## Fluids

#### Helping in module ABL with forest

• Ofir
Subscriber

Hey all,

I need to module an ABL before above and inside a forest canopy.

The module should be be the following steps: before the forest --> above--> inside.

my eq' are:

• DrAmine
Ansys Employee
Which diffusion? n
• Rob
Ansys Employee
One option is to model the forest as a porous media (to represent the trees), that should slow up the flow in that region. nI'm not sure what the resistance coefficients are for trees (I've always been given them) but have seen papers on the subject. n
• Ofir
Subscriber
Yes, I need to consider it as a porous media but what r my coefficients C1 and C2 ? ( lets consider that I know the thickness of the trees), the coefficient alpha I don't know it. nand before that, I should write in the ABL before the forest what TKE I want to converge. how I decide it? base on what? n
• Rob
Ansys Employee
You'll need to figure out the coefficients from either literature or experiment: I want to see pictures if you manage to get a tree into a wind tunnel! There's a lot of work on wind resistance due to trees for wind farms and field boundaries. nAs for whether to start the ABL at ground level or tree top height, I don't know. ABL tends to assume the ground (or water) is smooth, I don't know how it reacts to trees. What does the literature tell you? n
• Ofir
Subscriber
hehe I will show a pic of the tree in the wind tunnel, just wait....5 or 6 years.nI will explain my question:nI need to verify a several things:nhow can I check my Yplus ? Technically in ANSYS... which buttons? nWhen I initialized the simulation I choose TKE= 1 and specific dissipation rate =1 --> I do it as a default but I want to understand the reason and if I need to change how I should notice that.nWhen I put the drag force as a parameters and want to know how it act pver a plot - how am I doing it? n
• Rob
Ansys Employee
In the solver run the model and have a look under Turbulence in Post Processing. I'd advise turning node values off, and looking at one wall at a time. Note, for ABL type applications you also need to be careful about surface roughness:the roughness heights may be much larger than the near wall cell if you want a y+ of around 1. You may need to accept a somewhat higher value. nIt's 1 because someone set that as being numerically sensible about 25 years ago. It's sensible in that it's the mid point of 1e8 and 1e-8 but physically it's nonsense. If you initialise from the boundary you should get a more sensible value. nRe drag force, have a look at surface monitors and the actual definition of drag force to see if it's what you want.Only 5-6 years , I'll wait. Redwood would be a good one to try.....
• Ofir
Subscriber
nI don't if I did it right but how is it possible that the Y plus goes to numbers like this? n
• Rob
Ansys Employee
Where are you taking the data? y+ is a way to determine the near wall cell height. For environmental flows you may need a near wall cell of under 1mm on a km scale domain: however you also don't necessarily know exactly where the ground is. Given all of that getting y+ exactly right may not be computationally possible (you'd need a very big cluster) or worth the effort (how does a small numerical error compare to the uncertainty of the terrain)? n