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Inconsistent units

Posted Dec 22, 2010, 2:34 a.m. EST Computational Fluid Dynamics (CFD) Version 3.5a 5 Replies

SM Ananth
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Hi,
I prescribed an oscillating inlet velocity boundary condition of the form u*sin(w*t) [say for example 20*sin(20*pi*t)] to a simple 3D cylindrical geometry.Once prescribed in this form, in the boundary settings dialog box only it is indicating 'inconsistent unit' and the unit 'm/s' will turn into a red colour.Can anyone help to find what exactly the problem is..
Thanks,
S.M.Ananth
5 Replies Last Post Dec 28, 2010, 3:33 p.m. EST
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Dec 22, 2010, 8:17 a.m. EST
Hi

In v4 red is an error in the formula (might be your use of u,v,w that are already used internally by COMSOL => variable name conflict). orange is a unit error. note that most functions sin, cos analyitcal ... requires dimensonless inputs ad are dimensonless at output, so

sin(20*pi*t)

is wrong as it should read

sin((20*pi)[1/s]*t)

or

sin(20*pi*t[1/s])

--
Good luck
Ivar
Hi In v4 red is an error in the formula (might be your use of u,v,w that are already used internally by COMSOL => variable name conflict). orange is a unit error. note that most functions sin, cos analyitcal ... requires dimensonless inputs ad are dimensonless at output, so sin(20*pi*t) is wrong as it should read sin((20*pi)[1/s]*t) or sin(20*pi*t[1/s]) -- Good luck Ivar
SM Ananth
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Posted: 1 decade ago Dec 22, 2010, 1:48 p.m. EST
Dear sir,
Thanks a lot for the help.Eventhough it was a simpe mistake(but serious) I couldn't figure out.
S.M.Ananth
Dear sir, Thanks a lot for the help.Eventhough it was a simpe mistake(but serious) I couldn't figure out. S.M.Ananth
Debaditya Dutta
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Posted: 1 decade ago Dec 22, 2010, 4:20 p.m. EST
Hi Ivar,

I am using a function loaded from a .txt file. The name of the function is say fheat and in the .txt file it is given as:

%x y z fheat
0 0 0 1
0.1 0 0 1
..... etc.

Now, when I am using this function (fheat) in the COMSOL model, I get an 'inconsistent unit' warning. I am expecting that this function should be unitless because I haven't specified otherwise. Any thoughts?

Thanks much,

Deb
Hi Ivar, I am using a function loaded from a .txt file. The name of the function is say fheat and in the .txt file it is given as: %x y z fheat 0 0 0 1 0.1 0 0 1 ..... etc. Now, when I am using this function (fheat) in the COMSOL model, I get an 'inconsistent unit' warning. I am expecting that this function should be unitless because I haven't specified otherwise. Any thoughts? Thanks much, Deb
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Dec 22, 2010, 4:41 p.m. EST
Hi

how do you call it up ?
if you use yourFunction(x,y,z) the three x,y,z have units, but COMSOL would expect none so here to you should use:

yourFunction(x[1/m],y[1/m],z [1/m])

or whatever fits. Anyhow units orange warnings are not errors and Comsol solves further anyhow, it is just telling us that here there might be something wrong

--
Good luck
Ivar
Hi how do you call it up ? if you use yourFunction(x,y,z) the three x,y,z have units, but COMSOL would expect none so here to you should use: yourFunction(x[1/m],y[1/m],z [1/m]) or whatever fits. Anyhow units orange warnings are not errors and Comsol solves further anyhow, it is just telling us that here there might be something wrong -- Good luck Ivar
Debaditya Dutta
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Posted: 1 decade ago Dec 28, 2010, 3:33 p.m. EST
Ivar, Just wanted to let you know that your advice worked perfectly. Thanks!

DD
Ivar, Just wanted to let you know that your advice worked perfectly. Thanks! DD

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