External Fluid Flow -- Immersed Body Friction Drag

In summary, the coal barge 1000 ft long and 100 ft wide is submerged in a depth of 12 ft in 60°F Water. The barge is being towed at a speed of 12 mph. The friction drag on the barge is estimated to be DFC= (1/2)*ρ*∪2*b*L*CDf.
  • #1
pistolpete
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Hello all, I have browsed this forum occasionally when I had questions and threads showed up on a search. I have been stumped by a problem, so I decided to create an account and see if anyone could help me out!

1. Homework Statement

A coal barge 1000 ft long and 100 ft wide is submerged in a depth of 12 ft in 60°F Water. It is being towed at a speed of 12 mph. Estimate the friction drag on the barge.

Homework Equations


When treating the submerged part of the barge as Flat plate the equation for Drag Friction is :

DF=(1/2)*ρ*∪2*b*L*CDf

Since the submerged part of the barge would have three "faces" that are parallel to the flow direction (2 side faces of 1000ft*12ft, and one bottom face of 100ft*1000ft) would I add up the total area of the submerged faces parallel to the flow to use in the above equation?

The only examples I can find have only the bottom part of the plate, but they are floating so that seems logical to me.

The Attempt at a Solution



Converted the 12 mph to ft/s then Calculated Reynolds # to find laminar/turb regions and use appropriate eqns to Find the CDf term

Using found values and properties of water @ 60°F solve for DF
 
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That value for Cd would work to find the total drag force (due to skin friction and pressure drag), but this question asks just for the friction drag and the drag coefficient that would need to be used (or assumed and then iterated with Renolds # until convergence) is CDf not Cd I believe.
 
  • #5
Nidum said:
Pdf
Thanks for that, that makes thing a bit more clear! From that I see the area is selected depending on the application and in the case of the barge it implies that you would use wetted area (all submerged faces in th fluid) which would include the front and back faces (the two faces normal to the direction of flow). It makes sense to me to use this area if you were using a value of CD and finding the total drag.

Since friction drag is due to the shear at the wall, and these faces are normal to the direction of flow (cant support shear in the direction of drag force) so am I right in saying these faces should not be included in the Area when finding the frictional drag only?
 

FAQ: External Fluid Flow -- Immersed Body Friction Drag

1. What is external fluid flow?

External fluid flow refers to the movement of a fluid (such as air or water) over an object that is immersed in it. This can occur in various scenarios, such as the flow of air over an airplane wing or the flow of water over a ship's hull.

2. What is an immersed body?

An immersed body is an object that is partially or completely submerged in a fluid. This could be a solid object, such as a ship or a submarine, or a fluid-filled container, such as a water tank.

3. What is friction drag?

Friction drag is a type of drag force that is caused by the friction between a moving object and the fluid it is immersed in. In the case of external fluid flow, this drag is caused by the resistance of the fluid against the surface of the immersed body, which slows down its movement.

4. How does friction drag affect external fluid flow?

Friction drag plays a significant role in external fluid flow as it can greatly impact the speed and efficiency of an object's movement through a fluid. The higher the friction drag, the more energy is required to overcome it, leading to increased fuel consumption and decreased overall performance.

5. What factors affect the amount of friction drag in external fluid flow?

The amount of friction drag in external fluid flow is influenced by various factors, including the shape and surface roughness of the immersed body, the density and viscosity of the fluid, and the speed of the fluid flow. Additionally, the presence of any external disturbances or turbulence can also affect the amount of friction drag experienced by the immersed body.

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