problem in simulation of velocity field using mixture model
 

Hello everyone!
I am simulating a velocity field of a discharge from a pipe at the bottom of a lake in a 3d model. And this lake is of density stratification with high salinity and zero sediment concentration. But the discharge from the pipe has a low salinity and low sediment concentration.
After I used the mixture model to simulated the case of difference in density caused by salinity. I found that the variables fields( salinity, velocity, turbulent viscocity etc.) are different from those solved by single fluid method. (PS:scalar equation is used to solve the trasport of the salinity, and the density of fluid is defined using udf)
Does anynoe know why? Please share your knowlege! thank you!
if you need details about the case, please contact me!