1.0 



— 0.8 



or 

 o 

 I- 

 o 

 < 

 u. 



o 



0. 



en 



0.6 - 



0.4 



0.2 



5 10 15 20 25 30 35 



TIME AFTER NEAR STEP CHANGE OF TOWED SPEED (SEC.) 



Fig. 11. Rotor response to abrupt change "between zero and steady speed. 



40 



45 



deceleration) . Therefore, the test results 

 given below may be applicable only to the limit- 

 ing conditions of the special case; this possi- 

 bility is quite definitely supported by prelim- 

 inary results of tests in progress at the time 

 of this writing. 



Results of response tests employing a near 

 step change, i.e., one requiring a small but 

 finite length of time for the transition, from 

 stop to a steady towed speed (positive step) or 

 vice versa (negative step) are given in Fig. 11. 

 Mechanical difficulties of accelerating the tow 

 carriage limited the number of usable positive 

 step runs . The positive response deviates from 

 exponential and there is some overshoot (not 

 shown) . Several rotor revolutions are required 

 for the rotational frequency to settle down to 

 a reasonably steady value. 



From the response curves shown for negative 

 step changes it is obvious that the rotor decays 

 towards zero much more slowly than it accelerates 

 and that the decay rate is a function of step 

 change magnitude as is characteristic of so 

 strongly an inertial system. Time constant 

 values are compared in Table I . The times for 



Table I. Rotor CS-2 response to step changes 

 of towed speed. 



90% response are also given. In a non-inertial 

 system these values would be 2.3 times that for 

 63$ response. 



Another feature of the deceleration curves of 

 Fig. 11 is of interest. Immediately after stop- 

 ping the tow carriage it is noted that the 

 response factor goes above unity before falling 

 off as it should. This is due to the wake that 

 has been generated by the meter as it travels by 

 the stopped rotor, thus giving an additive com- 

 ponent to the omnidirectional sensor. The 

 greater the flow speed before the negative step 

 the more rapidly the wake moves past the meter, 

 consequently shortening the time during which 

 the rotation rate is significantly influenced, 

 as seen in Fig. 11. 



Fig. 12 gives the flow speed indicated by the 

 rotor as its speed through the water is varied 

 in a somewhat irregular fashion. Some of the 

 characteristic response features described above 

 are readily discernible. Note that when the 

 speed varied almost sinusoidally with a period 

 of roughly h seconds (between time of 160 and 

 170 seconds in Fig. 12) the indicated speed fol- 

 lowed the mean trend but failed to indicate the 

 variations . The plotted points represent single 

 rotor revolution averages; somewhat better 

 response might have been obtained by shorter 

 averages, but at low speeds the previously dis- 

 cussed angular rotor speed variability due to 

 non-uniform torque would introduce considerable 

 error. 



123 



