Modelling Aircraft Fuel Jettison Using Smoothed Particle Hydrodynamics (SPH)

Abstract

Aircraft that have a significant difference between the maximum take-off weight and maximum landing weight are often required to have a fuel jettison system installed and validated according to international regulations. The current method of validation is via flight testing, the expense of which gives motivation towards simulation, however conventional multiphase simulations have large computational requirements that discourage adoption.
Assuming a significant momentum difference exists between fuel and airflow, coupling in the multiphase model can be reduced to only affect the fuel phase, ignoring disturbances to the airflow, and greatly reducing computational complexity. Such an approach allows a precomputed aerodynamic solution, such as in the development of an aircraft concept, to be viable for modelling the fuel jettison system, saving regeneration of an entirely new mesh and solution, further minimising cost associated with simulating the fuel jettison case.
The method selected for modelling the liquid continuum is weakly compressible smoothed particle hydrodynamics (WCSPH), featuring adaptable equations that implicitly handle a free surface, and widespread development for single phase problems. The WCSPH model is coupled to an aerodynamic solution using a continuum correction to discrete droplet equations of motion, where two continuum models are considered: first, a pre-existing model that considers the continuum to exhibit drag of a flat plate with constant drag coefficient; second, a novel approach that considers the pressure induced by the interface geometry. Both use a linear interpolation, based on neighbourhood density, between discrete and continuum drag models to ensure retention of spherical drag in the event of a singular particle.
The models are validated using fundamental test cases: spherical droplet drag and breakup; jet in a crossflow; and jet in coflow, before being applied to fuel jettison cases including coupling to a vortex lattice method, and a range of two and three dimensional aircraft cases. Comparisons between the models are made to experiments where available, and fully coupled simulations, via volume of fluid (VOF), to provide comparison of the significance in neglecting one coupling direction. The original sphere-plate model is found to model breakup acceptably in flows where airflow is away from the surface tangent, such as droplet breakup or jet in crossflow, but is
severely limited in the jet in coflow case and fuel jettison cases. The induced pressure model performs better in these parallel flowing scenarios, inducing improved breakup across all cases, in agreement with VOF and experimental results, thus recommended as the preferred model for simulating fuel jettison. Significant cost improvement is observed, with 100 times less core time than an equivalent fully coupled simulation, with further improvement possible with code optimisation and transition to particle tracking, approximately 500 times faster again, when appropriate.

Date of Award	6 Dec 2022
Original language	English
Awarding Institution	University of Bristol
Supervisor	Thomas C S Rendall (Supervisor)