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December 27, 2011, 03:28 
minimum force to start a motion

#1 
Member
s Kumar
Join Date: Jul 2009
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Hello everyone,
I have a 2D elliptical solid in rectangular fluid domain. I want to know what is the minimum horizontal force required to start off the horizontal motion. The fluid is highly viscous. Reynodls number is very low. Stokes law is applicable. Any idea, how to find the minimum force? Thanks in advance Doctsh Last edited by doctsh; December 27, 2011 at 03:46. 

December 29, 2011, 03:00 

#2 
Senior Member
cfdkid
Join Date: Mar 2009
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Hi,
Is object suspended in the fluid i.e are the vertical forces balanced? 

December 29, 2011, 03:42 

#3 
Member
s Kumar
Join Date: Jul 2009
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Hi. thanks for the reply. It is immersed inside fluid and free to move. I want to know how much minimum force I have to apply horizontally so that it can start moving horizontally. vertically no force is acting on it. weight and other inertial forces are negligble.


December 29, 2011, 04:07 

#4 
Senior Member
cfdkid
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Consider your object to be freely suspended i.e vertical forces are balanced , and the fluid is stationary.
Now, whatever Re you take if at all the fluid moves or has velocity in any given direction Then the object must move. Now , how would you answer " what is F for moving the object?". Any bare minimum force is enough, i suppose. Given i understood your Boundary conditions properly. But if you plan to move object against the flow direction, then "the F required will have to be greater then Drag Force". Regards, Kid Last edited by kid; December 29, 2011 at 04:31. Reason: there can be two cases as problem discription was not self explaintry. 

December 29, 2011, 04:58 

#5 
Member
s Kumar
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Yes. I got the answer from you.
My problem is little deeper. This is where I am confused. Ok I will explain my problem. I have an ellipse solid in stationary fluid. It keeps oscillating up down (prescribed moving wall BC). Due to this there is thrust generated laterally. I want to know how much lateral force is required to initiate horizontal motion through vertical flapping motion. The outer BC for fluid is to denote infinite extent. Last edited by doctsh; December 29, 2011 at 05:20. 

December 29, 2011, 06:17 

#6 
Senior Member
cfdkid
Join Date: Mar 2009
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kumar,
Suppose you solve this problem for n number of oscillation i.e accordingly you need to check time for run. Now we require to calculate drag coefficient i.e drag force on your object. and also conduct another simulation for steady state calculate and its drag coefficient and drag force. Use this data to match from simulation one and find out where it matches or becomes grater then the steady state drag force. That should be the minimum force required to move object in either direction. Would like to know your comment and suggestions. Regards, kid 

December 29, 2011, 06:25 

#7 
Member
s Kumar
Join Date: Jul 2009
Posts: 50
Rep Power: 8 
OMG
This is what I wanted. I am doing that. you are not cfd'kid', you are cfd'expert' 

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