Tek-Tips is the largest IT community on the Internet today!

Members share and learn making Tek-Tips Forums the best source of peer-reviewed technical information on the Internet!

  • Congratulations TugboatEng on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!

Cd values in water

Status
Not open for further replies.

VoyageofDiscovery

Structural
Joined
Apr 7, 2002
Messages
617
Location
CA
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
 
Cd is based on dynamic similarity, so generally no.
 
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)
 
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.
 
correct me if i'm wrong, but isn't the equation

Cd = 24/Re

true only for ideal spherical bodies?
 
Status
Not open for further replies.

Part and Inventory Search

Sponsor

Back
Top