Cd values in water

[VoyageofDiscovery]

I am designing a cofferdam in a river and was wondering if the Cd (Drag Coef) in water changes from values given for wind?

Thanks

VOD

 

[hacksaw]

Cd is based on dynamic similarity, so generally no.

 

[lionelp]

Cd is a function of the Reynolds number (Re), and will not be anything like constant at low values of Re so you have to evaluate it at the appropriate Re, but if its highly turb the value will be approximately independent of Re and hence whether its water or gas ( a factor of 15 difference)

 

[DLANDISSR]

VoyageofDiscovery:
The Drag Coefficient(Cd) is based on the Reynolds Number(Re)
The calculation of the Drad coefficient is based on Stokes law. The formual is:
Cd = 24/Re
The same formula is used for gas or liquid.

Calculate the Reynolds Number for the fluid and shape of the immersed body and plug it into Stokes law formula and you will get the Drag Coffieicnt.

Mark's Standard Handbook for Mechanical Engineers published by Mcgraw-Hill gives the procedure and a table of Drag
Coefficients for various shapes and Reynolds Numbers.

 

[phex]

correct me if i'm wrong, but isn't the equation

Cd = 24/Re

true only for ideal spherical bodies?

 

[DLANDISSR]

PHEX:
IT IS FOR SPHERES AND CIRCULAR DISKS

 

[phex]

thanks, thought so.

phex.