A CFD comparison of the aerodynamic performance of two wing tips, a winglet and a raked tip, has been conducted. The first step of this study was the design of the wing and of the tip devices which was achieved to operate at a tr ansonic velocity (M=0.8).
The comparison was done in several parts. A near field flow study has shown the presence of interferences, which could lead to the formation of a strong shock wave at the junction between the wing and the winglet. A suction peak at the leading edge of the raked tip was observed, resulting in the production of thrust at the stations concerned.
A far field flow study exhibited the formation of a second vortex at the junction between the raked tip and the wing. This phenomenon must be due to a difference in the pressure characteristic over these two devices, resulting in instabilities in the vortex sheet. On the winglet, the flow rolls up over the tip device leading to the spreading of the tip vortex.
The comparison of the performances has shown great reduction in drag with the addition of the raked tip. The winglet has exhibited fewer advantages during this study, certainly related to the presence of a coarse mesh in the tip region resulting in a bad estimation of the strength of the vortex. Besides, the increase in performances related to the use of the winglet is highly dependant on the lift produced by the wing. Finally, the distribution of the loading over the wing mounted with the raked tip is more like triangular than elliptic, what is interesting as regards of the structure of the wing.
This study shows the advantages of using a raked tip on a wing. However, this study is clearly not complete because some inaccuracies have been introduced in the computations. First of all, the mesh should be refined in the tip region so as to be able to compute the tip vortex accurately and to carry out a more fair comparison of the two
tip devices. Then, the inner part of the wing should be added so as to see whether the bad data computed at the root disappear. The far field flow study has shown that the use of turbulence model was not the priority in this study and the Euler solver seems well adapted to deal with this sort of problems.
To end with, some problems were encountered when doing the swept wing design. It seems that not enough Weber stations are computed by the Sweptdes program and one should be aware of that when creating the database for a CFD purpose. This can be easily modified in the input data of the program, changing the default value (16 Weber stations) in 32 stations should be enough.
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