Supersonic compressors always held a special place in the heart of compressor designers. Some of them always tried to stay as far away as possible (too much losses!!!) or had to live with them (a fan got to be supersonic at the tip, accept it!), while others simply loved them and achieved some of the greatest understandings of compressible flows in general. Prof. Frans Breugelmans was one of them, together with Kantrowitz, Starken, …

In the 70s, Frans designed some of the most incredible machines. The apex was reached with his Mach 4 axial compressor (yes folks, inlet axial Mach number was supersonic too!) that then was tested (not without struggle) in the first version of the VKI R4 wind tunnel, running on Freon for the occasion. Then years went by, compressors turned into more and more subsonic machines and legislation forced to get rid of Freon. In other words, VKI lost its ability of testing supersonic compressors. Until today!

Going flow-wise, the first component of the tunnel is the modular inlet duct. Flat duct walls (for subsonic testing) or convergent-divergent ones can be swapped in a matter of minutes, still preserving the position of inlet instrumentation. An inlet turbulence grid can also be installed! Every wall of the duct is equipped with boundary layer suction. Each suction slot is independently connected to the facility vacuum tank through a regulation valve, hence allowing for a precise tuning of the suction mass flow on every surface. The top wall features a tuneable non-reflective surface.

The cascade is mounted on circular end walls together with most of the instrumentation. This allows for a quick variation of the inlet flow incidence (in subsonic conditions) while minimizing instrumentation alignment errors (perfect test-to-test back-to-back repeatability). The end walls of CT-2ss feature an optical access covering the whole set of blades pitch-wise and covering 1.5 chords of the cascade streamwise. This is currently employed for Background Oriented Schlieren (BOS) and is equipped with throat suction slots to promote, if needed, the starting of the cascade. One set of wall static pressure taps is placed upstream of the leading edge plane and one set downstream of the trailing edge plane, where a multi-hole probe is also traversed. The latter is operated by a high-accuracy high-speed traversing system. Motorized tailboards provide periodicity adjustment and throttling in a fully automatic way.

Stable supersonic conditions are maintained for about 0.7 s, in full Mach and Reynolds numbers similarity with respect to engine conditions.

In the next months to come, CT-2ss will be very busy in generating the experimental dataset for the validation of novel supersonic compressor airfoils, designed in partnership with Safran Aero Boosters within the WINGS initiative to support the development of the forthcoming open-rotor architectures, but bigger plans are already being prepared for the future!

This success could not be possible without some few and good people. A big thanks goes to Julien Clinckemaillie, the true father of CT-2ss, to Monica Veglio and Remy Princivalle for the great advices and support, and to the whole Facility team for the hard and efficient work: Abdellah Jbilo Kebbor, Louis Duculot, Alessio Ferro, Francesco Cospito and the VKI Design Office!

Let me thank you with a nice BOS visualisation at M=1.4, stay tuned for more updates!