130 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 1 



heater current or the voltage, and the current or voltage variations 

 are used as a measure of the gas flow velocity. The heat loss, and 

 hence the square of the heater current /, is approximately propor- 

 tional to the square root of the gas velocity v. 1 



P = BVv + c 



The resistance-variation method is usually preferred, although 

 the sensitivity obtained by the constant-temperature method is 

 superior. 



The method is applicable for flow velocities ranging from 0.5 cm/ 

 sec up to very high velocities in the supersonic range. 

 The application of the method in subsonic and super- 

 sonic flow is extensively discussed by Kovasznay. 2 



Temperature variations of the gas will cause changes 

 in the transducer output; the temperature influence 

 can be reduced considerably and readings practically 

 independent of the gas temperature can be obtained 

 if the temperature of the wire is high. Another 

 means to reduce the influence of gas-temperature 

 variations upon the transducer output consists of a 

 Fig. (1-6)12. bridge circuit in which one wire is exposed to the flow- 

 Direction-sen- • an( j a w j re m an a( j • acent bridge arm is exposed 



sitive hot-wire & & •' ° . 



p ro be. to the resting gas by mounting it in a shielding tube 



in the gas stream. 3 

 The response varies as the position of the probe changes with 

 respect to the flow direction; it is a maximum if the wire is perpen- 

 dicular to the flow direction. Arrangements with two wires set under 

 an angle permit the identification of the angle of flow. Figure (1-6)12 

 shows an arrangement for measuring airflow direction with two 

 wires set under an angle of about 10°. 4 The same authors also use 

 three- and four-wire arrangements for three-dimensional direction 

 analysis and temperature-independent flow measurement. Fay 5 de- 

 scribed an arrangement with six wires which measures velocity 



1 L. V. King, Phil. Trans. Roij. Soc. {London), (A) 214, 373 (1914); see also 

 J. M. Burgers, in W. Wien-F. Harms (eds.), "Handbuch der Experimental- 

 physik," 4th ed., part I, p. 637, Akadem. Verl. Ges. m.b.H., Leipzig, 1931. 



2 L. S. G. Kovasznay, Physical Measurements in Gas Dynamics and 

 Combustion, in R. W. Ladenburg (ed.), "High Speed Aerodynamics and 

 Jet Propulsion," vol. 9, sec. F2, Princeton University Press, Princeton, N.J., 

 1954. 



3 G. S. C. Thomas, Phil. Mag., (6) 39, 505 (1920). 



4 L. F. G. Simmons and A. Bailey, Phil. Mag., (7) 3, 81 (1927). 



5 R. D. Fay, J. Franklin Inst., 183, 785 (1917). 



