How to define porosity ?

Jaimin

I am doing flow analysis of reverse bag house.in that ther are some bags (consider some tubes ).I am instead modelling that tubes i am makking blockage of same thickness.MY problem is how to define blockage for that.In subdomain i can define loss coefficients but how to give values.i.e.e i have pressure drop from that how suppose i a finding loss coefficient.then in to which term i have to put this head loss coefficient? I also have t odefine porosity.i.e. in screen i have to define porosity.I have 40 percent area open.How to deind this porosity?Pls let me know this . Thanks