Size of air intake - dynamic losses

[nicog]

Hi alls,

We have to install a motorfan on the roof of a 250 kph train, wich fan will throw air troughout a radiator, the all system below a roof carenage.
This is the first time we use a roof carenage at those speeds.
1) What should be the right method to determine the size of the air intake ?
2) Did anyone has experience of estimating dynamic pressure losses in function of the speed ?
3) Is it possible using calculations/CFD softwares, without passing through wind tunnel ?

Thanks for your help.

Regards,

Nic