Wind tunnel with strange corner mesh

Hi, I have faced a problem with wind tunnel formation. The corners are out of the cubic shape and stretch out like the attached image. How can I fix it?

Hey sina ,The number of grids on your X-axis is too small. Please increase the number of grids

Thank you @ZhengrongTao, it has solved but now I can’t increase the cell size higher than 10 to have a bigger grid. The error is "Attempted to divide by zero ?!

Please enlarge your wind tunnel area. This is because the wind tunnel area is too small to automatically grid