The IUSTI's supersonic group has recently developed, in collaboration with the CNES, a new academic shock wave induced separation experimental configuration, so called the "Free Separation", referring to the “Free Shock Separation” occurring in overexpanded nozzles. This 2D configuration is using a secondary subsonic flow with a static pressure large enough to promote the separation of the supersonic turbulent boundary layer, generating a detached shear layer which doesn't reattach downstream. From this pioneering configuration, it has been shown that the system has some physical similarities with the classical shock induced separation (shock reflections, compression corner, etc.). Nevertheless, significant differences have been observed when the unsteady properties are considered. In this case, it seems that the system acts as an amplifier of the downstream perturbations, typically from the subsonic secondary flow. An important finding was that the system reacts to the whole range of frequencies for the upstream perturbations originating from the incoming supersonic turbulent boundary layer, but it seems that the interaction is not sensitive to downstream perturbations with frequencies larger than about 1kHz.
This project aims to confirm and to complement these first results. The most challenging improvement of the set up will be to develop a new secondary flow, with similar mean properties (velocity, density), but with acoustic modulation controlled in frequency and amplitude. From previous experiments, the amplitude and frequency ranges of this acoustic modulation are known. Two possibilities will be tested:
The first configuration will used classical Hartman-Springer tubes, known to be able to generate in low-speed flows (less than 60ms-1) strong acoustic fields with a frequency defined by their geometry. This system will be implemented in the recompression ramp installed downstream of the secondary flow injection. The second configuration will use corrugated pipes in the secondary flow injection system: such pipes are known to generate intense acoustic fields, again defined from their geometry. This will control the amplitude and frequency imposed to the system in the downstream subsonic region. The analysis of the response of the interaction to these controlled excitations will be the main objective of this project.
The IUSTI's supersonic wind tunnel is well adapted for these experiments. It is a continuous blowing installation (up to 5hours) with constant stagnation conditions (temperature and pressure). This allows to generate free separated configurations with well controlled aerodynamic parameters. The specific metrology developed during previous collaborations with the CNES will be used in this study. The specific dual-PIV system, associated with unsteady wall pressure transducers will allow to derive spectral map in the whole fields for a wide range of frequencies (100Hz<f<150kHz). Two points spectral analysis will allow to derive coherency and phase relationship. This will give a global overview of the space-time properties of the system, for the different acoustic excitations imposed on the secondary subsonic flow.
The last objective will be to generalize this analysis to the so called "constraint" configuration: this configuration includes, as in over-expanded nozzles, an expansion in the location of the separated shock. This configuration has been validated in the previous studies (Naima Demni's PhD granted by the CNES and AMU’s Labex MeC) and must be completed. It will benefit from the previously described controlled secondary flow.
The results associated to this study will make it possible to characterize the dynamics of this 2D free separation, and especially to address the source of unsteadiness observed in Free Shock Separation. The analysis of the low pass filtering mechanism observed on the downstream excitations will be the keypoint of this research work. It will be compared with available results on the classical separated shock wave boundary layer interactions, where the flow reattaches downstream. In particular, the links between the separated shear layer unsteadiness and the separated region will be characterized in this new configuration where the separated shear layer is not reattaching downstream.
These results will provide a framework for comparisons with axisymmetric configurations and will produce a database that will be available for Large Eddy Simulations and stability analysis studies.
