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

Transient results with periodic boundary conditions?

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

Like Tree2Likes
  • 1 Post By FMDenaro
  • 1 Post By FMDenaro

Reply
 
LinkBack Thread Tools Display Modes
Old   January 17, 2017, 14:20
Default Transient results with periodic boundary conditions?
  #1
New Member
 
Theo
Join Date: Mar 2009
Posts: 4
Rep Power: 10
holgerbre is on a distinguished road
Someone applies periodic boundary conditions to a turbulent channel flow to mimic an infinite domain size in spanwise and streamwise direction.

Does it make sense to analyse the flow during the transient stage, i.e. before a statistically stationary flow is reached?

A practical example: I see in literature simulations of particle-laden channel flows. It is analyzed how an initially uniformly distributed particulate phase obtains preferential locations close to the wall. I understand that the statistically stationary flow field has a physical meaning. But before that, during the transient stage, is that physical? Note that the particles in the simulation have all the same residence time. In reality that would mean that they are not at the same location...
holgerbre is offline   Reply With Quote

Old   January 17, 2017, 14:35
Default
  #2
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 3,250
Rep Power: 37
FMDenaro will become famous soon enoughFMDenaro will become famous soon enough
Quote:
Originally Posted by holgerbre View Post
Someone applies periodic boundary conditions to a turbulent channel flow to mimic an infinite domain size in spanwise and streamwise direction.

Does it make sense to analyse the flow during the transient stage, i.e. before a statistically stationary flow is reached?

A practical example: I see in literature simulations of particle-laden channel flows. It is analyzed how an initially uniformly distributed particulate phase obtains preferential locations close to the wall. I understand that the statistically stationary flow field has a physical meaning. But before that, during the transient stage, is that physical? Note that the particles in the simulation have all the same residence time. In reality that would mean that they are not at the same location...

My opinion is that you should distinguish between the transient of the velocity field and the transient of the particle laden problem.
Starting from the arbitrary initial condition, the velocity field develops only a numerical transient with no physical meaning. After that a statistically equilibrium is reached, you can set an initial uniform distribution of particles that develop a certain transient over a physically meaning unsteady velocity field. The question could be now in the fact if a uniform initial distribution of particles can or not be a good physical assumption.
holgerbre likes this.
FMDenaro is offline   Reply With Quote

Old   January 17, 2017, 16:56
Default
  #3
New Member
 
Theo
Join Date: Mar 2009
Posts: 4
Rep Power: 10
holgerbre is on a distinguished road
Quote:
Originally Posted by FMDenaro View Post
My opinion is that you should distinguish between the transient of the velocity field and the transient of the particle laden problem.
Starting from the arbitrary initial condition, the velocity field develops only a numerical transient with no physical meaning. After that a statistically equilibrium is reached, you can set an initial uniform distribution of particles that develop a certain transient over a physically meaning unsteady velocity field. The question could be now in the fact if a uniform initial distribution of particles can or not be a good physical assumption.
Thank you for that answer!

Can you please tell me why you think the transient of the velocity field (e.g. from an initially laminar profile to turbulent) has no physical meaning whereas the transient of the particles (e.g. from initially uniform distribution in a turbulent field to non-uniform) is physical?
holgerbre is offline   Reply With Quote

Old   January 17, 2017, 17:11
Default
  #4
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 3,250
Rep Power: 37
FMDenaro will become famous soon enoughFMDenaro will become famous soon enough
because using periodic BC.s you are not simulating a spatial evolving boundary layer over two flat plates... This flow problem is just a model for studying the wall turbulence, therefore we let the flow to forget the initial condition and study the turbulence only once it is fully developed...

the Lagrangian tracking of the particle depends only on the velocity field, the initial conditions for the particles never corresponding to a real experiment.

However, if you want, you can try starting from an exact Poiseuille velocity field without any superimposed perturbation and compute simultaneously the tracking of particles starting from an initial uniform distribution.
What is the physical meaning of such simulation is not clear...
holgerbre likes this.
FMDenaro is offline   Reply With Quote

Old   January 17, 2017, 17:15
Default
  #5
New Member
 
Theo
Join Date: Mar 2009
Posts: 4
Rep Power: 10
holgerbre is on a distinguished road
great, I got it, thanks!
holgerbre is offline   Reply With Quote

Reply

Tags
particle-laden, periodic bc, transient

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
3D Windturbine simulation in SU2 k.vimalakanthan SU2 13 June 20, 2016 05:27
PEMFC module + multiple periodic boundary conditions vkrastev FLUENT 2 December 22, 2014 05:15
Symmetry and Periodic boundary conditions CFD_Fluent_User FLUENT 1 October 16, 2014 02:18
Error finding variable "THERMX" sunilpatil CFX 8 April 26, 2013 07:00
Periodic boundary conditions for 2D pipe (water flow). thess FLUENT 3 May 31, 2012 13:38


All times are GMT -4. The time now is 03:50.