82 



OVERHEAT, S^l (°F) 



FIGURE 5. Transition Reynolds numbers measured at exit: 

 two extension tubes . 



number is far more pronounced here than for the 

 shorter extension tube length. The most reasonable 

 explanation of this lies in the fact that in the 

 second case the boundary layer has passed over a 

 much longer region of unheated wall , which should 

 have a destabilizing effect. This less stable 

 boundary layer is then more sensitive to external 

 perturbations such as the disturbances created by 

 the orifice plate exit. 



As the extension tube length is increased still 

 further, the transition Reynolds numbers obtained 

 with the smooth contraction begin to decrease. 

 Apparently the destabilizing effect of the long 

 unheated wall is felt even with the low disturbance 

 exit condition. These results indicate that, under 

 some conditions, a moderate length of unheated wall 

 can be used downstream with no measurable reduction 

 of transition Reynolds number. 



When transition is determined at a distance of 

 1.4 m upstream of the exit rather than at the exit 

 nozzle itself, the influence of the exit condition 

 is greatly diminished. Taking the case of the 2.44 

 m unheated extension as an example , there is a 

 factor of 2.3 difference in the maximum transition 

 Reynolds number obtained with the orifice and with 

 the smooth contraction when transition is measured 

 at the exit (Figure 5). However, when transition 

 is measured 1.4 m upstream of the exit, the corre- 

 sponding difference is only 15 percent in Reynolds 

 number. Clearly the disturbances present at the 

 exit nozzle can affect the transition process if 

 it occurs near the nozzle, but this influence 

 diminishes rapidly as transition moves upstream 

 of the exit. Since the highest transition Reynolds 

 numbers have consistently been obtained with laminar 

 flow over the full length of the tube, most future 

 measurements will be made using one of the two 

 laminar flow exit conditions. 



Although it is difficult to assess uncertainties 

 in transition Reynolds number in this experiment, 

 some effort should be made. Results for the highest 

 transition Reynolds numbers exhibit a large amount 

 of scatter, but most of this can now be attributed 

 to variations in the free stream particulate content. 

 The purity of the water supply varies considerably 



with weather conditions at the site, and these 

 changes in purity have been directly correlated 

 with changes in transition Reynolds number. Under 

 the most adverse conditions, this effect has reduced 

 the maximum transition Reynolds number to less than 

 15 X 10^ (compared with 42 x lo^ for "clean" water) . 

 If we compare results that were obtained during 

 periods of relatively high water purity, the stan- 

 dard deviation in transition Reynolds number is 

 about 10 percent of the mean. 



This extreme sensitivity of the results to water 

 purity was quite unexpected, and an effort has been 

 made to improve the water quality by filtering 

 upstream of the settling chamber. Measurements of 

 the particle concentration spectrum have been made 

 using a Coulter Counter, and some of the results 

 are shown in Figure 6. The bands on this figure 

 indicate the typical ranges of concentration that 

 are obtained in the present experiments, as well 

 as in the NSRDC towing basin and the ocean. Note 

 that the flow tube particle spectrum has a steeper 

 slope than either the ocean or the tow basin, which 

 implies that for particle sizes greater than 10 y, 

 the flow tube water is much cleaner than the other 

 two. The filtration system presently used in the 

 flow tube effectively removes all particles larger 

 than 100 y. 



The reason for the strong sensitivity of results 

 to relatively minor contamination of the water 

 supply is not understood at present. The most 

 likely mechanism seems to be a slight increase in 

 wall roughness due to the adhesion of particles 

 to the wall. Whatever the mechanism, this effect 

 will clearly be of importance in hydrodynamic 

 applications . 



Comparison with Theory 



Wazzan et al. (1970) have presented numerically 

 predicted transition Reynolds numbers for heated 



PARTICLE MEAN DIAMETER (microns) 



FIGURE 6. Particle concentration spectra: flow tube, 

 NSRDC towing basin, and open ocean. 



