Convection

Convection is observed when we heat fluid from below.  The two relevant parameters are Rayleigh number  R_a=gαd³(ΔT) ⁄ νκ  and Prandtl number Pr=ν/κ

For free-slip boundary condition convection starts after Ra>Rac≈ 657.8.  For no-slip, the corresponding number is approximately 1707.