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Old 03-05-2013, 12:17 PM
mitch10284 mitch10284 is offline
 
Join Date: Mar 2013
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Location: St Louis
Drag Forces on a Flat Face Barge

Hello. I'm working on constructing an anchoring system for Barges on the MississippI River in St Louis and need to determine the force the river will impose on an anchored barge. I'm using the Drag Force Equation

F = 1/2 x velocity^2 x density x Area x Cd

Assumptions

Drag Coefficients
Cd = 1.98 for the bow
Cd = .005 for bottom and sides

Density = 62.4 lb/ft^3 for water

Barge is 35'x195' and sits 11' in the water

I've been told by a few Barge Captains the river can reach 26 ft/s during a strong flood. I'm designing for worst case so I've used this number.

The problem I'm having is that the drag force numbers I'm getting seem to be way high.

Bow - 16,576,329 lb
Sides - 466,433 lb
Bottom - 742,053 lb

Hoping someone can confirm these numbers are incorrect and can help me out.

Thank You
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  #2  
Old 03-05-2013, 12:30 PM
Submarine Tom Submarine Tom is offline
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Isn't water density 1.0?

Don't discount windage.
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  #3  
Old 03-05-2013, 12:36 PM
mitch10284 mitch10284 is offline
 
Join Date: Mar 2013
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Location: St Louis
Using US Customary Units and the Mississippi is fresh water in St Louis.

Regards
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  #4  
Old 03-05-2013, 01:00 PM
TANSL TANSL is online now
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Can be Cd bigger than 1?
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  #5  
Old 03-05-2013, 01:39 PM
DCockey DCockey is offline
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Location: Rockport, Maine & Rochester Hills, Michigan
Quote:
Originally Posted by TANSL View Post
Can be Cd bigger than 1?
Yes. Nothing special about a Cd value of 1. The table in my old fluid mechanics textbook shows a drag coefficient of 2.00 for an infinitely wide flat plate normal to the flow, 1.20 for a rectangular flat plate with an aspect ratio of 5 normal to the flow, and 1.10 for a square plate normal to the flow. Those are drag coefficients for an object completely immersed in an "infinite" flow.
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  #6  
Old 03-05-2013, 01:44 PM
DCockey DCockey is offline
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Location: Rockport, Maine & Rochester Hills, Michigan
Quote:
Originally Posted by mitch10284 View Post
Hello. I'm working on constructing an anchoring system for Barges on the MississippI River in St Louis and need to determine the force the river will impose on an anchored barge. I'm using the Drag Force Equation

F = 1/2 x velocity^2 x density x Area x Cd

Assumptions

Drag Coefficients
Cd = 1.98 for the bow
Cd = .005 for bottom and sides

Density = 62.4 lb/ft^3 for water

Barge is 35'x195' and sits 11' in the water

I've been told by a few Barge Captains the river can reach 26 ft/s during a strong flood. I'm designing for worst case so I've used this number.

The problem I'm having is that the drag force numbers I'm getting seem to be way high.

Bow - 16,576,329 lb
Sides - 466,433 lb
Bottom - 742,053 lb

Hoping someone can confirm these numbers are incorrect and can help me out.

Thank You
You need to divide by the "density" of water of 64.4 lb/ft^3 by 32.2 ft/sec^2 to obtain the mass density of 2.0 slugs/ft^3. That will reduce the drag force numbers by a factor of 32.2.
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