FDS Vs StarCCM+ (or Ansys)

[Siva]

Hi all,

Some lengthy query ahead. Please go through. I hope it will be worth your while.

May I know what is the reason we do not simulate fire scenarios in Starccm or Ansys, instead of FDS? 

For this discussion, let us take the case of evaluating temperatures and heat fluxes during a fire development in a passenger train carriage. 

I can think of following reasons. Please add more and also please correct me if I am wrong.

1. FDS is a specific purpose solver. i.e., It is designed to solve for fire driven flows. Whereas Ansys or CCM are general purpose solvers. I am not sure what advantage we gain because of that?

2. The results, for instance, temp contours are smoothed out in Ansys/CCM due to averaging done by those solvers, whereas in FDS they are detailed.

3. There is fine meshing involved in Ansys/CCM and it inturn results in huge number of cells for a passenger car of approx 20m length with all the complex geometry.  Due to its simplicity, the FDS solver is at least 4 times faster than the Ansys/CCM solver.

3. This maybe very important one: One thing I learnt from some research paper is that, FDS does not solve for radiation as efficient as Ansys/CCM. So for radiation dominant fires, for e.g more than 400 deg C, FDS underestimates heat flux and temp. And the cases we get in passenger carriage fire exceeds 400 deg C. Inspite of that, FDS is widely used. Why is that? May I know if my understanding is wrong or am I missing something?

I really appreciate your inputs on this and if there are any lengthy explanations which is too much to type ;) please share a link or some direction I should look at.

Thanks in advance and much appreciated,

Siva

[dr_jfloyd]

In my view the major reasons are: FDS is free, FDS is relatively easy to learn how to use to model a fire (an advantage of having a model and its documentation focused on a specific type of problem), FDS is fast, and FDS is very well validated for fire simulation. 

In regards to your four numbered items:

1. Generally speaking, one would expect that a purpose built solver would be more efficient and/or have more capabilities for solving the problems it is designed for than a general solver. On the negative side, one would expect a purpose built code to have a much more limited application space than a general purpose code. For example, FDS cannot do compressible flow.

2. This isn't a completely true statement. Many general purpose CFD codes have multiple turbulence models. Some models inherently time average, and others (like LES) do not. 

3. I don't know that I would consider FDS to be simple. It's speed is because it is a purpose built solver focused on a specific class of problems. This allows FDS to be optimized for those specific problems. FDS may not have an a la carte menu of turbulence models, heat transfer models, etc. that a general purpose commercial code; however, FDS does have, I don't think it is correct to state they are any less rigorous than similar models in a commercial code.

4. Be careful drawing conclusions about FDS or any other computational tool from a single research paper. Papers typically focus on a small number of experiments which may not provide a true picture of performance, the FDS version used could be dated, and it may not be easy to how well FDS was used. I suggest you look at the FDS Verification and Validation Guides.

[Sivasankaranarayanan S]

Thanks a ton. I have briefly gone through the verification and validation guides. I will go in depth now.

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