35 



the assumption that the total change in resistance of the model is the same 

 regardless of the particular stimulator employed if the corrections for the 

 extraneous effects of the stimulator are made. Making use of the results of 

 the test with the trip wire, the corrected increase in resistance 



VM -&* U 



P) 



i rod L l x J rod L * J wire 



where D,, is the estimated wake and wave effect of the turbulence rod, and £ 

 W '« 



is the decrement of frictional resistance arising from the laminar flow for- 

 ward of the trip wire . 



The increase of model resistance 4C^/C„ obtained from drag measure- 

 ments with the rod is plotted in Figure 22 together with the corrected in- 

 crease |4C-/Ci, ) obtained from Equation [25]. At Reynolds numbers greater 

 than J x 10 6 the agreement of the corrected curve with the increase AC* f /C' f 

 derived from the results of the hot-wire survey is fair. The calculated cor- 

 rection to the drag data for the additional effects of the rod is seen to be 

 from 2 to 3 percent of the calculated model frictional resistance. 



In all three cases of turbulence stimulation the findings of the 

 hot-wire survey are in good agreement with the results of the resistance tests 

 corrected for the undesirable effects of the stimulators. 



CONCLUSIONS 



A general conclusion which may be reached as a result of these ex- 

 periments is that the hot-wire technique can be used in water to map qualita- 

 tively the regions of laminar, transitional, and turbulent flow in the bound- 

 ary layer of any model. With such a qualitative picture a close study of the 

 effects of various turbulence stimulators can be made and a good estimate of 

 the increase in resistance produced by each stimulator can be accomplished. 

 Even for this particularly large model the hot-wire investigation has shown 

 that the extent of laminar flow would lead to a sizeable error in the pre- 

 diction of the power required for the prototype. Moreover, it is now very 

 clear that stimulation should be employed on all full-form models and should 

 never be omitted in series tests where the effects of progressive geometrical 

 changes are sought. 



The selection of the best device for stimulating turbulence cannot 

 be made on the basis of these limited experiments. The turbulence rod has the 

 effect of converting the entire laminar region to turbulence at a speed lower 

 than was possible with either the trip wire or the sand strip used in these 

 tests. However, the resistance tests made with and without these stimulators 



