Technological optimization of an airfoil for minimum drag operation at low Reynolds numbers.
Key words: airfoil technology, airfoil optimization, aeroacoustic wind tunnel, numerical simulation.
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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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.
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
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|Author:||Rugescu, Radu Dan; Tache, Florin; Tulita, Catalin; Chiciudean, Teodor Gelu|
|Publication:||Annals of DAAAM & Proceedings|
|Article Type:||Technical report|
|Date:||Jan 1, 2007|
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