change ventilation coefficient placement in MM drop growth eq. (heat vs. mass diffusion)

[slayoo]

[slayoo]

@kaitlyn-loftus here's a "tiny" fix that changes how the ventilation factor (introduced in #1282) is used in the drop-growth formula - IIUC, the ventilation factor should multiply the diffusion coefficient, and not the density gradient. In an ordinary Fick's formula that is the same, but in Maxwell-Mason approximation, there is a difference. Essentially, the question is if we assume that the ventilation acts equally on mass and heat transfer. Previous logic assumed that it acts in the same way. This version assumes that there is no ventilation for heat, while the ventilation coefficient - which we compute using the Schmidt number - affects mass transfer only. IIUC, for including ventilation for heat, we would need to compute the Prandtl number.

So, there are three options:

• should we keep it as is (Schmidt number drives heat ventilation),
• should we merge this PR (Schmidt number drives mass ventilation, there is no heat ventilation),
• should we introduce Prandtl number (so that Sc and Pr drive mass and heat ventilation, respectively).

The last option is best and likely the goal, but what to do in the interim?

Also, do you have any hints where to look for a good (=simple) test case in literature which we could assert against?

[kaitlyn-loftus]

I think as you say the best option will be the last one: introduce Prandtl number (so that Sc and Pr drive mass and heat ventilation, respectively). In the interim, I think assuming the heat and mass ventilation coefficients are the same is preferable to neglecting heat ventilation wholesale.

When you ask for a test case in the literature, do you mean for the heat ventilation coefficient specifically or for evaporating drops with both mass and heat ventilation?

[slayoo]

Thanks for replying.

When you ask for a test case in the literature, do you mean for the heat ventilation coefficient specifically or for evaporating drops with both mass and heat ventilation?

Basically, anything that could help address the current situation - this PR changes how ventilation is computed, but there is not a single test that fails, proving that it is not covered.