The strategic use of computational fluid dynamics can speed the development of high-tech products.

BORIS MAROVIC,
JOHN PARRY
MENTOR GRAPHICS CORP.

TRADITIONALLY, analysis using computational fluid dynamics (CFD) took place late in the design process once changes to part geometries had pretty much stopped. But CFD is starting to be applied at earlier stages of development.

For an indication of the trend, consider recent events at Blue Origin, the spaceflight services company set up by Amazon.com founder Jeff Bezos. The firm’s BE-4 rocket engine, scheduled to power Blue Origin’s rockets by 2019, has undergone several million hours of CFD modeling to perfect preburner sizing and injector element designs. Bezos credits CFD with significantly shortening the engine’s design and testing schedule.

Of course, it should be noted that CFD is not something used just by rocket scientists. The latest tools let design engineers do CFD analysis. This means routine thermal simulation and analysis can take place earlier in the design cycle. The early application helps to reduce design reiterations, improve quality, and shorten the time to get products to market.

Here are a few steps that electronics companies can use to boost productivity when applying CFD while also making it easier for engineers to design complex systems and components.

Let designers do routine CFD — In a workflow where designers routinely hand-off design baselines to a CFD analysis department, it can take days, sometimes weeks, for results to come back. Meanwhile, the design will have evolved and, if there are fundamental changes necessary to address thermal issues, a lot of work will have been wasted. However, in many cases, designers can undertake routine CFD to check the effect each design iteration has on product performance without having to send it out of their department.

Let CFD analysts do the most complex CFD work — CFD specialists tend to use one high-end CFD tool for all simulations, regardless of how simple or complex the job. However, 80% of simulations don’t need an analyst’s in-depth expertise. Tying up analysts with routine work limits the usefulness of their highly valued experience. Also, the specialized tools they use are expensive and in short supply. Specialists tied up with pre- and post-processing tasks or dealing with mundane projects can’t help designers.

Keep up-to-date on work practices — Despite enormous advances in tools and technology, people are surprisingly slow to change their own work habits, including how departments are organized. Mature companies often have design and analysis departments set up years ago to manage a historical workflow in a way that is no longer optimum. With today’s market demands, these old structures can impede progress. Before a design engineer can hand off thermal analysis to the CFD specialist, he or she must create a formal specification of the analysis task, including the detail expected and reporting format of the results.

Even as technology has evolved, many designers still believe CFD tools are too difficult and time-consuming to learn and use. Many managements and/or in-house analysts also still believe designers can’t do a reasonably accurate analysis in a timely manner. This idea is outdated and wrong.

Modern tools give designers fast “directional” guidance about how to improve designs, and many smaller, more flexible, companies are embracing them. As well, some larger companies are designing new workflows using modern tools because they must reduce costs when developing new products.
Interact more, report less — One of the most time-intensive activities when doing CFD with traditional tools is the meshing. It can account for as much as 70% of the time spent on CFD. For example, in one case the meshing of an LED headlight spanned about 2.5 weeks, whereas a tool such as FloEFD can handle the task in 15 minutes. The solver time is then around 10 to 20% of the process — anywhere from hours to a few days — depending on the item simulated, the physics considered, and whether conditions are stationary or transient.

It also takes time for specialists to understand and interpret simulation results in the context of the desired product performance, then feed results and recommendations back to designers. Formal reporting bloats an already time-intensive process. The reporting process must be balanced against the value of the simulation results and the possibility of the design baseline used for the simulation becoming stale with lengthy delays.

As products become more complex, it’s harder to make design trade-offs between performance goals and considerations such as weight and cost. It’s crucial that engineering interact with other departments because real-time communication speeds the workflow. When formal reporting is needed, use tools that automate reporting from data such as Microsoft Excel and Word files, with customizable templates to save time.

Use CFD more often earlier in the design – In many companies CFD has become an activity that only happens late in design, with one detailed CFD study before the product goes into production or to physical prototyping. However, conducting routine CFD within the design workflow improves the quality of the product, often resulting in fewer or no physical prototypes. And it forces the CAD geometry for the final CFD analysis to be prepared well in advance.

Create consistent quality — CFD analysts craft meshes and can adjust many little-known functions in their traditional body-fitted CFD tools. So if the software has 20 different turbulence models, it can produce at least 20 different results, all other things being equal. In comparison, modern CFD tools that use Cartesian-based meshes are not only simpler but, because they intelligently provide guidance through the analysis workflow, give results with far less unnecessary variability, again saving time and resources.

Get quick simulation results — Timeliness is crucial in making good decisions. Thermal analysis should focus on getting good answers. A result that is sufficiently accurate and timely enough to support a proposed design change is more useful than a better answer delivered too late.

CFD tools should be able to turn around a simulation within 24 hours. That turn-around time should cover the use of modified CAD geometry, performing meshing, creating a solution, and post-processing the results to avoid slowing the overall design workflow.

Improve overall simulation accuracy — CFD traditionally is time-consuming, particularly for CAD preparation and meshing stages. It often takes too long to create a good-quality fine mesh, so engineers instead use coarse low-quality meshes for simulation. This practice results in high levels of mesh-generated error that degrade the quality of the simulation results.

However, new techniques let CAD geometry mesh quickly, reliably, and without over-simplification. These newer solutions are augmented by technologies that make simulations on coarse meshes more accurate by using, for example, empirical approaches to predict pressure drop and heat transfer in narrow channels that are not adequately meshed.

Liberate the design space — The earlier CFD can be used in design, the greater the chance engineers can confirm the product’s performance and make it even better. Exploring the product’s potential around a single design point almost always leads to the discovery of a better design or to improvements to the current design. These investigations can also reveal which aspects of the design are crucial to product performance.

Eliminate CAD data conflicts — When adopting new CFD tools, pay special attention to the CAD data format they can use. It’s best if the thermal simulation and analysis tool can use native CAD data; it is far easier to fix issues than when working with neutral file formats such as IGES and STEP. When non-native data is imported, it is as detailed as native data, but often the import results in interpretation errors. The problem manifests itself as missing surfaces and holes in the models that must be healed or fixed. This model mending takes time and sometimes is not possible at all if too much of the geometry is lost during import.

Traditional body-fitted CFD tools often use geometry healing tools or a surface wrapper to (hopefully) fix such imported geometries. But it is often a lot of work to produce a healthy geometry for the CFD simulation. Complex geometries are usually simplified to make meshing easier, and this simplification can degrade the realism of the simulation.

CAD data that must be simplified or re-imported through neutral file formats also introduces other problems. For example, it’s tough to investigate the use of parts that are outside general parameters or how changes to parts can affect the design when using CAD data in neutral file format. Working with designers who can use CFD in their process, we noticed a faster feedback loop with naturally built native CAD data, and better CFD results.

The future is interdependent — Today’s CFD software, such as Mentor Graphics FloTHERM XT and FloEFD, lets thermal, mechanical, and electrical engineers work more effectively with CFD analysts to create better products quickly. Design engineers can “front load” analysis, determine trends, accurately analyze and solve problems faster and earlier in the design process. They can complement what specialists must do in the later stages of verification. This close interaction and involvement can reduce thermal simulation and analysis time from weeks or days to hours.

CFD can let design engineers test various options using a design-of-experiments approach to arrive at a more competitive and/or reliable product. Both designers and CFD experts can use modern fast and accurate thermal simulation and analysis tools to more efficiently work together creating the complex electronics that modern products demand.