Air flow study on the NERVA space launcher aileron.

Tache, Florin ; Rugescu, Radu Dan ; Bogoi, Alina 等

1. INTRODUCTION

The NERVA-entitled project is a Romanian endeavour to place a small satellite in orbit and cope with everything related to this goal (Rugescu, 2008). A simple, modified, old military rocket can be a good alternative to the inability of reusable systems, like NASA's space shuttle, to acquire a low cost capability of launching scientific and commercial payloads in Low Earth Orbit (LEO). The scientific research project is well under way, having highly-trained participants from the Faculty of Aerospace Engineering in University "POLITEHNICA" of Bucharest, Romania, qualified engineers from related industry fields and military personnel for consultancy purposes.

The paper focuses on the study of the airflow around the launch vehicle guiding ailerons by means of CFD simulations.

2. CASE BACKGROUND

A simplified 3D model of the rocket has been constructed, paying a special attention to the point of interest for the current study, namely the movable red ailerons in the back of the space vehicle shown in Figure 1. These "small wings" help guide the rocket during its ascent through Earth atmosphere.

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By rotating the ailerons around their axes, the rocket can be conveniently guided while there is still sufficient air around it. Ascent parameters have been set and from the many different flight regims of the vehicle's journey into orbit, one has been selected for thorough study. For future research, an extension of the study to several other important ascent regims is envisaged.

3. ASCENT REGIM INTO CFD SIMULATIONS

The selected case finds the rocket at an altitude of 11km, travelling at a speed of Mach 2, while surrounding pressure and temperature are 22632 Pa and 216.65 K, respectively.

The aileron has a thin hexagonal root section that gradually transforms into a smaller diamond-shaped tip section, with "almost sharp" leading and trailing edges over its entire span. Aileron's height is 286 mm, while the chord varies between 445 and 110 mm from root to tip and the thickness from 21 to 8 mm.

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For CFD simulations, four different equaly spaced sections have been used, along with a 3D model of the complete aileron, thus being able to study the 2D air flow at different locations on the wing's span and also having an overall image of the three- dimensional flow parameters distribution.

4. COMPUTATIONAL GRIDS AND SETTINGS

For each 2D section, as well as for the entire 3D model of the aileron, six different cases have been studied. While keeping the flow parameters to the values presented earlier, the angle of incidence has been varied from 0 to 20 degrees by 5-degree increments. In addition, the maximum pitch of 28 degrees has been considered. In total, 24 two-dimensional cases and another 6 three-dimensional cases have been simulated with the commercially available software named Fluent.

As high-supersonic speeds are involved, the computational grids are of utmost importance (Seebass & Woodhull, 1998). Carefully constructed grids have been used to allow the simulation of the flow to be as accurate as possible. Around each 2D section, a structured grid made of 101075 quadrilateral cells has been constructed (average cell count; exact number depends on the 2D section). A special attention has been paid to the boundary layer region and to the leading and trailing edges of each airfoil. The 3D grid is also a structured one, but made of roughly 590000 tetrahedral cells. A more thorough grid, made of 3D quadrilateral cells is intended for future study, which may include the entire rocket model.

The k-omega turbulence model has been adopted for the simulations. The use of a k-omega formulation in the inner parts of the boundary layer makes the model directly usable all the way down to the wall through the viscous sub-layer. The model switches to a k-epsilon behaviour in the free-stream and thereby avoids the common k-epsilon problem that the model is too sensitive to the inlet free-stream turbulence properties. Authors who use the k-omega model often praise it for its good behaviour in adverse pressure gradients and separating flow. Nevertheless, the k-omega model produces a bit too large turbulence levels in regions with large normal strain, like stagnation regions and areas with strong acceleration, but this tendency is much less pronounced than with a normal k-epsilon model (CFD-Wiki, 2009).

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A low Courant number value was used to start the iterations, and it was gradually incresed during the simulations. A low Courant number does not allow a steep residuals decrease, but the solution that advances towards convergence is as stable as possible (FLUENT INC., 2009). The convergence criteria for the numerical residuals were set to [10.sup.-5], considering this sufficient enough for an accurate flow parameters description. As a confirmation, the Drag and Lift coefficients variations became horizontal by the time the residuals had reached the aforementioned convergence criteria.

5. RESULTS AND DISCUSSIONS

The computing power employed for conducting these simulations included several PC's. The 2D simulations were not so resource-needy, so systems with a single-core processor and 1 GB of RAM memory prooving sufficient to lead to convergence in a decent period of time (around 1200 iterations on average, performed in 3-4 hours for each considered case). However, the 3D aileron simulations called for a more powerful system with three-core processor and 4 GB of RAM, in order to produce a result in an adequate period of time for each incidence angle considered. For studying aileron-fuselage interference and the entire rocket model, the platform has been recently upgraded to a faster mainbord and memory modules, practically doubling the system's memory speed and capacity.

Following charts show Drag and Lift variations with the angle of incidence, and Lift variation with Drag for 2D sections and complete 3D aileron. Airfoil geometry influence is minimal, as differences between sections are hardly distinguishable. However, the numbers vary for the 3D model.

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Next pictures show examples of flow parameters (air speed and pressure) around the aileron (2D sections & 3D body).

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Simulations have shown that the fluid accelerates rapidly in the trailing edge region, especially around the tip section. As expected, the Lift coefficient of the three-dimensional aileron is lower than that of any of the 2D sections, mainly due to the effects of a finite span wing, unlike the infinite span considered in 2D cases. On the other hand, the Drag coefficient of the 3D object was expected to be higher than those obtained in the 2D simulations, but the curves depicted in Figure 4 show otherwise. This will be thoroughly investigated by considering aileron-fuselage interference in a future study.

6. CONCLUSIONS

Prior to practical application of the NERVA small orbital vehicle, understanding the rocket's behaviour during air flight and accurately predicting its performance is mandatory. The obtained numerical results are promising and encourage further study, extending the research to other flight regims, including very high altitudes and velocities and, eventually, incorporating the entire 3D model of the rocket into the CFD simulations.

Like any numerical study of fluid flows, these results will ultimately be validated by wind tunnel tests on a reduced-scale model of the NERVA rocket. In time, they will help in the development of a simple, yet low-cost and reliable system, capable of placing small-size payloads in Earth orbit.

7. REFERENCES

Haroldsen, D. J. & Sturek, W. B. (2000). Navier-Stokes Computations of Finned Missiles at Supersonic Speeds, 22nd Army Science Conference, Renaissance Harborplace Hotel, Baltimore, MD, 11-13 December 2000

Rugescu, R. D. (2008). NERVA Vehicles, Romania's Access to Space, Scientific Bulletin of U. P. B., Series D in Mechanics, 70, no. 3, pp. 31-44

Seebass, R & Woodhull, J. R. (1998). Supersonic Aerodynamics: Lift and Drag, paper presented at the RTO AVT Course on "Fluid Dynamics Research on Supersonic Aircraft", held in Rhode-Saint-Gendse, Belgium, 25-29 May 1998, and published in RTO EN-4.

***CFD-Wiki, the free CFD reference (2009). SST k-omega model, http://www.cfd-online. com/Wiki/SST_k-omega_model, accessed 2009-05-20

*** FLUENT INC. (2009). Fluent Help, accessed 2009-05-17