NO. 4 WIND TUNNEL EXPERIMENTS IN AERODYNAMICS 2)7 



IV. ADJUSTMENT OF VELOCITY GRADIENT ACROSS 

 A SECTION OF THE WIND TUNNEL 



By H. E. ROSSELL, Asst. Naval Constructor, U. S. Navy, and 

 D. W. DOUGLAS, S. B. 



In any wind tunnel experiments, it is important that the velocity 

 of the air striking different parts of the model shall be the same. 

 Consequently, after developing precise methods for measuring 

 velocity, the cross-section of the tunnel was explored to detect vari- 

 ations in velocity from point to point. 



The procedure was as follows: The side plate described above 

 (Report I, page lo) was connected to the Chattock gage. One 

 observer by regulating the motor field rheostat kept the velocity as 

 nearly constant as possible, indicated by keeping his gage reading con- 

 stant. A speed of about 28 miles per hour was selected as standard. 

 The Pitot tube (National Physical Laboratory tube) was mounted on 

 a standard and moved parallel to itself along vertical lines 6 inches 

 apart. Great care was taken to point the tube in the axis of the wind. 

 The Pitot tube was connected to the alcohol gage. A velocity reading 

 was taken every 6 inches by a second observer. The same two 

 observers made the entire test. 



The first preliminary tests showed the velocity over the section for 

 a constant static pressure, as shown on the Chattock gage to be far 

 from uniform. The velocity near the sides was higher than near the 

 center. Such a result could be caused by the honeycomb at the suction 

 mouth of the tunnel. The air entered the mouth in converging lines 

 of flow as was shown by the direction taken by fine silk threads. The 

 honeycomb was at the very end of the tunnel, and probably straight- 

 ened out this flow too soon to allow a sufficient volume of air to reach 

 the center. 



To assist the air to flow more to the center, the honeycomb was 

 shoved I foot into the tunnel. The effect was satisfactory, but not 

 enough. 



The honeycomb was then shoved i foot farther into the tunnel. 

 The exploration of velocity over the section showed that a fair result 

 had been obtained, and it was concluded that no advantage would 

 be gained from further movement of the honeycomb. 



Since models not greater than 18 inches in span are to be used, the 

 useful part of the tunnel is included within a square 2 feet on a side. 

 The parts of curves, drawn through the experimental points, which 

 passed through the 2-foot square, show an average velocity of 27.8 



