[Springroove]

Dear,

I am trying to calculate the pressure drag with an obtained pressure coefficient distribution according to: Cd_p(i) = Cp(i) * dy(i)

I have a geometry described by x and y coordinates, and dy is the difference in y direction between each point.
I therefore first calculate Cp*dy and calculate the area under the curve, which should be equal to the integral of Cp*dy. (Attached is the graph of the curve)


However, when I increase the number of points describing my geometry, automatically dy will be smaller as the distance between two points becomes smaller. Therefore, the whole Cp*dy curve shifts downwards leaving me with a smaller value of integral, hence smaller value of Cd which should not be the case.

I know I'm going wrong somewhere but can't seem to figure out where. Any help or advise is appreciated! Cheers!

[piu58]

I don't understand fully hat you try to calculate. But you have to use the same area (or line, in 2D) for calculation of the pressure. For a flow past a body you may use the whole surface. It doesn't count then how small the elements are.

[Springroove]

Thank you for your answer. I am trying to calculate the drag coefficient which is would be Cp*dy. But my area under the curve always changes with different dy, because I have Cp*dy on my y axis.

If I understand correctly, the drag coefficient is the integral of Cp*dy from the front to the back right

[piu58]

Originally Posted by Springroove

Thank you for your answer. I am trying to calculate the drag coefficient which is would be Cp*dy. But my area under the curve always changes with different dy, because I have Cp*dy on my y axis.

If I understand correctly, the drag coefficient is the integral of Cp*dy from the front to the back right

The numerical integral has smaller terms, but more of them.