For almost all engineering problems, solutions and calculations need to be reliable and accurate. Errors in calculations can lead to procedures not working as intended, resulting in costly problems, or even serious safety issues. For this reason it is essential to understand the limitations of any tools, including CFD, which are used as part of the design and validation process. Knowing these limitations allows an engineer to estimate the errors and provide reliable conclusions that do no promise more than they may deliver.

There is no doubt that the classic computer science saying of “garbage in, garbage out” applies to Computational Fluid Dynamics (CFD).  With some of the more user friendly CFD packages that exist in the current market it is relatively easy to set up a simple model, and produce colourful results that bear little resemblance to the modelled systems behaviour. For this reason some opponents of CFD like to refer to it as “Colourful" Fluid Dynamics. However, when used correctly, with accurate inputs, it can produce incredibly powerful and meaningful results.

Inaccurate CFD results can stem from many sources, each of which can be alleviated or reduced through experience and care. Potential errors include:

  • Inadequate mesh resolution in areas of high gradients or fine details. This can lead to excessive diffusion or unrealistic flow fields.
  • Using an inappropriate turbulence model. There is a wide range of turbulence models, each of which performs better under different conditions.
  • Improperly defined inputs and outputs. Without accurate input and output criteria you are solving a different problem!
  • Inadequately located and defined boundary conditions. Putting the boundaries too close can lead to inaccurate flow fields, and not defining the correct boundary velocity profiles in some cases can produce wildly inaccurate results.
  • Neglecting small, but important physical processes, such as heat fluxes, turbulent mixing and buoyancy. In many cases these effects are negligible, but sometimes they can have large impacts.

Knowing the potential for errors is the first step to preventing and minimising them. Once they are identified, experienced CFD engineers, like those at Synergetics, can work to isolate and circumvent them, leading to accurate, reliable simulations. This begins by taking a rigorous look at the problem, identifying the relevant physical processes and any missing data.  The necessary and optimal models are selected based on a combination of experience and relevant academic literature.  Where there is a lack of published literature, or a lack of agreement in the literature, two options are considered.

Firstly, where possible, onsite measurements are taken for existing or similar facilities. This is especially useful when developing models for very complex systems, which have not been modelled in the past. Such onsite measurements allow the CFD model to be validated, and can identify any deficiencies in the model. For example, when performing ground breaking modelling  of a multiple hearth furnace, Synergetics collected onsite measurements, and then used the CFD model to replicate those operating conditions, achieving validated errors of under 10% on almost all sensors. This provided both our engineers, and the client with confidence that our model was able to accurately resolve and predict the complex physics occurring within the furnace.

Secondly, the sensitivity of the solution to different approaches is considered and, were relevant tested. Sometimes two models will produce near identical results. In other cases, one may predict a more extreme result than another. In this case, if validation data is not available, the solution which minimised risk to the client is taken. Typically this will be the model that predicts the worst case behaviour, presenting a more conservative solution. This approach is especially useful with mesh resolutions, as a sensitivity study will quickly reveal any mesh dependence on the solution.

Using a combination of all these techniques, together with extensive experience, skilled CFD engineers can provide their clients with reliable solutions to their engineering problems. Knowing the details of any limitations allows both best and worst case scenarios to be identified, and provides the client with confiedence that the solutions can be trusted.

It is important to understand that CFD is not the right tool for every problem. However our staff can help you assess you engineering problems and determine if CFD is the right tool for you, and if it is we can assist you in getting an accurate solution that meets your requirements.