Computational fluid dynamics, usually abbreviated as CFD, is a branch of fluid mechanics (the study of fluid flow). The interaction of liquids and gases with surfaces, such as the flow of coolant through a pipe or the flow of air over an airplane wing, is extremely complex. It was not until the development of powerful computers in the 1960s, along with the necessary software, that numerical methods and algorithms could be used to analyze and solve design problems that involve fluid flows.
CFD has been in use for many years, primarily in aerospace engineering, but very long compute times and the need for a mainframe computer kept it in the research laboratory. Today, just as with finite element analysis (FEA), advances in computer power and software have greatly reduced the cost of applying CFD, to the point where it is ready for more mainstream use by design engineers and even designers. METAL-TO-PLASTIC At RL Hudson, we work primarily at the component or subassembly level. As a result, much of our engineering work is on “drop-in” replacements for existing components.With the current focus on cost and efficiency, many of our customers have been interested in converting metal components and subassemblies to plastic (M2P), but it’s rarely a matter of simply duplicating a metal component in plastic. The new component must perform as well as, or better than, the original, but it also has to be manufacturable.*
Since our customers’ component requirements typically involve pressure, mechanical loading and elevated temperatures as well as high production volumes, injection molding is often the best solution.However, the realities of injection mold design, and the molding process itself, often dictate large changes in flow geometry (the shape of the flow path) to ensure manufacturability in ducts, pipes and similar parts. CFD IN ACTION Here is an example from a recent project. The pipe shown above is typical of components used in cooling systems for heavy truck applications. The pipe is a formed metal tube that has a machined metal piece brazed onto each end to accommodate seals.While effective, this is an expensive way to make the part.The customer came to us for a solution.
The smoothly curved internal flow path cannot be made via injection molding, but to ensure that the engine continued to cool properly it was essential to maintain the original part’s flow characteristics. The most reliable, and least expensive, mold design for such an application uses two offset core pins that “kiss off” (make contact) in the middle to create a flow path.The model below shows an early design that was considered. Note in the cross-section how the internal flow path shape is affected.We were concerned that such a design would not be acceptable, even though it would be manufacturable.
Years ago it would have been necessary to make a prototype part and test it against the original component, then alter the design and test it again—perhaps multiple times. Today, RH Hudson’s continuing investment in computing power and software means that we could perform CFD on the original component and on this preliminary design. Using Star CCM+,we analyzed the models at the worst case flow rate. The images above show a comparison between the original flow path and the “first iteration” flow path. Obviously, the flow is not as smooth, but in less than a day we were able to alter the design three times to yield a manufacturable injection molded component that showed an increase in predicted pressure drop of only 0.0105 psi. Once we had this good internal flow geometry, we could performFEA (to confirm structural strength and materials selection) and MoldFlow™ analysis (to optimize the molding process) before presenting the concept to the customer. This new capability, combined with FEA, MoldFlow, and RL Hudson’s deep knowledge of materials and mold design,makes it possible to validate part designs virtually, saving time, minimizing part production cost, and effectively eliminating the risk of having to make a design change once the part is in service.
*For a design to be “manufacturable,” it must be possible to produce the part with consistent quality, at the required production rate, and at (or below) the quoted cost for the entire product lifecycle.