# Strange velocity direction in the rectangular channel

The attached file “wrong_v_direction.zip” is a simple example with only one flow boundary condition (Fig3). I use it to check the flow direction. However, the simulated flow direction contradicts common sense. Specifically, the flow is “into” the channel, however the velocity is “out” (Fig1 & 2). Could anyone explain it to me?
Fig1_wrong_v_direction.png
Fig2_View3D.png
Fig3_flow_boundary.png
wrong_v_direction.zip

This issue is related to the cell angle. Somehow you have rotated the cells all to 90 degree as attached. We could recommend you use a tool like CVLGrid to generate grids so that the cell orientation is correct when you generate your grid. There are also tools to adjust the angle or reverse the I J directin in EE but they didn’t really help in this case: https://eemodelingsystem.atlassian.net/wiki/spaces/EEREF/pages/2380337/Grid+Tools+and+UtilitiesYou can use CVLGrid to reverse the I or J direction however (edited for clarification).

Thanks! I also received your letter which is so helpful! Just like what I said in the write back, I was wondering: (1) why the grids will be in wrong rotations because I already use CVLGrid? (2)How to check the cell rotation before the happening of “wrong velocity direction”? I do not know how to distinct which “rotation” is correct.

This is a very good question. In my experiences, we need to check the I,J Map to define the rotation correctly. Note that in EE: - Increasing number of i direction means from West to East.- Increasing number of j direction means from South to North.You can check it in the 2D Viewing Plan/ Cell Indices … and Show I & J’s or tick to Show East and Show South.In case your i,j map is in a wrong direction, you need to use the Transpose I-J Map or Re-order I-J Map tools which are available in the main menu.Cheers,

Excellent！ Your suggestion hits the mark! The reason of the wrong velocity direction in my case is because of the wrong direction of “I map”. When reversing the I direction using CVLGrid, the model simulates the correct velocity direction.