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Expert CFD and Engineering Simulation Consulting

Fluid dynamics, CFD, science, HPC and industry views

Fit for purpose CFD

Understandably, a lot of effort goes in to making sure CFD gives accurate predictions, and of course there's a lot of scope for figuring out what accurate enough actually looks like. In this blog we'll discuss what we mean by fit for purpose CFD and how that cuts through a lot of work to get to answers that are useful in the shortest possible time.

For anything but the most straight forward of flows (in which case you may as well figure it out with pencil and paper), CFD takes some time and effort to provide useful output due to the complexities of the geometry involved, turbulence, multiple phases and so on. The important question is how much of that complexity is important you, and to know that you need ti be really clear about what information you are trying to get from a simulation. You might be faced with a flow induced vibration problem at certain points in your piping network and want to simulate how making some different pipe geometry choices will affect the vibration frequency. The flow is unsteady, turbulent and multiphase and you face some tricky choices of which models to use, and you don't really have time for thorough validation - you need to show your model captures the existing vibration frequency straight out of the oven and can therefore predict geometry changes effectively. If the phases in the flow are well dispersed, it may be sensible to model the flow as single phase with a representative density, removing the multiphase physics and simplifying the choice and implementation of turbulence model. The simulation can now deliver predictions in the timescales required. Now, the critical part here is to go back and look at the simulation data and analyse it to see if the single phase assumptions holds - are there any characteristics of the flow that suggest it will no longer be well dispersed, in the critical areas? Of course, if you don't match the measured vibration frequency then you know you've missed something, but if you do you need to be sure it's a well founded agreement, and that when you simulate the modified pipework you go back and check your data again for the validity of the assumptions.

For us, this is the essence of fit for purpose CFD - making helpful assumptions and constantly checking the data to see if they hold, and if they don't, working out how much they affect the information being used from the model - asking and understanding "what physics are important here?". In order to answer this it's critical to know whether the CFD is expected to give accurate quantitative information or qualitative shifts. In the vibration example above, the requirement might be to increase the vibration frequency above the structural resonance frequency, in which case a qualitative shift would suffice. It might be to increase the frequency away from the first structural resonance but not enough to run into the next mode, in which case the results must be sufficiently quantitatively accurate.

In the product development cycle, simulation delivers the best value early on, in the concept development and assessment phases where varied physical prototype testing is tricky and costly. A reduced set of physical tests linked to CFD through specific validation points allows the simulation to be well validated and deliver important performance characteristics while design philosophies are evolving. The timescales here are more favourable for detailed validation and the importance of real data for this purpose is often well appreciated given the nature of the decisions being taken early in the design cycle. Nonetheless, it's unlikely that the most physically complete simulation will be run for all design considerations, especially for complex products where the full set of physics would require long simulate times and extensive hardware performance. These days the cloud provides the hardware, but product development timescales are always pressured, so being able to run sub-models for specific aspects makes a lot of sense. Here our approach of constantly assessing the physics in light of the required information works well with regular referral to and modification of the experimental programme.

CFD is often required to solve problems later on in the design cycle, when architecture decisions have been made and there are some performance issues to resolve. This is challenging for simulation as the problems are often not well addressed by experiment, hence the requirement for simulation to provide some additional information not available from test. The opportunities for validation here are limited as it's the extra information from CFD that is required. Timescales are often tight as it's a problem solving scenario and fixes are required fast. The broad and flexible validation link to experimental programmes that's well placed in concept design and development often doesn't fit here. In this case its critical to understand exactly what information is required and use that to assess what physics are likely to be important and tailor the CFD to provide answers in as short a time as possible. It's in these sorts of cases where our experience in physics selection and continuous analysis adds real value to enable CFD to provide answers to difficult questions in challenging timescales.

If you have requirements for effective simulation applied at any stage of your product development process
get in touch and we'll use our experience of fit for purpose CFD to help.

Categories of CFD Software

There's a brief article on Engineering.com highlighting the Pros and Cons of 5 CFD Software Categories that makes interesting, if a little light, reading. The 5 categories are Open Source, through Open Source plus Wrapper, CAD-integrated, Specialised and Complete. There's quite a lot going on here, and worth digging a little deeper (and also worth having a look at that article first)….

The open source category, our particular favourite, is free at the point of use, assuming you have some hardware (or maybe just access to a web browser, which is all you need to use
SimScale, more of that later). It's not really free at the point of delivering some useful analysis, as you need to invest some time to learn it, or hire someone who knows how it works - of course assuming that you have chosen which software to use, which given the capability and complexity of some of the tool sets out there is not straight forward. Nevertheless, the lack of license costs is a clear advantage, although these can be offset by the cost of finding folk skilled in the art. For us one of the real value adds of open source is access to the source code and the availability of people skilled in its use and development (like us). This suits niche applications well where you can have some specific development performed for your particular application much faster and cheaper than through the other categories, which would almost always involve the software company doing the development, and prioritising it against their own roadmap - for SME's or bespoke requirements it's going to have to work hard to make it to the top of their list. The examples are CFD (finite volume: OpenFOAM and SU2) and also Lattice Boltzmann (LBM) based (Palabos). There's a lot of open source tools out there, so you can't mention them all, but the FEA open source equivalent to Comsol, Elmer, definitely deserves a mention. It could be labelled as "multi-physics" solvers as it can do fluids, solids, electromagnetics and a bunch of other stuff too, so maybe omitted for those reasons (although technically OpenFOAM is "multi physics" too).

We're not too sure about the interpretation of "wrapped" open source in the article, certainly in terms of suggesting support is poor. Taking the widely known and used finite volume differential equation solving toolset OpenFOAM as an example (it's
not just CFD), there are some GUI front ends such as HelyxOS from Engys that are very much do it yourself, but the commercial Helyx offering and Caedium from Symscape are provided by people who absolutely know their onions, and are able and willing to help you with yours. There's a subtlety here too: there's a lot of not wrapped, i.e. no GUI, developed flavours such as that from Caelus and the specific application versions from CFDSupport. Maybe OpenFOAM is a bit of a special case here, as it's had some good work for some time from a number of areas. It's interesting though that the examples provided are Caedium (already mentioned), SimScale, and Visual-CFD (from ESI). A particular mention here is deserved for SimScale which combines OpenFOAM, SU2 (a CFD specific, compressible flow and adjoint optimisation focussed delivery) and Calculix (a solid mechanics specific FEA solver with good contact modelling capability). All of these are open source, and SimScale "wraps" them up into a web browser. The SimScale guys also know their stuff, so maybe these examples are a bit off point.

CAD Integrated CFD gets a fair description in terms of pro's and con's, and very much on point with regards it not been the route of choice for analysts. There's another blog's worth of material on the topic of whether you need a designer, engineer or analyst to do your CFD for you - the subject of "Democratisation" of CFD - we'll get on to this in another blog.

The Specialised CFD category is a tricky one, which tries to identify a subset of what we would describe as commercial analyst level CFD. If there is a line it's certainly blurred; the Complete CFD category is described as being standard in the aerospace and auto industry, but Exa
PowerFlow is heavily used by Jaguar Land Rover in the UK, and that software is in the Specialised CFD grouping. Volkswagen and Audi are heavy users of OpenFOAM, too. The Commercial Analyst Level CFD certainly feels a better catch all description - granted within that you have tools that only offer specific functionality rather than the "everything you could need" level from the well know big players, but the only real disadvantage for the specific functionality offerers is just that. This category is worthy of some further detail, but any sub-categorisation doesn't really add much to the open source versus commercial benefits and limitations conversation, so we'll leave it there for now.

A finer point worth dropping in at the end is the use of "CFD Software" as a description, with some later mention of pre- and post-processing capability. The landscape across open source and commercial looks a little different when you start to think about pre- and post-processing tools - we'll be discussing these two other parts of the CFD process with regards open and commercial tools in another blog, coming soon….
Nov 2016
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