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Jared Barber (jared_barber)
Member
Username: jared_barber

Post Number: 20
Registered: 01-2007
Posted on Thursday, August 02, 2007 - 11:44 am:   

Hello,

I've attached the code "Boundary_Prob" which includes a 2-d cylinder being pushed by fluid through a 2-d tube. The cylinder is rather close to the edge of the tube and there is a gap there of about .0001 units. In this case it seems that FlexPDE assumes the cylinder and wall of the tube are connected as is evidenced by the plot elevation(u) around the cylinder. In particular, since the plot elevation(u) around the cylinder has only one point where u is zero (and everywhere else u has magnitude significantly larger than 0), it seems FlexPDE simply connects the very bottom point of the sphere to the tube and leaves everything else alone.

For the codes that I am running, inevitably the "cylinder" will get very close to the edges of the "tubes". When it does so, it is unrealistic for the velocities of the points nearest the wall to be zero.

It seems there is a tolerance which, when the gap between two boundaries is narrower than that tolerance, the two boundaries are joined (in some way). If there is, what is that tolerance and can it be changed? (In the problem I've sent you, the tolerance appears to be .001 but I don't know if that tolerance will change in my other codes...e.g. is it an absolute tolerance or a relative tolerance depending on the size of the geometry being considered.)

Are there any other suggestions that can be made for how to fix this sort of problem?

application/octet-stream
Boundary_Prob.pde (2.2 k)


Thanks,
Jared
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Robert G. Nelson (rgnelson)
Moderator
Username: rgnelson

Post Number: 924
Registered: 06-2003
Posted on Thursday, August 02, 2007 - 04:53 pm:   

In version 5.0.18, we implemented a controllable merge distance with the selector MERGEDIST.

Apparently, we did not get all the merge tests modified, and this control does not work in your problem (because the critical measurement is the closest distance between a line and an arc, and this test appears not to use the user mergedist).

However, if you break the bottom line of the outer boundary at a point directly below the circle, the test becomes joint-to-joint proximity, and works correctly.

(We will complete the implementation of MERGEDIST in version 5.0.19)

See the attached script.
application/octet-stream
Boundary_Prob1.pde (2.3 k)

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