Fluid Mechanics - Drag coefficient and Pressure coefficient

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SUMMARY

The discussion focuses on calculating the drag coefficient (Cd) from pressure coefficient (Cp) data obtained from airflow around a cylinder. The user, Chusslove Illich, has computed Cd using the integral formula Cd = 0.5 * INT(Cp * cos(x) dx, 0, 360), resulting in an unexpectedly high value of approximately 30. This discrepancy suggests a misunderstanding or misapplication of the relationship between Cp and Cd, as the expected value should be around 1.1. The conversation seeks clarification on the relationship between these coefficients and requests additional data points for comparison.

PREREQUISITES
  • Understanding of fluid mechanics principles, specifically drag and pressure coefficients.
  • Familiarity with integral calculus, particularly in the context of area under curves.
  • Experience with data analysis tools such as Excel for numerical integration.
  • Knowledge of airflow dynamics around cylindrical objects.
NEXT STEPS
  • Research the relationship between pressure coefficient (Cp) and drag coefficient (Cd) in fluid dynamics.
  • Learn about numerical integration techniques for calculating areas under curves, including Riemann sums and trapezoidal rule.
  • Explore the use of computational fluid dynamics (CFD) software for accurate drag coefficient calculations.
  • Investigate experimental methods for measuring drag coefficients in wind tunnel tests.
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Students and professionals in fluid mechanics, aerospace engineers, and researchers involved in experimental aerodynamics who are looking to understand the relationship between pressure and drag coefficients in airflow scenarios.

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?

--
Chusslove Illich (Часлав Илић)
 

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