I have to model the evaporation rate from a water surface (a lake) under several wind and geometry conditions, in Cosmofloworks (SW 2008)
The lake is a flat static surface (no two-phase problem, just air flow + moisture). I consider, at least for now, no thermal effect.
I started with a parallelepiped fluid domain (see attachment), with an air inlet and an outlet, two ideal walls on the sides. The upper and lower faces are enviroment-pressure openings.
I set the upper face to correspond to the environmental humidity (e.g. 30%, like for the inlet).
The lower face is in contact with the water surface, and I set humidity to be 100% (saturated layer).
I expect, in this basic situation, that, after a while, the system will get to an equilibrium: in particular, moisture distribution will be stable.
Then, I guess, if I take a control volume anywhere in the domain, the water mass content will be constant and, therefore, the total water flux through its surface will be 0.
My idea was, then, to take a control volume with the lowest face close to the water surface, and to consider the flux thorugh this face as the evaporation rate.
The problem is that, even after many trials and corrections, the flow through the surface of the control volume never becomes 0. So, I don't know how to interprete the results... in particular, I don't know if
- my "equilibrium idea" is wrong,
- my model is wrong, or it could be done much better,
(I already checked that there is no condensation in the control volume)
Do anybody has some clue or suggestion?