Resistance Model
Resistance Model is a Flow module option you can use to set a resistance in a selected domain. In the Flow Analysis Tree, under Domains, select a fluid domain to see the resistance models in the Properties panel. There are two types of resistance models:
Pressure Loss—Specifies the following parameters:
Material Structure
Linear Drag Coefficient
Quadratic Drag Coefficient
Darcy's Law—Specifies the following parameters:
Material Structure
Permeability
Quadratic Drag Coefficient
* 
Porosity is set in the Common module.