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Old   February 14, 2013, 09:08
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Originally Posted by RodriguezFatz View Post
So it's recommened to take the worst case (=smallest) time step to ensure that it is actually small enough. Ok, but if you double it afterwards and compare the results you can learn for future simulations what might be small enough...

if you use a dt that is suitable from the point of view of the stability constraint but is quite large compared to the characteristic time scales of your turbulent flow, then your solution is somehow implicitly filtered also in time, that is v=v(x,t, Delta_x, Delta_t). However, you do not add an sgs model that would take into account the unresolved time components...
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Old   February 14, 2013, 09:16
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From a book of Sagaut:

We follow the common interpretation of LES as result of an evolution equation with spatially reduced resolution, and thus we focus on the analysis of errors due to spatial discretizations
and spatial filtering. This implies that the time-step size chosen for time integration is always sufficiently small so that error contributions from temporal discretization and temporal filtering (if applied) are negligible as compared to spatial error contributions. From now on this will will be tacitly assumed.

That means that your dt should be formally at level of the Kolmogorov time-scale to ensure it does not produce filtering effects (or, form a numerical point of view, a time-contribution of the local truncation error)
The time varying motion at the Kolmogorov scales is not resolved in an LES but modelled with a turbulence model. If you want negligible numerical errors then you must resolve the motion in time but this will be the grid scale motion and not the viscous dissipation scales. In practice an LES model for the instantaneous sub-grid scale motion tends to be very poor and significantly worse than RANS models for the Reynolds stresses for example. LES works if the sub-grid stresses are small compared to the resolved stresses and if the sub-grid model behaves well in an average sense in transferring energy to/from the sub-grid modelled motion.
In practice there is little need for the grid scale motion to be fully resolved unless you are comparing sub-grid models.
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Old   February 14, 2013, 10:01
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The time varying motion at the Kolmogorov scales is not resolved in an LES but modelled with a turbulence model. If you want negligible numerical errors then you must resolve the motion in time but this will be the grid scale motion and not the viscous dissipation scales. In practice an LES model for the instantaneous sub-grid scale motion tends to be very poor and significantly worse than RANS models for the Reynolds stresses for example. LES works if the sub-grid stresses are small compared to the resolved stresses and if the sub-grid model behaves well in an average sense in transferring energy to/from the sub-grid modelled motion.
In practice there is little need for the grid scale motion to be fully resolved unless you are comparing sub-grid models.

This issue is quite controversial in literature, some authors explicitly consider the time-filtering as present in the simulation and propose some specific sgs model for the unresolved time-scales... however we can be off-topic
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Old   February 15, 2013, 03:25
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From my experience with Fluent and the pipe flow, your Re number might be too low to allow a static Smagorinsky model to work. Otherwise, if you are using a Dynamic version, the problem might be in the convection scheme if it is the bounded central one.

However, some instability is clearly present, and possibly it is just the effect of a bad initialization (the fact that it is suggested by Fluent means nothing)

My experience with this case at Re_D=10k is to use:

dz+ = 30 and (R*dtheta)+=15 with classical wall normal spacings

dt=0.1*nu/u_tau^2 should be always enough (there is no point in going below this value for LES)
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Old   February 15, 2013, 04:49
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I see there are different approaches for the time step setting. Thank you for this insight.

One additional question: Is there any reasonable way to judge if the LES model works in a somewhat proper range? Recently I made an ERCOFTAC workshop about DES and some of the lecturers recommended to check the ratio of resolved to total turbulent kinetic energy for the "LES" part of the DES, which is pretty easy since the RANS model has a modeled "k" anyway.

Is there any way for a real LES to do something like that?
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Old   February 15, 2013, 04:54
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using an eddy viscosity model allow you to compute the averaged radial distribution. Further, you should always do the computation on the same grid and same time-step without using any turbulence model (LES no-model) that give you a clear framework of the action of the sgs model
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Old   February 15, 2013, 05:08
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Radial distribution ok, but that only helps for types of flows where reference values are known. Comparing to DNS results or experiments does only mean you get the correct real results. That does not mean, that your SGS-model works as it is designed for.

Can you give a general rule how to implement your second suggestion?
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Old   February 15, 2013, 05:38
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just set your case to laminar flow, without turbulence model
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Old   February 15, 2013, 05:40
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just set your case to laminar flow, without turbulence model
Well I know how to start a DNS. I ment an advise how to use this information to judge whether the SGS works fine or not...
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Old   February 15, 2013, 06:09
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Well I know how to start a DNS. I ment an advise how to use this information to judge whether the SGS works fine or not...
The Reynolds number tells you this. There is no need to perform a simulation. Your example is for a low Reynolds number flow and so a turbulence model that assumes the Reynolds number is high like a standard LES model is not appropriate. If this is the Reynolds number of your flow of interest then that is good news because you can simply solve the Navier-Stokes equations without any assumptions about turbulence models.

Wanting to solve a steady state flow at low Reynolds number is a fairly common requirement and there are low Reynolds turbulence RANS models although they tend not to be particularly general like the high Reynolds number models.

Wanting to solve an unsteady flow at low Reynolds number and include a turbulence model is odd unless this constraint is coming from something else like being a part of a high Reynolds number flow. Why do you want to do this?
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Old   February 15, 2013, 06:21
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I guess I did not illustrate my thoughts clear enough:
You can run a simulation with LES turbulence model and a bad (too large) grid. Then, the level of modeling of the SGS model will be quite high. These models are pretty imprecise and made to model a relatively small amount of turbulent spectrum. That's what I mean by "That does not mean, that your SGS-model works as it is designed for" in my previous post. The question now is: How can I judge, if the amount of modeling by the SGS model is small enough to give good results.
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Old   February 15, 2013, 06:45
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compare modelled and no-modelled LES by means of statistics (rms, spectra), the effect of the eddy viscosity model is in the damping of the highest resolved wavenumbers
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Old   February 15, 2013, 07:13
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Originally Posted by RodriguezFatz View Post
I guess I did not illustrate my thoughts clear enough:
You can run a simulation with LES turbulence model and a bad (too large) grid. Then, the level of modeling of the SGS model will be quite high. These models are pretty imprecise and made to model a relatively small amount of turbulent spectrum. That's what I mean by "That does not mean, that your SGS-model works as it is designed for" in my previous post. The question now is: How can I judge, if the amount of modeling by the SGS model is small enough to give good results.
In which case you would seem to be confusing numerical resolution and modelling assumptions plus picking an inappropriate test case. To expand on my answer to your original question:

For an LES simulation good numerical resolution means accurately resolving the large energy containing energy scales. These are the RANS Reynolds stresses which are evaluated from the statistics of the flow. For numerical methods designed for LES these stresses tend to be too anisostropic and too large when under-resolved. This can be seen by grid refinement. For general purpose numerical methods the turbulence may go out or blow up depending on the details.

The sub grid scale stresses can also be evaluated from the statistics (with a modelling assumption for most models) and compared with the Reynolds stresses. In high Reynolds number regions they should be small but they will grow near, for example, walls which usually introduce additional modelling assumptions.

The viscous stresses can also be evaluated from the statistics and compared with the sub-grid stresses and the Reynolds stresses. These are exact terms and so the growth near walls is good rather than a cause of concern.

In addition there are "numerical stresses" which broadly follow from the leading truncation terms in the transport terms. These are usually comparable with the sub grid stresses and in LES orientated numerical schemes their type and behaviour is important and controlled.

The above would be implemented in pretty much all research LES codes but how much is already built into your commercial code I do not know.

Last edited by andy_; February 15, 2013 at 07:54. Reason: typo
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Old   February 15, 2013, 07:41
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Quote:
Originally Posted by RodriguezFatz View Post
I guess I did not illustrate my thoughts clear enough:
You can run a simulation with LES turbulence model and a bad (too large) grid. Then, the level of modeling of the SGS model will be quite high. These models are pretty imprecise and made to model a relatively small amount of turbulent spectrum. That's what I mean by "That does not mean, that your SGS-model works as it is designed for" in my previous post. The question now is: How can I judge, if the amount of modeling by the SGS model is small enough to give good results.
I have a further suggestion, that however implies some work...
your Re number is quite low to try a DNS with some effort. Then you can use your DNS solution by applying a volume filter on the velocity components, the width being the same of the volume grid you use for LES. You can then compare all data.
This test is not perfect but can give you further indications about the quality of your LES if coupled with the LES no-model
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Old   March 10, 2014, 10:44
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Hi, in your description, you said "y+=0.9 nearly everywhere, with y=6mm. I have a z and x of about 30mm, thus x+=z+=5". But what if the geometry is complex and you don't really know the size in Z and X direction? What software are you using? can your software plot the X+ and Z+ , just like Y+?

Thank you so much ! I am using StarCCM+ and feeling lost about X+ and Z+...

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Dear all,

I try to get some experience in LES and need some help.
My setup is a simple straight pipe of 4m length and 1m diameter. Periodic bc with a mass flow rate of 0.044 kg/s is applied, which results (for air) in a velocity of about 0.07 m/s and thereby Re(pipe)=4800. y+=0.9 nearly everywhere, with y=6mm. I have a z and x of about 30mm, thus x+=z+=5.
Attachment 19017

Initialization with RANS, then switched to LES (Smagorinsky-Lilly) with some disturbances from the terminal (as described in the Fluent manual). I set the timestep to something save dt=0.15s, time to cross one cell should be about 0.5s. Also solution methods are set as recommended in the manual. Now, from yesterday to today the simualtion ran and I got some first results.

z-velocity:
Attachment 19020
y-velocity:
Attachment 19019
x-velocity:
Attachment 19018

1) Simulation time is 261s, so the fluid traveled nearly four times through the entire pipe, which is a bit too less, I guess. (?)
2) I would like to let some monitor run during the simulation with some good predictor for convergency. Any ideas? What about a volume integral of vorticity magnitude?
3) Now, I want to check for numerical stability by means of grid size, time step,... What would be good for a judgement of that? Just the velocity profile?
4) Also I was wondering of a Re=4800 pipe is "turbulent enough" to see something in an LES at all.
5) Is there any other good way to see, if my simulation is actually a real LES (and not some VLES with a too large grid), then just do the grid / time step - independence study?
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Old   March 10, 2014, 10:50
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I use Fluent, and here, you need to evaluate x+ and z+ by yourself. "y" is allways the direction of the flow in fluent, even if it isn't actually the "y-direction" of your physical coordinates.
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Old   March 10, 2014, 22:59
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Hi, I know what does Y+ mean, and Y+ actually mean the direction normal to the surface of the model... But did you mean that X+ and Z+ can not be calculated precisely?
Thank you for your kind reply.


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I use Fluent, and here, you need to evaluate x+ and z+ by yourself. "y" is allways the direction of the flow in fluent, even if it isn't actually the "y-direction" of your physical coordinates.
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Old   March 11, 2014, 07:15
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I guess what RodriguezFatz was trying to say is that, as those parameters actually depend from your flow direction, there is no obvious way to compute them without knowing in some detail your flow.

And even in that case, your grid might not be aligned with the flow direction, then how would you say what is dx^+ (stream-wise grid spacing in viscous units) and what is dz^+ (span-wise grid spacing in viscous units)?

However, for a given cell near the wall, you usually know the instantaneous y+ or, if you already have your statistics, you can compute the average y+ trough the mean wall shear stress. Then, if you know the grid (in Fluent you have access to several cell-related geometrical parameters) you can compute whatever d+ you want.

I remember doing something like this for a conference, where i had to plot the delta+ in a swirler for different swirl numbers (= different mean flow directions). I did it quite easily in Fluent but there was some hard-coding related to the geometry in order to obtain the correct values.
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Old   March 11, 2014, 07:17
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Hi, I know what does Y+ mean, and Y+ actually mean the direction normal to the surface of the model... But did you mean that X+ and Z+ can not be calculated precisely?
Thank you for your kind reply.
Hi,

What I do is just calculating

x+ = (y+ / dy) * dx

with dy and dx the real physical coordinates.
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Old   March 11, 2014, 07:30
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Hi, thank you for your reply again. I agree with the equation you mentioned below. But how to get the "real physical coordinate" so that you can get dy and dx? My geometry is complicated and I don't really know what is the size of every cell, which means I don't know exactly what is dx and dy because they varies a lot along the geometry....

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Hi,

What I do is just calculating

x+ = (y+ / dy) * dx

with dy and dx the real physical coordinates.
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