Abrupt Pressure Drop Value in Mesh Independence test
 

Dear All,
I am simulating a Pipe flow with and without turbulators inserts, using water properties as function of temperature.
I'm doing grid independence test .
All the errors of parameters like Nusselts number , Heat transfer coefficient , bulk mean teperature , outlet velocity, outlet temperature etc decrease with increase in cell count even though after 36lac mesh count all become stabalize but the problem is with pressure drop whose error increases with increase in mesh count,
I am using realizable k-e scalable wall function model with Y+= 11.25. and tetrahedral mesh.
and I using prandtl eq{ (1/f^1/2) = 2.0 log (Re( f^1/2))- 0.8} to calulate friction factor analytically and then finding pressure drop with simple results of Bernoulli eq.
Please help me out with pressure drop abrupt dependence with cell count....
Thanks in advance