How to Avoid Sacrificing Accuracy for Speed When Prepping and Meshing CFD Geometry

I don’t know about you but I want the best I can get — whether I’m doing CFD prep and meshing or just living my life. It had better be good and speedy and easy — I want it all! Unfortunately, life doesn’t work always that way.

FenceFor example, I have a beautiful wrought iron fence in front of my house. The problem is that the fence is made of steel and as we all know, steel has the unfortunate property of rusting as the paint ages and loses integrity. So, this summer I have a big project to scrape, brush and repaint the metal. I’m not looking forward to the many hours of hands on time that will take!

IMG_1066I’m tempted to buy one of those rusty metal primers and just paint over the rust. But I realize that while rusty metal primers may seem quick and easy now, the rust will be back all too soon. If I want the railing to look good for years to come, there is no substitute for the hard work of actually removing as much rust as possible before painting.

Like the rusty fence, we are faced with a similar trade-off when preparing and meshing geometry for CFD simulation. Fidelity and accuracy are critical in CFD simulation. Physical prototyping and testing can only be reduced by CFD if one can expect accurate results.  Up to now, high fidelity, high-accuracy results came with a price. Complex geometries, required hours of manual effort to “clean up” the geometry and then prepare the mesh. It can be tempting to take shortcuts that speed up prep but take a toll on accuracy and fidelity. Unfortunately, there is no way of knowing just how those inaccuracies skew results to put your recommendations in doubt. For example, if you don’t resolve a boundary layer correctly then any aerodynamic drag figures could be highly inaccurate.

Speed & Simplify CFD Prep and Meshing Nine Ways

I won’t compromise on results and I don’t want you to either. That’s why I have been spotlighting model preparation and meshing over the last few releases. In recent months, we have released nine powerful new enhancements and tools that speed and simplify prep and meshing. As a result, pre-processing time for complex geometries has been reduced by 60 to 80%.

CFD deals with many application areas and industries whose models vary widely in size and complexity. For example, automotive engineers must be able to model  problems ranging from simple flow through a pipe to more complex components like alternators, manifolds and catalytic converters — all the way up to the complete vehicle. At the high end of the scale, conventional methods of importing, cleanup and meshing have a tendency to bog down. The following two enhancements are of particular interest for those preparing very large, complex geometries.

Importing Large, Complex Models

Importing large and complicated assembly models can be challenging. Many complex models benefit from different levels of faceting quality in order to achieve the required level of accuracy in areas with high transients while avoiding excessive computational time and cost. For example, in a combined aero and underhood simulation on an automobile, high faceting quality is usually needed on selected parts such as fans, grills, and the exterior shell but low quality is OK for the rest. Another problematic scenario is when you have a large baseline CAD model from which you need to extract various smaller models, all needing different levels of faceting accuracy. In the past, users had to import solid geometry with the highest uniform level of faceting quality, causing challenges with very large file sizes, long loading times and slow system response.

Very large, complex models can be imported and managed 36% faster with the new integrated CAD management module

Very large complex models can be imported and managed 36% faster with the new integrated CAD management module

A new CAD Assembly module makes it possible to represent each body, part or subassembly at any faceting level from low-level faceting to high quality CFD surface mesh, achieving the right geometry or mesh for each part or surface. The CAD geometry is converted to an internal format, resulting in a significant speedup in re-faceting and CFD surface meshing, and enabling the faceting quality to be adjusted later if needed. A link is maintained between the original CAD model and the engineering model for easy updates and design changes. Internal testing on a GM Cadillac external aero model showed a 36 percent reduction in hands-on time compared to the previous release. You will find that the CAD assembly tree maintains exactly the same hierarchy of subassemblies, parts and bodies as your CAD package, so users can easily replace a part with a modified part directly from CAD.

Scripted Automatic Meshing

Scripted automatic meshing can further simplify the process of mesh creation, physics pre-processing and solver setup on large, complex geometries where numerous variations must be simulated, for example when optimizing contours on an automobile hood. Certain features are critical to the simulation and yet are subtle enough to be easily lost by conventional tools. For example, Volvo needed to capture all the details of sculpting on an automobile hood designed to set air flow around the “A” pillar in a way that minimized wind noise. Heavily based on wrapping technology, scripted automatic meshing combines many solid parts of the same material into a single unit. The wrapping triangulates and projects an initial Cartesian grid onto the geometry. If the flow is internal, the approach is analogous to inflating a balloon from the inside until it touches all the walls and fills in the gaps. The wrapping operation then extracts the fluid volume and maintains all details of the geometry using a mesh-based rather than a CAD kernel-based approach. Scripting can reduce hands-on effort by 40 percent or more. It now takes as little as two or three days to prepare a high quality mesh for a complete automobile geometry using this approach.

Scripted automatic meshing greatly reduced hands on time required this automotive hood contour (Courtesy of Volvo)

Scripted automatic meshing greatly reduced hands on time required this automotive hood contour (Courtesy of Volvo)

Now You Can Have it All (at least for CFD prep and meshing)

Those were just two of our recent enhancements that speed time to CFD. We cover these and seven more in the webinar, Better, Faster Prep and Meshing in ANSYS CFD or check out the overview of fluids prep and meshing on ANSYS.com.

Now, you can have it all with today’s ANSYS CFD prep and meshing – unsurpassed gold standard accuracy with drastically reduced manual effort.  Hands-on time has been reduced by up to 80% when using these tools along with scripted pre-processing. If you have not tried ANSYS CFD lately, you have not tried ANSYS CFD.  Contact your local sales representative to learn how you can get a free trial license.

Now, if I could just get better, faster tools to finish my wrought iron fence!