Overset gridding is a technique that avoids the topological complexity of generating abutting, point-to-point connected, multi-block grids by allowing the component grid blocks to overlap. The price paid for that flexibility is the need to ensure a sufficient degree of overlap so the CFD solver can accurately share data between the component grids. The overlap is computed using overset grid assembly software. Users of OGA software typically rely on a circuitous collection of home-grown tools that are patched together. By integrating OGA (in the form of the PEGASUS and Suggar++ software packages) with Pointwise, the entire process is made more efficient
Other features in Pointwise Version 17.3 R1 include improved support for PLOT3D, ANSYS Fluent, STL, and NX files, a more robust T-Rex, anisotropic tetrahedral extrusion, hybrid mesher, and several user experience enhancements.
Pointwise, Inc. is solving the top problem facing computational fluid dynamics (CFD) today – reliably generating high-fidelity meshes. The company's Pointwise software generates structured, unstructured and hybrid meshes; interfaces with CFD solvers, such as ANSYS FLUENT, STAR-CCM+, ANSYS CFX, OpenFOAM, and SU2 as well as many neutral formats, such as CGNS; runs on Windows (Intel and AMD), Linux (Intel and AMD), and Mac, and has a scripting language, Glyph, that can automate CFD meshing. Large manufacturing firms and research organizations worldwide rely on Pointwise as their complete CFD preprocessing solution.
www.pointwise.com
The F-15E is a two-seat version of the United States Air Force's frontline fighter aircraft. It is used primarily for all-weather ground attack with the second pilot managing the complex sensors and weapon systems that differentiate the F-15E from the standard F-15. Adding the second seat changes stability and control slightly for most conditions, but can radically change flight characteristics in extreme maneuvers such as spins.
A spin is caused by asymmetric stall of an aircraft's wings and control surfaces. Stall occurs when the airplane reaches such a large angle of attack that the flow can no longer remain attached to the wings. This causes a sudden loss of lift and a rapid descent. If one of the wings stalls before the other, it induces a violent rotation in addition to the rapid descent. This is a spin.
Spins can be difficult to recover from and in fact some airplanes are impossible to pull out of a spin once they start. Obviously, it is important to find out if your airplane is one of these before you "take it for a spin". Traditionally, this is done through flight-testing. The aircraft is fitted with a small parachute in the tail that the pilot can deploy to hopefully break out of any unrecoverable spins. It is a risky business and does not always tell how to correct any problems that show up.
So spin analysis seems a natural place to apply computational fluid dynamics (CFD). An engineer sitting safely at her desk could model the flow around the spinning aircraft and thus gain insight into how it would behave. Unfortunately, the flow around a spinning fighter is separated and highly unsteady. Separated flow is difficult to model without properly applied turbulence models and an ample amount of grid points in the near-wake region. And the unsteadiness makes it expensive to analyze since many times steps have to be simulated.
At Cobalt Solutions, engineers have been working on this problem. They have developed Cobalt, an unstructured Navier-Stokes solver that works on hybrid grids. For analyzing separated flows they use an advanced turbulence modeling technique, Detached-Eddy Simulation, combined with hybrid grids, which provides a good balance between accuracy and computation time.
The engineers made an initial hybrid grid composed of approximately 6 million cells using Gridgen. This took about 3 days compared to the three to four weeks to make a structured grid. Near the surface of the airplane, where the boundary layer develops, they used prismatic, or wedge shaped, cells that align nicely with the boundaries to increase accuracy. Outside the boundary layer region, they clustered tetrahedral cells, which are easier and faster to build, in regions of highly unsteady flow. Thus they were able to make a grid that gave them the accuracy they needed in a relatively short time.
Using Gridgen for Unstructured/Overset Grid Generation
Unstructured grid generation simplifies CFD analysis of complicated geometries. Cobalt Solutions, LLC has used Gridgen®for many years to create unstructured grids around complex aircraft and several ground vehicles. Clients have used these grids for unsteady analysis at extreme conditions - such as massively separated flows and complicated flight maneuvers. In order to more accurately simulate flight maneuvers, even more geometrical detail is needed - specifically, control surface deflection.
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Several methods are available to model control surface deflections within a CFD analysis. Cobalt Solutions investigated the use of unstructured, overset grids to accommodate control surface deflection. In the past, overset grids have been used to model bodies in relative motion, e.g. weapons separation. In those cases, the position of the moving body is not known a priori, so clustering of grid cells is difficult. With control surfaces, the motion is prescribed and the placement of grid cells can be contained to areas near the control surfaces. Plus, the gaps between the aircraft body and the control surface, which are present on conventional aircraft, can be modeled. The overset capability in Cobalt has automatic hole-cutting which is useful for such a complicated set of overset grids.
Gridgen was chosen for grid generation because of its ability to create a high quality unstructured grid with prisms and tetrahedra within the confines of the problem - specifically, a complex aircraft geometry with extremely small gaps. Within Gridgen, the capacity to explicitly control grid cell size and the rate of cell size growth was helpful in this analysis. Additionally, the grid examination tools provided a quick and accurate means of inspecting grid quality in the overlapping regions.
The F-15 model with movable control surfaces is shown in Figure 1. Each control surface - left and right aileron, left and right flap, left and right elevon and left and right rudder - was treated as an independent grid in the overset system. Including the fuselage, nine grids were created and used for the CFD analysis. As seen in Figure 2, the eight blocks around the control surfaces overlap each other and the main F-15 grid.
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Gaps were approximately 0.5 inches and there were no physical connections between the control surfaces and the fuselage. The surface grids were generated using layers of anisotropic triangles to keep grid spacing equal on all sides of the sharp edges. This was useful especially at the sharp trailing edges and the sides of the control surfaces. The gaps between the wing and the front of the aileron and flap and the vertical tail and the rudder was curved to allow for rotation. A structured domain - one each on the front of the control surface and the rear surface of the wing or vertical tail - was diagonalized to create opposing surfaces of similar triangles. Doing this provided two grid attributes that are important for good overset interpolation - cell-size match-up and extent of overlap.
Each control surface grid consisted of a prism boundary layer and a tetrahedral inviscid region. The boundary layer prisms were created using block extrusion and each control surface block had approximately 20 layers of prisms. Beyond the prism layer, the grid block was filled with both anisotropic tetrahedra next to the prisms and isotropic tetrahedra on top of those. This allowed for a smooth transition in grid size spacing from the boundary layer region to the inviscid region. Grid sizes for the control surface grids ranged from 1.1 million cells to 1.7 million cells each.
The grid around the F-15 fuselage and wing was created in a similar manner as the control surface grids. Due to the complexity of the geometry, the boundary layer was limited to about 12 layers of prisms. Anisotropic cells were used to bring the boundary layer spacing out more for a better transition to the isotropic tetrahedra. The F-15 grid consisted of approximately 20 million grid cells for the complete aircraft.
The input to Cobalt consists of a file listing the grid files and their position relative to each other. Cobalt performs automatic hole-cutting during the solution process, which in this case, saves a lot of time. The motion of each control surface is provided by a motion file and each surface can move independently. Results are shown in Figure 3.
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With the ability to move control surfaces during a CFD simulation, engineers now are capable of simulating an aircraft flying through a maneuver complete with control surface movement replicating flight control movements.
One Of Larger CFD Models Ever Helps Optimize Advanced Fighter Aircraft
Mike Malone, Engineer/Specialist, Northrop Grumman Corporation, Pico Rivera, California
Silicon Graphics World, July 1998, page 14
One of the larger computational fluid dynamics (CFD) models ever developed helped to optimize the performance of an advanced fighter aircraft. The 5,000,000 grid point model was used by Northrop Grumman Corporation to investigate straight vertical landing for the Joint Strike Fighter, the U. S. Air Force's next generation combat plane. The model, which covered the entire exterior of the proposed aircraft, helped engineers investigate the effects of entrainment, which produces negative lift and must by counteracted by additional engine thrust. A special preprocessor automated most of the model creation process, including strategically distributing grid points for high accuracy while minimizing computation time.
Northrop Grumman Corporation, headquartered in Los Angeles, Calif., is a leading designer, systems integrator and manufacturer of military surveillance and combat aircraft, defense electronics and systems, airspace management systems, information systems, marine systems, precision weapons, space systems and commercial and military aerostructures. The company was formed in 1994 when Northrop Corporation acquired Grumman Corporation. Since then, it has acquired Vought Aircraft, the defense and electronics systems business of Westinghouse Electric Corporation, and Logicon Inc. Northrop Grumman employs about 52,000 people and reported sales last year of $8.1 billion.
Northrop Grumman is a principle member of the Lockheed Martin team in the competition to develop the Joint Strike Fighter (JSF). The JSF employs a direct lift system for short takeoffs and vertical landings, with uncompromised up-and-away performance. The JSF is an affordable, multi-service aircraft that will enter service in the next century with the U.S. Air Force, Maine Corps, Navy and the United Kingdom Royal Navy. America's armed forces will need as many as 3,000 JSFs to replace several different aircraft in service today.
The Advanced Short Takeoff/Vertical Landing (ASTOVL) design was a previous concept tested at near full-scale in the wind tunnel. One of the critical issues that arose during the development of the ASTOVL, and the JSF, was a concern over negative lift caused by close ground effects during vertical landing. When the plane is hovering close the ground, the jets at the front and rear of the craft 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.
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Hot gas ingestion is another related problem that can occur during vertical landing. It occurs when the jet engine inlet draws in hot gas from the fountain described above rather than clean air. As temperature of the gas ingested by the engine rises, the performance of the engine drops. this effect can interact with the negative lift phenomenon described earlier to cause serious problems during vertical landing. Another related concern is that the hot recirculating gas striking the bottom of the plane could raise its temperature high enough to damage its skin.
While a prototype of the ASTOVL had been built and tested in a wind tunnel, engineers felt that computer simulation would greatly streamline the design process. Wind testing provides flow and temperature information only at the limited number of points where sensors can be placed. Evaluating a different geometry requires an expensive and time-consuming modification of the prototype. A CFD analysis, on the other hand, provides fluid velocity, pressure, temperature and species concentration values throughout the solution domain. Engineers can usually modify the model in a matter of an hour or two in order to investigate a different geometry or boundary conditions. Simulation allows engineers to evaluate many more alternative designs in a short period of time and provides more information about each design they evaluate. The result is a better design in less time.
Simulating an object as complex as the ASTOVL, however, is a difficult challenge. The problem is the analytical model must include the entire aircraft exterior and at the same time capture many small details in order to achieve an accurate simulation. Conventional CFD preprocessors are not suited to the task. Meshing the entire aircraft is not difficult but maintaining the level of detail required to define such complex areas as the engine inlets would require a model with an enormous number of grid points. Such a model couldn't be solved in a reasonable period of time, even on the Cray C90 computers at NASA Ames Research Center that Northrop Grumman engineers have available.
But, Northrop Grumman engineers were aware of a preprocessor, Gridgen from Pointwise (http://www.pointwise.com), Bedford, Texas, that had been designed in cooperation with NASA for modeling tasks of this magnitude. A key advantage of this software program is its ability to divide a structure into contiguous subdomains called blocks that make it possible to use a fine mesh where needed to capture details and a coarse mesh elsewhere in order to minimize computational time. Gridgen also provides a number of powerful tools that automate many of the more difficult aspects of the meshing such as the asymmetrical allocation of grid points and smoothing the mesh to eliminate negative volume cells. Northrop Grumman engineers run Gridgen on an R8000 Indigo II workstation from Silicon Graphics, Inc., Mountain View, California.
To produce the mesh, engineers start with an IGES file defining the plane's geometry with NASA IGES surfaces, surfaces that conform to the IGES subset defined by NASA. After reading these surfaces onto Gridgen, engineers then decomposed the geometry into about 30 blocks. They defined block boundaries around areas that require close mesh spacing because of high pressure or flow gradients. Areas requiring close mesh spacing include areas of geometric complexity, such as the engine inlet and auxiliary inlet guide vanes, as well as the boundary layer around the surface of the aircraft. They drew connectors on the geometry to define the edges of the block domains, grouped the connectors to form surfaces (domains) and finally grouped the surfaces to form volumes defining blocks. In cases where the grids did not adhere precisely to the domains, the engineers used a Gridgen feature that automatically projects a grid onto the geometry.
The next step was distributing grid points along the connectors. Uneven spacing is desirable along most of these connectors. For example, fine spacing is usually needed at sharp and trailing edges of aerospace surfaces while the area in between can usually be quite coarse. Ideally, the mesh should start fine at the leading edge, then become continually finer. This type of mesh distribution is very tedious to produce by hand because of the need to define each grid point one by one on order to provide a smooth transition from a fine to coarse mesh. Instead, on this project engineers used a Gridgen feature that automatically distributes grid points along a connector based on any of a wide range of functions that can be specified by the user. In most cases, engineers used a hyperbolic tangent function to provide the spacing described above.
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The irregularity of the ASTOVL geometry meant that the initial grid had areas of negative and zero volume that would have made it impossible to analyze. 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. Fortunately, Gridgen provides an elliptic smoother that allowed the engineers to improve the quality of the mesh automatically applying elliptic partial differential equation methods. Engineers applied smoothness, clustering and orthogonality controls to improve the mesh. With each iteration of mesh improvement, the program provided a graphical display of negative and skewed volume cells. In only a few hours, they had produced an excellent quality mesh ready for analysis with Northrop Grumman's proprietary CFD solver.
The analysis took about 60 hours on a Cray C90 supercomputer. The results correlated very well with wind tunnel testing. Oil droplets placed on the floor of the wind tunnel were used to determine the flow streamlines at the ground plane. They matched up very well to the ground plane streamlines at the ground plane. They matched up very well to the ground plane streamlines produced as one of the results of the analysis. The most critical area, the stagnation zone where the two jets meet, was 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.
Gridgen Speeds F-35 Design
Lockheed Martin is currently developing the F-35 multi-role fighter aircraft in Air Force, Navy, and Marine variants. Each variant has different key performance parameters that must be met contractually, and many of the performance parameters are driven by aerodynamic considerations.
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Gridgen has played a significant role in the grids generated for aerodynamic design and drag evaluations of all three JSF variants. The flexibility of this tool for rapid structured grid generation was extremely important because it produced high-quality grids in the shortest time, which enabled tight schedules to be met.
In the case described here, CFD was used to redesign the STOVL variant of the F-35 for reduced drag and increased internal volume.
Figure 1 shows how the outer contours of the F-35 were changed from the baseline configuration. As you can see from the figure, the volume has been increased somewhat by pushing out the upper contour of the aircraft near the lift fan that is used for vertical takeoff and landing. In the CFD study, Lockheed Martin wanted to see how this would affect drag.
Lockheed Martin has refined its CFD analysis process over many years to achieve rapid turnaround times. It starts with a geometry database from CATIA V4 or V5. Gridgen is then used to build structured surface and volume grids which are exported to Falcon, a full Navier-Stokes CFD code developed in-house at Lockheed Martin. The results from Falcon are postprocessed with the Lockheed Martin Aero Loads Code and FIELDVIEW.
The Loads Code was used to determine the increments in lift coefficient, drag coefficient, and moment coefficient at each grid point. The results were then plotted in FIELDVIEW. (Figure 2)
Looking at the data in this manner shows a lift coefficient increase (blue) on the wing that was not apparent from looking at pressure distributions alone. Examination of the drag coefficient increment showed a problem area of increased drag (red) aft of the shape change that was not noticed before. This resulted in a subsequent modification to the shape change to reduce the problem area. This showed a trimmed drag coefficient reduction of 0.00062 compared to the baseline aircraft configuration at a cruise condition.
Based on AIAA 2006-3663, Use of CFD in Developing the JSF F-35 Outer Mold Lines, Perry A. Wooden and Jeff J. Azevedo, Lockheed Martin Aeronautics Company. This article is also available in PDF format.
