In 1981s, Gregory-Smith's study showed that Coanda Effect may occur in a certain ratio of jet width to local surface curve radius. the flow would separate from the ...
44th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit 21 - 23 July 2008, Hartford, CT
AIAA 2008-4593
Numerical Experiments on the lift generating mechanisms of the GFS UVA Haixin Chen1, Yu Sun2 School of Aerospace Engineering ,Tsinghua University, Beijing 100084, China
The flow field around the GFS UVA (Unmanned Vertical-taking-off Aircraft) is numerically simulated by CFD. The effect of the cowling is analyzed by comparison between configurations with and without it. The thrust enhancement mechanisms of the GFS UVA is explained.
I. Introduction n the year 2005, British engineer Geoff Hatton developed the so-called GFS (Geoff’s Flying Saucer) UVA (Figure.1). From the aerodynamics angle of view, such a novel UVA is essentially constructed by mounting a cowling under a lift fan. According to the GFS project’s web-site (http://www.gfsprojects.co.uk/index.html), this UVA is creating lift by “using the Coanda principle”, which makes it “have very little downwash and be aerodynamically stable”. The so-called Coanda effect was first proposed by Henry Coanda in early 20th century. It is a natural fluid dynamics phenomenon that if a stream is passing near a curve solid surface with not too large curvature, the stream will tend to leave its original direction and follow the surface. As to the GFS UVA, the axial flow driven by the upper ducted fan, because of the Coanda effect, will follow the curvature of the cowling mounted below the fan. But whether and why such a cowling can generate larger lift than a sole ducted fan? In this article, we try to explain these by a series of numerical experiments.
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Figure 1.GFS UVA and it’s designer (Picture from http://www.gfsprojects.co.uk)
II. Modeling and calculation A digital model is constructed to simulate the GFS UVA (Figure.2). The rotor of the ducted fan is NASA Rotor-67. We self-assigned an external curve, together with the original casing, to act as the duct for the rotor. As an extension of the Rotor-67’s hub, an inlet cone and a cowling are also designed. Such a model may not be a proper design for GFS UVA. However it can help us to understand its propulsion generation mechanism. The shape of the cowling below the fan is designed according to the geometry of Geoff Hatton’s lifting body model (Figure. 2). In 1981s, Gregory-Smith’s study showed that Coanda Effect may occur in a certain ratio of jet width to local surface curve radius. the flow would separate from the surface when the ratio is greater than a certain 1 2
Associate Professor, School of Aerospace Engineering, Tsinghua University, Beijing 100084, China. Master student, School of Aerospace Engineering, Tsinghua University, Beijing 100084, China. 1 American Institute of Aeronautics and Astronautics
Copyright © 2008 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
value. This research result is fully considered in the profile design of the GFS UVA’s cowling. In order to analyze the effect of tail cowling, comparison model without cowling is also constructed, as is shown in Figure. 3. A. Grid generation In the far-field grid generation, the meshes are created by IGG module of NUMECA. A hemisphere computational domain with a radius of five times the height of the fan blade is artificially generated at the entrance of the fan, while a cylinder computational domain with a radius of 10 times fan blade height and a length of 12 times fan blade height is constructed for the exit. The whole lifting system is enclosed by computational domain connecting import and export domain. Meshes in the fan channel are created by the AUTOGRID module of NUMECA. An H-I-H mesh topology is adopted. The grid node number for a single blade passage is 412,425. Only one blade passage is computed. The overall grid structure is shown in Figure. 4. The grid about the configuration without a tail cowling is shown in Figure. 5 .
Figure 2.The model with cowling
Figure 3.The model without cowling
Figure 4.Grid for the model with cowling
Figure 5.Grid for the model without cowling
B. Calculation The numerical simulation is conducted with the FINE module of NUMECA. The unsteady Navier-Stokes equations are solved, and the time format is for the unsteady state The cell-centered finite volume methods are used for spatial discretization. For temporal integration, NUMECA uses multi-step Runge-Kutta method. The one equation Spalart-Allmaras model is selected for turbulence stress closure. During the calculation, when the residual is less than 1.0 * 10-4, the convergence is considered to be achieved. The thrust in axial direction is calculated via integrating the forces on all the solid surface of the model. The fan’s total inner flux is obtained by integrating it on the fan grid’s export surface. As to the total axial flux caused by the UVA, including the ejector flow outside the fan, the integration is performed on the whole domain’s exit grid plane. The rotor shaft power is calculated by the torque integrated from the blade and other surfaces and the rotation speed. The whole UVA system is working in an open atmosphere. Different from the traditional fan computation in multi-stage condition, in which the working condition is usually defined by back pressure, the working condition in the present computation is set by changing the RPM of the fan.
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III. The results and analysis In Figure.6, the curves for axial thrust at different input power are compared between models with and without tail cowling for a hovering flow condition. It is very clear that with the cowling, the same input shaft power can generate higher lifting thrust, or in another word, the cowling produces higher propulsion efficiency.
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single ducted fan model
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Figure 6.The thrust vs. input power
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axial thrust(N)
Figure 7.The thrust vs. total axial mass flow ratio
In Figure.7, with the same thrust, the overall axial flux, from both inside and outside of the fan duct, of the GFS UVA model is much greater. The comparison in fig.8 further indicates that for the GFS configuration with the cowling, at the same mass flow through the fan duct, the overall mass flow is dramatically increased. As we all know, the turbo-fan engine get higher propulsion efficiency than the turbo-jet engine by increasing the mass flow rate and decreasing the exhaust speed. We suppose that the cowling increases the contact area of the ducted fan’s exhaust flow with its surrounding air. This makes a better ejector effect and caused a larger overall mass flow. In this ejector process, the kinetic energy left in the fan’s exhaust air flow is reused and the overall efficiency is therefore increased.
Figure 8. The thrust vs. mass flow ratio through the fan duct From the angle of force, people may naturally think that the cowling can provide additional thrust to the GFS UVA. However in Figure.9, the axial force in the direction of thrust integrated on the surface of the cowling, the additional lift, decreases with the increase of the input power. When the input power is large enough, the cowling’s additional lift can ultimately have a negative value. Even at this time, the total lift of the GFS configuration is still larger, as is shown in Figure. 10. The lift can only be generated on the surfaces of the fan blades and the cowling. It is obvious that with the cowling, the lift increase on the fan blade is overwhelming, not that on the cowling.
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8850
the foece on the cowling
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the total thrust
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Figure 9.Total thrust and the additional thrust on the cowling vs. power 1200 1100
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Fig 10.Torque vs. input power In Figure.10, at the same input shaft power, the torque of the GFS UVA’s fan is significantly larger than that of the sole ducted fan. This confirms that the load on the fan blades are increased by the cowling.
Figure 11.Pressure distribution when additional force is positive
Figure 12.Pressure distribution when additional force is negative In Figure.11 and Figure.12, static pressure contour are showed on meridian profiles for different cases in which the additional lifts on the cowling are positive and negative respectively. With the increase of the fan’s rotary speed, although the Coanda effect tends to increase the area of low pressure, due to the impact of the high-speed flow just
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from the duct on the near-axis region of the cowling, and also the decrease of the pressure on the lower side of the cowling, the total additional lift is decreased.
IV. Conclusion It has been confirmed that GFS UVA has a noticeable advantage of static thrust over traditional ducted fan. The cause of such an advantage is that the cowling increases the ejector effect on the surrounding air, and decreases the average jet speed of the whole system, in this way the propulsion efficiency is increased. The lifting force increment caused by the cowling is mainly from the fan itself. The additional lift from the cowling is very small and can even be negative.
References 1
Coppinger, Rob, 2006, “UVA PROJECT TAKES OFF”, Flight International. Vol. 169, no. 5022, pp. 24. 7-13 Feb. 2006. 2 Henri Coanda, 1936, “PROPELLING DEVICE”, United States Patent. 2,108,652, Feb.15,1938. 3 Charles A. Grotz, 1977, “BOUNDARY LAYER SCOOP FOR THE ENHANCEMENT OF COANDA EFFECT FLOW DEFLECTION OVER A WING/FLAP SURFACE”, United States Patent. 4,146,179, Mar.27,1979. 4 Lockwood, V.E., “LIFT GENERATION ON A CIRCULAR CYLINDER BY TANGENTIAL BLOWING FROM SURFACE SLOTS”, NASA Tech. Note D-244,1960.
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