(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)