Reply To: Age of Air

[Holzmann]

Hey there,nnmy name is Tobi. I do have some questions regarding the AoA calculation mentioned above. From my personal perspective, the equation given there is wrong. First of all, the air can exchange via advection and due to molecular transport phenomona. Hence, we do need both terms in our conservative equations. However, the diffusion of the air is basically the molecular viscosity of the air (at the cell temperature) plus the increase due t o turbulence. As we are using the Boussinesq Hypothesis + Reynolds Averaging method (bla blub), we end up with different turbulence models which at the end will just do one thing ... right, increase the viscosity. Therefore, in the diffusion equation we should have either:mu + mu_t or mu_eff. In the equation above we clearly find the effective viscosity (C_MU_EFF). The pre-term C_RHO * 2.88e-5 does not make any sense to me. However, it seems that it should be related to the molecular viscosity of air which is:density * kinematic viscosity. However, the kinematic viscosity is around 1.8 e-5. So I guess someone made a mistake here and everybody just re-used the same. nnNevertheless, as we already take the effecitve viscosity (which normally comes from the turbulence model), we should not include the first term and also there should be no need to divide the second part by the turbulent Prandtl number as is should be done in the turbulence class already. This seems to be a mistake that no-one resolved in public forums. Personally, I would identify the diffusion term that is given in this thread and also in the Ansys User-Guide to be wrong. It seems that the first term is the molecular viscosity of the fluid (with a wrong constant -> kinematic viscosity is 1.8e-5 rather than 2.8 e-5) and the second term should be the turbulent viscosity rather than the effective viscosity. At the end, the return function should be simply C_MU_EFF(c,t). As I cannot checkout the code of Fluent, it would be quite nice to get a feedback related to this topic.nnThank you in advance,nKind regards,nTobias Holzma