[Lemanes]

Hi,

I am simulating natural convection in an enclosure cavity. Can I evaluate Rayleigh number automatically? I am using the following equation:

Rayleigh#=((density^2)*g*(T1-T2)*L^3)/(k*mi)
It is a classical equations than can be found in all heat transfer books.

((ave(Density)@Fluido) *(ave(Density)@Fluido) *9.8[m* s^-2] *(ave(Specific Heat Capacity at Constant Pressure)@Fluido) *(1/Average)*Delta TOTAL *Espessura ^3)/((ave(Dynamic Viscosity)@Fluido)*(ave(Thermal Conductivity)@Fluido))

My problem is that I am using boussinesq approximation so that my ave(Density)@Fluido doesn't change. It states that density differences are sufficiently small to be neglected, except where they appear in terms multiplied by g, the acceleration due to gravity.
How can I get the correct value of density?

Thank you in advance!

[evcelica]

If you are only using one Density, then you already have it. Rayleigh doesn't compare densities, it only uses them in the thermal diffusivity and kinematic viscosity values.

( g * Beta * (T1-T2) * Lc^3 )
/ (ThermalDiffusivity * KinematicViscosity )

[Lemanes]

Hi, thank you for your help.


I did not find KinematicViscosity on CFX post. I only found dynamic viscosity. And in fact, Dynamic Viscosity = Kinematic Viscosity (cSt) x Fluid Density (kg/m3)


I also did not find ThermalDiffusivity (k/ ro *cp) on CFX post.


Then, In both variables fluid density appear. I don't know what I am doing wrong.

Thank you

[evcelica]

You are not doing anything wrong, It is all mathematically equivalent, and you are doing it the right way.

You can define new expressions and variables in CFX post if you want to.

Remember, Beta is only (1/Temperature) for an ideal gas. If you are modeling a liquid this will not be the case.