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why the inlet velocity magnitude differrent from the value i initialize?

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Old   July 1, 2009, 21:47
Default why the inlet velocity magnitude differrent from the value i initialize?
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i initialize the inlet velocity to 6.67m/s.
but after the iteration done,
why the magnitude showed at the inlets there is not same like the value i initialize(<6.67m/s)??
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Old   July 2, 2009, 01:31
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what kind of boundary condition did you set at the inlet (pressure-inlet/velocity inlet/...)?
If you choose velocity inlet, then the value at the inlet should be the one you set in the BC (independant from your initialization)
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Old   July 2, 2009, 08:06
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thx for reply.

the boundary type for the inlets are mass flow inlet.
and mass flow rate i used is 1kg/s and i initialize the velocity with the value of 6.67m/s.

should i change the boundary type to velocity inlet and set the value of the velocity magnitude to 6.67m/s? what value should i put during initialize?
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Old   July 2, 2009, 08:46
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NO, you can give whatever you want as Initialization... the value at your inlet will be the velocity computed from your massflow, eg: U = massflow / (Inlet_area * density)
The initialization gives a guess solution for all the cells in your domain, except the one with BC
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Old   July 2, 2009, 23:22
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Thanks for ur help.

i got the correct velocity magnitude as i initialized by change the inlet boundary type to "velocity inlet".
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Old   August 18, 2012, 14:02
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Dear friends,
i am doing steady analysis for my 3D wing model,with velocity is 5m/s. For that i give my

Boundary condition ie
velocity inlet
1. velocity specification method---->magnitude,normal to boundary
2. reference frame---->absolute
3.velocity magnitude----> 5m/s

After this, i go for initialization in that i select,
Compute from------>velocity inlet
Initial values
gauge pressure(pascal)----->o
x velocity-------> - 5m/s
y velocity-------->0
z velocity---------0

here my doubt is i give velocity is +5m/s in velocity inlet in boundary condition, but i got -5m/s after compute from velocity inlet, what that negative sign indicates? please help me
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Old   August 18, 2012, 20:57
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Dear hhh,
Your inlet velocity is in the opposite direction of the vector x
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Old   August 20, 2012, 01:04
Default Initialization doubt
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thanks toure, but opposite direction of the vector x means what excatly, i am get confused, because my coordinates are in +x direction,-y direction,-zdirection, please explain me little brief.
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Old   August 20, 2012, 01:46
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You inlet must be at x= Length and your outlet at x=0. That's why. Show a drawing.
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Old   August 21, 2012, 03:05
Default Initialization doubt
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Dear Touré,

please find the attached image, my boundary condition is that 1 refers my velocity inlet, 2 refers my pressure outlet, remaining faces are symmerty and wing is wall.
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Old   August 22, 2012, 14:12
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When you chose magnitude,normal to boundary your flow is from right to left in this case. magnitude,normal to boundary makes the velocity vector inward. Your velocity is in opposite direction with Gx. That's why you have a negative sign
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Old   August 23, 2012, 04:14
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Dear toure thanks for information,
for example if i choose velocity magnitude & direction means what? under cartesian coordinates, i choose x-component of flow direction is 1 or -1 ?
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Old   August 23, 2012, 06:26
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If you choose velocity magnitude & direction, you need to give the magnitude of the velocity which is positive and the vector direction which could has the value of -1 or -2 for the x-component. Only the sign of the x-component is important in this case.
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Old   August 23, 2012, 08:51
Default velocity inlet doubt
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dear toure,

1.The vector direction which could has the value of -1 or -2 for the x-component. what is -2 i didnt understand


2.Only the sign of the x-component is important in this case. for this case which is better?

3.magnitude is 5m/s
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Old   August 23, 2012, 11:52
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I want to say that it's the same giving the direction (-1, 0, 0) or (-2, 0, 0)
(-1, 0, 0) represent the vector -1i+0j+0k

The magnitude is the Euclidien norm of the velocity V = sqrt(Vx^2+Vy^2+Vz^2)
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Old   August 24, 2012, 10:33
Default boundary doubt
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Dear toure, thanks for your kind response

According to my problem, i am doing 3D wing analysis by using gambit & fluent software. I have the input is velocity is 5m/s, area is 0.0102 sq m, length is 0.065 m, for my case it is steady, laminar, incompressible flow, i want to do analysis, in fluent i don't exactly which boundary condition i have to give, please see the attached image, in that i consider wing as wall, one face is velocity inlet& opposite face is pressure outlet, remaining faces which boundary condition is more suitable (ie Top&Bottom and left side&right side faces for my domain), earlier i consider as symmetry for remaining 4 face, please tell me if you have idea.
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Old   August 24, 2012, 19:27
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Your boundary conditions are good. For the symmetry, the surfaces must be far enough (5 times the length of the wing) because it means that you don't a flow rate across the surfaces.
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Old   August 25, 2012, 11:42
Default boundary doubt
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thanks toure, sorry i didn't understand( it means that you don't a flow rate across the surfaces)if u don't mind explain briefly

what i am understanding is, here velocity is 5m/s, so the effect of the flow is near by wing, its not too far, that's why i choose symmetry boundary condition whether its correct or wrong?
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Old   August 26, 2012, 21:33
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Symmetry means "nothing crosses the surface" and you don't force the velocity to be 5m/s everywhere on that surfaces. You let the software compute the velocity on the symmetry boundaries from the inlet velocity that you gave.
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Old   August 27, 2012, 01:55
Default coordinates in gambit&fluent
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thanks toure, i have anonther doubt, i selected the coordinates is +y view in gambit (ie x is towards right, y is towards horizontal wing & z is downwards,) see image 1 xyz coordinates, after i finished my meshing i import in fluent in that, if i see grid in display option, the coordinates its shows (x is towards right, y is vertical & z is horzontal) see image 2 xyz coordinates, how i can change my coordinates in fluent according my gambit coordinates, anything is wrong with that? please find the attached images,

In gambit my wing looks towards bottom (ie actual geometry), but in fluent its change, see wing & xyz coordinates closely in both images.
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File Type: jpg image 2.jpg (1.8 KB, 48 views)
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