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February 14, 2013, 09:08 

#21  
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Filippo Maria Denaro
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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... 

February 14, 2013, 09:16 

#22  
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andy
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In practice there is little need for the grid scale motion to be fully resolved unless you are comparing subgrid models. 

February 14, 2013, 10:01 

#23  
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Filippo Maria Denaro
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This issue is quite controversial in literature, some authors explicitly consider the timefiltering as present in the simulation and propose some specific sgs model for the unresolved timescales... however we can be offtopic 

February 15, 2013, 03:25 

#24 
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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) 

February 15, 2013, 04:49 

#25 
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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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February 15, 2013, 04:54 

#26 
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Filippo Maria Denaro
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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 timestep without using any turbulence model (LES nomodel) that give you a clear framework of the action of the sgs model


February 15, 2013, 05:08 

#27 
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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 SGSmodel works as it is designed for.
Can you give a general rule how to implement your second suggestion?
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February 15, 2013, 05:38 

#28 
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just set your case to laminar flow, without turbulence model


February 15, 2013, 05:40 

#29 
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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...
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February 15, 2013, 06:09 

#30  
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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? 

February 15, 2013, 06:21 

#31 
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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 SGSmodel 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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February 15, 2013, 06:45 

#32 
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compare modelled and nomodelled LES by means of statistics (rms, spectra), the effect of the eddy viscosity model is in the damping of the highest resolved wavenumbers


February 15, 2013, 07:13 

#33  
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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 underresolved. 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 subgrid 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 

February 15, 2013, 07:41 

#34  
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Filippo Maria Denaro
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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 nomodel 

March 10, 2014, 10:44 

#35  
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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+... Aqua Quote:


March 10, 2014, 10:50 

#36 
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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 "ydirection" of your physical coordinates.
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March 10, 2014, 22:59 

#37 
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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. 

March 11, 2014, 07:15 

#38 
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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^+ (streamwise grid spacing in viscous units) and what is dz^+ (spanwise 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 cellrelated 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 hardcoding related to the geometry in order to obtain the correct values. 

March 11, 2014, 07:17 

#39  
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What I do is just calculating x+ = (y+ / dy) * dx with dy and dx the real physical coordinates.
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March 11, 2014, 07:30 

#40 
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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....
Aqua 

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