[oajor]

Hi!

I want to model a spillway weir with a vertical shaft and a tunnel system directly downstream. I expect including the air entrainment model in Flow-3D will cause an earlier backwater effect than without the model. My question is how to set up the parameters of the air entrainment model in Flow-3D for best possible results with no calibration data?!

In the basic air entrainment exercise (given by Flow Science) the following parameters are used (SI-units):
-Air entrainment rate coefficient = 0,5
-Surface tension coefficient = 0,073 N/m (water 20 degrees C)
-Drag coefficient = 0,95 (for buoyant spheres)
-Average particle radius = 0,0005 m (typical 0,5 mm - 2 mm)
-Richardson-Zaki coefficient multiplier = 1
-Viscosity of two-phase mix = 0,001 Ns/m^2 (assumed to be viscosity of continuous phase - water)
- Viscosity of water (phase 1) = 0,001 Ns/m^2
- Viscosity of air (phase 2) = 0,001 Ns/m^2 (assumed bubbles always separated by fluid film)
- Density of liquid = 1000 kg/m^3
- Volume fraction of air at inversion point = 1 (assumed bubbles always separated by fluid film for smaller volume fractions)
- Allow gas to escape at free surface = Yes
- Density of air = 1,2 kg/m^3

Does these parameters give reasonable results? Any better suggestions? I find it a bit weird to set the viscosity of air and the two phase mix to the one of water. I also wonder if there is any validation data to help me choose the air entrainment coefficient and or the drag coefficient?

I've also heard that the air entrainment model is really cell size sensitive. Any advice on setting the cell size for air entrainment (rule of thumb)? Will a cell size reduction study give better results (or convergence at all?)?

Is there any gain in also activating the adiabatic bubble model?

Any replies are welcome.