Wind simulation
 

Hello,
I have a problem in simulating wind in an open area. the geometry is a 3D cube (1000*100*100 m). the boundary condition is velocity inlet (for inlet wind :5 m/s constant), pressure outlet (for outlet wind), the bottom plane is wall, left and right planes are symmetry and top plane is wall (shear stress=0).
after simulation, velocity couture is as below.
as you can see, the velocity is not constant and there is a velocity profile in outlet! please help me to have a constant wind velocity profile. thank you so much. (wind direction is from right to left)
https://s6.uupload.ir/files/untitled34_3bh3.png