unexpected temperature fluctuation
 

Dear all,

I am trying to model the heat dispersion using Comsol. The first layer is a 100 um thick Lithium disk which deposits 30% heat, the second layer is a 20 um thick Ta disk which deposits 70% heat, and the third layer is an 11 mm thick copper plate with water-cooling channels.
I used the module 'Heat Transfer in Solids and Fluids' and the module 'Laminar Flow'.

However, the plotted peak temperature shows chaotic temperature fluctuation in the beginning and gradually reaches an oscillation state.

The oscillation state looks reasonable, because I used a rotation beam. But I can not understand the chaotic fluctuation at the beginning.

Could you please help me solve this problem?

Thanks!