{"id":34346,"date":"2019-03-29T20:31:51","date_gmt":"2019-03-29T20:31:51","guid":{"rendered":"\/forum\/forums\/topic\/udf-wall-velocity-trouble\/"},"modified":"2019-03-29T20:31:51","modified_gmt":"2019-03-29T20:31:51","slug":"udf-wall-velocity-trouble","status":"closed","type":"topic","link":"https:\/\/innovationspace.ansys.com\/forum\/forums\/topic\/udf-wall-velocity-trouble\/","title":{"rendered":"UDF wall velocity trouble"},"content":{"rendered":"

UPDATE: the initial question has been answered, but another problem arose.  See further down.  Thanks.<\/strong><\/p>\n

I was going off of this example:  https:\/\/www.youtube.com\/watch?v=neJyTxLla0E<\/p>\n

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I have a very simple geometry, as shown below:<\/p>\n

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\"\"<\/p>\n

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 <\/p>\n

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I wanted the wall to move forward, which would push the fluid out the top,which is an outlet. So I used a UDF like this:<\/p>\n

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\"\"<\/p>\n

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To accomplish this, I set the wall to a Rigid Body Dynamic Mesh with that UDF.  It worked, but not like I naively expected, as shown below:<\/p>\n

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\"\"<\/p>\n

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After the first time step it error with:<\/p>\n

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Error: Update-Dynamic-Mesh failed. Negative cell volume detected.<\/span><\/p>\n

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Error Object: #f<\/span><\/p>\n

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I understand that I needed to have the adjacent wall shrink along with the movement of the moving wall so that the corner stays the same.<\/strong><\/p>\n

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Questions:  How do I do that?  Another UDF?  Which macro to use and how?  My ultimate goal is to have the bottom wall and the two adjacent walls all move towards the center of the fluid domain to create the pressure that drives the fluid out the top, for a much more complex geometry.  Is this possible and am I on the right track? 
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Thanks<\/p>\n","protected":false},"template":"","class_list":["post-34346","topic","type-topic","status-closed","hentry"],"aioseo_notices":[],"aioseo_head":"\n\t\t\n\t\n\t\n\t\n\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t