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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.

Figure 1: F-15 with control surfaces +
F-15 control surfaces

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.

Figure 2: Grid blocks on the eight control surfaces +
F-15 control surface blocks

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.

Figure 3: Pressure contours on the surface of the F-15 show the effect of control surface movement. +
F-15 control surface blocks

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.

Ken Wurtzler
Director of Operations
Cobalt Solutions, LLC
May 2009