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flow at 15 deg. AoA over a duct - 3D case

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Old   February 21, 2013, 13:07
Default flow at 15 deg. AoA over a duct - 3D case
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Sandeep
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hi friends ,

i am simulating flow which takes place over a duct that is protruding out of a wall .
Pl find attached the picture of the flow domain and also the dimensions in 'mm' as indicated there.

here are the BCs i have used :
‘Inlet’: u = 0,
v = 50.83m/s,
w = 200.91m/s ( as the flow is in the YZ plane at an angle of attack)
‘Outlet’ = 0 psi
Top face = ‘No-slip wall’
Face 1 = ‘Opening’ at 0 psi
Face 2 = ‘Opening’ at 0 psi
Shaded face = ‘Symmetry’

the problem i face is that pressure which i am monitoring just upstream of the duct , where the flow hits is, comes equal to 15 psi (gauge)where as it should be around 3- 3.5 psi (g) as per calculation ?

suspecting something wrong with the BCs on the Faces 1 and 2 , i have used 'opening with entrainment' , ' opening with cartesian vel. components' and also 'free slip wall' but each time the pressure has been on the higher side

could you please guide me as to where i am going wrong in the AoA case , which i have tried to illustrate in the attached figure.

[ for 0 deg AoA, i get good match with the analytical value.]

thanks

Sandeep
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Old   February 21, 2013, 16:18
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Edmund Singer P.E.
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How is this AOA? You have a wall at the top and your V velocity will just impact it.

Also, is this air? Are you running compressible? You are running this at M~0.7.
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Old   February 21, 2013, 17:33
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Glenn Horrocks
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This sounds like a FAQ to me: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F
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Old   February 22, 2013, 07:57
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Sandeep
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@singer 1812

Fluid is 'Air ideal gas'

the duct is a part of aircraft fuel system , hence the 15 deg AoA and yes the flow will impinge on the 'wall'

@glenn : mostly i have tried all that the 'FAQ' tell , only afterwards i am posting this doubt here , hoping to gain from someone's experience on solving a similar problem
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Old   February 23, 2013, 05:48
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The flow might impinge on the wall but modelling it using an inlet with a fixed AOA across the entire face is not going to be very representative. You are going to have to make the domain bigger and more representative, and allow air to spill around the side, and spall out from the impingement site - these are what really happens when a jet impinges on a wall, but your setup will not allow this to form.
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Old   February 28, 2013, 10:55
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"....using an inlet with a fixed AOA across the entire face is not going to be very representative"

@glenn : 1. how to make the inlet more representative ?
2. what should i keep so as to represent the 'farfield' boundaries - which are the side and bottom faces of the box i have shown in fig. ?

thanks

sandeep
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Old   February 28, 2013, 17:46
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Extend the simulation domain so you include this impingement effect. The boundary of the domain should be placed somewhere that simple flow conditions exist, like constant pressure, a simple flow in a certain direction or something like that.

You can use a pressure opening, or could use a wall. Either way do a sensitivty analysis to check it is far enough away that it is not affecting results. A Wall will need to be further away than a pressure opening.
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