Fabula 



BRIDGE AMPLIFIER FILTER 



DUAL 



TRACE 



OSCILLOSCOPE 



MAGNETIC 



TAPE 



RECORDER 



Fig. 4 - Equipment for towing-tank 

 grid-turbulence recordings 



approximately prewhitened signal in order to record at a suitable signal-to- 

 noise ratio over the entire frequency range of interest. 



Ordinary methods were used to obtain the power spectral density functions 

 of the grid-turbulence samples. The noise-test recordings were analyzed in the 

 same way, and noise corrections were made to the grid-turbulence spectra, by 

 assuming that the "signal" and "noise" were uncorrelated. Because of the na- 

 ture of the results, it was generally inappropriate to convert the frequency 

 spectra of recorded bridge voltage fluctuation to turbulent velocity spectra. 

 Thus the results will be presented as curves of "relative spectral level" in db 

 units versus frequency, after correcting for signal conditioning, but not for sen- 

 sor sensitivity. 



RESULTS 



Signal Waveforms 



It was important in this work to show that certain spectral differences were 

 not due to a distortion of the turbulent eddy structure but rather to a fairly vmi- 

 formly distributed "raggedness" of the signal. This was done by photographs of 

 the bridge output oscilloscope trace. Many oscilloscope sweeps were put on 

 each picture by shifting the film between sweeps while the run was in process. 

 The sweeps are thus spaced at roughly equal intervals along the length of each 

 rvin. The time scale is 10 ms per major division, so that a major division cor- 

 responds to 0.36 cm of sensor travel, and each total sweep covers about one 

 quarter of a mesh length. A positive deflection of bridge output voltage (in- 

 creased heat transfer) produces a downward deflection on the photograph. The 

 variation of the vertical scale nominal velocity calibration, due to variation in 

 sensor sensitivity, is not large and is unimportant to the discussion. 



46 



