The standard deviations. 0"(x), of the concentration curves 

 are just the quantities V y^(x) . Consequently, the last formula 

 be come ss 



V'(0) = U Jcr(o) , 

 dX 



i„e« the transverse intensity of turbulence at the end of the in- 

 jector tube may be obtained from the slope of the curve «r (x) 

 at that point. Figure 6 shows a as a function of x together with 

 the points determining the curves for the two cases under consid- 

 eration, L/M s 15 and L/M 5 22. In the case L/M & 15 it appears., 

 that the curve would not extend back through the origin. It 

 seems likely that this is an effect of the dye injection method 

 u«ed« The finite size of opening of the injection tube will 

 have the effect of starting the dye wake with a positive value 

 of er right at the exit. However, this is in the wrong direction 

 to explain the behavior mentioned above Q A discrepancy of this 

 type could also be caused by the following facto The rate of 

 dye injection used was equivalent to an average velocity at the 

 exit of 2.2 ft/sec, that is, about 1.4 times the free-stream ,-- 

 velocity. The higher velocity of the jet will tend to postpone, 

 as a function of x, its diffusion. In particular, the substitu- 

 tion x s Ut used above will no longer be true, even approximately, 

 near the injector. Although correction for the finite size of 

 the injector would be relatively easy to carry out, correction 

 for the velocity discrepancy would be more troublesome and could 

 easily be avoided by more careful control of the injection rate. 

 In the present case, the slope was computed from the lower end 

 of the curve. Corrections would apparently have the effect of 

 changing the value of L/M somewhat but perhaps not very greatly 

 the actual slope at the sampling stations. In any case, it is 

 clear that more careful experimentation is needed. The two 

 slopes so obtained were used to compute the values of U/v" 

 shown in Figure 7. On the same figure are shown values of 

 U/u' obtained with a hot-wire velocity meter in the same channel 

 with the same grid (M s 3/4") and with a smaller grid (M s 1/2"). 

 The agreement seems better than the crudeness of the experiment 

 leads one to expect. 



CRITIQUE OF METHOD 



It seems clear from the analysis of the data obtained in 

 these experiments that the method will be more reliable as the 

 necessary corrections become smaller,, > This can only be accomplish- 

 ed by having the dimensions of the injector and sampler tubes 

 small In comparison with the scale of the turbulence . This will 

 generally be attained if the tube diameters are small compared 

 with the mesh width M. From dimensional considerations, increas- 

 ing the mesh width M while preserving the mesh Reynolds number 

 UM/v will not change the slope do/dx but will increase the dis= 

 tance from the end of the injector in which c is nearly a linear 



