Modelling the vortices in jet engines

Supersonic jet physics is hellishly complicated, but Chinese researchers are making gains in understanding it. Andrew Masterson reports.

Illustrations showing the modelling of forces operating within a jet engine at supersonic speed.
Illustrations showing the modelling of forces operating within a jet engine at supersonic speed.
Zhaoxin Ren, Bing Wang and Longxi Zheng

The physics of jet engines becomes significantly more complicated at supersonic speeds, presenting serious challenges for designers and engineers.

Now, however, the task of understanding the complexities of the relationships between airflow, temperature and fuel has become a little clearer thanks to modelling work done by researchers led by Zhaoxin Ren of China’s Northwestern Polytechnical University.

At slowish speeds, jet engine functions are reasonably simple. Optimum performance arises from adjusting the airflow in order to increase temperature and pressure. This permits the ideal level of combustion: burning the right ratio of fuel and air produces appropriate energy to overcome drag and produce acceleration.

At supersonic speeds above Mach 1, and especially at the super-high speeds achieved by modern scramjet technologies, the picture becomes much more complex. Gravity and drag are joined by supersonic shockwaves. The combination affects the development of the dynamic features created by turbulent flow – whirling masses of air called vortices – that in turn multiply the possible behaviours of all the particles within the system.

The result is a very large number of variables, including things such as the mass fuel load, the intensity of shockwaves and the reflective properties of metal casings, all of which can affect the efficiency of the engines.

Real-world analysis of all these possible permutations is by its nature a long, loud and expensive business. In an attempt to truncate the process, therefore, Ren and his colleagues turned to computer modelling.

Using a combination of custom-made simulation codes and an approach to mapping two-phase flows in continuum mechanics known as the Eulerian-Lagrangian method.

Despite the current and increasing importance of supersonic scramjet engines, the research comprised the first time such modelling had been attempted.

“Currently, no commercial software can simulate the supersonic combustion problem because it requires high-order numerical schemes to compute supersonic flows with complicated evolved shocks, as well as corrected models to describe the droplet dynamics, both of which we carefully consider in our in-house simulation codes,” says co-author Bing Wang.

“Direct numerical simulation can capture the full scales of flows involved in the shock-vortex interaction.”

The results of the exercise provide unparalleled detail for understanding induced combustion modes and the behaviour of refracted waves combined with chemical refractions. The findings will help inform new engine designs.

“The scramjet engine is the most favourable option for high-speed flight at Mach 6 or more,” adds Wang.

“Understanding the complicated physical mechanism of supersonic combustion and the impact of incident shock waves could help engineers choose the best combination of mixing and combustion through installing movable components in the combustor.”

The research is published in the journal Physics of Fluids.

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