unsteady feild changes to steady one
 

hi every body i'm modeling flow around a rectangular bridge's pier by fluent. there are many vortexes around this pier and they are unsteady and they change with time alternatively. they have their own special frequences. but fluent gives the average flow field and average drag factor.average drag factor have good accordance with average experimental results but average lift factor is exactly 10 times bigger than average experimental results.
why fluent changes unsteady phenomenon to steady one?
and why average drag factor is Ok but average lift factor is 10 time bigger than average experimental results.

nobody knows?!