36 



on this tanker model indicate that the turbulence rod produces a somewhat 

 smaller increase in resistance than expected from the hot-wire survey. This 

 anomaly may possibly be attributed to the wake set up by the rod. The wire 

 and sand strip, while not as effective at lower speeds have the advantage that 

 their own drag tends to balance out the drag deficiency of the laminar region 

 forward of the usual position of these stimulators. 



As a result of this investigation two additional sources of turbu- 

 lence stimulation were revealed. The free surface appears to give rise to 

 additional disturbances which stimulate early transition. Hence the zone of 

 laminar flow is reduced by this free-surface effect. Also, it was found that 

 minute model- surface irregularities such as paint roughness are effective in 

 producing a turbulent boundary layer. This suggests that a few isolated but 

 judiciously positioned small roughnesses may be as effective as the other 

 stimulators. 



A valid decision on the optimum technique for stimulating turbulence 

 must await basic measurements of the shear stresses set up in the artificially 

 stimulated boundary layer. Such measurements may be possible with the future 

 developments of the hot-wire system for quantitative determination of the char- 

 acteristics of turbulence in water. 



APPENDIX 

 THEORY OF THE HOT-WIRE TECHNIQUE AS APPLIED TO THE FLOW OF WATER 



The basic theory of the hot wire has been given in the literature 

 (see, e.g., References 2 and 16) but little discussion has been given to the 

 particular aspects of the technique when the wire is used as a velocity meter 

 in water . 



The basic equation of hot-wire anemometry may be written in the fol- 

 lowing form for a wire under equilibrium conditions: 



i 2 RRa 



g_ = a + b VV sin [26] 



R - R a 



where i is the heating current, 



R is the mean resistance of the wire at operating temperature, 



R is the resistance of the unheated wire at 0° C, 



o 

 R is the wire resistance at the temperature of the ambient fluid, 



a is the temperature coefficient of resistance of the wire material 

 referred to 0° C, 



