Change in velocity at inlet
 

Hi all,

please let me tell you that I am real newbie in CFDs topics, so bear with me if the following it's a stupid question.

In air sampling the concept of isokineticity (i.e.: the fact that the ration between free stream velocity (U0) and velocity within an inlet (U) is equal to 1) is damn fundamental and my question is:

is there any way to estimate U knowing U0, temperature and pressure of external air and the inlet diameter and length?

To make it extremely clear: let's assume I have a 20 cm long circular pipe of 10 cm of internal diameter which is standing isoaxially in air flowing at 40 m/s (at 303 K and 101 kPa) what would be the velocity inside that pipe?

I know that the Darcy-Weisbach equation is used to calculate pressure-drop (aka head loss) inside a pipe, but the equation assumes I know the average flow velocity INSIDE the frickin' pipe...

...this thing is driving me mad! :mad:

Please, any help would be very very appreciated...