CFD Online Logo CFD Online URL
Home > Forums > FLUENT

Mixture model problem - could someone please advise?

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

Like Tree1Likes
  • 1 Post By matlab_monkey

LinkBack Thread Tools Display Modes
Old   July 25, 2012, 12:11
Default Mixture model problem - could someone please advise?
New Member
Join Date: Jun 2012
Posts: 3
Rep Power: 7
matlab_monkey is on a distinguished road
Hi Guys,

I'm trying to put together a two phase model using the mixture model, but have run into some difficulties. I've gone right back to basics and constructed a simpllified problem, I thought this may help me work out where I'm going wrong.

Unfortunately, I'm getting nowhere fast, could someone please offer some advice?

My simplified problem consists of a 2D room. The room 10m long and 2.5m tall. On the left wall there is an inlet on the centre of the wall (flush with the wall) which is 0.2m in height. On the right hand wall I have an outlet which is 0.5m in height. The outlet is 0.75m away from the top edge of the domain.

My mesh appears good, I've done the quality checks etc and don't see any problems there.

For this initial problem I have effectively switched off the secondary phase (setting inlet volume fraction to 0). When I have a satisfactory flow established with a single phase then I'll add in the second phase.

I am using the k-epsilon turbulence model, with default parameters for now.

My inlet (mixture) conditions are a turbulent intensity of 5% and hydraulic diameter of 0.2 (inlet diameter). The air phase enters the room at 1m/s. The outlet condition is a pressure outlet. I have specified a turbulent intensity of 5% and hydraulic diameter of 0.5 to account for any backflow.

When setting my operating conditions I activate gravity (y=-9.81) and (as done in the Ansys multi-phase tutorial) set the operating density to be 0.

When I run the calculation I get reversed flow at the outlet - which doesn't appear physical. In an attempt to avoid this I modified the domain, to add a 'pipe' leading to the outlet. I thought this may give the flow some time to develop before reaching the outlet edge, without altering the flow within the room. Unfortunately this doesn't fix the problem.

If I plot contours of static pressure then I find (almost) horizontal stripes, as expected the pressure is higher at the floor than the ceiling (i.e. hydrostatic).

If I switch off gravity (or the specified density) then I lose the reversed flow but my pressure field is no longer hydrostatic.

My intention is to use the mixing model to simulate heavy particles dropping out of an air stream. But first I need to get this simple problem fixed.

I've only been using Fluent for a few weeks and must admit that I'm a little bit confused over the way that Fluent handles pressures and densities. I suspect that this is somehow responsible for the reversed flow I'm seeing at my pressure outlet.

I should point out that the flow seems to converge well. There doesn't seem to be any issues with the solver settings.

Could someone please advise me how to fix this problem?


rajann_786 likes this.
matlab_monkey is offline   Reply With Quote

Old   July 26, 2012, 07:27
Senior Member
Join Date: Jul 2012
Location: France
Posts: 122
Rep Power: 8
Marion is on a distinguished road
How long have you run the case?
When you say it is converged, do you wait until the residuals are very low and stable (as opposed to reached 10-5 or something like that)?
Sometimes you can have reversed flow at the outlet at the bieginning of the run and then it disappears after more iterations.
It is strange that the reverse outlet flow doesn't disappear with the pipe at the outlet.. except if it is not long enough, or if the model has not really converged.
Marion is offline   Reply With Quote

Old   July 26, 2012, 08:20
New Member
Join Date: Jun 2012
Posts: 3
Rep Power: 7
matlab_monkey is on a distinguished road
Hi Marion,

Thanks for your response.

The calculation is converging in the sense that the residuals for continuity are small and all of the residuals are very stable (give or take a small wiggle). The backflow is still present at the outlet after many iterations.

I think it must be something to do with the operating conditions. If I uncheck the specified density option then the reverse flow disappears (after just a few iterations) but I lose the stripey (hydrostatic) static-pressure field.


matlab_monkey 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
Modelling Sand erosion using Mixture model and DPM karlchapman FLUENT 3 July 5, 2012 08:55
Mixture model validation cases Antoine Main CFD Forum 1 January 24, 2011 02:02
How to include lift forces/wall lubrication forces in the MIXTURE multiphase model? hohomm FLUENT 0 December 17, 2009 06:45
mass balance problem in multiphase model mdsanij1 FLUENT 0 July 28, 2009 16:01
Multiphase Mixture Model fluent5.4 Julia Kullmann FLUENT 0 March 9, 2001 06:13

All times are GMT -4. The time now is 22:20.