Boundary Resistant
 

Dear expect:

I have a question that want to ask, the description might a little long, hope someone who can help, please.

Here is a rectangular channel like figure(1), these is two 1m width wall at the side, with a 20C water and no roughness at side wall.
Attachment 28538
figure(1)

I give a steady flow about 1400(cms) from the left boundary through this channel then outflow from right boundary

Then use two methods to make meshes, first is to make meshes without blue wall ( fig(2) ), another is two make meshes include wall ( fig(3) ).

Attachment 28539
figure(2)

Attachment 28540
figure(3)

Then cut the downstream cross-section and watch the Y-Z 2D free-surface plot( fig(4) and fig(5) )

Attachment 28541
figure(4)

Attachment 28542
figure(5)

Obviously, method-1's free surface plot is very smooth, but method-2's plot has a drop at right side.

Could some answer me why these two plots are different? Cause I have closed the wall roughness so there might no drag.

Hope someone could help me, thank you so much.