NO. 4 WIND TUNNEL EXPERIMENTS IN AERODYNAMICS 9 



Static pressure should l)e constant across a section, for if pressure 

 differences existed there would be a transverse flow of air created. 

 Tests in our wind tunnel showed constant pressure across a section 

 to a good approximation. Incidentally the constancy of this static 

 pressure across a section is a measure of the uniformity of flow.^ 



A small hole in the side of the tunnel can then be used to measure 

 the static pressure, but the dynamic pressure measured by the impact 

 end of the Pitot tube is 



where 



p+^- — =po' by Bernoulli's equation. 



/? = pressure at any point in a stream line. 

 t.' = velocity at any point in a stream line. 

 p = density at any point in a stream line. 

 ^g = pressure where v is zero. 

 In our wind tunnel a fan sucks air through the tunnel which is 

 therefore all under suction. The air is discharged by the fan through 

 a strainer into the building at one end, whence it returns at low 

 velocity to the other end to pass again into the tunnel. At a point in 

 the room the pressure transmitted by an impact tube would be 



Pr+ -^^ -Po- 



But the room is 30 times as large as the section of the tunnel, and 

 when a wind of 30 miles is blowing in the tunnel there is only a gentle 



draft in the room of about i mile per hour. Thus the ratio -'V = ^ 



^ r- 900 



and the pressure in the room can be taken as 

 neglecting Vr'. 



Pr = P. = P+% 



^ The static holes of the National Physical Laboratory Pitot tube were con- 

 nected to an alcohol gage, and the velocity being kept constant, the tube was 

 moved along the vertical center line of the tunnel. The following readings 

 were taken : 



Head in I mm. alcohol Distance from wall 



440.2 3" 



438.0 6" 



439-5 12" 



439.8 18" 



439-5 24" 



441.2 30" 



44 1 -0 36" 



441.0 42" 



441-2 45" 



