Investigation on supersonic high-speed internal flows and the tools to study their interactions

Air-breathing vehicles are characterised by a high level of integration between the propulsion system and the vehicle frame. Since the peculiarity of this type of aircraft is the absence of moving parts, before the air flow arrives at the combustion chamber, it must be slowed down to lower supersonic speeds, in scramjets, or to subsonic speeds, in ramjets, by means of a shock wave structure, called a shock train. The prediction and control of the shock train is important for the evaluation of the engine performance. This work aims to improve the understanding of the flow mechanisms occurring in the shock train as a consequence of the interaction of shock waves with the boundary layer in long and narrow ducts. A full pressure sensitive paint system was developed. Polymer-based and ruthenium-based compounds were identified as suitable for the investigation of the shock train in the wind tunnel. Before being able to collect experimental data, the design and manufacture of an indraft supersonic wind tunnel able to operate at mach numbers M= 2 and M= 4 was accomplished. The air at ambient conditions is drawn into the tunnel and then discharged into a vacuum tank with a volume of 34 m^3. Preliminary attempts to run the wind tunnel have identified the presence of leakages between the vacuum tank and the wind tunnel that prevented the establishment of the pressure difference required to obtain a supersonic flow in the test section. In support of the experimental approach, different flow configurations are numerically studied using the RANS equations. The k-w Wilcox model provided the most accurate results for such a complex flow field. Sensitivity studies are carried out since the characteristics of the shock train depend on several variables, including the duct geometry and the back pressure. The numerical findings revealed that the location of the shock train strongly varies with the grid size. Transient simulation is used to reproduce the shock train oscillation due to the pressure fluctuations that occur in the combustion chamber of an air-breathing aircraft. Under a sinusoidal forcing, the shock train executes a motion around its mean position that deviates from a perfect sinusoidal profile depending on the oscillation amplitude, frequency, and whether the pressure is first increased or decreased. With large oscillation amplitudes the shock train is greatly influenced by a pressure increase rather than a pressure drop, but the opposite is observed at small oscillation amplitudes. With varying forcing frequency, the shock displacement around its mean position decreases as the forcing frequency increases