Considering the flow through a dense net / grid
 

I have a grid like structure in the flow path and I'd like to consider it's effect with out actually meshing it.

The attached image shows the sructure / surface im talking about.

I'd like just keep it as a plane surface with out actually meshing those small holes but considering their effect by some means.

Could some one please tell me how to do this.

use a porous domain to represent the pressure drop WRT flow. you can calculate the pressure drop curve with orifice calculations or actually model a single representative area of it.

The solver theory manual has a lot of documentation on this, I'd suggect you check out all the options and how the pressure drop is calculated.

Could you please let me know if there is some thing like a area porosity as it is just a surface with such holes over it.

volume porosity is just one of the options, but all this does is increase your velocity through the domain if you choose the true velocity option. You also have to define the curve of how the pressure decreases with velocity. for this you define the permeability or loss coefficient or both, as per the equation below:

DP per unit length = -(Viscosity/k.perm)*Velocity - (density/2)*Loss.Coefficient*Velocity^2

Where the velocity is either the true velocity or superficial as set in the porous options tab.

There are many ways to do this though, I'd check the theory manual.