Hello Everyone:

I have a question on the dominance of body forces. In a given problem, Rayleigh number gives the strength of free convection.

When the Rayleigh number is more than 1.0e+7, we say that the turbulent effects need to be accounted. That does not necessarily mean that even the body forces are dominant too. Under what limit, the effects of body forces need to be accounted ?

Thanks,

Thomas

Hello Thomas,

1. Rayleigh No = Gr*Pr, physical meaning is the ratio of "Natural Advection" to diffusion terms in the energy (temperature) equation. Natural advection being the motion driven by the body force of bouyancy.

2. The analogy in forced convection is the Peclet No = Re*Pr and as you stated the influence and onset of turbulence is related to Pe for forced and Ra for natural convection.

3. How can one have a flow with a large Ra if the effect of bouyancy is not large (ie Gr is not large)??

To quote Eckert from p525 "Analysis of Heat and Mass Transfer": "A flow situation is called free- or natural convection flow when it is created solely by body forces."

Have a read through Eckert's discussion!

good luck!....................................Duane