Model in hours rather than days or weeks

One method of speeding up product development is to use mathematical modelling software to evaluate physical and operating parameters without having to build a prototype, writes Dean Palmer

One method of speeding up product development is to use mathematical modelling software to evaluate physical and operating parameters without having to build a prototype, writes Dean Palmer

"Mathematical modelling software can save design engineers valuable time and cost by reducing or eliminating the need to build costly prototypes," said Jukka Tarvo, managing director at mathematical modelling software company Comsol. "Even if a prototype has to be built at some stage, the designer can do so with a much higher level of confidence that the device or process will operate as expected," he continued.

Tarvo added that the modelling process should be as streamlined as possible and integrate with other software already in place. He explained that Comsol's 'Femlab' software can accelerate the modelling process because it imports files from leading CAD and meshing programmes. "Many large companies have standardised on certain CAD programmes; their engineering staff have years of experience using these packages and have also collected considerable intellectual property from past projects.

"Clearly, these users don't want to work with a modelling package that forces them to go through the time-consuming task of recreating a complex geometry with the modelling package's own CAD editor. Femlab offers interfaces that read in all the major formats including SAT, IGES, STEP, ProE, Catia, VDA/FS and Inventor as well as DXF, GDS, STL and VRML formats."

He also suggested that many large companies have bought or developed their own special meshing programmes, but virtually all of these firms generate output files in the Nastran format, which Femlab also supports.

Once a user imports a geometry or mesh into Femlab, the next step is to define the physics, boundary conditions and initial conditions - all of which are application specific. Here, the design engineer takes his deep knowledge of the product or process and formulates it with the help of the software. Tarvo said there was no need for engineers to write a single line of code, but by "entering values and making selections in a variety of dialogue boxes".

The software also enables multiphysics modelling, so fluid flow, heat transfer and structural mechanical effects can all be calculated.

Tarvo provided an example of how users are benefiting from using Femlab. Staff scientist, Dan Smith, at MKS Instruments has been using Femlab for the design of a throttle valve used in chemical vapour deposition (CVD).

One of the key parameters in the CVD process is the pressure in the reactor, which operators control with a throttle valve. The valve chokes off the flow so that a huge pressure difference can exist on different sides of the valve - it can be as high as 10 torr (10mm/Hg) in the process chamber upstream of the valve, compared to only a several hundred millitorr downstream. Such a dramatic pressure drop in a distance of just millimetres can lead to a significant temperature drop in the carrier gas. Also, a vortex can form downstream of the valve because its opening is not exactly symmetric, so the flow area is larger on one side than the other. The vortex, in turn, leads to an increase in the local partial pressure of any precursor.

"Before using Femlab, engineers were unable to explain why condensate was forming and clogging the valve," explained Tarvo. "But with the model, the staff at MKS could see these effects and take appropriate countermeasures."

The most difficult part of the simulation, according to Tarvo, was resolving the shock downstream of the valve blade. Streamline diffusion and a high mesh density were necessary to accurately resolve the shock and therefore observe the large resulting temperature drop.

"The simulations clearly showed that heating the blade was not sufficient to prevent the gas condensation, which results from the expansion of the carrier gas. Given this information, MKS engineers developed a novel way of eliminating this problem, a method they are now testing and plan to incorporate into future designs."

Tom Shelley

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