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jylee4 May 5, 2020 07:14

How to plot the specific catch ratio for wind-driven rain simulations?

If I understood this paper ( accurately, then the formula to calculate the specific catch ratio is as follows?

(alpha_d * |V_n(d)|) / (R_h * f_h(R_h,d))
  • alpha_d is the volume fraction of rain phase d,
  • |V_n(d)| is the velocity magnitude of the rain phase in the direction normal to the building facade. Would V_n(d) be the rain phase velocity in the streamwise/air flow direction?
  • R_h is the horizontal rain intensity through the horizontal plane. This seems to typically be in units of [mm/h], and so is kind of a velocity? With limited domain knowledge, I failed to understand what exactly R_h is. Is R_h the rain phase velocity in the downwards/vertical direction which is perpendicular to the streamwise direction? Which horizontal plane would this be?
  • f_h(R_h,d) is said to be the raindrop size distribution through the horizontal plane. I would greatly appreciate any explanation one can offer here on what f_h(R_h,d) is. If a constant raindrop diameter was used for simplicity, what would the value of f_h(R_h,d) be?

I am aware of this page (, which recommended the DPM with Eulerian Wall Film. I got to a point of injecting particles (water-liquid) via surface into the flow of air around a simple building, but nowhere close to knowing how to plot the specific catch ratio or quantify wind-driven rain. With the Eulerian Wall Film enabled, is there a way to translate film thickness to specific catch ratio?

Or, would you recommend the Eulerian model as a potentially less computationally intensive and simpler approach compared to DPM? I tried using the Eulerian model with air as the primary phase and water-liquid as the secondary phase. Then in CFD-Post, tried to use an Expression to calculate the specific catch ratio, to plot as a new Variable on the wall boundaries of the building and the ground:

(Phase 2.Volume Fraction*abs((Phase 2.Velocity u) ) )/(abs(areaAve(Phase 2.Velocity v)@top)*1)
Unfortunately, the values turned out to be zero, because the values of Phase 2.Velocity u at the walls are zero.

Could you please kindly share some advice? Thank you.

The Ansys Workbench file in .wbpz archive format can be found here:

vinerm May 6, 2020 15:44

Not many people work in this field, hence, you will have to read more literature.

Rain intensity, though looks like velocity in terms of dimension, is not velocity. It is actually about the height up to which water will be filled in an hour if collected on a flat vessel with an area of 1 m^2. In other words, how much volume of rain passes through an area of 1 m^2 in an hour, so, volumetric flux, hence, units of velocity.

As far as rain drop size distribution is concerned, it is also not that simple as just the diameter itself. It's units could be m^{-1} or m^{-3} mm^{-1} depending upon the model. Usually, it is given in terms of mass or volume (or mass or volume fraction) of the droplets below a certain size d out of total mass.

So, I'd suggest you read more about it since the field although not esoteric yet is rather less studied.

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