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Old Dec 15th 2014, 09:34 AM   #1
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Flow to move object in pipe

I've been hitting my head against a wall trying to figure out how to begin tackling this problem, maybe someone can help out.

I have an object in a pipe filled with fluid. I am tasked with finding the flow rate required to move this object initially (overcome the weight and static friction). I'm thinking I would just use a force diagram and somehow incorporate pressure into it... where would I begin to think about solving this... could someone help out.
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Old Dec 15th 2014, 10:33 AM   #2
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Too General to Begin!

Hi Jessica O,

Your premise is a little too general to begin.
Is any manner of sketch available? Is the object a "plug"
occupying nearly the entire cross-section?

Sorry, I can't help.

Good Luck TSH for PHF
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Old Dec 15th 2014, 11:16 AM   #3
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You're on the right track - from a free body diagram you can see that the force of fluid on the object must overcome static friction and the object's weight (if the intent its to move the object vertically). The force of the fluid may be diffiicult to determine, as it depends on the object's geometry and the nature of the flow around it (laminar, or not). But a good starting point is to use a general formula for drag caused by fluid flow:

F = (1/2) C_d A rho v^2

where C_d is the coeffeicient of drag, which depends on the object's shape,
A is the cross-sectional area of the object facing the direction of fluid flow,
rho is the density of the fluid
v is the velocity of the fluid.
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Old Dec 16th 2014, 07:30 AM   #4
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the problem i'm having is that I need to find the force that the fluid is exerting on the object.

Specifics: i have a large stainless steel bolt resting on the bottom of a carbon steel pipe in the horizontal position. The flow in the pipe will be way into the turbulent range eventually (but initially i guess it will start from zero velocity before i begin to pump, so it'll have to transition from laminar to turbulent). The interior diameter of the pipe (Dp) is greater than the diameter of the bolt face that the fluid is acting against (Db). Dp is 4.5" and Db=2" (bolt length is like 10" with a weight ~10lbm). If it were plugged flow I would be ok, but I know that the flow is going around the top of the bolt, so the force of the fluid acting against the face of the bolt is going to be different across the face of the bolt . I'm assuming this will take some sort of integral somehow... The flow will be going around the bolt like you see in textbooks for fluid moving past a body, but resting on a surface.

So how would i figure out what force is needed to begin moving that bolt (I'm reallly shooting for the pressure needed, but i can back that out easily)? Also, I'm using water, so won't buoyancy or lift come into play somehow too? Also, the contact surface area of the bolt coming into contact with the pipe? This problem is not exactly a textbook case, so I'm a little stumped...

Last edited by JessicaO; Dec 16th 2014 at 08:23 AM.
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Old Dec 16th 2014, 12:59 PM   #5
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"O" Jessica

Your thinking includes facts and your knowledge of flow regimes. The bolt is BIG and Dp small. its a complicated flow past that bolt.
It seems the only thing you can ask is... "for bolt static initially (with flow = 0) then thereafter as flow increases what "least flow" puts movement to the bolt. (How else can we pursue this?)

Use an Open system, momentum of steady-flow equation (speed unknown):

Consider "IN" (well upstream) and "OUT" (well downstream). Being water, V, A rho and are the same "IN" and "OUT."
I think only pressure would be different.

Now you have to represent the drag force on the bolt. Guess its projected area, Cd.. to be X V^2/2. Set the value so side wall friction (that's a guess.) is defeated.

The answer to your question is statistical not analytical. For there to be an "analytic" response needs massive approximation.
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