Negative UDS value occurs in 3D concentration field simulation
 

Hi there,
Here are the details about my simulaiton:
This case is to simulate the concentration field in a flume with a small inlet connecting with a 45 degree slope, two outlet at the lateral side, and wall on the oppsite side. 1 UDS equations is define, and zero boundary condition is difine for the outlet and wall, and no source term is defined. But a small negative value occurs in most domain at the beginning of the iteration and grows up with the proceding.
Can anyone help with this problem?
Thank you!
Edison