discussion of this problem which begins with the interpretation of Equation [1] for the case 

 of boundary-layer flow. 



There are also fundamental reasons for being interested in the pressure fluctuations 

 associated with a turbulent boundary layer. There is considerable uncertainty as to how 

 energy is produced, convected, diffused, and dissipated in the turbulent boundary layer. In 

 particular, the role of pressure fluctuations is but vaguely understood. Although this work 

 does not measure the pressure fluctuations inside the boundary layer, it does constitute a 

 start on this very challenging problem and it is believed that a further extension of this work 

 will prove valuable. 



To obtain the data for this report, the boundary layer on the wall of a subsonic wind 

 tunnel was studied. Pressure transducers with a very small active area were mounted flush 

 with the wall. Using conventional sound analysis equipment consisting of a narrow tunable 

 filter and an rms meter, it was possible to obtain the spectral density of the pressure fluc- 

 tuations at a point on the wall. 



By measuring the pressure fluctuations in the total frequency range, it was possible 

 to obtain the mean square value for the pressure fluctuations. This value was also checked 

 by integrating over the spectral density. 



By using two pressure transducers and studying the correlation and the cross spectral 

 density* between the pressure fluctuations at the two points, it was possible to study the 

 spatial pattern of the pressure fluctuations and how rapidly it evolves as it is convected 

 downstream. By measuring the time delay necessary to maximize the correlation between the 

 two points it was possible to measure the velocity at which the pattern was convected. 



These, and other results, are presented and discussed. 



EXPERIMENTAL APPARATUS 

 THE WIND TUNNEL 



The wind tunnel was a closed circuit subsonic tunnel. The velocity range of the 

 tunnel was from 50 to 200 ft/sec. The transverse dimensions of the working section were 

 20 by 15 inches. The measurements were performed on the wall of the working section, 5 feet 

 downstream from the entrance nozzle. At this point the displacement thickness of the 

 boundary layer was 0.105 inch at a velocity of 100 ft/sec. By comparing the mean velocity 

 profiles with those given by Klebanoff^ it was judged that the boundary layer was nearly well 

 enough developed to show similarity. Unfortunately, the working section was not long enough 

 to permit working further downstream so as to prove that the boundary layer was exhibiting 

 similarity. 



This term will be defined in the section giving the results of the measurements. 



