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December 8, 2011, 10:21 
Transient solution not oscillating

#1 
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anonymous
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Hello,
I searched the forum but didn't find something so i do my first post: I have a Problem with my transient Solution for which I expect the Forces to oscillate. If I use a converged steadystate solution as initial condition for the transient run, no oscillation apears. If I start on the other hand directly with the transient Run the forces oscillate but with no sufficient convergence. (Convergence of the oscillation of course) The advection scheme is HighRes and the time discretization is 2nd order. Turbulence is still 1st order and i have smal time steps. My Question: Is it the right way to start such a transient run with a converged steadystate solution? Or should i start with a transient 1storder solution? I appreciate your advice. Paul 

December 8, 2011, 17:32 

#2 
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Glenn Horrocks
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Is this an FSI simulation or is the geometry fixed? What is the geometry?
To get oscillations you need high order numerics and good convergence. Can you attach your CCL file? If the oscillation is real then any realistic initial condition should end up oscillating. 

December 9, 2011, 11:30 

#3 
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anonymous
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Sorry, cant post any files now.No FSI but i plan to do it later on.Geometry is a parabolic trough solar collector in 2d near the ground. In addition i have a logarithmic wind profile as initial condition. Reynolds Number is about 4e6. Using SST model but only 1st order numerics for turbulence. I try running with highres then. I think in ansys docu i read that advection and time always need to be 2nd order (or highres) but turbulence can remain at 1st in most cases. I will also try blendfactor=1 for high order numerics then.
Im still not sure about the starting solution. Should i start with an converged steadystate solution with highres schemes or with a transient 1st order solution. Or start directly with transient high order numerics? And does my starting solution only have influence on the time until im converged or can it change the character of the flow? Thanks for your help and have a nice weekend Paul 

December 10, 2011, 07:00 

#4 
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Glenn Horrocks
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The whole idea of turbulence models is they average out the turbulent fluctuations and leave a steady state bulk flow. This may be what is stopping the oscillations.
What oscillates anyway? The air flow around the collector? In this case you will probably need a SAS, DES or LES approach to get something realistic. 

December 12, 2011, 04:48 

#5 
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anonymous
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Yes, the air flow around the collector should be oscillating and therefore the pressures and forces. Wind tunnel results are conform with that. The strange thing is that at some angles i have the osscilation but at some i don't. Maybe at some critical angles the osscillating is so strong that the RANS approch don't can smooth it?
Im not familiar with LES or DES. Do I need special licences to use them? Thanks again Paul 

December 12, 2011, 18:21 

#6  
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Glenn Horrocks
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Quote:
If the oscillations are what you want then you should use a model designed for that sort of thing. I think the LES/DES/SAS approaches require an advanced turbulence license. Also they are not topics for beginners, if you are not already proficient in CFD then I would recommend you get some experience with standard CFD models before progressing to them. At the very least read up about them from turbulence textbooks like "Turbulence Modelling for CFD" by Wilcox  then you will have a good idea about the complexity of these models. 

December 13, 2011, 13:58 

#7 
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anonymous
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Ok. Thank you for your help. I think i will use the time average results then.


December 16, 2011, 12:12 
SAS SST model

#8 
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anonymous
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Hello again,
I'm currently trying the SASSST model for turbulence now. I hope to get a "LESlike behavior" of the oscillating flow with less complexity and effort. Therefore i still want to use my twodimensional grid (with only one element in the depth). My question is: Is it acceptable to do a ScaleAdaptiveSimulation with such a 2d grid. The examples i found were all 3d and I know that turbulence is a threedimensional problem, but maybe it's still possible to get good qualitative results? Thanks again Paul 

December 16, 2011, 13:15 

#9 
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Kailash
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I have performed LES simulations on Aerofoils in the past. I was advised to use a 3D grid with depth 10% of the characteristic length (chord of the aerofoil) and to use delx=dely=delz (cells with equal dimensions in all three dimensions) to take into account of isentropic turbulence.


December 16, 2011, 16:13 

#11  
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Glenn Horrocks
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Quote:


December 19, 2011, 03:55 

#12 
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anonymous
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All right. Thanks for the comments. I will try that but I'm not sure if I have enough hardware power to simulate in 3d. Maybe I can use a more coarse grid or just compute over christmas.


December 19, 2011, 06:45 

#13 
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Glenn Horrocks
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Christmas was invented as a gift to CFD engineers to run those really long simulations you never get enough CPU time to run normally. Just make sure the cleaners don't knock the power cord.


April 20, 2015, 03:29 

#14  
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Mina
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Quote:
I have to model standing wave in a thermoacoustic heat exchanger, but for the beginning to make problem as simple as possible I modeled flow between two parallel plate with the air at the iso condition. At the inlet I used opening with velocity signal of u=0.5 sin (2*pi*f*t) (velocity untinode)and at the outlet I have wall (velocity node). Frequency of signal is f=13 Hz. One time I consider the length of the domain to be 27 m which is almost equal to the wavelength . The number of elements in the y and z axis is one. So one can assume it as a 1D acoustic model. I monitored dynamic pressure inside the domain it was oscillating sinusoidaly around the mean value of zero . So it was ok. pressure 27 m.jpg Then I reduce the length to 6.6 (equal to one quarter wave). And again I used unti node in the inlet (opening with the same velocity signal)and node (wall Bc) at the oulet. But this time at the monitoring point I had kind of pressure drift. pressure was not oscillating around zero. so results show kind of mean pressure in the system. while we don’t expect mean pressure . I don’t know what the reason is for this deviation from zero mean pressure? Is that kind of numerical error? i changed the convergence criteria from 1e6 to 1e9. but it was still the same. pressure (6 m).jpg 

April 20, 2015, 05:41 

#15 
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Glenn Horrocks
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If it is convergence it will be the imbalances which are better convergence criteria then the residuals. So I would recommend activating the imbalances as a convergence criteria. But if you are converging to 1e9 then it does not sound like convergence.
I suspect your inlet velocity BC is the cause. I suspect the reflected wave is not interacting with the boundary condition in the manner you expect. This means that initially the inlet pushes air into the domain, but when the reflected wave hits the inlet it is not behaving as expected and this leaves you with a residual positive pressure due to the initial flow. Just a guess, but it sounds plausible. You might need to be more realistic with your inlet boundary. Either use a wall and moving mesh, or expand the inlet out to a far field condition and apply the inlet boundary at a far field. 

April 20, 2015, 08:53 

#16  
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Mina
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Quote:
Thanks for your advice, i agree with you about the possible cause, when i am monitoring the velocity at the inlet (inlet is an opening boundary with u=0.5 sinwt) it is not sinusoidal neither. but then if i use the moving wall at the inlet i am not sure from acoustic point of view what kind of reflection we will have in the lnlet. when i am using opening with specified velocity it is velocity untinode and wall is velocity node so it must create one quarter wave length standing wave in the domain. but using the moving wall in the inlet and fixed wall at the outlet, which Independence will create? Last edited by Mina_Shahi; April 20, 2015 at 10:27. 

April 20, 2015, 09:05 

#17 
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Mina
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Also in the starting of the solver, i have this message
++  Reference Pressure Information  ++ Domain Group: BODY This is a transient run with at least one compressible fluid and no boundary pressure set. The pressure level is set through the transient term in the continuity equation. To accelerate convergence, the pressure level will also be shifted dynamically to satisfy global mass conservation. could it be the cause for creating non zero mean pressure in the domain? 

April 20, 2015, 09:44 

#18 
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If you have a nonclosed system with a compressible fluid, you are always better off using a pressure (decide if total or static based on boundary condition guidelines) based boundary condition. Otherwise, you may have an illposed problem.
The software is warning you about it, and taking some corrective action. I rather be explicit if possible than leaving it to chance. 

April 20, 2015, 09:58 

#19  
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Mina
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Quote:
well i am not using a pressure based boundary condition, i am giving the velocity in inflow and wall at outlet boundary. however in the basic setting i am giving the reference pressure which is 1 bar. i don't know how to get rid of this warning. I guess (not sure) that this may be a reason for having a pressure drift when i am monitoring the pressure in the domain. 

April 23, 2015, 09:18 

#20  
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Mina
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Quote:
the channel originaly has velocity untinode in inlet and velocity node at the end. if i use the domain which is two times longer and use the outlet instead of wall, it will be velocity untinode, velocity untinode at both ends so i suppose in the middle (which is in reality a wall) i get the velocity node as of in the original case. 

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