Calculating pressure drop from drag

[nalasimbha]

Hi,

I am interested in calculating the pressure drop across rectangular cylinder. I know the coefficient of drag of the cylinder from the Cd vs Re plot and the drag force across the cylinder is given by

Fd = Cd * 0.5 * rho * V^2 * frontal area.

How do i calculate the pressure drop from the drag force? Is

dp = Fd / frontal area

a correct equation to use?

Thank you in advance

 

[BrianPetersen]

That will indeed give the average difference in pressure between the leading and trailing sides of the object in a free-flow stream (neglecting a minor amount of shear force along the sides), but for an object in a free-flow stream, the actual pressure distribution will be nowhere near uniform, particularly on the trailing side.

 

[Cockroach]

No, you need to do the control volume balance and figure out the stream velocity as a consequence of boundry inputs. Then you apply Bernoulli Equation.

The difference in pressure with the drag force and without the drag force would be dP.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[btrueblood]

Above about Re~5x10^4, the trailing edge will shed vortices, and pressure measurements made there will show strong fluctuations.

 

[nalasimbha]

Thank you for your inputs. If I am interested in a first order approximation of the pressure drop across the rect cylinder. Also I dont think the Re would go beyond 5x10^4.

In that case would the above equation be sufficient.

Also I would like to ask you if there is a way to convert Cd to fanning friction factor?