## Fluids

Topics relate to Fluent, CFX, Turbogrid and more

#### Tree canopy settings

• anhvu lee
Subscriber

Hi,

I'm going to set the tree canopy as a porous zone and add a source terms. I have some related questions:

1. Is there any reference for the Porous zone settings (Viscous and Inertial resistance, Relative viscosity,...) ? I've tried to found out but still get stuck
2. In the source term category, I found the model for the Momentum but how to define the X Y Z momentum?
3. The H20 in source term is the hourly equivalent evapotranspiration or the evapotranspiration in total ?

Thanks

• C N
Ansys Employee

Hello Anhvu lee,

1) For the porous zone settings I recommend you to check the default setting of relative velocity resisitance formulation . An alternate way to calculate the viscous resistance is Darcy porous media law where it states that the pressure drop is directly proportional to velocity and the inertial term of pressure drop equation can be considered to be zero. but this is applicable only for laminar fows.

So in such cases the viscous resisitance can be calculated by

where 1/alpha is the viscous coefficient. So to calculate the viscous resistance coefficient we have to know the pressure drop , dynamic viscosity , superficial velocity and  thickness.

I am attaching the user guide link . Kindly refer this 3.4.7 user guide link of cell zone conditions to understand how the porous settings are given.

Chapter 3: Modeling Flow Through Porous Media (ansys.com)

2)In source term set the momentum in particualr direction as source by multiplying the mass*velocity of the fluid.

3)By setting specific dissipation rate option you can simulate the hourly equivalent evatranspiration.

I am attaching the screenshots for your reference.

I hope this helps you in your simulation.

Thanks,

Chaitanya Natraj

• Rob
Ansys Employee

To add, tree canopies also have the fun distinction in that the leaves move with wind speed so the resistance coefficients may also change with flow rate! I think Ben Gurion University or the Technion (Israel) did some work on this - I vaguely remember the presentations from around 15-20 years ago. The secondary conclusion of the work was to use blown wind tunnels as the type with the fan downstream didn't like the amount of leaves that came off the test samples....

• anhvu lee
Subscriber

Thanks CN and Rob so much,

I would like to ask the different of "Turbulent Kinetic Energy & Turbulent Dissipation Rate" in the Source Terms for Fluid settings and the Momentum for the Boundary Conditions? Or they're the same one?

Sincerly

VU

• Rob
Ansys Employee

The boundary value will set the value at that surface. The source term will add the value to whatever is in the cells. So, same scalar field but slightly different effect.