Just a few years ago, the use of computational fluid dynamics (CFD) in most aerospace companies was restricted to pure research or troubleshooting problems with existing designs. But in the past few years, newly available CFD tools are fully embedded in the mainstream mechanical design environment and hence much easier, faster and less expensive to use.
The trend toward using CFD early stages in the design process has resulted in a large number of new users of this powerful analysis technique. Bell Helicopter technical specialists have developed a set of CFD best practices with the goal of guiding users in following company design guidelines and optimizing the analytical processes. The tools are based on guidelines developed for use by engineers working on propulsion systems. The guidelines are based on the application of Mentor Graphics Mechanical Analysis Division’s FloEFD suite of CFD software used by Bell Helicopter for internal flow simulations.
These best practices empower design and analysis engineers to analyze concepts during the first gates of the design process, so that problems can be corrected early and performance can be optimized at the lowest possible cost. These CFD tools utilize native 3D data and provide automatic gridding of the flow space, minimizing the need for engineers to expend significant effort on the numerics of CFD.
Verifying Solid Model Quality
The utilization of native 3D data places a premium on the quality of the solid model. For an internal flow model with minimum mesh requirements, the solids must form a sealed internal space with no leak paths outside the internal flowfield. The flow boundary conditions should be applied to “lids” that form the inlet faces and outlet faces of the computational domain.
Minute details of the geometry should be eliminated wherever possible to keep the CFD model size to a minimum. For example, latches, fasteners, and brackets on the inner surface of cowling should be eliminated unless they are critical to the flow field in order to reduce the CFD solution time.
After the geometry is imported, it should be checked for problems using the “check geometry” feature in the CFD software. Any invalid contacts listed must be resolved prior to analysis. Typically, invalid contacts occur when two parts in the assembly share an edge. Also, for internal flow problems, the flow volume value at the end of the check must be a positive number. A zero flow volume indicates a leak path out of the internal flow field that should be resolved.
Thin-walled solids can result in irregular cells if the mesh density is too low. Irregular cells computationally represent a hole in a thin solid. If there are numerous irregular cells in a mesh, then a solid wall will no longer provide a physical barrier in the simulation. Irregular cells can be remedied by increasing the mesh density in the area with the irregular cells. This can be accomplished by increasing the global mesh level, shrinking the minimum wall thickness in the mesh settings, or applying a local mesh in the affected zone to impose a higher level of cell refinement.
Transient simulation is particularly challenging from a computational standpoint because the flow field must be solved for a number of time steps. These guidelines have been developed by Bell technical specialists to overcome the challenges of transient analysis:
- Minimize geometry detail; exclude volumes without flow boundary conditions; keep cell count at a minimum.
- Ensure simulation initial conditions are accurate – these are critical for transient simulations.
- Double-check boundary and free - stream conditions – transient simulations potentially may take days to complete, so ensure everything is absolutely perfect before launching a simulation.
- Establish an equation goal to track and record physical time at during every iteration.
- Establish a solution file output period consistent with the file size, flow gradients, and physical time duration of analysis.
- Use the default physical timestep size initially to accelerate the solution ramp up to larger time steps as the simulation progresses if initial high flow gradients dissipate.