text: For free convection near a vertical wall, the Rayleigh number is defined as:
  where:
  *x is the characteristic length
  *Ra is the Rayleigh number for characteristic length x
  *g is acceleration due to gravity
  *β is the thermal expansion coefficient (equals to 1/T, for ideal gases, where T is absolute temperature).
  * is the kinematic viscosity
  *α is the thermal diffusivity
  *T is the surface temperature
  *T is the quiescent temperature (fluid temperature far from the surface of the object)
  *Gr is the Grashof number for characteristic length x
  *Pr is the Prandtl number
  In the above, the fluid properties Pr, ν, α and β are evaluated at the film temperature, which is defined as:
  For a uniform wall heating flux, the modified Rayleigh number is defined as:
  where:
  *q&Prime; is the uniform surface heat flux
  *k is the thermal conductivity.