Fabul2 



Fig. 1 - Towing tank during a grid-turbulence run 



A better view of the sensor carriage with the sensor -carrying, surface- 

 piercing strut, is given in Fig. 2. The strut is faired to reduce vortex-shedding 

 excitation and is mounted on a simple vibration-isolation moimting of very low 

 natural frequency. The sensor is connected to its constant-temperature bridge 

 by the long cable seen in Fig. 1. 



Two types of hot-film flowmeters were used with constant-temperature 

 bridges. The thermistor type developed by Lumley (4) was initially emphasized. 

 An advantage of thig type is its small size, with the diameter of the sensitive 

 area being about 0.006 in. In water, the sensor must have a coating to keep wa- 

 ter out of the thermistor film, but the glass coatings used for this purpose have 

 been tinreliable." Thus most of the work was done with quartz- coated platinum 

 film sensors manufactured by Thermo-Systems, Inc., of Minneapolis, Minne- 

 sota. Figure 3 shows the form and relative size of the sensor types used in this 

 work. Names were usually assigned to the various sensors as a convenient 

 means of identification. 



Due to the low frequency range in this work, the frequency response of the 

 bridges used is flat, and only the frequency response of the sensors is of con- 

 cern. Their comparative frequency response in water was evaluated by meas- 

 urements of a standard grid-turbulence spectrum. Significant frequency re- 

 sponse drop-off was found for circular-cylindrical sensors and for a glass-coated 

 thermistor. For the circular cylinders, Lighthill's analysis (5) was used to 



*Modifications made since the completion of this work have eliminated this 

 difficulty. 



42 



