# Symmetry Boundary Condition

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 November 17, 2005, 17:57 Symmetry Boundary Condition #1 Noname Guest   Posts: n/a when we model flow past a cylinder how do we mathematically express the symmetry boundary condition when we want to do calculations only for the half domain.

 November 17, 2005, 18:43 Re: Symmetry Boundary Condition #2 boling Guest   Posts: n/a keep all the physical variables same except reverse the normal velocity component (normal to the symmetry plan)

 November 17, 2005, 21:57 Re: Symmetry Boundary Condition *NM* #3 Mani Guest   Posts: n/a

 November 18, 2005, 14:27 Re: Symmetry Boundary Condition #4 ramp Guest   Posts: n/a The flow regimes for the flow across a circular cylinder are: (a) steady flow without separation (b) steady flow with symmetric vortices behind the cylinder, (c) laminar vortex shedding flow (d) 3-D unsteady flow, etc Upto to the regime (b), the computations can be carried out using half domain because the flow is symmetric in the upper and lower half of the domain. In case of Newtonian flow, the occurance of laminar vortex shedding is approx @ Re=46 (few have reported 49) and so the half domain can be used for Re < 46 ......

 February 18, 2014, 03:54 symmetry boundary condition #5 Member   Join Date: Feb 2014 Posts: 56 Rep Power: 3 Hi all, I have the following geometry: 3 cylinders (2 concentric and the remaining excentric). So, the geometry has a symmetry plane. Have I define each plane of each cylinder as symmetry? Thanks in advance, vut

 February 18, 2014, 16:55 #6 Senior Member   lore Join Date: Mar 2010 Location: Italy Posts: 463 Rep Power: 9 Not sure I understand your question as I did not understand what you are trying to simulate. Anyways, as ramp already said, the fact that a cylinder has a symmetric shape doesn't mean that the resulting flow will also be symmetric. In most of the cases it will not actually.

February 19, 2014, 05:43
Geometry of the injector
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Join Date: Feb 2014
Posts: 56
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You can see in the attachment the geometry of the injector that I want to simulate.

In fact, I hope to be able to consider a half of the geometry. Thus, I define three surfaces as geometry planes. The simulation is converged. However, the mass flow rate I obtained is quite small comparing to that measured.

Let's assume that I define a pressure inlet and a pressure outlet.

My first question is:
do you have an idea to mesh this geometry?

And the second one is:
what are possibly the reason of low mass flow rate in this case?

Attached Images
 InjectorWithThreeDisks.png (20.1 KB, 10 views)

 February 19, 2014, 14:11 #8 Senior Member   lore Join Date: Mar 2010 Location: Italy Posts: 463 Rep Power: 9 I guess it is ok to use a symmetry plane in this case. What do you mean with "how to mesh this geometry?" Can you set the inlet condition with a specified massflow instead of pressure?

 February 20, 2014, 17:48 Mass flow rate not matched #9 Member   Join Date: Feb 2014 Posts: 56 Rep Power: 3 Hi, I mesh each cylinder sepately. Then I assembly them by tmerge. Everything seems OK. The problem I have is the following: I set the inlet as pressure inlet and the outlet as pressure outlet. However, the mass flow rate given by Fluent does not match the experimental one!!! Some body has a great idea to solve this stuff? Thanks in advance,

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