[blockMesh] When should I use mergePatchPairs?

yadongh · April 5, 2016, 19:49

Dear friends,

I am quite new to OpenFoam, and self-training myself via the user guide templates.

In the first template example, the 'Lid driven cavity', there is a part on the 'Graded mesh', where the the domain is divided into four block with different grading in each block. I can understand the 'blockMeshDict' file well except the last part, which is the 'mergePatchPairs'.

Why is this part blank? According to the later part of the manual, since we have four blocks, shouldn't the mergePatchPairs be used to deal with the connections between blocks? Or this means under some circumstances, this is not necessary?

Thanks a lot for any help!

Yadong

aow · May 31, 2018, 21:58

I'm assuming by now you figured it out, but for anyone else running across this post I'll give a short explanation.

The mergePatchPairs option is an alternative to the default method of "face matching" where nodes and mesh cells line up exactly (i.e. the numbers of cells in x and y directions on each face that is touching are the same for both faces and the corner nodes are the same too). In such a circumstance, you don't need to specify any additional face merging. This means you only need to use the mergePatchPairs when either the cell numbers don't match or the nodes aren't the same from one block (or hex as they're called in blockMeshDict) to another.

April 5, 2016, 19:49	When should I use mergePatchPairs?	#1
yadongh New Member Join Date: Apr 2016 Posts: 1 Rep Power: 0	Dear friends, I am quite new to OpenFoam, and self-training myself via the user guide templates. In the first template example, the 'Lid driven cavity', there is a part on the 'Graded mesh', where the the domain is divided into four block with different grading in each block. I can understand the 'blockMeshDict' file well except the last part, which is the 'mergePatchPairs'. Why is this part blank? According to the later part of the manual, since we have four blocks, shouldn't the mergePatchPairs be used to deal with the connections between blocks? Or this means under some circumstances, this is not necessary? Thanks a lot for any help! Yadong

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May 31, 2018, 21:58		#2
aow Member Andrew O. Winter Join Date: Aug 2015 Location: Seattle, WA, USA Posts: 78 Rep Power: 10	I'm assuming by now you figured it out, but for anyone else running across this post I'll give a short explanation. The mergePatchPairs option is an alternative to the default method of "face matching" where nodes and mesh cells line up exactly (i.e. the numbers of cells in x and y directions on each face that is touching are the same for both faces and the corner nodes are the same too). In such a circumstance, you don't need to specify any additional face merging. This means you only need to use the mergePatchPairs when either the cell numbers don't match or the nodes aren't the same from one block (or hex as they're called in blockMeshDict) to another.