Technological optimization of an airfoil for minimum drag operation at low Reynolds numbers.

Rugescu, Radu Dan ; Tache, Florin ; Tulita, Catalin 等

Abstract: Debates were produced around the Dolphin airfoil, to prove the efficiency of this design concept at supersonic speeds. The application for aircraft low take-off and landing speeds or enhanced operational efficiencies of air compressors were only mentioned, while the manufacturing technology was not considered at all. The goal of the present work is to introduce design and technological improvements of dihedral profiles for aircraft low speed operation. The attention is also focused on vanes and blades applications, e.g. in the heat exchanger of the aeroacoustic wind tunnel WINNDER, a new concept of the authors for aeroacoustics and energy generation. Due to a common research of the University "POLITEHNICA" in Bucharest and of Texas A&M University, USA, computer simulations are presented in support of the newly optimized profile with good drag characteristics at very low speeds. Conclusions are drawn for incorporating these airfoils in various industrial applications.

Key words: airfoil technology, airfoil optimization, aeroacoustic wind tunnel, numerical simulation.

1. INTRODUCTION

Technological optimization of the airfoil by numerical and wind tunnel runs is performed to remove the impractical shape of the Dolphin design. Two classical airfoils have been simulated through a commercial CFD code and the airflow around corresponding dihedral profiles was computed for comparison. As a result, technological enhancements are proposed to improve the low Reynolds numbers operation of the optimized airfoil, with realistic manufacturability.

2. CLASSICAL AND DIHEDRAL AIRFOILS

Passing from a round leading edge airfoil to a sharp edge is not as easy as it may sound (Rugescu 2002, Tulita 2002). There are two major aspects that need to be carefully considered. The first, inherent challenge is to manufacture a truly sharp leading edge. More and more wings incorporated in actual airplanes have a composite structure, where the airframe is basically composed of beams, stringers and ribs, usually made of carbon fibers - epoxy resin composites. The profile shell is commonly made of aluminum alloys or glass/carbon fiber composites that could also be stiffened by filling the shell with a honeycomb structure. So the leading edge and trailing edge of the wing can be either round, if a single part is used for both the lower and upper sides of the wing, or blunt if two different parts are used to materialize the envelope. In the latter case the two parts are stuck together, to form a single leading or trailing edge. In either case, the result is an easy to manufacture dihedral edge, with a certain thickness.

A second important aspect is related to the complications that appear when calculating the airflow around a dihedral airfoil. The computational grid can no longer be similar to a grid used for calculating a common, round, leading edge airfoil. So an entire series of grid adaptations have to be implemented in order to obtain convergence to a solution that would eventually be validated by experimental results.

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Figure 1 compares the classical, round leading edge airfoil the dihedral and the Dolphin one.

3. NUMERICAL SIMULATIONS

In order to confirm the advantages of a dihedral airfoil over a classical one at very low air speeds, two known, classical airfoils have been chosen for comparison, namely the NACA-0009 and NACA-0012 airfoils, with their corresponding dihedral counterparts. Due to their symmetry and widespread use, the selected airfoils constituted a very good test bed for the optimization.

Experimental results were available for Mach 0.15, therefore the numerical simulations involved boundary conditions with air speed of approximately 50 m/s. The tables below show CFD results obtained after several thousand iterations for each particular case. The structured grid used for numerical integration of PDE-s that describe the airflow around airfoils is split into up to 105000 quadrilateral cells, depending on the profile design (Bucur et al., 1983).

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The following charts depict the drag and lift coefficients for the two classical airfoils. Corresponding charts for the dihedral airfoils are to be computed during further research.

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The next pictures illustrate the flow around the NACA-0009 airfoil and its dihedral counterpart for an upstream air speed of Mach 0.15 and 0o angle of incidence. Differences between the air flow around a classical airfoil and its dihedral counterpart can be easily observed.

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4. CONCLUSIONS

The flow fields around the rounded and dihedral airfoils are strongly different, confirming the markedly different aerodynamic behavior. The location of the inflexion points on the dihedral profile is important, with emphasis on the continuity of the generating curve. A second order continuity was here used by cubic splines (Micula, 1978). The lift to drag ratio is improved, but further simulations and eventual tests in a wind tunnel will need to be carried out in order to confirm the optimum profile characteristics. As it is well known, the focal point of most airfoils lies between the leading edge and the first 25% of the wing chord. This may completely change in case of the dihedral airfoils.

5. REFERENCES

Trapp, J, (2002), Aerodynamic Profiles Geometry Program, DLRpage, Available from: www.pagendarm.de/trapp/programming/java/profiles/NACA4.html Accessed: 2005-06-25

Micula, Gh. (1978), Spline functions and applications, Ed.T., Bucharest

Bucur, C. M., Popeea, C. A., Simion, Gh. (1983), Numerical Calculus, Ed. D.P., Bucharest

Rugescu, R. D. (2002), Design Enhancements for Noise Suppression in Aircraft Compressors, Proceedings of the Anniversary Scientific Session ISBTeh-2002, Bucharest, June 07-08, 2002

Tulita, C., Benard, E., & Raghunathan, S. (2003), Transonic Periodic Flow Subjected to Adaptive Bump, The Queen's University Belfast, Great Britain, Paper AIAA-2003-444, 41st Aerospace Sciences Meeting and Exhibit, Reno, Nevada, Jan. 6-9, 2003

Tulita, C., Raghunathan, S. & Benard, E. (2002), Control of Transonic Periodic Flow on NACA-0012 Aerofoil by Contour Bumps, Proceedings of IUTAM Symposium Transsonicum IV, Goettingen, Germany, 02-06 September 2002