[Martin Hegedus]

Originally Posted by lakeat

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.

[tfuwa]

Originally Posted by lakeat

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?

[FMDenaro]

Originally Posted by tfuwa

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

[tfuwa]

Originally Posted by FMDenaro

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.

[FMDenaro]

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

[anandsudhi]

Originally Posted by Martin Hegedus

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

[FMDenaro]

Originally Posted by anandsudhi

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.

[azna]

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

Thanks

[Esther Jin]

Originally Posted by pete

2D LES is physically incorrect. Trying to model the large edies in 2D is not possible since their behaviour and development is very much three dimensional. 2D RANS is not physically incorrect in the same way, but requires certain criteria or assumptions that are often not fulfilled

Hello,

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

[flotus1]

Originally Posted by Esther Jin

Hello,
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?

There is quite a lot to untangle in these two short sentences...

"turbulence is 3-dimensional"
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 time-averaging. 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 time-averages at one position or if you take spatial averages along the cross-flow 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 3-dimensional. 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 3-dimensionality 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.

[Esther Jin]

Hello Alex,

Thank you for your reply

Best regards,
Esther

[lynncheng]

Originally Posted by lakeat

Dear all,

I found my simulation of an square cylinder, external flow, the 2D simulation always comes with a low Cd value, like 30%. Is this normal?

Any experience?

Hi Wei, Have solved this problem? I am a new openfoamer. And I am simulating the flow around a 2D square cylinder at Reynolds number equals 22000.

I have similar problem in gettting the correct Cd values from literature.

Can you give me some suggestions in solving this problem?

[lzhaok6]

This thread has been very helpful. My question is solved.

[Bodo1993]

Originally Posted by azna

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

Thanks

Hi, I am wondering if you have found any related references. Thanks.

[aero_head]

Originally Posted by Bodo1993

Hi, I am wondering if you have found any related references. Thanks.

Hello,

There's quite a nice breakdown in this thread:
https://forum.ansys.com/discussion/4...utation-domain

[Bodo1993]

Originally Posted by aero_head

Hello,

There's quite a nice breakdown in this thread:
https://forum.ansys.com/discussion/4...utation-domain

Hi, thanks. What I am missing at the moment is how to make these two cases comparable? In other words, if we have the results of 2D planner simulation, what should be changed in the problem settings to make a comparable axisymmetric case?