Fluid Mechanics - Drag coefficient and Pressure coefficient

AI Thread Summary
The discussion focuses on calculating the drag coefficient (Cd) from pressure coefficient (Cp) data obtained from airflow around a cylinder. The user has successfully plotted Cp against angle (Theta) and computed the integral for Cd, but their result of around 30 is significantly higher than the expected value of approximately 1.1. They seek clarification on the relationship between Cp and Cd, as well as input on their (Theta, Cp) values. The conversation highlights the importance of accurate integration methods and potential discrepancies in experimental data. Further insights into the relationship between pressure and drag coefficients are requested to resolve the issue before the lab deadline.
hammacks
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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?
 
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The expression for drag coefficient looks good. What are your (theta, Cp) values?

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