cherry-pick 56b49f3

(cherry picked from commit 1c1c12968df801d995d39e0aaa4d2950eee7a73a)

(cherry picked from commit 4d49e04593989485e1c3710fe497fb45ae41d588)

(cherry picked from commit 77aca325)

before only one channel side could have a velocity boundary condition (bc). When setting a bc at a second side, the first bc was overwritten.
Velocity bcs were fied in the previous commit (bda2c544271657adfabb63e9ae974e77692566a6). This commit fixes the boundaryQs for velocity.

(cherry picked from commit 811e344b)

before only one channel side could have a velocity boundary condition (bc). When setting a bc at a second side, the first bc was overwritten.

(cherry picked from commit daa787a3)