GEM Research in the U.S. 285 
Ve 
Ca opt oe 
EA WSS WN 
IS Seat OI Y Cr Test 
Nozzle @-—— Sr WY ‘ 
TITITTTIRFIV IVI TIT IVIFITITIT?. Sa Section 
Gap h 
Diffuser-Recirculation Open-Jet 
Vehicle Wind Tunnel 
Simplified Idea] Theory: Wind Tunnel Analogy: 
Fig. 7. Diffuser-recirculation system 
Most of the research and development effort in the diffuser-recirculation area is con- 
cerned with modifications and variations of the basic system described. The practical out- 
look for the basic diffuser-recirculation system is not encouraging, primarily because of the 
following considerations: 
1. The geometric relationship between the nozzle G and gap 4 is rather critical. This 
relationship can be established properly only for a single operating height; and variations in 
h from wind and surface disturbances cannot be avoided. Particularly disconcerting is the 
occurrence of an unstable condition, wherein lift force decreases with decreasing height, at 
heights below the optimum. 
2. In practice, as in theory, the performance is totally dependent upon efficient diffu- 
sion (deceleration and static pressure recovery) of the flow as it moves inward between the 
vehicle base and the ground. Irregularities in the surface over which the vehicle moves 
would make efficient diffusion impossible. 
An interesting analogy can be drawn between the diffuser-recirculation vehicle and the 
open-jet wind tunnel, also sketched in Fig. 7. The wind tunnel is also a closed system in 
which the maximum velocity occurs at a section (the test section) where the flow is exposed 
to the atmosphere. The power dissipation of the wind tunnel would be zero with inviscid 
flow; and, like the vehicle, its actual performance depends upon an efficient diffusion proc- 
ess. Tunnel performance is often expressed by the “energy ratio” E, defined as the ratio of 
the kinetic energy flux through the test section to the power dissipation. Applying this 
terminology to the vehicle, taking the gap area AC as analogous to the test section area, 
gives 
646551 O—62——20 
