









































/ 

















zX-'^T-y^ 





















/ 





















/ 





















/ 





^ 



0.05^ 



■^ 













/ 





^ 

















/ 



X 





^ 



-^ 



0.10 











A 





-^ 



'^ 







0.20 











^ 







-^ 















^ 



^ 







■ — ' 









0.40 





A 



^ 





















Figure 4 - Effect of a Surface Film on the Sensitivity of a Hot Wire 

 The response of a constant-temperature wire becomes 



[a 

 L_ 

 \/v~ 

 i+|/-s- 



\u «Ar 



V 1 +aAr. 



If a is small in these equations the wire may be just as sensitive to temperature fluctuations 

 as to velocity fluctuations. In the towing tanks at TMB there is sometimes a temperature dif- 

 ference of about 10 F over a 20-ft depth, and some thermal microstructure may be expected 

 when the water is disturbed. A hot wire must be used with caution in such facilities. 



The response of a bare constant-current hot wire to a step-like change in convective 

 cooling is given to a very good approximation by a simple exponential function 



^T. 



AT* [1 - e 



-t/M 



12 



