"How Does Your Fluid Flow?" Feature Article, December 2003

how does your fluid flow?

Thirty years ago, CFD was academic; now doctors and engineers wonder how they lived without it.

by Jean Thilmany,
Associate Editor

Computational fluid dynamics, the method of analyzing fluid flow in structures and designs, is enjoying a burst of interest. The analysis method was introduced in the 1960s, but wasn't much sold through software vendors until the 1980s. Twenty years later, manufacturers across a myriad of industries are licensing the technology from a pool of vendors who now market CFD packages of many stripes.

Those vendors are continually tweaking their CFD offerings to be of greatest use to engineers. Many developers of finite element analysis software, for instance, are now incorporating CFD code into their structural analysis tools. Recently, makers of computer-aided engineering programs have jumped into the fray, linking their tools to CFD analysis capabilities as well.

Engineers use CFD to predict how fluids will flow and to predict the quantitative effects of the fluid on the solids with which they are in contact. Airflow is commonly studied with
the software.

Now, a number of companies in the CAD and computer-aided engineering markets are working to bring analysis capabilities closer to the earliest stage of product design, almost always carried out in a CAD package, according to Vincent Harrand, director of software technology at software developer CFD Research Corp. of Huntsville, Ala. The company recently introduced software that lets engineers design a part, make a quick CFD analysis of the design, then tweak the design to correct for any flaws found during analysis.

Many software developers are now marrying their fluid-flow software with CAD technology, which helps engineers analyze while they design.

The move toward combined CFD and CAD packages mirrors a similar change in the engineering technology market seen only a few years ago, as vendors worked to link finite element analysis and computer-assisted design. Prior to the early 1970s, FEA could be run only on mainframe computers owned mainly by the aerospace, defense, and nuclear development companies. Until only a decade ago, FEA had been performed exclusively by specialized analysts who held Ph.D.s in the subject and had devoted their careers to the discipline, according to Charles Foundyller, chief executive officer at Daratech, a market research firm in Cambridge, Mass.

But with the rapid decline in the cost of computers and the increase in computing power, today's personal computers can easily produce accurate FEA results. Many engineering technology vendors stepped up to the plate, marketing packages that married simplified analysis to CAD. The software walks users through a series of steps that allow them to define the analysis they want to run and then interpret the results, Foundyller said. This is the same way today's CFD packages work.

Because the analysis programs are packaged with CAD software, engineers can analyze as they design, and they can change and update models to reach a workable design much earlier in the process. Now, makers of engineering software see a chance to add fluid analysis to the analyzing-while-designing niche.

"Historically, analysis has been relegated to the research and development departments, or the tail end of the design process," Harrand of CFD Research said. "Imagine the design engineer who, with minimal training or experience, can assess a series of geometrical variants or a series of unique approaches and determine within minutes fluid flow, heat transfer, thermal or structural stress, or any number of other physical phenomena."

The CFD industry could be said to mirror FEA industry growth in many ways. Though its beginnings were in the early 1960s at academic institutions, CFD became more commonly used in the early 1970s, just as FEA systems were beginning to make the jump from mainframe to smaller computers. CFD software developers began offering commercial engineering technology in the 1980s, and the industry expanded significantly in the next decade, according to Daratech. As CFD packages are more easily connected to CAD packages, the industry is set to take off even further.

Lost in Translation

The U.S. Navy, for example, is interested in stepping up CAD and CFD integration. Last summer, the U.S. Naval Air Warfare Center granted a research contract to Pointwise Inc. of Fort Worth, Texas, to study software techniques for converting CAD data into solid models that can be easily meshed, the first step in performing CFD analysis. The software vendor makes the mesh generator Gridgen.

CAD geometries don't always fit together properly, though it may look as though they do during design. Before the part can be analyzed, the designer needs to heal the design. In industry parlance, this means designers must ensure that geometric surfaces fit together properly and don't contain gaps or overlaps.

Navy engineers cite this lack of integrity in CAD geometries as one impediment to the use of CFD to analyze their engineering problems, said John Steinbrenner, vice president of research and development at Pointwise. For instance, naval engineers recently wanted to simulate how aircraft flying near a ship are affected by the ship's wake, which is a fluid-flow problem. They compiled CAD designs for both the ships and the aircraft they were studying, Steinbrenner said.

A linked CFD and CAD package from CD Adapco Group includes its own solid modeler, for very tight design and flow-analysis integration.

Because the craft were made up of thousands of surfaces, the engineers were overwhelmed not only by the amount of CAD data, but the amount of data that also had to be healed before it could be analyzed, he said.

The current way for engineers to run CFD analyses of CAD designs is for them to manually repair, or heal, the CAD file as best they can, and then to deal with any remaining geometry problems as they mesh the part in preparation for analysis. The cumbersome process keeps engineers from easily running what-if scenarios on designs that include fluid-flow issues.

But if CAD and CFD programs were integrated and easy to use, an engineer could create a design, see how it operated during a CFD simulation, switch back to the CAD program, and change any part of the design that failed to work during the simulation.

Pointwise engineers are using the grant, awarded under the Navy's Small Business Innovative Research program, to look at creating a software application that automatically makes CAD geometry suitable for CFD meshing. With such technology, engineers would no longer have to manually repair their CAD files and the software application would ensure that CAD geometries are watertight, according to Steinbrenner.

"Problems with CAD geometry are the biggest problems with applied CFD today," he said. "Watertight surface geometry frees the engineer to focus on the simulation results and the fluid dynamics phenomena."

Getting to CAD

Although they nearly all use CAD, many mechanical engineers don't need access to all the bells and whistles an advanced CFD program can provide. Advanced analysis programs are usually the purview of a user trained on a particular CFD package. But for the many industries that don't require a full, advanced CFD program for their designs, a smaller program accessed via the CAD system will do.

For instance, the infrastructure-design consulting company Halcrow in London is using a software called Star-Works that ties the CFD code from CD Adapco Group of New York with the CAD system SolidWorks from SolidWorks Corp. of Concord, Mass.

The package lets design engineers carry out simple flow simulations while they design. They can access more advanced CFD code from CD Adapco from within the software when needed. Halcrow uses the software to study ventilation in buildings and to look at airflow and heat characteristics within a nuclear reactor vault, said Paul Carey, an engineer at the company. Carey and his team modeled the vault, ran the analysis, then changed airflow conditions by making changes to the design.

"We could change the flow conditions and run any number of cases, which made it easy to see the effects of changing airflow on the temperature distribution within the vault," he said.

To use the software, Carey moves the model from SolidWorks to the CFD package for analysis. Other offerings from CD Adapco, like its Comet-Design software introduced in August, are more tightly integrated. The package includes its own solid modeler, from the CFD developer, closely married to a CFD analysis tool. Engineers can also import designs from a CAD system, but they may then need to modify, or heal, the geometry.

Other developers' products, like CFX from Ansys of Canonsburg, Pa., also allows for direct CAD import. CFX includes a direct CAD link that helps engineers rapidly clean up CAD models for analysis.

At Home in the Hospital

The move toward integrating CAD andCFD more tightly comes at a time when fluid analyses are more commonly conducted across engineering industries. Engineers in the aerospace and defense industries were about the only ones to study fluid flow at the dawn of the CFD era 30-some years ago. The automotive industry came on board slightly later.

Now, however, fluid flow is finding a home in nearly any industry in which engineers work—and in many lines of work that don't include engineers. Officials at Concentration Heat and Momentum Ltd., a CFD vendor in London, predict that within a few years surgeons will operate on a person only after analyzing the flow of fluids within that patient's body and looking at a number of what-if surgical scenarios for that particular case. CFD software is already used in many hospitals. The human body, after all, can be said to be one large fluid-flow system that includes blood, urine, air, and the fluid cushioning the brain.

Meanwhile, the home appliance industry has come to CFD fairly recently, said Valter Diurno, senior CFD engineer in the corporate technology and engineering refrigeration department at Whirlpool Europe in Italy. Like many manufacturers, home appliance makers are looking to CFD because they find it saves money by allowing engineers to work out kinks in a design before the product is prototyped.

Engineers use CFD to predict how fluids will flow. But airflow, like that surrounding airplane engines, can be studied with the same packages.

"For engineers in our industry, it's new territory to be conquered," Diurno said. "CFD experts in home appliance companies have to work hard to sell the benefits of CFD to departments that aren't often aware that thermal flow analysis can provide a fast route to more competitive product design."

Many products are proposed at brainstorming sessions, he said. But engineers can't know, without doing a preliminary CAD design and analyzing it deeply, whether a proposed product could be successfully made. Diurno said his company uses Star-CD CFD technology from CD Adapco to quickly analyze these preliminary designs. They can be analyzed within a few hours, he said.

At General Domestic Appliances in Peterborough, England, engineers used CFD technology from Fluent Inc. of Lebanon, N.H., to help develop a new line of frost-free refrigerators. To reduce the amount of energy the refrigerators draw, engineers had to use the smallest fan that generated the greatest flow of air within the unit. Small fans would also help the unit run more quietly, said Graham Sands of General Domestic Appliances.

Engineers used CFD to determine how to best position the fans so that air flowed inside the refrigerator and the freezer in the most efficient way. After studying fluid flow simulations, they made prototypes of the most promising modeled designs to see if the prototypes matched CFD simulation results. They did.

As for the future of CFD, in addition to predicting its widespread adoption in the medical industry, officials at software vendor Concentration Heat and Momentum expect CFD to follow other engineering technologies that have adopted a pay-as-you-go licensing scheme.

Some CFD software providers are seeking to make their software available online. That is, vendors host software on their own servers and, for a fee, offer engineers access to programs via the Internet. This eliminates the need for engineers to pay for the software any longer than they need it. And they don't have to download it to their own computers. That arrangement would make CFD available even to small companies or single users, like consultants who can't afford to buy advanced software packages.

The ability to model fluid flow easily and quickly is revolutionizing many industries. And as it becomes more commonly used in hospitals, it may touch us personally as well.