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August 8, 2019, 22:07 |
Tackling Backflow using extended domain
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#1 |
Member
Soumitra Vadnerkar
Join Date: Aug 2018
Posts: 70
Rep Power: 8 |
I have seen somwhere that, if I want to completely avoid the backflow, I can extend the domain length wise as well as c/s area wise.
However, in that case don't you think I am messing with the domain of my interest by inducing pressure/velocity changes with the artificial (extended) domain due to Venturi effect? Can somebody please share their insights for avoiding backflow condition? If I just extend the domain lengthwise keeping c/s area (and other BCs) same, it does not make any difference, other than moving the backflow influenced area away from the main domain of interest. So is this the correct approach to just extend the domain length wise? P.S I am using ANSYS Fluent evaporation-condensation model for solving a condensation problem of a tube fluid flow with mass flow rate inlet, known pressure outlet and const. temp tube wall BC. I have backflow warnings through out the simulation calculations |
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August 9, 2019, 12:41 |
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#2 | |
Senior Member
Lucky
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 5,747
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Quote:
For example, if you were using a pressure outlet BC and had reversed flow and then decided to tackle it by extending the domain. You need to provide now a slightly different pressure (lower if it's a constant extrusion). Usually the small difference is not an isssue and negligible because you at least get a better solution without reversed flow that is much more acceptable than getting the wrong flow with the correct BC. For a mass-flow outlet, you don't really need to do anything. The only problem arises when the outlet extension causes problems in the upstream solution. This is quite rare because the point of the extension is to fix these problems. |
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August 9, 2019, 22:09 |
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#3 | |
Member
Soumitra Vadnerkar
Join Date: Aug 2018
Posts: 70
Rep Power: 8 |
Quote:
I understood that I should extrude the domain as extension (without changing cross section) and use slightly different pressure than the actual know value that I have. (For my case, may be I will need higher value as the outlet is downside and in the exp. it is observed to have higher pressure than inlet). |
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August 10, 2019, 19:10 |
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#4 |
Senior Member
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,849
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The real key of the question is if the backflow is a numerical issue or is due to vortical structures transported towards the outlet.
That strongly depends on the formulation, on the flow geometry and so on. Extending the lenght of the domain should be discussed in the framework of the physical BCs. at the outlet whe you prescribed a fixed pressure |
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August 10, 2019, 19:16 |
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#5 |
Member
Soumitra Vadnerkar
Join Date: Aug 2018
Posts: 70
Rep Power: 8 |
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August 10, 2019, 19:28 |
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#6 | |
Senior Member
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,849
Rep Power: 73 |
Quote:
Generally, if the physics you are simulating does not produce a vortical structure at the outlet, the first problem to check is for a wrong setting of the BCs. Is your flow model fully compressible or you set an incompressible formulation? |
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August 10, 2019, 19:38 |
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#7 | |
Member
Soumitra Vadnerkar
Join Date: Aug 2018
Posts: 70
Rep Power: 8 |
Quote:
P.S. the tube is vertical c-shaped (>3m long) with inlet at higher level. |
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August 20, 2019, 22:45 |
Do backflow warnings cause hurdle for convergence?
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#8 |
Member
Soumitra Vadnerkar
Join Date: Aug 2018
Posts: 70
Rep Power: 8 |
I tried simply extruding (extend) the outlet domain BC. With application of slightly higher pressure at the new extended outlet, I could achieve approax. required actual outlet pressure at the actual outlet location.
But backflow issue is not yet solved. With this extend -extruded domain setup I just may have controlled the pressure and moved the backflow affected region little far from the actual domain. I am doing a steady state simulation. And it is not at all converging. Do you think the unsolved backflow warnings could be hurdle for convergence ? |
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August 21, 2019, 08:02 |
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#9 | |
Senior Member
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,849
Rep Power: 73 |
Quote:
Without convergence there is no meaning in the solution you get and the problems can be due to many reasons. Have you tried first to check in a simpler controlled case problem, for example lower Re number, no condensation, etc? |
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August 21, 2019, 08:12 |
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#10 | |
Member
Soumitra Vadnerkar
Join Date: Aug 2018
Posts: 70
Rep Power: 8 |
Quote:
I have given inlet steam mass flow rate at a given pressure and temperature for a const. temperature C-shape tube. At outlet, I know pressure from the experiment. My simulation should be able to match with the experimental temperature profile that I have along the tube length. Can you please guide me how can I simplify the case to check and verify if my setup is right? |
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September 11, 2019, 16:52 |
Backflow issue solved!
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#11 |
Member
Soumitra Vadnerkar
Join Date: Aug 2018
Posts: 70
Rep Power: 8 |
For me it was Eureka! I found it!!
The real reason for backflow at outlet of my geometry was NOT the geometry, boundary conditions or the physics itself. It was a tiny parameter in the problem Setup. In Fluent, in the Cellzone assignment section, there is option for Operating Conditions. In Operating Conditions unchecked Variable-Desity Parameters, which earlier I had just checked it incorrectly. After unchecking the Variable-Desity Parameters, I got rid of the backflow completely (except for the beginning- which is acceptable)backflow_solved_uncheck_variable_density_parameter.PNG |
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Tags |
backflow, cfd, condensation, fluent, multi phase |
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