At what moment do you have unacceptable accuracy when using Fluids?

[jarek_o]

Hi,

Is there a thumb rule available on when the Fluid module starts to show inaccuracy problems? With a tick time of 1.00.

Thanks,

Jarek

[dj_moens]

Hi.

This depends also on the ratios between rates and the contents of objects. We have encountered almost always accuracy issues using time slicing and sometimes these issues are hidden.

For instance, in the tick system you can only move material to the next object (within one tick). If that object can hold less content then the rate times the tick, your network seems to run OK, but in fact you are losing transfer rate as not all material was moved downstream in the tick.

We also have had issues regarding tanktop and tank empty calculations (triggers not firing) if rates are for instance defined in decimals (like 0.333).

Below an example where you already loose 2% with a tick time of 0.1.:

Tank of 10 liter filling at 3 l/min.
After filling, empty at 3 l/min. Repeat 10 times.
Dots = fluids, line = theoretical and FloWorks calculation!

Tme slicing (ticks):

682 events (calculations)
Error in final amount: 4 liters (2%)
Error in final time: 1.33 minutes (2%)


Optimum: 20 events (As is calculated by FloWorks)

Rgds,

Dirk-Jan

[dj_moens]

fillingandemptying.png

[anthony_johnsonT83CM · Accepted solution]

Unfortunately there's no perfect rule of thumb on when accuracy becomes an issue in fluid objects. The tick time is only one factor among many that may impact accuracy, such as flow rates, the number of and connection configuration of the objects in the system, and even the accuracy "tolerance" that users are willing to accept. In relation to DJ's reply, in some situations a 2% tolerance might be "good enough," whereas in others it is obviously unacceptable.

I think what DJ was getting at in his answer is that Talumis' FloWorks module simulates fluid systems in a much faster and more accurate manner. In reality, the question of "how much inaccuracy is acceptable" shouldn't really need to be a question at all. As such, we at FlexSim have discontinued our development of the fluid library, and instead suggest FloWorks, and use it ourselves in-house, as the best high-accuracy fluid simulation solution in FlexSim. We have left the legacy fluid library in the product so that users who do not want to pay the extra add-on cost of FloWorks may still have a solution, albeit a less accurate solution, for simulating fluids.

So, my suggestion is, if you really just want to get a rough estimate out of your simulation, then perhaps the fluid library will be good enough. You can do a sensitivity analysis (try different tick times) to see how much results are affected. But if you don't want to even worry if the accuracy is good enough, to me it is worthwhile to bite the bullet and get a license of FloWorks.

[dj_moens]

Anthony, thanks for this clear addition.

[jarek_o]

@dj.moens and @anthony.johnso, thanks for the bright answer.

Jarek