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andrea chan (andreachan)
New member
Username: andreachan

Post Number: 2
Registered: 12-2009
Posted on Friday, January 15, 2010 - 11:52 am:   

I want to set one side of the boundaries like this:

It is a spline.
the spline goes through three points: A,B and C.
the points A and C are fixed. But point B moves over time. That is, the displacement of point B is a function depending on time.

Is such setting in FlexPDE possible?
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Marek Nelson (mgnelson)
Moderator
Username: mgnelson

Post Number: 172
Registered: 07-2007
Posted on Friday, January 15, 2010 - 04:20 pm:   

No and Yes...

The spline boundary is only used in the initial grid generation and once computation starts, the spline is not recalculated.

But, if you can construct an expression that describes the node velocities you want along that boundary, you can use FlexPDE's moving mesh capability to do this.

Also note that three points in a spline is likely insufficient to give you a reasonable shape. Since the FlexPDE script language does not have a mechanism for defining the end slopes of the spline, FlexPDE uses the end pairs of points to determine the end slopes. You should use two close points at each end of the spline to define the end slopes.
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andrea chan (andreachan)
Junior Member
Username: andreachan

Post Number: 3
Registered: 12-2009
Posted on Saturday, January 16, 2010 - 03:19 pm:   

Thanks Marek,
I am a FlexPDE beginner.
I want to know how to set the node velocities on the boundary.
I have seen the moving mesh examples in the manual. But it seems that we can only set the side velocities on the boundary, but not the node velocities.
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Marek Nelson (mgnelson)
Moderator
Username: mgnelson

Post Number: 174
Registered: 07-2007
Posted on Saturday, January 16, 2010 - 08:35 pm:   

When you define a moving mesh, FlexPDE only applies velocities to nodes of the finite element mesh. The FlexPDE scripting language only has provision for defining velocities along boundary paths. So you will be defining the velocity of the nodes on a specific boundary. You can do this in terms of local physics or an explicit function of position.
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andrea chan (andreachan)
Member
Username: andreachan

Post Number: 4
Registered: 12-2009
Posted on Thursday, January 21, 2010 - 08:09 am:   

thanks Marek,

now the problem is solved.

But I want to confirm a question. When I use moving mesh in FlexPDE, then should I change the equations in the EQUATION part?
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Robert G. Nelson (rgnelson)
Moderator
Username: rgnelson

Post Number: 1320
Registered: 06-2003
Posted on Thursday, January 21, 2010 - 02:46 pm:   

FlexPDE assumes that all equations are stated in the stationary frame, and correction terms are automatically applied if the mesh is moving.

You can force the equations to be interpreted as moving-frame equations by declaring
LAGRANGIAN EQUATIONS
instead of EQUATIONS
This supresses the automatic correction terms.

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