Why there is fluctutaion in residual. Did somebody has observed this?. Pratap

Typically this is a result of trying to solve an unsteady problem with a steady-state solver, like when there's vorticy shedding. Sometimes the model will be able to average out the shedding, but that's dependent on the solver settings, the turbulence model, and your mesh.

Goodluck, Jason

Yes infact I am trying to model flow behind bluff body. I only get this results with k-omega and SST k-omega model for certain mesh. But how can I avoid this kind of problem.?

Like you said... "for certain mesh". This type of flow is sensitive to your grid. Further refinement downstream of the body should help. You may need to refine the surface mesh... take a look at your pressure gradients (adapt->gradient) as well as your y+ values. If the gradient seems high, or you're not in the appropriate range for y+ (for the k-omega I'm pretty sure you want to be between 30 and 200, or less than 1 if you use wall functions) then you'll have to adjust your mesh. Also, I've heard that running 2ddp (or 3ddp) sometimes helps.

Hope this helps, and goodluck, Jason