MIDENGRAD

High speed air flows generate density variations whose intensity increases rapidly with Mach number. In the flow region near the wall (the boundary layer), the appearance of intense local density gradients is the common cause of two issues around which the MIDENGRAD project focuses: acoustic radiation and aero-optical effects. The first of these two topics has implications for our ability to carry out hypersonic tests in ground facilities that can correctly reproduce the in-flight flow physics. There are very few hypersonic wind tunnels in the world that are said to be ‘silent’, none is in Europe, and there is every reason to believe that most of the data available to us will continue to bear the signature of the noise emitted by the boundary layers of the wind tunnel nozzles. In this project, we will develop a methodology for getting round this difficulty that is not based on eliminating noise - which is what the silent wind tunnel strategy hinges upon. Instead, we will experimentally characterise the noise of ‘noisy’ wind tunnels in order to incorporate it into our numerical models. This will remove a major obstacle in the process of validating models using noisy ground tests, allowing us to continue working with the vast majority of experimental data that will be available in the short and medium term. The second topic covers a new field at the intersection between fluid mechanics and photonics. It involves developing a capability for generating low-cost density fields that are representative of turbulent and hypersonic boundary layers. These synthetic fields will then be used to develop and calibrate methods to compensate for the aero-optical effects induced by the boundary layer on the light rays passing through it. The ability to see and communicate from hypersonic vehicles by controlling the hurdles to the propagation of wave fronts on these vehicles opens up new possibilities. Before that, however, a few scientific challenges need first to be tackled and overcome. They will be within the present project.