B,32 • EFFECT OF SURROUNDING AIR ON JETS 



of the jet was about the same as that of the surrounding air. The jet was 

 therefore cold, the initial density being 1.5 times that of the surrounding 

 air. The density and velocity were examined across the mixing zone from 

 2 inches to 7|- inches from the nozzle. The distributions were similar at 

 each cross section, and the velocity distribution could be represented by 

 ToUmien's theoretical curve for incompressible flow [131] in the subsonic 

 portion of the mixing region. Such distributions have the typical s-shape 

 of the Gaussian integral curve, and they reduce to a common curve for 

 different values of x when plotted against a-y/x, where cr is a scale factor. 

 The width of the mixing region is thus inversely proportional to o-. The 

 value for incompressible flow is generally around 12. Gooderum, Wood, 

 and Brevoort found a = 15. The rate of spreading into the jet core and 

 into the surrounding air was therefore less than that for incompressible 

 flow. This would appear to disagree with the trends found by Abramovich, 

 which of course apply to subsonic flow, but is what would be expected 

 from the density effect found by Corrsin and Uberoi in the round jet. 



Similar results were reported by Bershader and Pai [132] from meas- 

 urements on the discharge from a rectangular nozzle 1 by 2 cm at a Mach 

 number of 1.7. Density measurements were made with an interferometer 

 at several closely spaced stations within one nozzle width from the orifice. 

 The density distributions were found to be similar, and a was found to 

 be 17. The mixing zone was thus narrower than that for incompressible 

 flow. The profile of density ratio is in reasonable agreement with a curve 

 based on Pai's theory [133] which employs the concept of a constant 

 coefficient of eddy kinematic viscosity of the form of Eq. 29-1. 



These experimental results on supersonic jets do not distinguish be- 

 tween the effect of a denser jet and the heating effect resulting from 

 internal dissipation. We might assume, however, as pointed out by Pai 

 in relation to laminar flow, that the greater momentum associated with 

 higher density causes the stream to carry farther and thus decrease the 

 divergence of the mixing zone. Evidence to substantiate this assumption 

 is afforded by the work of Keagy and Weller [134] who found wider ve- 

 locity profiles for helium jetting into air and narrower profiles for carbon 

 dioxide. It may be concluded from this that the observed effects are pri- 

 marily density effects. Taken as a whole, the observations disagree with 

 the theoretical predictions of Abramovich. 



B,32. Effect of Axial Motion of Surrounding Air on Jets. When 

 a jet is projected rearward from a vehicle moving through the air, it effec- 

 tively emerges into a surrounding medium in motion in the same direction 

 as the jet. Some attention is now given to the effect of this motion on the 

 characteristics of the jet. We do not consider other cases, Hkewise of im- 

 portance, where the jet is projected forward or at an angle to a moving 

 stream, 



< 179) 



