Computational optimisation and analysis of a truncated hypersonic nozzle for X3 expansion tunnel

Large or full size model testing capability is essential for ground test facilities to replicate key flow features encountered in real flight. The X3 expansion tunnel at the University of Queensland is undergoing upgrades to accommodate both full-scale Hayabusa re-entry vehicle model testing and high Mach number scramjet combustion experiments. The former necessitates a core flow of 400mm in diameter, and the latter entails flow angles below 2° within a core flow diameter of no less than 200mm. To meet the requirements, a Mach 12 nozzle was designed. With conventional full-length nozzle design methodologies, the length of the nozzle would be too large to be feasible, due to not only spatial constraints but also the fact that a significant proportion of the nozzle exit flow would be the boundary layer. Therefore, computational optimisation of a truncated nozzle was carried out, and without enough flow-straightening, it became more difficult to achieve small flow angles. A Navier-Stokes flow solver with a 5-species air finite-rate reaction scheme was employed for the objective function evaluation, with the initial design input a truncated inviscid nozzle contour derived using the method of characteristics. To suppress contour contraction and the generation of shock waves and centreline disturbances in the nozzle flow, associated penalty functions were implemented. CFD simulation of nozzle exit flow development in the dump tank indicated that the nozzle could be further shortened since free expansion in the test section is equivalent to that within the nozzle over the truncated section of the same length, resulting even lower cost and smaller axial vacuum force.