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

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 July 1, 2009, 21:47 why the inlet velocity magnitude differrent from the value i initialize? #1 New Member   chu eyu hock Join Date: Jul 2009 Posts: 3 Rep Power: 10 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)??

 July 2, 2009, 01:31 #2 Super Moderator     Maxime Perelli Join Date: Mar 2009 Location: Switzerland Posts: 3,232 Rep Power: 34 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) __________________ In memory of my friend Hervé: CFD engineer & freerider

 July 2, 2009, 08:06 #3 New Member   chu eyu hock Join Date: Jul 2009 Posts: 3 Rep Power: 10 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?

 July 2, 2009, 08:46 #4 Super Moderator     Maxime Perelli Join Date: Mar 2009 Location: Switzerland Posts: 3,232 Rep Power: 34 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 __________________ In memory of my friend Hervé: CFD engineer & freerider

 July 2, 2009, 23:22 #5 New Member   chu eyu hock Join Date: Jul 2009 Posts: 3 Rep Power: 10 Thanks for ur help. i got the correct velocity magnitude as i initialized by change the inlet boundary type to "velocity inlet".

 August 18, 2012, 14:02 Initialization doubt #6 Senior Member   kunar Join Date: Nov 2011 Posts: 117 Rep Power: 7 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

 August 18, 2012, 20:57 #7 Member   Guiliguili Join Date: Aug 2010 Location: Montréal Posts: 94 Rep Power: 9 Dear hhh, Your inlet velocity is in the opposite direction of the vector x

 August 20, 2012, 01:04 Initialization doubt #8 Senior Member   kunar Join Date: Nov 2011 Posts: 117 Rep Power: 7 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.

 August 20, 2012, 01:46 #9 Member   Guiliguili Join Date: Aug 2010 Location: Montréal Posts: 94 Rep Power: 9 You inlet must be at x= Length and your outlet at x=0. That's why. Show a drawing.

August 21, 2012, 03:05
Initialization doubt
#10
Senior Member

kunar
Join Date: Nov 2011
Posts: 117
Rep Power: 7
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.
Attached Images
 image.jpg (21.3 KB, 32 views)

 August 22, 2012, 14:12 #11 Member   Guiliguili Join Date: Aug 2010 Location: Montréal Posts: 94 Rep Power: 9 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

 August 23, 2012, 04:14 Initialization doubt #12 Senior Member   kunar Join Date: Nov 2011 Posts: 117 Rep Power: 7 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 ?

 August 23, 2012, 06:26 #13 Member   Guiliguili Join Date: Aug 2010 Location: Montréal Posts: 94 Rep Power: 9 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.

 August 23, 2012, 08:51 velocity inlet doubt #14 Senior Member   kunar Join Date: Nov 2011 Posts: 117 Rep Power: 7 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

 August 23, 2012, 11:52 #15 Member   Guiliguili Join Date: Aug 2010 Location: Montréal Posts: 94 Rep Power: 9 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)

August 24, 2012, 10:33
boundary doubt
#16
Senior Member

kunar
Join Date: Nov 2011
Posts: 117
Rep Power: 7
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.
Attached Images
 image.jpg (65.4 KB, 8 views)

 August 24, 2012, 19:27 #17 Member   Guiliguili Join Date: Aug 2010 Location: Montréal Posts: 94 Rep Power: 9 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.

 August 25, 2012, 11:42 boundary doubt #18 Senior Member   kunar Join Date: Nov 2011 Posts: 117 Rep Power: 7 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?

 August 26, 2012, 21:33 #19 Member   Guiliguili Join Date: Aug 2010 Location: Montréal Posts: 94 Rep Power: 9 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.

August 27, 2012, 01:55
coordinates in gambit&fluent
#20
Senior Member

kunar
Join Date: Nov 2011
Posts: 117
Rep Power: 7
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.
Attached Images
 image1.jpg (65.4 KB, 8 views) image 2.jpg (1.8 KB, 64 views)

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