//Consider a low subsonic wind tunnel.
A1=2; //reservoir area,m^2
A2=0.5; //test section area,m^2
P2=1.01*10^5;//test section pressure,N/m^2
V2=40;//flow velocity in test section
//from continuity equation
V1=V2*(A2/A1)//velocity  before test section
D=1.23; //density of flow equals standard sea level,Kg/m^3