Fluctuating Scaled Residual Plot
 

I am doing the flow analysis of Francis turbine(hydro turbine).

While iterations, the scaled residual plot as well as static pressure curve at inlet to the turbine, both comes out as fluctuating curves (like sinusuidal curve)continuously for more than 6000 iterations, and solution do not converge.

Can anybody let me know what can be the probable reasons for this?

Thanks in advance.

Regards.