Understanding Thrust Reversers

How we used the latest techniques in computational simulation to create world class engineering models for the design of thrust reversers for next generation aircraft engines.

The next generation of passenger jets will need much more efficient engines with a larger diameter for improved propulsive efficiency but also shorter to save weight and reduce friction.

Engine manufacturers have made a lot of progress in developing new technologies for the fan and turbine – however, a critical part of the engine is the thrust reverser which creates a powerful jet, directed forwards to help the aircraft slow down quickly and safely during landing. How do we maintain the effectiveness of thrust reversers with less available space on the engine?

In addition to making thrust reversers more compact but without losing efficacy, there is another important consideration which is whether the air from the thrust reverser on a shorter engine has an increased likelihood of being sucked back into the front of the engine – also known as “re-ingestion”. This could create unsteady flow conditions for the engine fan which in turn could lead to premature fatigue with possible safety and maintenance cost implications.

The TRUFLOW project was setup to create and test a new generation of computational simulation methods and tools – often referred to as computational fluid dynamics or “CFD” – to better understand and improve the design of thrust reversers. The project also conducted what may be the world’s first industrial scale wind tunnel test to better understand the aerodynamics and performance of thrust reversers – and crucially to validate the computational methods and tools which we developed.

The flow around and through an engine with the thrust reverser deployed is extremely complex and requires a very advanced computational model to give useful results. In particular, the cascade of turning vanes which direct the jet forwards to generate the braking thrust requires a lot of detail and computational power.

ARA selected a traditional model of engine reflecting the designs commonly in use today. The computational simulation of this engine with thrust reverser deployed predicted that, for a condition very close to maximum thrust, there may be a small amount of re-ingestion of flow from the thrust reverser jet back into the engine fan.

The next step was to create a physical model of the engine to test in a wind tunnel and find out how accurate these computational simulations were.

We created a design for the scale model engine including a section of wing and a support strut which would suspend the assembly inside the wind tunnel.

The engine was detailed and included a rotating fan as well as the cascade of turning vanes for the thrust reverser.

The fan was the most challenging component to machine due to the delicate nature of the fan blades and the high precision and tight tolerances necessary to ensure it delivered the required pressure ratio for the wind tunnel test.

The model engine assembly and each constituent component were carefully inspected to ensure that they accurately represented the design used for both the physical model and the computational simulation.

The completed engine model was sent to the wind tunnel facility where a hydraulic motor was installed which would drive the fan. The engine model was positioned close to a false floor inside the wind tunnel which simulated the runway.

UV illuminated oil flow and particle image velocimetry or “PIV” measurements were taken to capture the behaviour of the flow around the engine with different power settings for both the engine and the thrust reverser jet.

PIV works by seeding the airflow upstream of the test model with tiny particles whose speed and direction can be tracked when measured in a sheet of laser light projected in a region of interest.

The experimental results were found to agree well with ARA’s computational simulation methods and tools, in particular the prediction that there would be re-ingestion of flow from the thrust reverser jet into the engine fan for a small number of high engine thrust conditions.

Computer Prediction (CFD)

Experimental Result (PIV)

Arrows indicate local flow direction. Careful examination of the flow underneath the nacelle at the front shows that in both the CFD and PIV image a small amount of flow travels forwards along the surface of the nacelle before being sucked back into the engine intake.