How do we make RANS practical?
Given the long preparation and run times typical of RANS, how can it be made to play a central role in design? Designers need to assess performance of a large number of options. They need to evaluate candidate designs over a range of operating conditions or assess their behavior under the influence of different initial or environmental conditions. They might even need to optimize a design. And yet at the same time, before RANS can prove reliable as a design tool, it needs to satisfy two distinct accuracy requirements. First, each RANS solution must converge to a point sufficiently close to the “right” answer that it is trusted to properly represent real-world performance. Second (and perhaps even more important), successive solutions must reliably predict design trends.
How can RANS meet the seemingly disparate goals of accuracy and efficiency? AFT’s unique approach satisfies these goals by combining the accuracy of RANS with a concept known as “overset” gridding. Overset grids allow arbitrary connectivity and communication between grid blocks, and mimic the simplicity of unstructured grids without giving up the high accuracy of structured grids. The resulting flexibility leads to a huge simplification in problem setup, and enables the use of automation to reduce overall turn-around time.
Arbitrarily complex geometries can be handled without the usual exponential growth in grid generation time, and even multi-body and moving-body problems are easily handled. Overset methods also allow for more efficient utilization of grid points, and therefore enable shorten runs times and more accurate solutions. An example of overset gridding is demonstrated at right where the grid and RANS solution near a mast and sails of an America’s Cup class yacht are shown.
AFT utilizes two RANS codes for its work. First is the Applied Fluid Technologies Incompressible Navier-Stokes (AFTINS) software package. The second is NASA’s OVERFLOW code. Both are overset based, and both offer a choice of turbulence models. AFTINS is a purpose-built incompressible code, and can perform time-accurate simulations at very low Mach number. OVERFLOW is based on the artificial compressibility method, and is actively maintained by NASA. This reliance on two independent codes is a significant advantage for AFT. It not only provides a means to check problem setup (grid quality, etc.), but also for building trust in the accuracy of RANS in general.