B • TURBULENT FLOW 



made that the highly irregular and sharply defined character of the bound- 

 ary was revealed. An intermittency in the turbulence recorded from a 

 hot wire probe in the outer regions of a round jet was first observed and 

 studied by Corrsin [112]. It soon became apparent that this effect was 

 due to a sharp and irregular boundary convected past the hot wire. The 

 phenomenon was studied in considerable detail by Townsend [113,114, 

 115,116] in connection with his studies of the plane wake. Corrsin and 

 Kistler [117] later made an exhaustive study of free stream boundaries, 

 and this together with studies in the boundary layer by Klebanoff [118] 

 has resulted in a reasonably clear understanding of the character and 

 meaning of the free boundary. 



In Art. 17 attention has already been directed to the outer boundary 

 of a turbulent boundary layer, and the situation has been depicted sche- 

 matically in Fig. E,17b. The character of the free boundary and the sharp 

 separation between turbulent and nonturbulent fluid is shown in actual 

 reality by the photograph of the turbulent wake of a bullet, displayed by 

 Corrsin and Kistler, and shown here as Plate B,28. No turbulence and 

 no other property transported by the shear flow, except some energy as- 

 sociated with potential motions, has penetrated the surrounding medium 

 beyond the boundary. Moreover, the boundary is a connected surface; 

 there are no disconnected parcels of fluid. The billows and hollows are, 

 of course, three-dimensional. All motion in the nonturbulent fluid outside 

 the boundary is irrotational, and the velocity there is that accompanying 

 the potential motion of a free stream. 



These phenomena are reproduced at all free boundaries, differing only 

 in degree. An "intermittency factor" has been adopted as one of the 

 criteria of the irregularity of the boundary. If a hot wire probe, capable 

 of following the fluctuations, is placed so that, as the flow passes by, it is 

 alternately in and out of the turbulent fluid, a record of the signal will 

 show intermittently turbulent and nonturbulent sections. From such a 

 record, or by other instrumental means, the fraction of the time that the 

 flow is found to be turbulent may be determined. This is defined as the 

 intermittency factor. As the probe is moved from the center of the flow 

 outward, the intermittency factor goes from unity to zero. The custom- 

 ary symbol for the intermittency factor is 7. This symbol when used here 

 is not to be confused with the same symbol for the ratio of specific heats 

 used earlier. 



It is instructive to compare 7 distributions for several types of flow 

 along with their mean velocity distributions. These are given for the 

 boundary layer, the round jet, and the plane wake in Fig. B,28a and 

 B,28b. In Fig. B,28a lOebanoff's data for a smooth wall and Corrsin's 

 and Kistler's data for a very rough wall are compared. While there is 

 considerable dispersion in the observations of intermittency, the differ- 

 ence between the curves for smooth and rough walls is believed to be real. 



< 164 ) 



