Numerical simulation of unconventional aero-engine exhaust systems for aircraft

2014-11-26T10:52:30Z (GMT) by Tim Coates
This thesis investigates the impact of upstream duct convolution on the plume development for high speed jets. In particular, investigations are carried out into an unconventional aero-engine exhaust systems comprised of a modified convergent-divergent rectangular nozzle where the converging section of the nozzle includes an S-bend in the duct. The motivation for this work comes from both the military and civilian sectors of the aerospace industry. The growing interest into highly efficient engines in the civilian sector and increasing complexities involved in stealth technologies for military applications has led to new design constraints on aero-engine exhaust systems that require further research into flows through more complex duct geometries. Due to a lack of experimental data into this area in the open literature validation studies are undertaken into flows through an S-bend duct and exhaust plume development from a rectangular convergent-divergent nozzle. The validation work is simulated using RANS CFD with common industrial turbulence models as well as LES with artificial inlet conditions. Subsequently, a CFD investigation into three unconventional aero-engine exhaust systems, with over-expanded conditions, with differing angles of curvature across the converging S-bend is undertaken using both RANS and LES methodologies governed by the validation work. As the curvature of the S-bend was increased it was found that the thrust and effective NPR both decrease. Whilst these changes were within acceptable levels (with some optimisation) for a circumferential extent of up to 53.1 the losses became prohibitive large at extents. For the ducts with a greater circumferential extents separation was seen to occur at the throat of the nozzle; this changes the design parameters of the nozzle leading to a higher Mach number and could potentially be harnessed to improve performance of the engine creating a `variable throat' nozzle. The impact of using different numerical solvers to simulate the flow through an unconventional aero-engine exhaust system has also been considered. The use of LES has shown that the octagonal, hexahedral and trapezoidal shapes initially observed in the development of the plumes of the RANS cases are likely to be an artifact caused by the RANS solver, as would the transverse total pressure gradients observed in the RANS cases at the nozzle exit as they are both absent from all of the LES results. Likewise the implementation of realistic inlet conditions has a significant impact on the development of the plume, particularly in the length of the potential core and the number of shock cells.