
[Sponsors] 
Is it usual in CFD community to guarantee the results? 

LinkBack  Thread Tools  Search this Thread  Display Modes 
November 5, 2014, 06:31 
Is it usual in CFD community to guarantee the results?

#1 
Senior Member
OJ
Join Date: Apr 2012
Location: United Kindom
Posts: 473
Rep Power: 18 
I work in industry and we use CFD studies to assess the pressure drop characteristics of our products. But lately there is something that is bothering me.
Often clients are interested in the guarantee of the pressure drop values we provide. Now we have done experimental testing of few of our products and typically our CFD results are within 20% of experimental pressure drop values. But then it is not possible to benchmark every product and every size of of that product, and which is the reason CFD is used to fill this gap. But then, I was wondering, if a product of say 6" size is benchmarked with CFD, how to assess whether the same accuracy can be expected in say 30" size of the same product. Or, for that matter any other type of the product. I have never worked in consultancy, but I am curious to know, if CFD community typically guarantees the results they give for all sizes of the product, based on representative benchmarking done for a single size of the product. Or if the commitment about the accuracy is expected, how it is handled. 

November 5, 2014, 07:05 

#2  
Senior Member
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,198
Rep Power: 66 
Quote:
Size is not the only relevant parameter... let me do an example for a flow around a cylinder of radius R. The flow is characterized by the nondimensional Reynolds number Re=U*L/ni, so you can fix L=2R as reference lenght. Now, if the group U/ni can be varied in such a way that Re=constant for different R sizes, you have the same flow features and the same quality in the CFD solution. But if the Re increases, the quality of the CFD solution can be much more difficult to be ensured, at present Direct Numerical Simulation for complex geometries and flows is possible at moderate Re number. Using turbulence model you have more parameters that introduce problems in guarantee the quality of the solution 

November 5, 2014, 07:19 

#3 
Senior Member
OJ
Join Date: Apr 2012
Location: United Kindom
Posts: 473
Rep Power: 18 
It is probably easy to assess what conditions would imply similar non dimensional results for simple geometries, but for an industrial product, like say a filtration equipment, it is difficult to judge these criteria.
DNS (if done correctly) would be obviously more accurate than typical RANS models given that the closure assumptions are not involved, but then, one needs a humongous computational power to do this, along with patience and time to find the correct lengthscales/timescales. This is not possible in industry where there is a budget for everything and it needs to be justified. And most importantly, results are needed yesterday! The trickiest part is, when management thinks.. "Oh well, we have invested in expensive CFD packages, manhours of CFD engineers and experimental testing. The CFD guys take 23 days for every result. So we should get results with 0% inaccuracy, otherwise what is the point in spending such money!" They do not understand the uncertainties involved, nor do they posses the CFDvocabulary/patience to understand the explanation given. So I was curious to know how others handle this 

November 5, 2014, 07:28 

#4  
Senior Member
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,198
Rep Power: 66 
Quote:
Yes! This is the real problem... speaking to people that have no experience in CFD applications!! Unfortunately, RANS in industry is still the standard and you cannot reach 0% inaccuracy even if you produce gridrefinement study... 

November 6, 2014, 14:44 

#5 
Senior Member

In my experience, even if i was never involved in actual consultancy, this is not the usual practice of serious people. But simply because it's impossible to give such ensurance. Notice also that the actual developers of turbulence models also wouldn't.
What should enter the CFD practice is the Uncertainty Quantification, but that usually means running several tens, or hundreds, simulations, to only understand how a single turbulence model affects a single flow case. No way it is gonna be everyday practice but, with that, you could give confidence intervals. However, besides the turbulence modeling effects (which are huge in practical CFD), this is not different from the experimental practice. No windtunnelist would ever give you ensurance if your Re number is different from the one tested. Without direct experimentation he can, at most, give you an advice based on experience. That's all. Also, notice that most consultancy firms use blackbox commercial CFD tools. They cannot give ensurance over something which is not produced by them. In practice this might also be applicable to the users of OpenSource tools, which i never found to be more prepared on CFD or Fluid Dynamics than generally experienced people not using such codes. It is actually more probable that they are more programmers than actual CFD experts (otherwise they should already have their own code instead of putting efforts in recompiling/repacking OpenFoam in all the possible flavors). 

November 11, 2014, 10:41 

#6 
Member
Join Date: Mar 2013
Posts: 42
Rep Power: 11 
there's also the grid convergence index and richardson extrapolation, but that will only estimate the error in your grid resolution, not the error inherint in your models ie turbulence and what not. and this also requires you to do a grid study, which i'm sure becomes difficult in industry


November 12, 2014, 05:20 

#7  
New Member
Garrison, Liang
Join Date: Feb 2012
Posts: 15
Rep Power: 12 
Quote:


November 12, 2014, 07:03 
Is it usual in CFD community to guarantee the results?

#8 
Member
Join Date: Mar 2009
Posts: 41
Rep Power: 15 
Hi All,
You can read relative answer at 1.4.4 Potential Problems in the book "Computational Fluid Dynamics in Industrial Combustion". Regards, Mehulkumar 

November 12, 2014, 08:05 

#9 
Senior Member
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,198
Rep Power: 66 
Some paragraph about industrial application with LES is in the Book of Sagaut


Thread Tools  Search this Thread 
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Matching CFD results and wind tunnel testing  huskerwong  Main CFD Forum  0  July 16, 2009 15:24 
CFD vs Experimetal Results for Aerofoil  aceofharts414  Main CFD Forum  0  April 22, 2009 08:14 
Export CFD Solver results to text file format  Maik  Main CFD Forum  2  May 23, 2008 02:34 
Results saving in CFD  hawk  Main CFD Forum  16  July 21, 2005 21:51 
STARWorks : Mainstream CAD with CFD  CD adapco Group Marketing  Siemens  0  February 13, 2002 13:23 