The Problem:
The flow-field around a jet-borne vertical landing aircraft, such as the Harrier or Joint Strike Fighter, is extremely complex and highly unsteady. Re-ingestion of hot gas into the engine can result in lift loss for the aircraft, and in extreme cases the engine may surge. This problem is referred to as Hot Gas Ingestion (HGI).
The Challenge:
Historically, HGI has been assessed and reduced through scale model rig testing. With the desire to reduce costs, rigs have been de-commissioned with the challenge of modelling HGI being passed to computational methods. The calculations carried out within this element demonstrate the potential of unsteady CFD to predict Hot Gas Ingestion and have approached the problem in two ways: unsteady Reynolds Averaged solutions for a model Harrier aircraft descending towards the ground (Cambridge) and a Large Eddy Simulation of the worst case of the aircraft being close to the ground (Loughborough). The two sets of calculations use a common CFD code (Hydra) and a common mesh. In addition there was close collaboration with researchers at Surrey University who carried out the optimization of the code for HPCx.
Use of HPCx:
This flow problem is highly challenging due to two key issues: i) the need to represent geometrical detail such as auxiliary intake doors, flaps and strakes; ii) the disparity of time scales between the high speed jet and the low speed onset flow. The former issue leads to a large number of nodes and the latter means that the computational time step is very small leading to large numbers of timesteps to capture the longer time scales. The combination results in an extremely challenging computation. The mesh contains 17.3 million nodes and Large Eddy Simulations were run using 128 processors for an equivalent of 14 days. The mesh was the same as that used by Cambridge University to compute Unsteady RANS solutions of the aircraft descending towards the ground using a moving mesh.
Outcome:
Although the calculation was run for an insufficient time to achieve meaningful statistical information, instantaneous visualisation allowed a better understanding of the flow behaviour leading to Hot Gas Ingestion. For example the strakes and dam could be seen interacting with the upwash fountain, and vortical structures were observed entering the lower portion of the intake.
Future:
The methodologies developed here are being further developed for LES of compression systems under the CFMS SMURF programme.
References:
- Li, Q., Page, G.J. and McGuirk, J.J., Large Eddy Simulation of Twin Impinging Jets in a Cross-Flow, Aeronautical Journal, 111 (1117), March, 2007, 195-206
- Page, G.J., Li, Q., Richardson, G.A., McGuirk, J.J., Large Eddy Simulation of a Harrier Aircraft at Touch Down, 25th AIAA Applied Aerodynamics Conference, AIAA 2007-4294, June 2007
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