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Is this the correct approach to my problem? (UDF for object in motion)

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Old   October 25, 2013, 07:45
Question Is this the correct approach to my problem? (UDF for object in motion)
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Dean Smith
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I need to simulate a small object in a still pool of liquid. A force is exerted on the object pushing it downwards into the fluid a certain distance and eventually stops.

I am currently looking into writing a UDF for this motion with a dynamic mesh. Is this the correct approach?
The UDF essentially defines the motion of the object correcT?
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Old   October 25, 2013, 12:11
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I don't use dynamic mesh but I was simulated couple of moving objects. For example for moving heat source you must add for your equation "dx" element. In my case dx=v*t, where v is velocity and t is current time step. Now you need to put that into your equation:
original equation - l=(p/a)*pow(2,x)
with dx l=(p/a)*pow(2,(x+dx));
That is of course example You must adapt that for your case.
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Old   October 26, 2013, 05:19
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Quote:
Originally Posted by vekh View Post
I don't use dynamic mesh but I was simulated couple of moving objects. For example for moving heat source you must add for your equation "dx" element. In my case dx=v*t, where v is velocity and t is current time step. Now you need to put that into your equation:
original equation - l=(p/a)*pow(2,x)
with dx l=(p/a)*pow(2,(x+dx));
That is of course example You must adapt that for your case.
I don't understand.

You didn't use a dynamic mesh?
Wouldn't this result in an inaccurate mesh when simulating the movement?

What equation do you mean? From what I can tell FLUENT only allows very simple movement in the boundary condition settings without coding a UDF
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Old   October 27, 2013, 06:39
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Quote:
Originally Posted by charlestan88 View Post
I don't understand.

You didn't use a dynamic mesh?
Wouldn't this result in an inaccurate mesh when simulating the movement?
In my opinion dynamic mesh is needed only when you move physical object i.e. rock in water. In my case I move only heat flux so I don't need that.
Quote:
Originally Posted by charlestan88 View Post
What equation do you mean? From what I can tell FLUENT only allows very simple movement in the boundary condition settings without coding a UDF
Yes - without UDF It's difficult to move anything I used UDF with DEFINE_PROFILE function to define moving heat source along determined path in my model.
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Old   October 27, 2013, 06:52
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Quote:
Originally Posted by vekh View Post
In my opinion dynamic mesh is needed only when you move physical object i.e. rock in water. In my case I move only heat flux so I don't need that.

Yes - without UDF It's difficult to move anything I used UDF with DEFINE_PROFILE function to define moving heat source along determined path in my model.

Any idea where I can learn how to write a udf for motion?
The tutorials from abyss fluent seem to be of little use.
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Old   October 27, 2013, 07:09
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I've learn it from this and other CFD forums. Youtube contains some helpful videos too. That take some time but there is no other way. If you know how to write simple C or C++ code you should learn UDF's quite fast. You must also be familiar with problem you want to solve. Equations which are included in UDF must be write properly. Additional options (like move or smth like that) must be done with precautions. To 'move' something there is often only one condition - to add some time dependent variable to equation.
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