CFD: When, how, and for how long?

17 Oct 2011 4min read

Team Discussion

Multiple authors

When determining the performance boundaries of a design, or even if just looking at the feasibility of a concept, a certain level of analysis needs to take place. When it comes to a fluid handling device, which in the medical world could be anything from a pen injector to a blood analysis machine, the triangle of physical testing, ‘heavy’ analysis (in the form of Computational Fluid Dynamics or CFD), plus math modelling, can be a very powerful analytical combination. However, it is essential to understand both the benefits and limitations of the tools at your disposal.

CFD can help you test the effects of a number of design changes without having to prototype and test all the variations in reality. By comparing a real-world test to the CFD and math models, you can validate the accuracy of those models, and from there run a series of virtual experiments.

With CFD you can also determine the critical parameters, dimensions and geometries, by altering them in the CFD model and observing the effects. This can help real-world experiment design, by helping you decide which parameters to experiment with, and which aren’t important to the performance of your fluid system. You can then run these experiments in the virtual world first, to see if they are likely to teach you anything. Taguchi style experiments reveal a great deal about a design with minimal testing, maximising the learning-to-cost ratio; CFD can help you determine which parameters to put into your Taguchi experiment.

However, you do not have to commit to modelling the entire fluid system in CFD. By taking bite-size chunks of the system, maybe just modelling the area of particular interest (such as a specific orifice or chamber), you can learn all that is needed. Math modelling can then be used to model the rest of the system. Keep a track on CFD modelling, and know when to stop; this will probably be driven by budget, but know when your results are ‘good enough’, and do not get stuck in analysis paralysis.

Before embarking on CFD, it is important to know what to expect from it, and whether it could help you or not. Too many assumptions in the modelling may invalidate the results, and it is possible to spend a lot of time and money fully modelling an entire system, without learning anything useful about the design.

Also ensure that you understand the limits of CFD. For example, if the fluid is not well understood then you will get inaccurate predictions from the CFD model.

A good relationship with a CFD service provider can help ensure success. CFD experts can advise on how well a system can be modelled, and which elements of a system to focus on – in other words, how best to spend your money. The provider can also highlight what to expect from the analysis, and what it can be used for.

CFD is a powerful tool that can save weeks of time in the lab and can teach you about the critical parameters of your system. However, it does have its limitations, and it can be costly. You have to make a decision as to whether a full – or even a partial – understanding of the system is critical enough to warrant spending the time, effort and money on CFD. Finding a highly capable CFD service provider can help you make that decision and then keep the analysis on the right track.

*Another article from Philip on ‘Strategies for designing a reliable, repeatable fluid handling system’ is due to appear in the November/December issue of European Medical Device Technology(EMDT).

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