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

Get Mesh Acceleration

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

Like Tree3Likes
  • 1 Post By singer1812
  • 2 Post By singer1812

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   July 22, 2015, 11:48
Post Get Mesh Acceleration
  #1
New Member
 
Pietro
Join Date: Jul 2015
Posts: 9
Rep Power: 10
Pietro Giorgio is on a distinguished road
Hi everyone, i am looking for mesh acceleration values in ansys cfx. I have searched in help and in forums but no one seem to know how can i compute this value.
My problem is: i need compute the add mass due the heave motion of a ship's transversal section. For this, i need know the mesh accelaration.
Pietro Giorgio is offline   Reply With Quote

Old   July 22, 2015, 13:31
Default
  #2
Senior Member
 
Edmund Singer P.E.
Join Date: Aug 2010
Location: Minneapolis, MN
Posts: 511
Rep Power: 20
singer1812 is on a distinguished road
Is this a 6DOF problem (rigid body or immersed solid)? You can use rbstate to get this.
singer1812 is offline   Reply With Quote

Old   July 22, 2015, 14:03
Default
  #3
New Member
 
Pietro
Join Date: Jul 2015
Posts: 9
Rep Power: 10
Pietro Giorgio is on a distinguished road
Quote:
Originally Posted by singer1812 View Post
Is this a 6DOF problem (rigid body or immersed solid)? You can use rbstate to get this.
My friend, i thank you response. I dont set a rigid body. As can be seen by the figure that i have attached, i set the surfaces of the hull as wall boundary conditions and forced her to make an oscillatory movement in the Z axis, which is defined by a function that I called "Osc=A.sen(w.t)".
I already have the acceleration of the surface(hull), and get this only by deriving the expression "Osc", the concept of additional mass implies look for fluid particles that have a close enough acceleration if compared with hull acceleration, for that, I need the vertical acceleration information of fluid particles throughout the domain.
I tried to get the velocity field in the previous time step, and could make a linear approximation of the vertical acceleration just subtracting the speed at the current time step less speed in the previous time step divided by time step. But I failed to get this data as well. Do you have any idea?
Attached Images
File Type: jpg osc.jpg (29.2 KB, 17 views)
Pietro Giorgio is offline   Reply With Quote

Old   July 22, 2015, 14:38
Default
  #4
Senior Member
 
Edmund Singer P.E.
Join Date: Aug 2010
Location: Minneapolis, MN
Posts: 511
Rep Power: 20
singer1812 is on a distinguished road
You are doing it the way I would suggest.
What do you mean by you are not able to get this data?

Do you mean you dont know how to take the previous time step data and subtract it from the current timestep? I can help you with this, if that is the problem.
Pietro Giorgio likes this.
singer1812 is offline   Reply With Quote

Old   July 22, 2015, 14:53
Default
  #5
New Member
 
Pietro
Join Date: Jul 2015
Posts: 9
Rep Power: 10
Pietro Giorgio is on a distinguished road
Quote:
Originally Posted by singer1812 View Post
You are doing it the way I would suggest.
What do you mean by you are not able to get this data?

Do you mean you dont know how to take the previous time step data and subtract it from the current timestep? I can help you with this, if that is the problem.
Yes my friend, I really tried and I could not do it. Could you teach me? I am deeply grateful!
Pietro Giorgio is offline   Reply With Quote

Old   July 22, 2015, 15:01
Default
  #6
Senior Member
 
Edmund Singer P.E.
Join Date: Aug 2010
Location: Minneapolis, MN
Posts: 511
Rep Power: 20
singer1812 is on a distinguished road
You need to load in the previous time step and create a new variable. That variable will be a FROZEN copy of the velocity field you want to get the accelerations from (you can create a separate new variable for each u,v,w).

That variable will retain the old time steps values and not change with a new time step. Load in the new time step. Create a new variable of your acceleration. It will be an expression variable of (velocity-old_velocity_variable)/timestep.

Now if you want to do this over a lot of time steps you will either have to create the FROZEN copy by hand at each time step, or use Power Syntax to do this for you. Power Syntax is a whole another topic that I will allow you to look into, but for now, you know the process.
ombvalves and Pietro Giorgio like this.
singer1812 is offline   Reply With Quote

Old   July 22, 2015, 15:26
Default
  #7
New Member
 
Pietro
Join Date: Jul 2015
Posts: 9
Rep Power: 10
Pietro Giorgio is on a distinguished road
My friend, really Thanks! I will have a look at it. You saved my project haha !
Pietro Giorgio is offline   Reply With Quote

Old   July 23, 2015, 18:50
Default
  #8
New Member
 
Pietro
Join Date: Jul 2015
Posts: 9
Rep Power: 10
Pietro Giorgio is on a distinguished road
I have a problem about it. The frozen copy is not really freezing the previous time step. I can not see what I did wrong, i consulted the tutorial, but apparently the method is correct. I went to the previous time step and created a new variable using the "Frozen Copy", called it "Vel Old" (Figure 1). Went to the next time step. I made expression called "Acc Mesh" and made the linear approximation to the acceleration (Figure 2), have created a variable called "Mesh Acceleration Z" and I related with "Acc Mesh" expression, however, it is always giving 0 (Figure 3), this means that the Frozen copy is updating his value, because as can be seen from figures 4 and 5, the speed varies greatly around the hull over time. Could anyone tell me what i am doing wrong?
Attached Images
File Type: jpg a1.JPG (21.7 KB, 11 views)
File Type: jpg a2.JPG (23.8 KB, 11 views)
File Type: jpg a4.JPG (23.5 KB, 10 views)
File Type: jpg a5.JPG (40.0 KB, 12 views)
File Type: jpg a6.JPG (38.0 KB, 7 views)

Last edited by Pietro Giorgio; July 23, 2015 at 19:57.
Pietro Giorgio is offline   Reply With Quote

Old   July 24, 2015, 01:04
Default
  #9
New Member
 
Pietro
Join Date: Jul 2015
Posts: 9
Rep Power: 10
Pietro Giorgio is on a distinguished road
Hi! I found the problem, the CFX 13.0 does not work well with frozen copy . I tested the version 15.0 and it worked perfectly.
Pietro Giorgio is offline   Reply With Quote

Reply

Tags
added mass, mesh acceleration

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
sliding mesh problem in CFX Saima CFX 46 September 11, 2021 08:38
Star CCM Overset Mesh Error (Rotating Turbine) thezack Siemens 7 October 12, 2016 12:14
[ICEM] surface mesh merging problem everest ANSYS Meshing & Geometry 44 April 14, 2016 07:41
[ICEM] Problem making structural mesh on a surface froztbear ANSYS Meshing & Geometry 1 November 10, 2011 09:52
[snappyHexMesh] snappyHexMesh won't work - zeros everywhere! sc298 OpenFOAM Meshing & Mesh Conversion 2 March 27, 2011 22:11


All times are GMT -4. The time now is 04:54.