[robbo]

If the pressure equation that potentialFoam solves is not laplacian(p)=0 then the manual should be corrected!

From the relation I've posted after some rearrangements it turns:

laplacian(p)=-rho*(d^2(phi)/(d(x_i)d(x_k)))^2

wich is different from the equation that you have posted. Also laplacian(p)=div(phi) is not dimensionally correct! and also: is it possible to define the divergence of a scalar field?

nice weekend to you!

Roberto

[ngj]

Hi

I am wondering if we are chasing something which is not present in the solution. Have found the analytical solution of the potential around a cylinder, and it writes:

phi = -u_0 x (1 + R^2 / (x^2 + y^2))

Thus in increases for decreasing x and varies linearly in the farfield of the cylinder. Could it be that the solution is actual for phi and not p?

/ Niels

[robbo]

The pressure field shown varies linearly in the whole field, not only in the farfield. I don't know exactly what is the solution for the potential flow but if it is the one that you write you can write it in polar coordinates as:

phi=-u_o*(1+R^2/r^2)

where r^2 is the generical radius far from the cylinder. In this way the potential contours must be concentrical circles around the cylinder... and this is not the case of the field shown. So the pressure field shown is not the potential field...

Roberto

[ngj]

Please note the 'x' after u_0.

- Niels

[robbo]

i missed that parsicular! i've plotted the function you posted and it shows good agreement with the pressure field shown... but why potentialFoam plots the potential field instead of the pressure field?

Roberto

[kev4573]

Did this issue ever get resolved? Both tutorials for potentialFoam seem to give incorrect pressure distributions if you run the simulation with the '-writep' argument. It does seem the pressure field resembles phi more than anything.

[kev4573]

Also, the field phi is a scalar field - taking divergence of that field seems nonsensical.. What am I missing?

[Aelsawis]

I' am trying to model the behavior of a non newtonian fluid inside an eccentric annulus. I need to get a profile of the annular velocity of the fluid for the whole cross section. I just discovered the OpenFoam and i was wondering if the "nonNewtonianIcoFoam" solver would manage a problem like this? Since i am new in this area any further help or suggestions is appreciated. What should i read to get started? Is there another solver that is more specialized in Annular flow? Will i need a mesh generation program(Boundary conforming grid generator) or Will the Openfoam handle the whole problem?

Really appreciate any help

[sega]

Originally Posted by Aelsawis

I' am trying to model the behavior of a non newtonian fluid inside an eccentric annulus. I need to get a profile of the annular velocity of the fluid for the whole cross section. I just discovered the OpenFoam and i was wondering if the "nonNewtonianIcoFoam" solver would manage a problem like this? Since i am new in this area any further help or suggestions is appreciated. What should i read to get started? Is there another solver that is more specialized in Annular flow? Will i need a mesh generation program(Boundary conforming grid generator) or Will the Openfoam handle the whole problem?

Really appreciate any help

I'm pretty sure your question is in not related to this thread.
Open you own discussion!

[Aelsawis]

yeah maybe ur right i am sorry for the disturbance.i just thought since the discussion began with the cylinder example ..that maybe someone would know something about the annular flow between two cylinders...

[shirazbj]

The p in the program has a dimension of m^2/s^2, so it is not simple a pressure. I think it is p/density. Becasue:

p - kg/(ms^2)
density - (kg/m^3)

p/density=kg/(ms^2)/(kg/m^3)=m^2/s^2

For potential flow, usually we start from solving phi, then get U by differentiation of phi. Then get pressure from Bernolli's equation. The U is correct here, so we could try to get pressure from it.

BTW, There is an analyticalFoam folder under the cylinder tutorial folder. Where this output is saved?

Cean

[kev4573]

Cean - That's what I ended up doing - since my case had a uniform free stream velocity it was relatively trivial to modify the potentialFoam solver to calculate pressure.

[shirazbj]

Originally Posted by kev4573

Did this issue ever get resolved? Both tutorials for potentialFoam seem to give incorrect pressure distributions if you run the simulation with the '-writep' argument. It does seem the pressure field resembles phi more than anything.

phi is the flux through the cell surface.

Does the equation means flux due to pressure change in the cell?

[kev4573]

By phi I really meant potential, sorry for the confusion.

[tamsilian]

Hi
Could you send me your code about cylindrical boundary condition?
TANX
Joseph

[ata]

Hi
I think that in that file which I have send for you, is a link to cylindrical B.C.
Best regards

[Ben UWIHANGANYE]

Originally Posted by ngj

Hi Roberto

You need to program a new boundary condition (at least that is what I think). The programming is rather simple, and I have something at another computer, which might help you. The short story is, that you know the point on the surface, x_S, and the center of the cylinder, x_0. Then specify

v = x_S - x_0.

Say that the angular frequency is omega, then the boundary condition for the velocity for the cylinder surface in any given point is given as (assume 2D)

U = - omega * vector(-v_2,v_1,0)

where v_1 and v_2 are the first and second component of v. Assumed that the cylinder rotates clockwise, if anti-clockwise just remove the minus in front.

Have fun, and if you need my source, just send me an email.

- Niels

Hello Niels,


I want to map a cylinder in OpenFoam with the following boundary conditions:


n(x,y,t)={n=1, when (x-xt)^2 -(y-yt)^2 <= R^2

n=0, when (x-xt)^2 -(y-yt)^2 > R^2 }


where: x,y are the Cartesian coordinate of the computational cell center
and xt, yt are the center of the cylinder at time t. R is the radius of the cylinder.

It means that the cylinder will be oscillating as n alternate from 0 to 1.
I am using OF 4.1.


May you advise how i can do that ?


I would appreciate!

[Ben UWIHANGANYE]

Originally Posted by ngj

Hi Roberto

As you said in the email, you are not completely sure how to compile, so I will make a small walk-through:

First here is the boundary condition.

rotatingCylinder.tar.gz

Unpack the archive and from the command-line you go into the directory and type:


This creates a dynamic library which is placed in the folder $FOAM_LIBBIN. The library is called librotatingCylinderFvPatchVectorField.so.

Then unpack the following

viscosityTest.tar.gz

which is a small test case with to concentric circles, where the large one rotates with a given cyclic frequency. The frequency and the center of the rotating cylinder is given in case/0/U. The library is only for 2D-case in the xy-plane, as I have not extended the code. Notice that the linking of the new boundary condition to icoFoam happens through /case/system/controlDict, where the library is added at the bottom.

Hope it works, otherwise to not hesitate to ask.

/ Niels

BTW: The setup is actually how viscosity was measured, because the flow is a couette-type of flow for large radii and small gaps. Thus the velocity profile is known, and therefore by measuring the force acting on the still cylinder, the viscosity can be determined as the velocity of the 'lid' is known.

wmake libso



Dear Niels,


I have tried to go through this exercises and when i run icoFoam got the following fatal error:


--> FOAM FATAL IO ERROR:
keyword pFinal is undefined in dictionary "/home/ben/OpenFOAM/ben-4.1/run/visicosity/system/fvSolution.solvers"

file: /home/ben/OpenFOAM/ben-4.1/run/visicosity/system/fvSolution.solvers from line 0 to line 38.

From function const Foam::dictionary& Foam::dictionary::subDict(const Foam::word&) const
in file db/dictionary/dictionary.C at line 642.

FOAM exiting


Can you help to correct that error?


I would appreciate your help



I would appreciate you support.!

[Ben UWIHANGANYE]

Originally Posted by ngj

Hi Roberto

You need to program a new boundary condition (at least that is what I think). The programming is rather simple, and I have something at another computer, which might help you. The short story is, that you know the point on the surface, x_S, and the center of the cylinder, x_0. Then specify

v = x_S - x_0.

Say that the angular frequency is omega, then the boundary condition for the velocity for the cylinder surface in any given point is given as (assume 2D)

U = - omega * vector(-v_2,v_1,0)

where v_1 and v_2 are the first and second component of v. Assumed that the cylinder rotates clockwise, if anti-clockwise just remove the minus in front.

Have fun, and if you need my source, just send me an email.

- Niels

Hello Niels,


May you help to know where this program should be written?


Indeed I want to map a cylinder on a Cartesian grid and move it with a simple program.
I have been advised to learn about vector form programming in Open Foam but i don't know where to start.


Thank you!

[Ben UWIHANGANYE]

Originally Posted by Ben UWIHANGANYE

wmake libso



Dear Niels,


I have tried to go through this exercises and when i run icoFoam got the following fatal error:


--> FOAM FATAL IO ERROR:
keyword pFinal is undefined in dictionary "/home/ben/OpenFOAM/ben-4.1/run/visicosity/system/fvSolution.solvers"

file: /home/ben/OpenFOAM/ben-4.1/run/visicosity/system/fvSolution.solvers from line 0 to line 38.

From function const Foam::dictionary& Foam::dictionary::subDict(const Foam::word&) const
in file db/dictionary/dictionary.C at line 642.

FOAM exiting


Can you help to correct that error?


I would appreciate your help



I would appreciate you support.!

Solved.


Thank you!