CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > OpenFOAM > OpenFOAM Running, Solving & CFD

Problem with very high gradients

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   November 27, 2012, 14:12
Default Problem with very high gradients
  #1
Member
 
Join Date: Nov 2012
Posts: 83
Rep Power: 13
Henning86 is on a distinguished road
Hi Foamers,


i'm trying to implent a new boundary condition for which i need the second derivative of U in normal direction. (slip velocity)

In order to get a second order derivative (which isn't laplacian) i'll do:

Code:
    volScalarField Ux(U.component(0));
    dU=fvc::grad(Ux);//VolVectorField
    partial2UT=fvc::grad(dU);//VoltensorField
in partial2UT only components 0,4,8 are of interest i store these values in another tensorfield and repeat it for Uy and Uz so that i finally get.

| Ux,xx Uy,xx Uz,xx |
d2U=| Ux,yy Uy,yy Uz,yy |
| Ux,xx Uy,xx Uz,zz |

This works great but only to a certain dimension of the mesh. Attached to this post there are 2 cases and a modified rhoPimpleFoam solver.

the first case "microKanal2"(Kanal:channel in german) has a height of 1.68e-3 meters. The velocity field result in a parabola with an increasing maximum for higher x values. Therefore the first derivative dU shows in y direction a linear function. The tensor compoent(3) Ux,yy should be a a constant value for a cross section of the channel, but changing with the position of x (see picture).

the second case "microKanal" has a height of 1.68e-4 meters. For component 3 of the tensor there are following values (see attached picture)

The field isn't smooth at all it suddenly jumps to another value. A possible reason for that could be that double isn't accurate enough.

Does anyone know what the problem is and how it can be solved?

Best regards

Henning

http://dl.dropbox.com/u/9273087/microKanal2.tar.gz
http://dl.dropbox.com/u/9273087/microKanal.tar.gz
http://dl.dropbox.com/u/9273087/rhoPimpleFoam.tar.gz
Attached Images
File Type: jpg microchannel2.jpg (41.2 KB, 29 views)
File Type: jpg microchannel.jpg (42.6 KB, 26 views)
Henning86 is offline   Reply With Quote

Old   November 28, 2012, 17:23
Default
  #2
Senior Member
 
immortality's Avatar
 
Ehsan
Join Date: Oct 2012
Location: Iran
Posts: 2,208
Rep Power: 26
immortality is on a distinguished road
hello
Where did you write these codes?
immortality is offline   Reply With Quote

Old   November 29, 2012, 18:05
Default
  #3
Member
 
Join Date: Nov 2012
Posts: 83
Rep Power: 13
Henning86 is on a distinguished road
the code is in the rhoPimpleFoam.C after the pimple loop

or what do you mean exactly?
Henning86 is offline   Reply With Quote

Old   February 13, 2013, 16:22
Default
  #4
Senior Member
 
Mieszko Młody
Join Date: Mar 2009
Location: POLAND, USA
Posts: 145
Rep Power: 17
ziemowitzima is on a distinguished road
Hello Henning
I am facing the same problem.
I need to calculate second derivative e.g for field F i need F_xx, F_yy.
I did it the same way as you.
gradF = fvc::grad(F) // vector of first derivatives
gradgradF = fvc::grad(gradF) // tensor of second derivatives

as far as gradF is fine/smooth, gradgradF is not anymore ...

Were you able to solve this problem ?

Best
ZMM
ziemowitzima is offline   Reply With Quote

Old   February 15, 2013, 18:27
Default
  #5
Member
 
Join Date: Nov 2012
Posts: 83
Rep Power: 13
Henning86 is on a distinguished road
sadly not.

for what do you need it?
Henning86 is offline   Reply With Quote

Old   February 15, 2013, 18:40
Default
  #6
Senior Member
 
Mieszko Młody
Join Date: Mar 2009
Location: POLAND, USA
Posts: 145
Rep Power: 17
ziemowitzima is on a distinguished road
Dear Henning,
please see my post located at:
http://www.cfd-online.com/Forums/ope...erivative.html

where I am writing about other way of dealing with second order mixed derivative.
What do you think about that ?

Best
ZMM
ziemowitzima is offline   Reply With Quote

Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Combustion problem, Temp to high. Devin FLUENT 7 June 8, 2017 10:49
rhoCentralFoam - Problem high difference pressure Nicolas_OF OpenFOAM Running, Solving & CFD 0 June 11, 2012 10:29
Problem about 3D blunt body high Re simulation David FLUENT 0 September 27, 2002 11:59
High temperature problem Julie Polyakh Siemens 1 January 5, 2002 17:24
Problems of numerical instability because of high gradients Xiangyang Ye Main CFD Forum 4 September 28, 1998 04:48


All times are GMT -4. The time now is 15:24.