I have read in manual that in Single frame of referance flow field which is unsteady in stationary frame becomes steady in rotating frame.i counld not understand it why?pls any body know it then let me give some information. Thanking you in advance. john

In single frame of reference(SRF) Fluent uses rotating frame of reference. Inour simple modelling you may be aware with inertial frame of reference. This inertial frame of reference is converted to rotating frame of reference. For e.g. Consider a circular tank and a impeller moving inside it. If you are standing outside for you the case will be unsteady as impeller blades will be moving form your point of location. Now say if you stand on the centre of the cylinder, and you are moving with impeller, you look at the flow as steady. This is the same example given in fluent documentation. I hope I have added some information which may help you to understand.

Hi, Akshay thank you, you mean to say that coordinates transformation takes place and so coordinates are also rotating, so the system becomes steady. is it right?

Right. The transformations are also mentioned in the manual. i.e you use rotating frame of reference, rather than inertial one.