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Old   January 11, 2012, 14:17
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  #21
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Martin Hegedus
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Quote:
Originally Posted by lakeat View Post
Thank you guys,

Hybrid would be definitely better than (U)RANS, but it is still too costly for large Re. Many hybrid still require a low y_1st_plus be well set, around unity or similar.

Low-Re turb model is very promising, but I am not sure which one is more stable. I am testing.

Yes, the 3D simulation I mentioned is LES, I will never do 3D URANS, I don;t think it worth the effort.

And btw, in 3DURANS, the vortices do break in the wake a little bit, but 2D rings persist for a quite long distance, this is what I've seen from many published papers.

These days, I am so interested in 2D URANS' performance, is that I am doing some project, where we have thousands of cases to run, or we have body motion dynamics mesh etc., all of these demand the single run to be as quickly as possible.
Since you are dealing with a separation bubble at the top and bottom of your square, which is driven by the sharp leading edge, I'm not really sure I know how y+=1 translates into a real grid spacing and how dependent the solution is on it. You may find that the required grid spacing for low and high Re number, at least in regards to your pressure forces, is similar. However, your viscous drag will be off. But, viscous forces are probably a smaller percentage of your overall loads and you can tweak it during post processing to get better results.

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.
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Old   September 1, 2014, 04:18
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Albert Tong
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Originally Posted by lakeat View Post
Yet see how many papers were published with 2D-LES...


I am more interested in the lift coefficient RMS values than in the Cd.

But I got impression that ALL URANS would give sine wave time history, and low RMS(Cl) values.

Is there any better solution within the scope of 2D (2D-mesh) simulations? That what I am looking hard for.
Hi All,

I have a simple question that why 2-D les is not physically right?
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Old   September 1, 2014, 05:22
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Filippo Maria Denaro
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Originally Posted by tfuwa View Post
Hi All,

I have a simple question that why 2-D les is not physically right?

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
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Old   September 10, 2014, 01:40
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Originally Posted by FMDenaro View Post
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
Hi Filippo,

Thank you for your promote reply. That is to say 2-D LES is similar to 2-D RAS? I read through this thread and some people seem to suggest 2-D LES is not mathematically right.

I am simulating pipeline sitting on seabed, and the pipeline is very long compared to its diameter. Is 2-D LES appropriate for such application, or 2-D RAS is more appropriate?

Many thanks.
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Old   September 10, 2014, 03:21
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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
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Old   December 7, 2015, 06:14
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Originally Posted by Martin Hegedus View Post
On aspect of this question is how much does one believe in 3D URANS? I don't do a lot of unsteady calculations so I can't really comment. However, IMO, RANS equations throw so much eddy viscosity at a problem I don't know how a vortex would be able to break apart by twisting on itself. For example, take the wake behind a truck or axially aligned circular cylinder, I assume the vortex coming off (for a URANS calc) is going to want to be a ring vortex, and stay that way. Yes, it will be unsteady in the down stream direction, but probably not around the axis. In other words, URANS would be unsteady in a 2D sense only. Again, these are conjectures on my part.

Take rotating helicopter blades for example. I've seen many times where the grid around the blade (these are Chimera grids) use the RANS equations and then there is a cartesian outer grid which is run with Euler (i.e. zero viscosity) One advertised reason they do this is to increase the turn around time of the solution. However, I wouldn't be surprised to learn if there are other benefits, i.e. good bye eddy viscosity. Unfortunately, I never asked them.

Question, the original 3D runs of the square cylinder to which you were comparing your 2D runs to, were they LES or URANS? Sorry, I just assumed they were LES. If they were URANS, I would expect the 2D and 3D URANS runs to be similar.

Maybe another modeling choice for you would be to do it like the helicopter people. Create an inner grid and use RANS/URANS on it and create a cartesian outer grid which uses Euler. This would have to be 3D. But, maybe, it runs faster than LES.
Hi

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
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Old   December 7, 2015, 06:58
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Originally Posted by anandsudhi View Post
Hi

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

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 time-dependent force, for example the piston movin in a cylinder.
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Old   January 26, 2016, 01:05
Default 2D, 2D axisymmetric
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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
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