A grid fin is a lifting and control surface that differs from a typical solid fin. The grid fin uses a honeycomb design that allows air to pass through the fin rather than bypass around it. One advantage of grid fins is the ability to maintain lift at higher angles of attack since grid fins do not have the same stall characteristics as planar fins. Another advantage is their very small hinge moment, which can reduce the size of control actuator systems. A third advantage is that their curvature has little effect on performance, so that folding the fins down onto the missile body is a storage design bonus.

The U.S. Army Research Lab’s viscous CFD analysis method has created a visual representation of airflow around the missile and through the fins. This method also provides numerical parameters such as axial forces and drag coefficients that are needed to accurately predict the trajectory. One challenge in developing a viscous CFD model for this purpose was creating an analysis mesh of the grid fin geometry.

During the creation of the analysis mesh, DeSpirito realized he needed two different scales of mesh in this problem, a high-density mesh for the fin region and a lower density mesh for the rest of the missile. Joining these mesh regions would have taken a prohibitively long time to do manually. Instead, DeSpirito used a GAMBIT capability to build a nonconformal interface, one where two adjacent (and different) mesh regions can be joined. The FLUENT solver allows flow variables to be passed across this kind of interface, despite the fact that the meshes on either side are different.

DeSpirito used the implicit, compressible (coupled) solver to calculate the 3D flow field. The calculations were made at Mach numbers of 2, 2.5, and 3 and at several angles of attack. The analyses were run using six processors on an SGI Onyx 2 computer with R12000 processors at the ARL Major Shared Resource Center in Aberdeen Proving Ground, Maryland. The grid fin calculations took about 5 minutes per iteration. The solution converged in about 1500 iterations.

The analysis provided results that had not been available previously by computational means, such as the axial force and drag coefficient on the missile and individual fins. It also provided a visual image of the flow field, which has the potential to be very helpful in understanding issues including the effect of the flowfield on the fins or the impact of the shock waves coming off the fins. "CFD lets us gain insight into what the flow is doing," says DeSpirito. "We can see the interaction of recirculation regions and shock waves. These are things we could only speculate about before and they have improved our understanding of how the fins work."

The work that DeSpirito did indicates that viscous CFD analysis offers an accurate method for calculating the flow field and aerodynamic coefficients for missiles with grid fins. It gives designers more information than experimental or other numerical simulation methods, providing a tool that can help them optimize the use of grid fins. According to DeSpirito, this work has spurred new interest in grid fins, and the CFD method is currently being used to determine the flowfield and aid in the design of a canard-controlled missile.