Can a non streamlined body have lower Cd value?

I am trying to calculate the coefficient of drag from a simple geometry that can be seen as an projectile.

As I understood the more elongated a body is the better (so that there is no wake).

 

I simulated two different projectiles one as you can see in the mesh and one where the rear pyramidical end is cut off.

 


Apart from that I used the same settings. Including fmg-initialization from the video on flow over a airplane wing.

 

 

The second last is the full body, the last one is the cut projectile. As you can see in the editor the coefficient of drag is higher (worse) in the complete body and lower better where the end of the projectile is cut off.

Is the solution bad?

Or is there an effect at play that I don’t understand?

Answers

  • DrAmineDrAmine GermanyForum Coordinator

    Parasitic drag contains form and viscous drag components. Just double check convergence of drag forces and run for more iterations before judging the results again. Also please add pictures of the near wall mesh and whether you are resolving boundary layer properly: yplus and number of cells across boundary layer plus the wall grid density.

  • So i read forum entries on y+ and figured my mesh is not too great as it had no elements close to the wall. Now with the refined mesh the solution is not stable anymore.

    This is the new mesh. element size on the body is 0,01 m if that answeres your question. If not please show me where i can calculate wall grid density and co.

    Cd is now unstable (0.508677 printed)

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