CFD Online Logo CFD Online URL
Home > Forums > STAR-CCM+

Free surface bubbling/air gap problem

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

Like Tree1Likes
  • 1 Post By ping

LinkBack Thread Tools Display Modes
Old   November 3, 2011, 12:58
Default Free surface bubbling/air gap problem
New Member
Join Date: Mar 2009
Posts: 14
Rep Power: 10
TWaung is on a distinguished road

I am attempting to model a flume tank (rectangular channel with free surface flow), with a wall and an actuator disk. Basically a dam with a large turbine. The attached images show a side view of the domain, with the white rectangle representing the wall, and the rectangular outline below the wall representing the extents of the actuator disk.

I am finding that initially, the water surface looks like the first image. Smooth, with no bubbles or gaps on the upstream side of the turbine. However, after running the simulation for a long time in order for the velocity through the actuator disk to reach a steady value, these bubbles/air gaps start appearing in the free surface upstream of the turbine (seen in the second image). This behaviour has also shown up even if I round the edges of the barrage.

I am primarily interested in the head difference across the wall, so this is a problem for me.

Does anyone have any idea what is going on?

The details of the sim are provided below:
Domain: 3D rectangular channel with a wall and actuator disk

Inlet BC: Mass flow inlet for water phase
Outlet BC: Pressure outlet with user function defining hydrostatic pressure
Initial Water level: Defined using a field function
Tank side walls: symmetry planes

Turbulence Model: k-epsilon

Thanks for any suggestions!

TWaung is offline   Reply With Quote

Old   November 5, 2011, 16:51
Senior Member
Join Date: Oct 2009
Location: Germany
Posts: 637
Rep Power: 15
abdul099 is on a distinguished road
Do you run steady state or transient? Have you checked your mesh?
abdul099 is offline   Reply With Quote

Old   November 5, 2011, 18:37
New Member
Join Date: Mar 2009
Posts: 14
Rep Power: 10
TWaung is on a distinguished road

I am running a transient simulation. I have a pretty fine mesh around the free surface. Do you have any mesh advice/ rules of thumb?

TWaung is offline   Reply With Quote

Old   November 6, 2011, 04:29
Senior Member
Join Date: Mar 2009
Posts: 321
Rep Power: 12
ping is on a distinguished road
you are hiding important information in your VOF scalar plot - how well the air and water are being separated at the interface - use the default number of colours and ideally for VOF you should see non red and blue across one maybe two cells. zoom in and look at the problem area - you might need a finer mesh and/or finer timestep - also plot convective courant number and try to keep below 1.
abdul099 likes this.
ping is offline   Reply With Quote


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
[ICEM] surface mesh merging problem everest ANSYS Meshing & Geometry 44 April 14, 2016 06:41
CFX convergence issues with free surface adenlan CFX 3 September 2, 2011 06:43
Linear analytical solution oto the 2D free sloshing water surface elevation bearcat Main CFD Forum 7 August 5, 2011 20:13
Free surface problem Luk CFX 2 July 27, 2006 01:07
CFX4.3 -build analysis form Chie Min CFX 5 July 12, 2001 23:19

All times are GMT -4. The time now is 03:39.