Oscillating Result

Nirupam Rohatgi

Hello everyone on this lovely forum. I am currently facing a problem and hope someone of you get me out of this.

The Problem: I am trying to do unsteady simulation of a closed room (3mx4mx5m)which is having an inlet of cold air and exhaust of mixed room air.

I am using following formulation:

Solver: Pressure Based, Implicit, unsteady formulations= 1 st order unsteady,

Discretization: Pressure = Std, and all else 1st order upwind.

The trouble: I am plotting temperature in room at a point. I observe that while residuals are behaving very nicely, temperature is oscillating about an expected value. The average value of temperature is very realistic, but it oscillates around this value.

Other important observations are that temperature is following a sinusoidal curve very closely and the time period of oscillation is not just one time step but varies from few seconds to 300 seconds while time step is of the order of 1 second.

I am not able to understand, why temperature is oscillating.

Can anybody give some clue on this?

thanks

nirupam