A parametric study on ambient pressure effects on ...

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

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]

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 the mechanisms of super circulation work. Thuslly, in order to generalize the applications of lift achieved by super circulation, parametric tests have to be made to insure at least a semi-empirical set of basic design equations. Perhaps one of the most famous equations used to describe super circulations is the momentum coefficient :

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

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 the times 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 in determining 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 the application. Another aspect that make parameterization of this aerodynamic effect difficult is numerically modeling the detachment 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 yield various 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, considering the same injector velocity using various viscosity models by Computational Fluid Dynamics (CFD).

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INTRODUCTION

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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 effect that 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 a favorable 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 conventional tube-wing aircraft, the most famous examples are the Antonov An-72 and An-74 and the Boeing YC-14. These aircraft used the cold by pass flow of their turbofan engines to generate a combined Upper Surface Blow (USB) that provides significant lift, yielding lower take off and landing velocities. Even if such aircraft have proved their commercialand often military- use, the lack of publicly available

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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.

Cμ=T/qS

(1)

Where

T represents the static thrust of the engine providing the USB system q is the dynamic pressure of the free stream S is the super circulated surface aria

@ 2011 http://www.ijaest.iserp.org. All rights Reserved.

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THE CFD SIMULATION

K-epsilon RNG with pressure gradient effect near wall treatment The flow immediately begins to split in two regions: a region that attaches itself to the curved ramp and another one, clearly affected by the pressure outlet boundary proximity. On first glance, the re-attached flow may be neglected giving the false impression that this turbulence model predicts flow separation quicker than the k-omega, which is counter intuitive. Upon closer inspection we can observe that the re-attached flow has significant 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 the flow is the fact that the boundary effect is quite intense, hence the 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 pressure drop 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 pressure gradient effect near wall treatment

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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 to the 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 is the k-omega. This model has a lower turbulent production than the k-epsilon and therefore can capture more accurately the 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 ramp 2.if we were to consider that there is an even pressure distribution across the ramp’s span, the resulting force will not be useful, as its lateral components will 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 likely obtain, however an optimum will be reached because of the fact that a fast flowing jet will become detached quicker on a highly 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 the circumferential length of the attached fluid. Knowing the influence of the ambient pressure over the pressure gradient obtained over the span of the ramp trough super circulation is important in two key aspects: 1.Calculating the effectiveness and efficiency of a super circulation system with altitude 2.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 the working 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 dimensional study, was chosen to be h=10 cm and the ramp radius R=50 cm, being close to 12.7% of the ramp span. Four viscosity models were initially tested and briefly analyzed before the full test in order to decide which one would most likely give the best approximation for the flow separation.

B. Discutions on the viscosity models

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The pressure plot indicates multiple vortices forming in the immediate vicinity of the injector. It is the sensible thing to assume that vortices are an expression of the KelvinHelmholtz interaction that manifests when two fluids with different velocities have a common interface. This is the prime noise generating mechanism for jet engines. Experience has shown that a time dependent nonstationary analysis yields more accurate results. Positive aspects of this model are the lack of influence both from 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 velocity of the injected air is critical in achieving lift trough super circulation, lowering it further from 100 m/sec makes little sense. Spalart Allmaras strain/vorticity based production In this model, the boundary effect is virtually nonexistent, the fluid gets practically no parasitic influence from the pressure outlets. The underside of the ramp has also no parasitic effect, leaving the flow unaltered. Also, it is remarkable that the fluid remains completely attached to the ramp for it’s entire span and that the drop in static pressure generated almost identical to that predicted by the k-omega SST model. Apart form the fact that it cannot predict flow separation, this model is very close to the k-omega.


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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 eventual separation from it. In this case we can observe some degree of boundary effect on the 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 the tangent line to the ramp’s curve at the separation point but rather 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 domain influences the exit trajectory in a converse manner. RESULTS

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A first observation that can be made is that the ramp accelerates the injector fluid in a region of approximate one third 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 by the thickness of the injector jet, primarily because of the inertia of the other two thirds of the flow. An interesting aspect of super circulation is the high lift obtained, i.e. the lift of the super circulated ramp is higher than the thrust provided by the injector jet – which is considered to be fully expanded and hence has only impulse component.

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by the ambient pressure variations. It also has shown to be independent of the ambient pressure both at low pressures and at high pressures. Another remarkable fact is that the lift calculated for this super circulated ramp is significantly greater than the thrust of the injector bare flow. The significance of this finding can potentially be greater than just augmenting aerodyne 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 non stationary analysis, thinner injector jets, ramp rugosity effects, temperature effects.

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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 gradients across the super circulated ramp with ambient pressure. IV.

Figure 1. Computational domain discretisation

REMARKS 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 geometry calculated for one particular ambient pressure will, most likely, 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 circulation lift 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

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Figure 2. Injector-ramp discretisation refinement


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Figure 3. Injector disctretisation

Figure 4. Lift distribution over a 180° ramp

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Figure 7. K-epsilon velocity plot

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Figure 5. K-epsilon pressure plot

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Figure 6. K-epsilon static pressure detail

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Figure 8. K-epsilon velocity detail


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Figure 11. Reynolds stress velocity plot

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Figure 9. Reynolds stress pressure plot

Figure 12. Reynolds stress velocity detail

Figure 10. Reynolds stress static pressure detail

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Figure 13. S-A stress pressure plot

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Figure 15. S-A stress velocity plot

Figure 16. S-A velocity detail

Figure 14. S-A stress pressure detail

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Figure 19. K-omega SST velocity plot

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Figure 17. K-omega SST pressure plot

Figure 20. K-omega SST velocity detail

Figure 18. K-omega SST pressure detail

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Figure 21. Ramp static pressure distribution by viscosity models

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

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Figure 23. Pressure drop at various ambient pressures perfectly overlap

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A. Authors and Affiliations

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

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

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ACKNOWLEDGMENT

The work has been funded by the Sectoral Operational Programme Human Resources Development 2007-2013 of the Romanian Ministry of Labour, Family and Social Protection through the Financial Agreements POSDRU/88/1.5/S/60203.

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REFERENCES

[1]

[2] [3]

[4] [5] [6] [7] [8] [9]

Phelps, Arthur E.; Letko, William; and Henderson, Robert L.: „LowSpeed Wind- Tunnel Investigation of a Semispan STOL Jet Transport Wing-Body With an Upper- Surface Blown Jet Flap‖ NASA TN D7183, 1973. M.H.Roe, D.J.Rensealer, R.A.Quam et al, ― STOL Tactical Aircraft Investigation-Externally Blown Flap- Vol.2, Design Compendium‖ Daniel White, 2010, ―Analysis of curvature effects on boundary layer separation and turbulence model accuracy for circulation control‖applications . in partial fullfiment of the requirements for the degree Bachelor of Science – California Polytechnic State University 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 US3971534 Skavdahl, Howard; Wang, Timothy; and Hirt, William J.: „Nozzle Development for the Upper Surface - Blown Jet Flap on the YC-14 Airplane.‖ Automot. Eng., Apr.-May 1974. [reprint] 740469, SOC. Spence, D. A.: „The Lift Coefficient of a Thin, Jet-Flapped Wing.‖ Proc. Roy. SOC. (London), ser. A, vol. 238, no. 1212, Dec. 4, 1956, pp. 46-68. T.Welsh (Boeing) 1984 US4426054 J. Glass (Cava Industries) US3589382 .


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