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Mark January 19, 2001 07:09

Symmetric problem - Assymetric results
I am using PHOENICS to look at the influences of the inlet and outlet geometries and configurations on the ambient flow field in a mechanically ventilated enclosure.

I am looking at a simple cuboid configuration of dims 2*1*1 (X*Y*Z) and have an inlet 0.1m high located on the X=0 boundary supplying air to the space at 1.0m/s. The location of the supply is half way up the X=0 wall. Opposite to this is an extract i.e. located on the X=L wall. Same dims as supply.

The configuration is such that the flow is essentially left to right.

The mesh configuration and the boundary condition specification suggests that the flow is essentially 2d.

The mesh is set at 40*1*20 and is uniformly distributed throughout the domain.

For now I am ignoring temperature influences and the flow is assumed laminar. Hybrid differencing is applied and gravity is switched off.

The results are puzzling and although the configuration is essentially symmetric the results are not. i.e. the inlet 'jet' is predicted to be drawn towards the floor of the domain!

I have also re-set up the case, with the same details except the inlet and outlet suggest the flow is now 'top to bottom'. I have changed the dimensions of the enclosure so that essentially I am asking PHOENICS to re-solve the original problem but with the configuration 'rotated' 90 degrees clockwise. The flow is again predicted asymmetric and the flow moves towards the west wall. i.e. if the plot is rotated back to its original position (90 anticlockwise ) degrees the flow is again predicted to move towards the floor.

After a long intro' my question is simple. Why would I get an asymmetric flow prediction to a symmetric problem definition.

Kike March 8, 2001 13:07

Re: Symmetric problem - Assymetric results
Dear Mark

Have you tried to run your case in a 2D (XY plane) domain with a grid of 40x20 instead of 40x1x20. The axis of coordinates are not equivalent in PHOENICS.

I mean, there are some details in Phoenics resolution procedure which make differences between the axis. If you are interested into 2D solutions you should allways try with XY plane before to use XZ or YZ planes. Your execution will be faster (that is sure) and, deppending some particular characteristics, maybe correct in this case.

Best wishes


David Glynn July 2, 2001 09:04

Re: Symmetric problem - Assymetric results
I would agree with Kike - you might do better using the x-y plane.

Also, it would be as well to try refining the grid, particularly in the lateral direction.

However, is there not a possibility that the actual flow situation is not symmetrical? Jets are attracted to walls (Coanda effect). If your jet is attracted to the sidewall, it seems more likely that the whole jet will attract to one or other side wall (at random), rather than half the jet going each way in a symmetrical fashion.

Something rather like this is the basis of the fluidic oscillator, in which the geometry and the boundary conditions are both symmetrical, but the flow oscillates from one side to the other through time.

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