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January 11, 2012, 14:17 

#21  
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Martin Hegedus
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If your separation is smooth body separation (i.e. a circle), then the pressure and viscous solution is dependent on initial grid spacing. Yes, as flow moves much further away from the body, I do expect the URANS flow to become unstable as eddy viscosity diminishes and the grid (in general) becomes coarser. 

September 1, 2014, 04:18 

#22  
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Albert Tong
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I have a simple question that why 2D les is not physically right?
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September 1, 2014, 05:22 

#23  
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Filippo Maria Denaro
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the key is to understand what you mean for 2d turbulence... 2d turbulence is a real model for large scale flows, such as atmospheric or oceanic turbulence becose you have a 2d plane of large scale and the third dimension (the vertical direction) is much smaller compared to them. For such flows you can perform 2d LES computation. On the other hand, if you have a 3d problem where all the characteristic scales in the three dimensions are comparable each other, then it makes no sense to perform an LES computation in 2d 

September 10, 2014, 01:40 

#24  
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Albert Tong
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Thank you for your promote reply. That is to say 2D LES is similar to 2D RAS? I read through this thread and some people seem to suggest 2D LES is not mathematically right. I am simulating pipeline sitting on seabed, and the pipeline is very long compared to its diameter. Is 2D LES appropriate for such application, or 2D RAS is more appropriate? Many thanks.
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September 10, 2014, 03:21 

#25 
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Filippo Maria Denaro
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No, that's not a mathematical lack, LES can be formally performed even in 1D...
The key is in the physics of the flow problem.... as happen in geophysical flows, when you have turbulent structures extending several order of magnitude more in two dimensions compared to the third dimension, a 2D LES can be a reasonable approximate model. Pipe and channel flows are typically flow problems with 3D turbulent structures and you have to use 3D LES. RANS is used in 2D as it implies a statistical averaging, that's very different idea from LES 

December 7, 2015, 06:14 

#26  
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anand sudhi
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I have seen many papers trying to do 3D URANS to capture the mean properties, even through the draw back you mentioned persists. So my questions is # Is it unreliable on properly describing the flow structures like in the wakes? # Also is there any application where URANS can give reliable results, in your experience, like calculation of Drag and other mean quantities. I am hoping you would find this thread as its an old one. Thank you 

December 7, 2015, 06:58 

#27  
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Filippo Maria Denaro
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You should always consider your goal and then the suitable tool will be defined URANS can provide some statistical unsteady details but if you want to study the structure of the wake in its details with the complete range of produced frequancy, than you have only LES/DNS. The better framework I can see to define correctly the URANS formulation is to study the flow produced by an external timedependent force, for example the piston movin in a cylinder. 

January 26, 2016, 01:05 
2D, 2D axisymmetric

#28 
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azna
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Hi ,
I was wondering that if anybody has any refrences regarding comparison of 2D ( planer) midel domain with 2D axisymmetric model. Thanks 

October 24, 2016, 10:18 

#29  
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I am a beginner in terms of CFD I am wondering why turbulent flow can be simulated in 2D, as I know, turbulence are 3 dimensional. How to understand that 2D RANS is not physically incorrect? Thank you very much in advance. Looking forward to your reply Best regards, esther 

October 24, 2016, 17:07 

#30  
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Alex
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Quote:
"turbulence is 3dimensional" It is not exclusively 3D. There are phenomena in 2D and in 3D that are characterized as turbulence. Yet they behave quite differently which is why a 2D DNS/LES simulation for a 3D turbulent flow is the wrong approach. "2D RANS is not physically incorrect" I like the double negative. RANS is often reduced to timeaveraging. But in fact averaging along a direction of flow symmetry is also a valid Reynolds average. For example: the flow over a backwards facing step (a 3D turbulent flow) will yield the same average flow quantities, no matter if you take timeaverages at one position or if you take spatial averages along the crossflow direction. For the same reason you can carry out a RANS simulation in 2D although the flow (and the turbulence you are trying to capture) are 3dimensional. Given that the geometry and the boundary conditions are symmetrical of course. So 2D RANS is physically correct in the way that it yields the same result as a 3D RANS under these circumstances. The 3dimensionality of turbulence has already been taken out by using Reynolds averaged equations in the first place. Of course, RANS with a turbulence model is a huge approximation itself, so be careful with the "physically correct". "I am wondering why turbulent flow can be simulated in 2D" To recap, you are only on the safe side with this assumption if you are using a Reynolds averaged set of equations and the average flow quantities are 2D. And if you consider RANS with a turbulence model good enough. With a LES or DNS formulation you have to go 3D to capture 3D turbulence correctly.
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October 30, 2016, 13:22 

#31 
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Hello Alex,
Thank you for your reply Best regards, Esther 

February 14, 2017, 04:25 

#32  
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lynn Cheng
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I have similar problem in gettting the correct Cd values from literature. Can you give me some suggestions in solving this problem? 

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