question of physic ? Fans/flow and opening size...

It isn't a plc question but i'm sure someone here know the answer.

I have to cool some drive and i would like to know how to calculate resulting flow vs fan free airflow installed into in enclosure with a limited opening.

For exemples, i have a 105cfm 4inch fan that is 16square inch in size.
I could use a 16in2 opening to keep restriction relatively low but what would happend if i make the inlet just 8square inch in size for exemple?
I would like to have the euation behind to estimate the needed opeining.

Their won't be any filter to make resistance (nema1 in a clean room)

Calculate the various factors yourself. The flow rate on the downstream side of the restricted opening is dependent on temperature, size of orifice, and pressure drop. This calculator has the equations listed. You may want to switch to Imperial units. Your 16 square inch opening would be equivalent to approximately a 4.51 inch diameter orifice [Diameter = 2x{Square Root of (Area/Pi)} = 2 x Sq Root of 5.096]. For the same pressure drop, reducing the opening by 1/2 reduces the air flow rate by about 1/4.

http://www.tlv.com/global/TI/calculator/air-flow-rate-through-orifice.html