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July 15, 2016, 02:32 |
Issue of divergence
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#1 |
New Member
hemant mittal
Join Date: Feb 2016
Posts: 21
Rep Power: 10 |
Hello every one,
I m simulating wind flow around rectangular buildings using LES on Ansys/Fluent.the problem of divergence is detected during the simulation after flow time of 3 sec. time step size = .0005 Vortex method (no. of vortices = 190) for turbulence generation Polyhedral mesh is being used So please guide me how to resolve this issue. Thanks |
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July 15, 2016, 12:02 |
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#2 |
New Member
Jeff
Join Date: Jun 2016
Posts: 20
Rep Power: 9 |
Your time step may be too long to capture the vorticies forming at the building edges depending on flow speed and mesh density. Without further info and assuming your mesh is good, its my best idea.
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July 15, 2016, 12:29 |
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#3 |
Senior Member
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,760
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Depending on the SGS model, cell dimension, cfl, the added eddy viscosity can drive to numerical instability for that time-step....
Could you address all the details of your LES setting? |
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July 15, 2016, 13:31 |
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#4 |
New Member
hemant mittal
Join Date: Feb 2016
Posts: 21
Rep Power: 10 |
Hello sir.
thanks for reply, I m using dynamic subgrid scale model. first cell height is 0.000125m wind speed at building height=9.5 m/s vortex method is used to add turbulence and of vortices =190 CFL condition for most of cell around building is <1 no. of polyhedral cells are about 1.9 million |
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July 15, 2016, 13:36 |
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#5 |
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Filippo Maria Denaro
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what kind of method are you using? did you set second order in time and space? is the numerical instability onset after many time steps?
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July 15, 2016, 13:37 |
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#6 | |
Senior Member
Arjun
Join Date: Mar 2009
Location: Nurenberg, Germany
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Quote:
Refine the mesh a bit further and see, how it behaves. I guess to make things worse times step is too low too (which also reduces dissipation). |
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July 15, 2016, 13:47 |
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#7 |
New Member
hemant mittal
Join Date: Feb 2016
Posts: 21
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spatial discretization for Pressure is second order and for momentum it is bounded central differencing.
time discretization scheme is second order implicit. Numerical instability occurred after 6880 time steps. |
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July 15, 2016, 13:58 |
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#8 |
Senior Member
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,760
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have you tried setting a smaller dt?
you should check some typical variable such as rms and spectra to see if you get energy pile-up. Just as check of validation, try the same discretization setting the static smagorinsky model but with Cs=0 (no-model) and reduce the Reynolds number. See if you get a stable unsteady solution. |
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July 15, 2016, 13:59 |
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#9 | |
New Member
hemant mittal
Join Date: Feb 2016
Posts: 21
Rep Power: 10 |
Quote:
I will do this.......... |
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