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

Simulation of flow past a cylinder

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

Like Tree5Likes
  • 1 Post By ghorrocks
  • 2 Post By Gert-Jan
  • 2 Post By ghorrocks

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   March 10, 2021, 15:19
Default Simulation of flow past a cylinder
  #1
Member
 
Ashkan Kashani
Join Date: Apr 2016
Posts: 46
Rep Power: 10
Ashkan Kashani is on a distinguished road
Hi everyone,

I want to use CFX to simulate the transient 2D flow past a stationary rectangular cylinder crossing the free surface (see Figure attached). The main objective is to accurately predict the lift and drag. Assuming that the free surface dynamics would probably have a minor effect, I want to treat the free surface as a free-slip wall (rather than take an involved multiphase approach), thereby speeding up the simulation and avoiding convergence issues associated with the explicit modelling of the free surface. With this in mind,
(1) What is the best practice for the boundary conditions here? Particularly, how can I impose zero pressure on the free surface in order to get accurate force prediction?
(2) Is it possible to have a more efficient boundary conditions arrangement whereby the free surface dynamics could be resolved only over a small distance upstream and downstream of the cylinder?
Note the fact that the inlet and outlet must be far away from the cylinder leads to a long narrow domain; so the free surface dynamics is absolutely of no interest over the major part of its span.

I would appreciate any comments.
Attached Images
File Type: jpg Figure.jpg (27.8 KB, 30 views)
Ashkan Kashani is offline   Reply With Quote

Old   March 10, 2021, 16:06
Default
  #2
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,699
Rep Power: 143
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
1) To impose zero pressure at the interface just make it a pressure boundary set to zero pressure. You will probably have to use the entrainment option to handle the cross flow. But pressure boundaries do not work well with lots of cross flow like this so you will probably have problems with convergence.

2) This model should be tractable modelling it as a proper multiphase model with the free surface modelled properly. I see no reason to do dubious handling of the free surface, especially as the free surface is going to have waves propagate out a very long way (like the wake of a boat), so handling these with a boundary close by is going to be difficult.

Just model it properly in full free surface multiphase. It is not that expensive these days and you will be doing real results quickly rather than trying to get some contrived boundary condition to work.

A side comment: The gerris CFD code (open source) would do this model very quickly, nicely and more accurately than CFX. There is a quite a learning curve on gerris but it should do well on this model.
aero_head likes this.
__________________
Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.
ghorrocks is online now   Reply With Quote

Old   March 10, 2021, 16:54
Default
  #3
Member
 
Ashkan Kashani
Join Date: Apr 2016
Posts: 46
Rep Power: 10
Ashkan Kashani is on a distinguished road
Thank you so much Glenn.

Quote:
Originally Posted by ghorrocks View Post
1) To impose zero pressure at the interface just make it a pressure boundary set to zero pressure. You will probably have to use the entrainment option to handle the cross flow. But pressure boundaries do not work well with lots of cross flow like this so you will probably have problems with convergence.
I have already imposed Opening with a relative pressure = 0 at the outlet (i.e. rightmost end of the domain). Is it mathematically/physically correct to set relative pressure = 0 at multiple boundaries? Could you please give further details?

Quote:
Originally Posted by ghorrocks View Post
I see no reason to do dubious handling of the free surface, especially as the free surface is going to have waves propagate out a very long way
Quote:
Originally Posted by ghorrocks View Post
Just model it properly in full free surface multiphase. It is not that expensive these days and you will be doing real results quickly rather than trying to get some contrived boundary condition to work.
I am not sure if I understand exactly what you mean here. Are you recommending multiphase modelling? Because I suppose you previously mentioned there's no point in dealing with the free surface dynamics in this problem.
Ashkan Kashani is offline   Reply With Quote

Old   March 11, 2021, 01:32
Default
  #4
Senior Member
 
Gert-Jan
Join Date: Oct 2012
Location: Europe
Posts: 1,827
Rep Power: 27
Gert-Jan will become famous soon enough
If you want to close your eyes for the free surface effects, then apply a wall with free slip instead of a pressure opening at the top.
Ashkan Kashani and aero_head like this.
Gert-Jan is offline   Reply With Quote

Old   March 15, 2021, 03:37
Default
  #5
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,699
Rep Power: 143
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
..... which is then effectively a single phase 3D simulation with a symmetry plane. In other words, this is no different to a normal 3D, single phase, cylinder simulation; with the addition of a symmetry plane in the middle.

I would check your initial assumption that the free surface effects have no effect. For any moderate velocity or higher it will create a cavity in the water which will have a major effect. And if you decide it has no effect, then just look up the cylinder drag numbers from a fluids textbook as it has been measured to extreme accuracy many times by other researchers.
Ashkan Kashani and aero_head like this.
__________________
Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.
ghorrocks is online now   Reply With Quote

Old   October 5, 2022, 20:23
Default
  #6
Member
 
Ashkan Kashani
Join Date: Apr 2016
Posts: 46
Rep Power: 10
Ashkan Kashani is on a distinguished road
Hi all,

Could anyone please give me some tips on the appropriate turbulence models for this problem? Could it be simplified as laminar flow by any chance?
Ashkan Kashani is offline   Reply With Quote

Old   October 6, 2022, 03:48
Default
  #7
Senior Member
 
Gert-Jan
Join Date: Oct 2012
Location: Europe
Posts: 1,827
Rep Power: 27
Gert-Jan will become famous soon enough
That depends on the Reynolds number......
Gert-Jan 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
[Gmsh] 3D Free surface flow past circular cylinder meshing using GMSH arieljeds OpenFOAM Meshing & Mesh Conversion 7 January 14, 2017 12:57
modeling flow past a cylinder Zeinaby FLUENT 0 June 16, 2016 02:19
Strange flow partern (Reverse Flow) in fluid past circular cylinder problem at exit HectorRedal Main CFD Forum 9 June 9, 2016 18:14
How to compute lift and drag coefficients for flow past a fixed cylinder? antonella.longo@ingv.it Main CFD Forum 2 May 11, 2016 17:26
Flow Simulation : air around an rotating cylinder using Solidworks Flow Simulation Wyrold Main CFD Forum 0 October 22, 2015 08:48


All times are GMT -4. The time now is 19:30.