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March 3, 2006, 05:25 
Unsteady flow around a rectangular building

#1 
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Hi,
I want to use fluent to model vortex shedding effect around a rectangular building. The detail are as follows building size: B=0.1125m D=0.075m H=0.45m domain size: B=3m D=6m H=1.8m coordinate x=1.5 to 4.5 y=1.5 to 1.5 z=0 to 1.8 The base of the centre of the building is at the origin velocity profile: power law u=14.7*(z/1.5)^0.15 turbulent model: RNG kepsilon I used fixed time step = 0.004s with SIMPLEC for velocity coupling and QUICK for covective transport. I used the same mesh to calc steady flow and there is no problem and I just changed the setting in fluent from steady to unsteady. I failed to obtain a periodic fluctuation in lift force. Is there an error in setting? Thanks 

March 3, 2006, 12:47 
Re: Unsteady flow around a rectangular building

#2 
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You will benefit from reading the tutorial (6) Flow past a circular cylinder www.studentfluent.com QUICK scheme is excellent if the stability is maintained


March 4, 2006, 02:19 
Re: Unsteady flow around a rectangular building

#3 
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Thanks for your reply. I used hex element to mesh the grid. Should I use SIMPLEC or PISO for pressurevelocity coupling?


March 4, 2006, 12:32 
Re: Unsteady flow around a rectangular building

#4 
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Use Piso for unsteady simulations. But it will take more time compared to Simplec.
Anindya 

March 5, 2006, 01:41 
Re: Unsteady flow around a rectangular building

#5 
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maybe you should consult the HELP, there are many useful information i think.
changkiang 

March 5, 2006, 23:32 
Re: Unsteady flow around a rectangular building

#6 
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I have tried PISO with time step=0.004s but continuity still does not converge after 100 iterations at the start
Should I further reduce the time step size? Thanks. 

March 5, 2006, 23:45 
Re: Unsteady flow around a rectangular building

#7 
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Should I use "Use Frozen Flux Formulation" also?
Thanks 

March 6, 2006, 07:38 
Re: Unsteady flow around a rectangular building

#8 
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I would suggest to stay with PISO algorithm and definitely reduce your time step, at least 2 orders of magnitude.
Hope this helps Edi. 

March 6, 2006, 11:14 
Re: Unsteady flow around a rectangular building

#9 
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reduce your time step to around 10^5 range.


March 6, 2006, 23:23 
Re: Unsteady flow around a rectangular building

#10 
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I tried to reduce to 0.001s but got a malloc error. What should I do?


March 7, 2006, 00:31 
Re: Unsteady flow around a rectangular building

#11 
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I changed the time step to 0.001 sec. It converges in 30 iterations but I got a out of memory error. Should I change back to SIMPLEC or use a single precision solver instead? If I use a single precision solver, is the convergence limit of 1e5 appropriate?
Thanks for your help. 

March 7, 2006, 05:35 
Re: Unsteady flow around a rectangular building

#12 
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The memory error should be related much more with the dp solver than that with the algorythm, so try to keep PISO (that reduces the iterations per time step a lot, believe me...) and move to single precision. The convergence limit of 1e05 (at least for continuity) sounds appropriate.
Hope this helps Edi. 

March 7, 2006, 09:36 
Re: Unsteady flow around a rectangular building

#13 
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Thanks for your suggestion. I started unsteady calculation with steady solution. In order to 'start' vortex shedding, I modify the inlet B.C. such that v=0 for y<0. But then I got the message "Reversed flow in ### faces on outflow". Is this message important? How to avoid this message?
Thanks again for your help. 

March 7, 2006, 10:55 
Re: Unsteady flow around a rectangular building

#14 
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It simply means that in some cells of the outlet boundary the flow is coming in instead of going out (as it is supposed to be for an outlet boundary). Basically it is due to the position of the boundary, if you have put a pressure boundary too close to the building the flow could not be fully developed and in order to satisfy the boundary condition reversed flow appears.
If it disappears after some iterations (before proceeding to the next TS) it's acceptable, otherwise consider to extend your domain past the building. Hope this helps Edi. 

March 8, 2006, 04:32 
Re: Unsteady flow around a rectangular building

#15 
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Thanks for your reply. I found out that when I change the velocity field at inlet, the first few timestep did not converge. Would it affect the solution of subsequent timestep?


March 8, 2006, 05:51 
Re: Unsteady flow around a rectangular building

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Yes. First time steps are usually a bit more difficult to converge. Consider augmenting the n° of iterations per TS until a converged solution is reached.
Hope this helps. Edi. 

March 13, 2006, 11:01 
Re: Unsteady flow around a rectangular building

#17 
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I tried PISO with single precision solver. But I found out that the residue continuity did not fall below 1E05 but oscillate around 2E05. I know that it may due to numerical diffusion. I also tried SIMPLEC with double precision solver. I started with steady state solution and patch the static pressure on one side face to start vortex shedding. Although the lift force is sinusoidal with decreasing amplitude, the result is wrong. The Strouhal number is 0.01, which is 10 times smaller. The amplitude is also very small, about 0.00014N. What should I do? Thanks for your advice.


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