[guillaume74]

Hello all,

I am running a transient thermal analysis and I would like to understand the results given in CFD post and get those results also by simple hand calcs.

I am interested in the internal energy of the fluid.

From a hand calc point of view it is based on the equation


If, in my case, from one tiime point to another, my average temperature varies from 40K, then can have my internal energy.

From the CFD-post point of view, the internal energy is not directly given but you can have the static enthalpy which is defined as


Then, by taking the two time points of interest and substracting those, you have the and thus the .

However, i don't find the same results at all. Not even close...

Am i missing something?

Thanks for your help

[Opaque]

You have not described the material type you are using; however, from your description of the Hstat, I assume it is a constant density material, i.e. incompressible.

If you are using the Thermal Energy heat transfer mode for incompressible materials, the variable Static Enthalpy is really the Internal Energy if I recall correctly.

From the documentation:

This equation can be derived from Equation 1–90 with two different sets of assumptions:

If is actually interpreted as internal energy,


(1–92)

then Equation 1–90 can be written as


(1–93)

which is equivalent to Equation 1–91 if we neglect and interpret as . This interpretation is appropriate for liquids, where variable-density effects are negligible. Note that the principal variable is still called 'Static Enthalpy' in CFD-Post, although it actually represents internal energy. Note also that, for liquids that have variable specific heats (for example, set as a CEL expression or using an RGP table or Redlich Kwong equation of state) the solver includes the contribution in the enthalpy tables. This is inconsistent, because the variable is actually internal energy. For this reason, the thermal energy equation should not be used in this situation, particularly for subcooled liquids.

On the other hand if and are neglected in Equation 1–90 then Equation 1–91 follows directly. This interpretation is appropriate for low Mach number flows of compressible gases.

The thermal energy equation, despite being a simplification, can be useful for both liquids and gases in avoiding potential stability issues with the total energy formulation. For example, the thermal energy equation is often preferred for transient liquid simulations. On the other hand, if proper acoustic behavior is required (for example, predicting sound speed), or for high speed flow, then the total energy equation is required.

Hope the above helps