Gauge pressure - Op. Conditions How to fill it ?
 

I'm a student in a french engineering school. I'working on an air foil.

I'm working under stanfard atm (101325 Pa). My teacher told us to fill in the "Gauge Pressure" with 0 Pa when at Mach 0.15, but to change it when at Mach 0.8... The reason I'm looking for help is that he doesn't knoww why changing this parameter beetween sub and transonic...

Does anybody know the answer ??

(Same questin qith Operating Contitions)

Thanks !! ;-)