CFD with FreeCAD!

[Chuck Rogers]

Nice. Let us know how you calculate CP from the CFD data, as it is a very abstract concept!

Integrate the forces acting on the surfaces of the rocket. For CNalpha and CP, primarily the pressure forces acting on the rocket. Of course you have to include the vector angle of each surface (relative to the centerline of the rocket) when applying the pressure to the surface. The CFD of course provides the pressure on the surface.


Charles E. (Chuck) Rogers

 

[Lt72884]

so here is my 2 cents. At the end of the day, if whatever software you use gives you accurate results, then thats all that counts. Its like the never ending battle between paintball and airsoft.. I dont care which one someone plays, i just like the fact they are out playing paint or plastic..

So for me, i LOVE cfd because of pretty pictures, BUT I LOVE openrocket for the mass amounts of data i get.

 

[Lt72884]

Integrate the forces acting on the surfaces of the rocket. For CNalpha and CP, primarily the pressure forces acting on the rocket. Of course you have to include the vector angle of each surface (relative to the centerline of the rocket) when applying the pressure to the surface. The CFD of course provides the pressure on the surface.


Charles E. (Chuck) Rogers

Depends on which solver is used. If simple foam is used, then this method works, but as soon as you use a high mach number, such as 0.6, then the solver HISA is used and that yields the REAL pressure and has to be done another way... found that out last night haha

 

[Buckeye]

Integrate the forces acting on the surfaces of the rocket. For CNalpha and CP, primarily the pressure forces acting on the rocket. Of course you have to include the vector angle of each surface (relative to the centerline of the rocket) when applying the pressure to the surface. The CFD of course provides the pressure on the surface.


Charles E. (Chuck) Rogers

Yep, that is all understood. More specifically, I am trying to figure out how to properly calculate CP when alpha=0. The pitching moment and normal force are zero in this case.

The answer probably lies in CNalpha. I need to brush up on this.

 

[drewnickel]

Yep, that is all understood. More specifically, I am trying to figure out how to properly calculate CP when alpha=0. The pitching moment and normal force are zero in this case.

The answer probably lies in CNalpha. I need to brush up on this.

Cp can be defined by either the pitching moment divided by normal force, or as the slope of the pitching moment vs alpha curve divided by the slope of the normal force vs alpha curve. The latter helps to get past the divide by zero error at alpha = 0, but requires you get a few points at some positive/negative but non-zero angles of attack and interpolating Cp location based off slopes.

 

[boatgeek]

Cp can be defined by either the pitching moment divided by normal force, or as the slope of the pitching moment vs alpha curve divided by the slope of the normal force vs alpha curve. The latter helps to get past the divide by zero error at alpha = 0, but requires you get a few points at some positive/negative but non-zero angles of attack and interpolating Cp location based off slopes.

It would be interesting to see what CFDOF comes up with for CP at a very small angle of attack, like 0.1 degrees.