Effect of water vapor on the AEDC Tunnel 9 Mach 14 nozzle flow

The Air Force's premiere aerodynamics and aerothermodynamics test facility is the AEDC Hypervelocity Tunnel 9. This wind tunnel uses a high-pressure and temperature supply of nitrogen to produce long-duration flows at a wide range of Reynolds numbers at Mach numbers up to 14. It is well known that the flow in the Tunnel 9 Mach 14 nozzle is not of optimal quality, with several apparent waves crossing through the test section. Presently, the source of these waves is unknown. In collaboration with AEDC personnel, we are working to determine the source of the flow non-uniformities using advanced computational fluid dynamics codes. The current assumption in the literature and within the Air Force Test and Evaluation community is that these waves are caused by small imperfections in the nozzle geometry, particularly at the joints between nozzle sections. Furthermore, the effects of vibrational relaxation are also not fully understood in the Tunnel 9 facility. Careful CARS measurements in the Tunnel 9 test section cannot detect the presence of vibrational nonequilibrium, while state of the art simulations show that nitrogen vibrational modes freeze at the throat temperature. Therefore, in this paper we evaluate whether low levels of water vapor could cause rapid vibrational relaxation in the nozzle, and we introduce a novel grid generation approach that may make it possible to more accurately model small irregularities in the as-built Tunnel 9 Mach 14 nozzle. In the longer term, this work will provide a new validated hypersonic nozzle flow simulation tool for future Air Force facility designs and upgrades.