Chapter 4
Transonic Wind Tunnels
4.1 Definition of the Transonic Regime
As the air flow around an aircraft is not uniform, locally supersonic zones may exist
even under subsonic upstream flow condition, this defines the transonic regime, a
condition where both subsonic and supersonic regions coexist. This situation significantly modifies the local behaviour of the airflow. Transonic phenomena were
responsible for the numerous failures and crashes during the first attempts to cross
the sound barrier, that is, to fly at a speed greater than that of the sound (Mach number greater than 1). Similarly, in a supersonic upstream flow, there could be subsonic
regions that influence the overall flow if they are significant in volume. The aerodynamic phenomena induced by the coexistence of subsonic and supersonic regions
are complex and justify the definition of a so-called transonic flight regime. It is
considered that transonic phenomena can be significant in flight conditions between
Mach 0.7 and Mach 1.2. This is currently the cruise condition for all commercial jets
in operation.
Transonic wind tunnels, in which freestream velocities close to the sound velocity
are reached, have been the subject of many developments because of the criticality
of the transonic phenomena, their strategic importance, and the particular design
problems they address. As shown in Fig. 5.1 of Chap. 5, in the vicinity of Mach one,
a very small variation of the test section area accelerates the flow from subsonic to
supersonic with drastic changes in its behaviour. Accordingly, the presence of the
model in the test section causes a local decrease in the cross sectional area available
to the fluid which then becomes a throat, so this results in a sonic blockage of the
flow due to the Mach number being sonic at the model location. The un-choking of
the flow can be achieved by various techniques that were intentionally kept secret in
the 1950s.
© Springer Nature Switzerland AG 2020
B. Chanetz et al., Experimental Aerodynamics,
Springer Tracts in Mechanical Engineering,
https://doi.org/10.1007/978-3-030-35562-3_4
97
Transonic Wind Tunnels
4.1 Definition of the Transonic Regime
As the air flow around an aircraft is not uniform, locally supersonic zones may exist
even under subsonic upstream flow condition, this defines the transonic regime, a
condition where both subsonic and supersonic regions coexist. This situation significantly modifies the local behaviour of the airflow. Transonic phenomena were
responsible for the numerous failures and crashes during the first attempts to cross
the sound barrier, that is, to fly at a speed greater than that of the sound (Mach number greater than 1). Similarly, in a supersonic upstream flow, there could be subsonic
regions that influence the overall flow if they are significant in volume. The aerodynamic phenomena induced by the coexistence of subsonic and supersonic regions
are complex and justify the definition of a so-called transonic flight regime. It is
considered that transonic phenomena can be significant in flight conditions between
Mach 0.7 and Mach 1.2. This is currently the cruise condition for all commercial jets
in operation.
Transonic wind tunnels, in which freestream velocities close to the sound velocity
are reached, have been the subject of many developments because of the criticality
of the transonic phenomena, their strategic importance, and the particular design
problems they address. As shown in Fig. 5.1 of Chap. 5, in the vicinity of Mach one,
a very small variation of the test section area accelerates the flow from subsonic to
supersonic with drastic changes in its behaviour. Accordingly, the presence of the
model in the test section causes a local decrease in the cross sectional area available
to the fluid which then becomes a throat, so this results in a sonic blockage of the
flow due to the Mach number being sonic at the model location. The un-choking of
the flow can be achieved by various techniques that were intentionally kept secret in
the 1950s.
© Springer Nature Switzerland AG 2020
B. Chanetz et al., Experimental Aerodynamics,
Springer Tracts in Mechanical Engineering,
https://doi.org/10.1007/978-3-030-35562-3_4
97