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Rapid meshing tool
Keywords Applied Computing and Engineering, Mesh generation, Landing, Safety
Applied Computing and Engineering announce Gridgen, a rapid meshing tool for CFD analysis. According to ACE, engineers have, until now, been unable to generate CFD meshes that accurately reflect a components complexity; it just took too long.
Using Gridgen, engineers can reportedly generate accurate rate meshes in minutes rather than hours, days rather than months. A key advantage claimed of this software program is its ability to divide a structure into contiguous sub-domains, called blocks, that make it possible to use a fine mesh where needed to capture details and a coarse mesh elsewhere in order to minimise computational time.
ACE reports that Gridgen also automates many of the more difficult aspects of meshing such as the asymmetrical allocation of grid points and smoothing the mesh to eliminate negative volume cells. The company points out that it also provides an elliptic smoother that allows engineers to improve the quality of the mesh automatically applying elliptic partial differential equation methods.
Safer landings for Joint Strike Fighter
The tool has been used to optimise the straight vertical landing of the Joint Strike Fighter (JSF), multi-service, aircraft that is scheduled to enter service with the UK Royal Navy and US Air Force, Marine Corps and Navy.
The JSF employs a direct lift system for short take-offs and vertical landings. A 5,000,000 grid point model covering the entire exterior of the JSF was used by Northrop Grumman Corporation to optimize the engine thrust needed to achieve a safe vertical landing.
One of the critical issues that arose during the development of the Advanced Short Takeoff Vertical Landing (ASTOVL) design for the JSF was a concern over negative lift caused by close ground effects during vertical landing. When the plane is hovering close to the ground, the jets at the front and rear of the aircraft hit the ground, move towards each other along the surface, then form a fountain when they meet that rises to hit the bottom of the aircraft. The result is a recirculation flow that creates a low-pressure zone around the bottom of the aircraft, often producing negative lift that would cause the aircraft to drop to the ground if it were not offset by sufficient engine thrust.
Meshing the entire aircraft is not difficult, we are informed, but maintaining the level of detail required to define such complex areas as the engine inlets would have required a model with an enormous number of grid points. In ACE's opinion, conventional CFD meshing software is not up to the task. Such a model they claim could not be solved in a reasonable period of time, even on the Cray C90 computers at NASA Ames Research Centre.
The irregularity of the ASTOVL geometry also meant that the initial grid had areas of negative and zero volume that would have made it impossible to analyse. Moreover, it is thought that with a conventional grid generator, Northrop Grumman engineers would have been forced to modify the grid element by element to improve its quality, a process that would have taken months or even years.
Gridgen reportedly allowed the engineers to produce an accurate mesh - one that could be modified quickly - and using simulation enabled them to evaluate many more alternative designs in a short period of time and provide more information about each design they evaluated. The result is believed to be a better design in less time.
The CFD analysis, we are informed, took about 60 hours on a Cray C90 super computer, and the results are said to have correlated very well with wind tunnel testing. The most critical area, the stagnation zone where the two jets meet, was also said to be precisely predicted by the analysis. Confident in the accuracy of the model, engineers used the results to determine the amount of thrust required to achieve a safe vertical landing.
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