# Reproduce COMSOL simulation in CFX

 User Name Remember Me Password
 Register Blogs Members List Search Today's Posts Mark Forums Read

 November 12, 2013, 07:25 Reproduce COMSOL simulation in CFX #1 Senior Member   Join Date: Feb 2011 Posts: 461 Rep Power: 11 Hello! I try to reproduce COMSOL's Airlift Loop Reactor example model in CFX and cannot obtain similar results (gas volume fractions, pressure and so on). So I have a couple of questions, maybe someone could answer: 1. Which multiphase model in CFX corresponds better to COMSOL's Bubbly Flow models (algebraic slip or something)? The assumptions of Bubbly Flow model are: a) gas density is negligible compared to the liquid density; b) the motion of gas bubbles relative to the liquid is determined by a balance between viscous drag and pressure forces; c) the two phases share the same pressure field. 2. By default in COMSOL it's assumed that gas volume fraction is less than 0.1, therefore continuity equation has form div(v_liquid)=0. Is there such option in CFX? 3. It seems that pressure variable (which is used in governing equations) in COMSOL includes hydrostatic contribution (if gravity is added), while in CFX hydrostatic pressure is included in Absolute Pressure but not just pressure (if buoyancy is activated). Is it so and may it be the cause of the problem? 4. How can I implement COMSOL's Gas Flux and Gas outlet boundary condition in CFX? Should I use boundary source and degassing/deposition respectively?

 November 12, 2013, 17:55 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 14,196 Rep Power: 109 This COMSOL model sounds like a standard inhomogeneous eularian flow model in CFX. But the devil is in the detail - what drag model? And corrections for things like pressure? Additional bubble mass from liquid dragged with the bubble? You can only compare solutions between software after you have done a verification/validation exercise on both codes. It is meaningless to compare inaccurate simulations. Some tips are here: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F I have no idea what the gas flux and gas outlet boundary in COMSOL are. CFX has a degassing boundary, this may be similar. I have

November 13, 2013, 03:17
#3
Senior Member

Join Date: Feb 2011
Posts: 461
Rep Power: 11
Quote:
 Originally Posted by ghorrocks This COMSOL model sounds like a standard inhomogeneous eularian flow model in CFX. But the devil is in the detail - what drag model? And corrections for things like pressure? Additional bubble mass from liquid dragged with the bubble? You can only compare solutions between software after you have done a verification/validation exercise on both codes. It is meaningless to compare inaccurate simulations. Some tips are here: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F I have no idea what the gas flux and gas outlet boundary in COMSOL are. CFX has a degassing boundary, this may be similar. I have
The drag model used in COMSOL is Hadamard-Rybczynski. This is not present in CFX. Gas flux BC is -n(gas_vf*gas_density*gas_velocity)=Gas Flux, where gas_velocity = liquid_velocity + slip_velocity + drift_velocity. Gas outlet BC means no condition for gas at boundary and if slip condition is used for liquid at this boundary it seems that it similar to degassing in CFX.

 November 13, 2013, 06:52 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 14,196 Rep Power: 109 You can specify your own drag coefficient either by CEL or a user fortran routine. So you can implement the model yourself if you wish.

 Thread Tools Display Modes Linear Mode

 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 OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post Lacerlacer CFX 11 March 12, 2012 10:32 Anil CFX 3 August 25, 2010 14:18 xulixian OpenFOAM Running, Solving & CFD 2 April 14, 2009 15:00 Ben Makhal CFX 5 April 11, 2007 08:44 Jonas Pedro Caumo CFX 0 December 9, 2006 14:54

All times are GMT -4. The time now is 22:42.

 Contact Us - CFD Online - Privacy Statement - Top