CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > CFX

Velocity u.Gradient X

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

Reply
 
LinkBack Thread Tools Display Modes
Old   April 4, 2006, 06:07
Default Velocity u.Gradient X
  #1
Mgtripple
Guest
 
Posts: n/a
Hi friends,

I tried to use the command "Velocity u.Gradient X" in order to extract the velocity gradient in X direction. Then this value enters a formulae to calculate viscosity. However it is not working in SOLVER. It gives an error: "Velocity u.Gradient X Velocity u.: read successfully, and then error found at item: Gradient X syntax error"

Is there anyway to solve this?

Thanks

Serdar
  Reply With Quote

Old   April 4, 2006, 09:18
Default Re: Velocity u.Gradient X
  #2
opaque
Guest
 
Posts: n/a
Dear Mgtripple,

The ANSYS CFX solver does not support access to miscellaneous/suffixed variables such Velocity u.Gradient X or the like via CEL..

However, you may write a User CEL Function where you can call USER_GETVAR for Velocity.Gradient and use it as needed. The pointer to the stack that USER_GETVAR provides includes all the components of the Gradient for all velocity components, i.e a matrix..

Good luck, Opaque..

  Reply With Quote

Old   April 4, 2006, 09:34
Default Re: Velocity u.Gradient X
  #3
CFXuser
Guest
 
Posts: n/a
Sorry to interupt. May I some question about this USer CEL thing? Would that be possible to use this function to define a plane in the flow domain and then output the area-averaged static pressure of this plane when the solver is running? Thank you for advice.
  Reply With Quote

Old   April 4, 2006, 09:43
Default Re: Velocity u.Gradient X
  #4
opaque
Guest
 
Posts: n/a
Dear CFXuser,

The ANSYS CFX solver can only compute averages on region/locations already defined as a boundary in CFX-Pre.. An option in 10.0 is to remesh, or define a surface (2D) region and create a domain interface at such region.. Then, you can use a areaAve(Pressure)@New 2D region..

It may work if you only need a single plane. However, if you need more than 1, I would not even try it..

I understand the upcoming 11.0 will allow to use average functions on already defined regions even if they are not boundaries, or domain interfaces..

Good luck, Opaque..

  Reply With Quote

Old   April 5, 2006, 09:36
Default Re: Velocity u.Gradient X
  #5
Mgtripple
Guest
 
Posts: n/a
Thanks for the answer

Please correct me if i am wrong but what i understood from the manuals that i need FORTRAN program installed on my computer to use CEL function ?

Regards,

Serdar
  Reply With Quote

Old   April 5, 2006, 10:00
Default Re: Velocity u.Gradient X
  #6
opaque
Guest
 
Posts: n/a
Dear Mgtripple,

You need to write your own FORTRAN routine, compile it (you need a supported compiler), and setup the proper details in CFX-Pre..

It is a workaround; otherwise, there is no mechanism to get what you want..

Opaque..

  Reply With Quote

Old   April 5, 2006, 11:13
Default Re: Velocity u.Gradient X
  #7
Mgtripple
Guest
 
Posts: n/a
thanks again

at least now i know where to start.

  Reply With Quote

Old   January 26, 2014, 15:00
Default how to use velocity gradient as a boundary codition
  #8
New Member
 
Chandrasekhar
Join Date: Oct 2013
Location: new jersey
Posts: 24
Rep Power: 3
chandrasekhar is on a distinguished road
Hello opaque
i would like to know how to input a velocity gradient( say in the x direction) as a boundary condition. my case is very simple, it is a 2d box with inlet and outlet. the bc's are 1m/sec at inlet and i want to input du/dx=0 at outlet. can u please tell me how to do this. Many thanks for replying.

with regards
Chandra Sekhar



Quote:
Originally Posted by opaque
;75830
Dear Mgtripple,

You need to write your own FORTRAN routine, compile it (you need a supported compiler), and setup the proper details in CFX-Pre..

It is a workaround; otherwise, there is no mechanism to get what you want..

Opaque..
chandrasekhar is offline   Reply With Quote

Old   January 27, 2014, 08:04
Default
  #9
Senior Member
 
Bruno
Join Date: Mar 2009
Location: Brazil
Posts: 236
Rep Power: 12
brunoc is on a distinguished road
Why do you want to fix du/dx at the outlet? For almost all cases that should be a part of the solution. Fixing that would affect the velocity fied upstream, which is not something you'd want unless you're testing something.

That being said, if your channel is long enough with a constant shape before the outlet, your solution should be du/dx=0 (developed flow).
brunoc is offline   Reply With Quote

Old   January 27, 2014, 11:01
Default
  #10
New Member
 
Chandrasekhar
Join Date: Oct 2013
Location: new jersey
Posts: 24
Rep Power: 3
chandrasekhar is on a distinguished road
Hi
first of all thanks for the prompt reply. i wanted to use du/dx=0 because i want to see how it is different from that of a pressure outlet at the outlet. I would like to know how to input such a boundary condition. Many thanks for replying.

with regards
chandrasekhar is offline   Reply With Quote

Old   January 28, 2014, 09:31
Default
  #11
Senior Member
 
Bruno
Join Date: Mar 2009
Location: Brazil
Posts: 236
Rep Power: 12
brunoc is on a distinguished road
Well, it's very different. On one you're imposing a prescribed pressure (velocity field is part of the solution), while on the other you're saying velocity u shouldn't change.

What is your application?
brunoc is offline   Reply With Quote

Old   January 28, 2014, 11:01
Default
  #12
New Member
 
Chandrasekhar
Join Date: Oct 2013
Location: new jersey
Posts: 24
Rep Power: 3
chandrasekhar is on a distinguished road
Hi
my professor had told me to see the velocity and pressure contours by applying different boundary conditions. so i thought of comparing three different scenarios by keeping the bc at the inlet constant. they are outflow at outlet, pressure outlet at outlet, and du/dx=0 at the outlet. I would like to know if i am doing some thing wrong. Also i tried using du/dx=constant in my case, fluent is showing an error. my case is a simple 2d rectangle with inlet and outlet at the opposite sides. I think i am doing it wrong. will be grateful for any help. Many thanks for replying.
with regards


Quote:
Originally Posted by brunoc View Post
Well, it's very different. On one you're imposing a prescribed pressure (velocity field is part of the solution), while on the other you're saying velocity u shouldn't change.

What is your application?
chandrasekhar is offline   Reply With Quote

Reply

Thread Tools
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 On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
help with UDF for contact angle based on contact line velocity gandesk Fluent UDF and Scheme Programming 14 October 29, 2012 14:58
How to calculate Velocity? Kali Sanjay Phoenics 1 May 1, 2006 13:57
Neumann pressure BC and velocity field Antech Main CFD Forum 0 April 25, 2006 02:15
Terrible Mistake In Fluid Dynamics History Abhi Main CFD Forum 12 July 8, 2002 09:11
what the result is negatif pressure at inlet chong chee nan FLUENT 0 December 29, 2001 06:13


All times are GMT -4. The time now is 15:06.