injector fluid is accelerated by the curved

ramp at higher velocities than those of the injector, providing

more leads for further refinement of our understanding of the

phenomenon itself.

Keywords-super circulation, Coandã effect, k-omega SST

I. I NTRODUCTION

In 1932, the Romanian aerodynamicist Henri Coandã proposed a new heavier than air lift concept, the ―lenticular

aerodyne‖. The principle used to achieve lift is now called―super circulation‖ and it is, in part, owed to the Coandã effectthat helps maintain a stream of fluid to a nearby wall.

Although the Coandã effect is necessary to achieve super circulation, it is not sufficient, i.e. in order to achieve afavorable pressure gradient we need to use curved surfaces

such as cylinders.During the years, many attempts have been made to

blend the lenticular aerodyne’s concept into conventionaltube-wing aircraft, the most famous examples are the Antonov

An-72 and An-74 and the Boeing YC-14. These aircraft usedthe cold by pass flow of their turbofan engines to generate acombined Upper Surface Blow (USB) that provides significantlift, yielding lower take off and landing velocities.

Even if such aircraft have proved their commercial-and often military- use, the lack of publicly available

parametric studies prevents the use of concept to it’s fullest

potential.Data provided in [1] and [2] are of significant value

as starting points, however they can only model one aircraft

configuration and offering little details on how themechanisms of super circulation work. Thuslly, in order togeneralize the applications of lift achieved by super

circulation, parametric tests have to be made to insure at leasta semi-empirical set of basic design equations.

Perhaps one of the most famous equations used todescribe super circulations is the momentum coefficient :

Cμ=T/qS (1)

WhereT represents the static thrust of the engine

providing the USB systemq is the dynamic pressure of the free streamS is the super circulated surface aria

The denominator includes the dynamic pressure of the free stream of air, which means it is more suitable for describing aircraft landing and taking off than the hovering

capability of Coandã’s original demonstrator – a lot of thetimes this equation proves very valuable when dimensioning a blown flap system, per se.

Key aspects such as injector stream velocities,

curvature radii, ambient pressure must be taken into account indetermining weather or not a super circulation application is preferable to a conventional lift system and under what

circumstances it is viable over the flight envelope of theapplication.

Another aspect that make parameterization of this

aerodynamic effect difficult is numerically modeling thedetachment of the boundary layer from the cylindrical ramp.

It is common knowledge that a turbulent boundary

layer is less likely to detach from a wall than a laminar boundary layer, therefore various viscous models will yieldvarious points of flow separations.

In this paper we will try to investigate the influence

of the ambient pressure, at zero true air speed (TAS) on the pressure decrease over the super circulated ramp, consideringthe same injector velocity using various viscosity models byComputational Fluid Dynamics (CFD).

Valeriu Drãgan et al. / (IJAEST) INTERNATIONAL JOURNAL OF ADVANCED ENGINEERING SCIENCES AND TECHNOLOGIESVol No. 5, Issue No. 1, 094 - 104

ISSN: 2230-7818 @ 2011 http://www.ijaest.iserp.org. All rights Reserved. Page 94



II. THE CFD SIMULATION

A. The setup of the CFD tests

Although it is not the intended purpose of this paper,a brief analysis of the viscosity models has been made prior tothe full test in order to insure as much accuracy as a

conventional viscosity model can offer.In the literature [3], [4]. the viscosity model most

commonly considered to predict boundary layer separation isthe k-omega. This model has a lower turbulent productionthan the k-epsilon and therefore can capture more accuratelythe detachment of the boundary layer.

The general layout of the test can be seen in Fig.1,

where we can observe the pressure outlet walls, the injector defined as a velocity inlet and the cylindrical ramp.

A 90° arc was selected due to practicality reasons:1.from past experience, a 100m/sec flowing jet will

not stay attached for much longer than the 90° of the ramp2.if we were to consider that there is an even pressuredistribution across the ramp’s span,

the resulting force will not be useful, as its lateral componentswill nullify each other as seen in Fig.4.

A sensible argument can be made that the higher the

curvature, the higher the pressure gradient we will most likelyobtain, however an optimum will be reached because of thefact that a fast flowing jet will become detached quicker on ahighly curved wall than on a lower curved wall therefore the

parameter to be optimized in this case will have to be the product between the pressure gradient obtained and thecircumferential length of the attached fluid.

Knowing the influence of the ambient pressure over the pressure gradient obtained over the span of the ramptrough super circulation is important in two key aspects:

1.Calculating the effectiveness and efficiency of asuper circulation system with altitude2.Calculating the prospect of having the super

circulation effect used by high pressure pneumatic systems

such as fluidic actuators as described in [9].The injector inlet velocity was intended as high as

possible while within the incompressible domain of theworking fluid-which generally is though to be below a Mach

number of 0.3, resulting in our case in an injector velocity of 100 m/sec. Hence a pressure based solver was employed.

The high of the injector for this, two dimensionalstudy, was chosen to be h=10 cm and the ramp radius R=50cm, being close to 12.7% of the ramp span.

Four viscosity models were initially tested and brieflyanalyzed before the full test in order to decide which onewould most likely give the best approximation for the flowseparation.

B. Discutions on the viscosity models

K-epsilon RNG with pressure gradient effect near walltreatment

The flow immediately begins to split in two regions: a regionthat attaches itself to the curved ramp and another one, clearly

affected by the pressure outlet boundary proximity. On firstglance, the re-attached flow may be neglected giving the falseimpression that this turbulence model predicts flow separation

quicker than the k-omega, which is counter intuitive. Uponcloser inspection we can observe that the re-attached flow hassignificant effects, remaining attached to the ramp for its

entire span – which was to be expected from this model.One indicator that shows this is not the best way to model theflow is the fact that the boundary effect is quite intense, hencethe necessity to generate a larger domain which in turn implies

a higher time expense.A final remark that needs to be made is that the rapid pressuredrop visible at 72 cm of the ramp’s span is caused by a vortex

meaning that, perhaps a more precise result may be obtained by a nonstationary simulation.

Reynolds stress 5 equation model with pressuregradient effect near wall treatment

The pressure plot indicates multiple vortices forming in the

immediate vicinity of the injector. It is the sensible thing toassume that vortices are an expression of the Kelvin-Helmholtz interaction that manifests when two fluids withdifferent velocities have a common interface. This is the prime

noise generating mechanism for jet engines. Experience hasshown that a time dependent nonstationary analysis yields

more accurate results.Positive aspects of this model are the lack of influence bothfrom the boundaries and from the underside of the ramp.However, the model does not predict attachment to the curved

wall at this velocity of 100 m/sec. Knowing that the velocityof the injected air is critical in achieving lift trough super circulation, lowering it further from 100 m/sec makes littlesense.

Spalart Allmaras strain/vorticity based production

In this model, the boundary effect is virtually nonexistent, thefluid gets practically no parasitic influence from the pressure

outlets.The underside of the ramp has also no parasitic effect, leavingthe flow unaltered.Also, it is remarkable that the fluid remains completelyattached to the ramp for it’s entire span and that the drop in

static pressure generated almost identical to that predicted bythe k-omega SST model.Apart form the fact that it cannot predict flow separation, this

model is very close to the k-omega.





K-omega Menter (shear stress transport SST)

This viscosity model is the most commonly used to model the

Coandã effect and related phenomena because it can predict both the attachment of the flow to the ramp and it’s eventualseparation from it.In this case we can observe some degree of boundary effect onthe flow as it exits the ramp however it has no practical

influence on the pressure gradient obtained on the ramp itself.The trajectory of the fluid as it exits the ramp is not along thetangent line to the ramp’s curve at the separation point butrather slightly diverted away from the ramp. This is because of the influence of the jet thickness that will be discussed further.

Also the vortex near the right hand side of the domaininfluences the exit trajectory in a converse manner.

III. R ESULTS

A first observation that can be made is that the ramp

accelerates the injector fluid in a region of approximate onethird of the total jet thickness. The velocity increase is of

approximate 5-7% of the initial injected velocity.Close observation of the pressure and velocity plots

shows close correlation between the velocity increase and the pressure drop on the ramp.

Detachment of the flow is most likely influenced bythe thickness of the injector jet, primarily because of theinertia of the other two thirds of the flow.

An interesting aspect of super circulation is the highlift obtained, i.e. the lift of the super circulated ramp is higher than the thrust provided by the injector jet – which isconsidered to be fully expanded and hence has only impulsecomponent.

In this particular case, the calculations give:

Lift by super circulation=1690.2172 [N]Injector thrust=1202.03125 [N]

As the title suggests, the focus of this study was to

appreciate the impact of the ambient pressure on the super circulation effect. The results are shown in Fig.22, 23 and24,clearly indicating the independence of pressure gradientsacross the super circulated ramp with ambient pressure.

IV. R EMARKS AND CONCLUSIONS

The main goal of this investigation was to determine

weather or not ambient pressure has an effect on the lift

obtained by super circulation at zero velocity of the far field.There were no significant differences between the pressure drop obtained across the ramp at any of the tested

ambient pressures, meaning that any optimal geometrycalculated for one particular ambient pressure will, mostlikely, be an optimum for any other ambient pressure.

This is a remarkable property because it opens the

way for large aeronautical applications as the super circulationlift is proven not to fluctuate its efficiency with altitude.

Total pressure plots have been made in order to

insure that the energy within the injected flow is not affected

by the ambient pressure variations. It also has shown to beindependent of the ambient pressure both at low pressures andat high pressures.

Another remarkable fact is that the lift calculated for this super circulated ramp is significantly greater than thethrust of the injector bare flow. The significance of thisfinding can potentially be greater than just augmentingaerodyne lift, it could open the way for new types of jet engine

nozzles, although experimental confirmation will be required before further speculating on this last prospect.

Further studies are required and may include nonstationary analysis, thinner injector jets, ramp rugosity effects,temperature effects.

Figure 1. Computational domain discretisation

Figure 2. Injector-ramp discretisation refinement





Figure 3. Injector disctretisation

Figure 4. Lift distribution over a 180° ramp





Figure 5. K-epsilon pressure plot

Figure 6. K-epsilon static pressure detail

Figure 7. K-epsilon velocity plot

Figure 8. K-epsilon velocity detail





Figure 17. K-omega SST pressure plot

Figure 18. K-omega SST pressure detail

Figure 19. K-omega SST velocity plot

Figure 20. K-omega SST velocity detail





Figure 21. Ramp static pressure distribution by viscosity models





Figure 22. pressure distribution over a supercirculated ramp k-omega SST

Figure 23. Pressure drop at various ambient pressures perfectly overlap


ISSN: 2230-7818 @ 2011 http://www.ijaest.iserp.org. All rights Reserved. 3



Figure 24. Pressure drop plots at low ambient pressure also overlap

A. Authors and Affiliations

―POLITEHNICA‖ University Bucharest, Faculty of Aerospace Engineering Str. Gheorghe Polizu, nr. 1, sector 1,011061,Bucharest, Romania

ACKNOWLEDGMENT

The work has been funded by the Sectoral OperationalProgramme Human Resources Development 2007-2013 of theRomanian Ministry of Labour, Family and Social Protectionthrough the Financial Agreements POSDRU/88/1.5/S/60203.

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degree Bachelor of Science – California Polytechnic State University

[4] DANIELA BARAN, NICOLAE APOSTOLESCU, ―ALOAD - a code

to determine the concentrated forces equivalent with a distributed pressure field for a FEM analysis ―INCAS Bulletin no 4 2010

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[6] Skavdahl, Howard; Wang, Timothy; and Hirt, William J.: „Nozzle

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[8] T.Welsh (Boeing) 1984 US4426054

[9] J. Glass (Cava Industries) US3589382

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