B,32 • EFFECT OF SURROUNDING AIR ON JETS 

 the basis of Pabst's results. The computed examples cover the range 



■^ = 0.5, 0.25, 0.05 



U 



Ti 



= 1, 1.75, 2.5 



where Ui, Ti = outer stream velocity and absolute temperature respec- 

 tively 

 f/o^ Tq = jet exit velocity and absolute temperature respectively. 

 Pabst's measurements of velocity and temperature distributions in a 

 round jet were made with C7o = 400 m/sec, C/i = 18, 101, and 188 m/sec, 

 and To = 300°C (To/Ti ^ 2). Since Szablewski's account of this work 

 appeared to cover the significant points, the original work of Pabst was 

 not consulted. 



The following are the major conclusions: 



1. The theoretical predictions for small density differences agree with 

 other work regarding the direction of the effect of an outside stream, 

 namely to decrease the rate of spreading of both velocity and temper- 

 ature. The predicted asymptotic boundary (jet so far from nozzle that 

 nozzle size has no effect) varies as x^ for any value of U\ except zero. 



2. An outside stream reduces the rate of velocity and temperature fall 

 along the axis. This is qualitatively confirmed by experiment, but there 

 is some question about the accuracy of Pabst's temperature measure- 

 ments. The asymptotic variation predicted by theory is x-^ for any 

 value of Ui except zero. 



3. When Ui 7^ the asymptotic velocity profile is given by 



U - Ui 



[-(•r) 



where U^ is the velocity at the center, ri is the radius of the jet bound- 

 ary, and c is a shape factor. The temperature profile is 



T - T 



T..- T 



1 \U, - UiJ 



where Pn is taken as 0.5. These distribution functions when fitted to 

 Pabst's measurements at 16, 20, and 24 nozzle diameters downstream 

 show reasonably good agreement. 



4. Reduced jet density, due to elevated temperature, increases the rate 

 of velocity and temperature fall along the axis. Apparently Pabst's 

 work does not provide any test of this effect. However, this is con- 

 sistent with the findings of Corrsin and Uberoi for Ui = 0. 



5. Reduced jet density, due to elevated temperature, decreases the rate 



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