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July 27, 2003, 05:00 
fully developed velocity profile in 3D

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July 27, 2003, 15:12 
BC at the inlet of the tube

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Hi all,
I've had a problem of specifying the boundary condition at the inlet of an openedend tube. I use CFD to simulate flow inside the tube having an embedded momentum source. The momentum source is located at the center of the horizontal tube and both ends (inlet and outlet) of the tube is opened to the atmosphere. I specify zero static pressure at the outlet (reference to ambient). The flow inside the tube is driven by the given momentum source. However, I don't know what the boundary condition at the inlet of the tube should be. Could anybody please give me some suggestion? Thank you very much Jackie 

July 27, 2003, 16:00 
Re: BC at the inlet of the tube

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B.C. at inlet might be fully developed, i.e. \vec{n} \cdot \nabla \vec{v} = \vec{0} (in latex form).


July 27, 2003, 16:05 
Re: fully developed velocity profile in 3D

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The fully developed flow velocity profile for a rectangular channel is a serial solution; following is the analytical solution written in latex form:
\[ u(x,y) = \sum_{n=1}^{\infty} u_n(y) \phi_n(x) \] where \[ u_n(y) = \frac{a_n}{\lambda_n} \left( 1+\frac{sinh\sqrt{\lambda_n}y+sinh\sqrt{\lambda_n} (by)}{sinh\sqrt{\lambda_n}b}\right) \] \[ \phi_n(x) = \sqrt{\frac{2}{a}}sin(\frac{n \pi x}{a}), \;\; n=1,2,... \] where $a$ is the length in the $x$ direction, $b$ is the length in the $y$ direction, $\lambda_n = \frac{n^2\pi^2}{a^2}$, $a_n = \sqrt{\frac{2}{a}}\alpha \frac{a}{n\pi}[(1)^n1] $ and $\alpha =  \Delta p/(L\mu)$, $\Delta p$ is pressure drop in the $z$ direction with length $L$, and flow viscosity $\mu$. 

July 27, 2003, 16:47 
Re: fully developed velocity profile in 3D

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Yes, but can it be expressed in terms of u_max (maximum velocity)? In 2D, there is a nice relation for u(y) which is given in terms of u_max (maximum velocity which occurs at the centerline of the parabola), and the ycoordinates. This expression is very convenient to use as velocity inlet b.c. profile in CFD codes (provided that u_max is known). Is there anything like that in 3D?
Also, can someone give me a reference where the detailed derivation of this exact 3D solution can be found? Thanks. 

July 27, 2003, 23:07 
Re: fully developed velocity profile in 3D

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Hi,
If u have a nice expression for fullydeveloped velocity profile (parabola) in 2D, I think it is easy to expand it to 3d, just let the profile rotate around the longitude axis let's say if x is the longitude axis, and at the inlet, u = U(y) is a function of y. Now at 3D, again you set the x' as longitude axis. At inlet, you will have coordinates like (y',z'), take r = sqrt(y'^2 + z'^2), and let u = U(r). Now, u expand the 2d velocity field to 3d 

July 29, 2003, 12:02 
Re: fully developed velocity profile in 3D

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The detailed derivation of this exact 3D solution can be found in book titled as Mathematical Methods in Chemical Engineering; the authors are: Varma, A. & Morbidelli, M.; Oxford University Press, New York Oxford, page 494.


July 6, 2013, 15:36 

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Ugur Goktolga
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I know it has been decade since this thread was opened, but just in case somebody googles and finds this page, I wanted to help.
You can check the equations 335338 from Shah and London. Generating parabolic velocity profile for 3D flows might not be a good idea since it does not represent fully developed flow for especially high aspect ratio channels. So, better check the equations that I have written. Actually, I have written a MATLAB script that generates fully developed velocity profile for rectangular channels and uploaded it to MATLAB file exchange forums, but it is not approved yet. If somebody replies this thread, I can supply the link for the file, or send as an attachment. 

January 10, 2017, 01:48 
fully developed profile at the inlet of a duct of rectangular section

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vidyadhar
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Quote:
I have seen this thread more than a decade after you have posted it. I want to have fully developed velocity profile at the inlet of a duct with rectangular cross section. I request you to help me in executing the same. Thank you, vidyadhar 

January 10, 2017, 02:23 

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Lane Carasik
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January 11, 2017, 13:39 
Equation used to generate fully developed profile in the rectangular duct

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vidyadhar
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Quote:
Thank you for the quick reply. I have seen the code provided in the link. May I request you to please let me know the equation used in the code.Also, what is um in the equation given. Though the reference mentioned in the code is a book by Shah and London, I am unable to access it. Thanks & Regards, Vidyadhar 

January 11, 2017, 13:44 

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Filippo Maria Denaro
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Quote:
the solution for a laminar steady developed flow in a rectangular duct satisfies the Poisson equation Lap u = Re*dp/dx with u=0 on the boundaries. 

January 11, 2017, 13:49 

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vidyadhar
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Quote:
Thank you for the reply. Since there are many analytical expressions available in various sources for the Poisson equation, I would like to know which form was used by the coder to develop the code. I request you to please let me know the same. Thanks & Regards, vidyadhar 

January 11, 2017, 14:00 

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Filippo Maria Denaro
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January 11, 2017, 14:06 

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I will try to find the equation in the book, if it is not hidden. Regards, vidyadhar 

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