Why am I getting fluctuating temperatures in cooling simulation?

[VJ0085]

I have shaft with disc object. I got heat distribution by applying necessary boundary conditions. Now I wish to perform and see the cooling down temperatures in it. So I applied max temperature obtained in previous simulation as initial temperature and other radiation and convection coefficients. But the temperatures first suddenly drops and later starts increasing. Why?

VJ
Mech-Aeroengineer

 

[corus]

In some transients you can get some instability in the solution depending on the thermal shock. This can produce some initial oscillation in the solution which then disappears. If this occurs then use a smaller initial time step and/or a finer mesh nearer the surfaces. If it's just a case of the temperatures decreasing and then increasing in a steady manner then it could be just a product of the boundary conditions which causes an initial temperature drop at the surface before heat from the remainder of the body conducts through to raise temperatures back up.