problem in large scale model
 

my model is 5000*5000*100000 (mm*mm*mm), and there is a hole (diameter 50mm *100000mm) in the center, the hot fluid flows in the pipe. the mesh is very coarse at the outside of the model about 100mm, and about 10mm in the innerside. the velocity distribution is abnormal, that is the speed at the part of inlet is right, but it becomes zero slowly at about 10 meters. the velocity at the inlet is about 100 m/s, the k-e model is high renolds, what's the matter?