Fig. k. Rotor units CS-2 and ST-I4- ready for calibration run in Hytech tank, July I962. 



Earlier tests failed to reveal any measurable 

 influence of one rotor on the performance of the 

 other with this tandem setup compared to towing 

 one rotor at a time. 



It is important to note that all of these 

 tests are restricted to still (non-turbulent) 

 water. Evidence that results under this condi- 

 tion may be significantly different from those 

 obtained under natural conditions was first 

 offered by Savonius 2 who found some 10$ to 15$ 

 increase in rotational frequency for the same 

 air speed in the natural wind compared to the 

 wind tunnel. It follows that performance is 

 probably dependent mainly on the frequency and 

 scale spectra of turbulence superimposed on the 

 mean flow. 



THRESHOLD AND AMGULAE SPEED VARIABILITY 



Savonius rotor has no clear-cut threshold that 

 fits this definition because the rotor turns 

 non-uniformly due to a dependence of torque on 

 vane position relative to orientation of flow 

 past the rotor. Fig. 5 is a polar torque diagram 

 for one of Savonius' early rotors. This is 

 "static" torque; its magnitude relative to other 

 torque components decreases as rotational speed 

 increases and the rotor gains inertia. There- 

 fore, if the rotor gains enough angular momentum 

 while being driven in its high torque region it 

 can stay in motion even though the torque would 

 be insufficient to start it from stall in the low 

 torque position. It is rather difficult in the 

 presence of variations in tow carriage speed and 

 residual turbulence in the tank to accurately 

 establish threshold. With good bearings, clean 

 surfaces and moderate manufacturing care the 

 threshold for any flow orientation should not 

 exceed the range of 0.02 to 0.05 knots. 



The threshold of a current meter, i.e., the 

 minimum current speed it is capable of detecting, 

 is of major importance. Unfortunately, the 



The exact value of threshold may be somewhat 

 academic since the rotor continues to turn non- 

 uniformly for flow speeds considerably above the 



117 



