# Fluid Mechanics - Drag coefficient and Pressure coefficient

1. Jun 26, 2008

### hammacks

Hey guys,
I'm working on a lab here about pressure variation normal to streamlines. For the last experiment, there is airflow around a cylinder with a static pressure tap. The pressure tap records Cp (divides by dynamic pressure) and angle of rotation.

I have data points for Cp vs Theta (10° increments) that looks great, just like it should. Now I need to calculate Cd. After looking on the web a lot I found something that said

Cd=.5*INT(Cp*cos(x) dx, 0, 360(or 2pi))

I have computed this integral as an area under the curve both in Excel as Riemmen Sums and with a fit line on my calculator. Both ways I get answers around 30. I thought I was fine...

Now I have a lab due tomorrow and found out Cd should be more like 1.1 or so. Any ideas? Is there another relationship between Cp and Cd?

2. Jun 26, 2008

### caslav.ilic

The expression for drag coefficient looks good. What are your (theta, Cp) values?

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Chusslove Illich (Часлав Илић)