102
4 Transonic Wind Tunnels
Fig. 4.6 Reflection of disturbances on the top and bottom walls of the test section in supersonic
flow
in free flight. Techniques consisting of local deformation the wall as in transonic
tunnels to compensate for the wave reflections are difficult to implement because of
requirements for local curvature in the wall to achieve supersonic speed.
4.2.4 Double Throat Diffuser
Transonic wind tunnels are often equipped with a throat located downstream of
the test section or sometimes in the diverging section, a second throat. It normally
constitutes of movable walls made out of adjustable plates, creating a convergentdivergent supersonic nozzle and starting-up the tunnel involves the passage through
two flow regime of interest. First by setting the Mach number in a starting supersonic
nozzle depending on the ratio of the local section and the throat (see Sect. 5.1)
and an adjustment of the section of the second throat makes it possible to fix the
Mach number in the subsonic part of the test chamber with a very high precision
(a thousandth). The supersonic flow in the diverging part of the downstream throat
prevents the upstream propagation of disturbances generated downstream of the test
section. The flow in the test section is thus isolated from slight fluctuations in the
operation of the wind-tunnel fan. The higher power required to achieve a slightly
supersonic flow in the converging-diverging nozzle of the second throat must be
accounted for. Figure 4.7 shows the test section of the S8Ch transonic wind tunnel
equipped with a second throat, at ONERA, Meudon.
4 Transonic Wind Tunnels
Fig. 4.6 Reflection of disturbances on the top and bottom walls of the test section in supersonic
flow
in free flight. Techniques consisting of local deformation the wall as in transonic
tunnels to compensate for the wave reflections are difficult to implement because of
requirements for local curvature in the wall to achieve supersonic speed.
4.2.4 Double Throat Diffuser
Transonic wind tunnels are often equipped with a throat located downstream of
the test section or sometimes in the diverging section, a second throat. It normally
constitutes of movable walls made out of adjustable plates, creating a convergentdivergent supersonic nozzle and starting-up the tunnel involves the passage through
two flow regime of interest. First by setting the Mach number in a starting supersonic
nozzle depending on the ratio of the local section and the throat (see Sect. 5.1)
and an adjustment of the section of the second throat makes it possible to fix the
Mach number in the subsonic part of the test chamber with a very high precision
(a thousandth). The supersonic flow in the diverging part of the downstream throat
prevents the upstream propagation of disturbances generated downstream of the test
section. The flow in the test section is thus isolated from slight fluctuations in the
operation of the wind-tunnel fan. The higher power required to achieve a slightly
supersonic flow in the converging-diverging nozzle of the second throat must be
accounted for. Figure 4.7 shows the test section of the S8Ch transonic wind tunnel
equipped with a second throat, at ONERA, Meudon.