ASM-bottom variation

antonio · December 19, 2011, 11:50

Dear all,
I am modeling a density current composed of a mixture of water plus sediments using the algebraic slip model (ASM).
I would like to compute the bottom variation (due to the settling of the particles). Is there any way of doing this using ASM (or at least to infer this)?
Regards

ghorrocks · December 19, 2011, 17:05

Just for clarification - due you mean the ASM turbulence model or a particle slip model?

antonio · December 19, 2011, 17:09

I mean algebraic slip model.

ghorrocks · December 19, 2011, 17:17

OK, thanks. What do you mean by the "bottom variation"? Are you trying to work out how the particle settle at the bottom of the thing?

What type of particles are they?

antonio · December 19, 2011, 18:02

Please imagine a mixture of water and sediments (by definition the algebraic slip model uses spherical particles) entering in a smooth flume with "clean water". You will have a wave of sediments that will "travel" along the flume and some of these sediments will settle in the bottom. This rate of deposition will be different along the flume.My main purpose is to see how the bottom longitudinal configuration has changed along the flume from time t=0 (where the bottom was smooth) to a certain time (where some sediments have settled in the bottom).

ghorrocks · December 19, 2011, 18:33

CFX does not have good models for what you want to do.

If you consider the lagrangian particle approach - CFX can model particles falling out of the flow, but does not have deposition models or scouring models. So the particles will fall out of the flow and hit the bottom wall, but will disappear and not build up a bed.

If you consider the eularian approach (which appears to be what you are looking at) - you can use maximum packing fractions to build up something which looks like a "bed", but it does not have the proper physics of particle bedding and scouring so would not be very accurate in many applications.

I think Fluent has some recent stuff on this area in the V14 release. Also other codes such as Flow3D have some models here. But if you want to use CFX then I would consider linking it to a DEM code (eg EDEM) so the DEM code handles the bedding and scouring models and CFX does the fluid flow.

antonio · December 19, 2011, 18:46

Please can you explain in more detail what you mean by "you can use maximum packing fractions to build up something which looks like a "bed""?Are you saying to consider a certain upper limit of the sediments mass fraction in order to "predict" the "new" bed configuration?

ghorrocks · December 19, 2011, 18:50

It depends on what you are trying to do. If you only want to see where the particles drop out of the flow then do not worry about it and what you have described is fine. But if you want to form a bed or a bed which then can be scoured then it is more complex.

The issue is that there is no limit to the volume fraction of particles by default. The particles will keep building up assuming they take no space until the volume fraction reaches 1. But in reality there is a maximum packing fraction where all the particles are packed together, but the fluid exists in the gaps between the particles. This maximum packing fraction is a volume fraction less than 1.

December 19, 2011, 11:50	ASM-bottom variation	#1
antonio Senior Member Join Date: Jan 2010 Posts: 110 Rep Power: 16	Dear all, I am modeling a density current composed of a mixture of water plus sediments using the algebraic slip model (ASM). I would like to compute the bottom variation (due to the settling of the particles). Is there any way of doing this using ASM (or at least to infer this)? Regards

December 19, 2011, 18:33		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	CFX does not have good models for what you want to do. If you consider the lagrangian particle approach - CFX can model particles falling out of the flow, but does not have deposition models or scouring models. So the particles will fall out of the flow and hit the bottom wall, but will disappear and not build up a bed. If you consider the eularian approach (which appears to be what you are looking at) - you can use maximum packing fractions to build up something which looks like a "bed", but it does not have the proper physics of particle bedding and scouring so would not be very accurate in many applications. I think Fluent has some recent stuff on this area in the V14 release. Also other codes such as Flow3D have some models here. But if you want to use CFX then I would consider linking it to a DEM code (eg EDEM) so the DEM code handles the bedding and scouring models and CFX does the fluid flow. Flowdy likes this.

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December 19, 2011, 17:05		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	Just for clarification - due you mean the ASM turbulence model or a particle slip model?

December 19, 2011, 17:09		#3
antonio Senior Member Join Date: Jan 2010 Posts: 110 Rep Power: 16	I mean algebraic slip model.

December 19, 2011, 17:17		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	OK, thanks. What do you mean by the "bottom variation"? Are you trying to work out how the particle settle at the bottom of the thing? What type of particles are they?

December 19, 2011, 18:02		#5
antonio Senior Member Join Date: Jan 2010 Posts: 110 Rep Power: 16	Please imagine a mixture of water and sediments (by definition the algebraic slip model uses spherical particles) entering in a smooth flume with "clean water". You will have a wave of sediments that will "travel" along the flume and some of these sediments will settle in the bottom. This rate of deposition will be different along the flume.My main purpose is to see how the bottom longitudinal configuration has changed along the flume from time t=0 (where the bottom was smooth) to a certain time (where some sediments have settled in the bottom).

December 19, 2011, 18:46		#7
antonio Senior Member Join Date: Jan 2010 Posts: 110 Rep Power: 16	Please can you explain in more detail what you mean by "you can use maximum packing fractions to build up something which looks like a "bed""?Are you saying to consider a certain upper limit of the sediments mass fraction in order to "predict" the "new" bed configuration?

December 19, 2011, 18:50		#8
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	It depends on what you are trying to do. If you only want to see where the particles drop out of the flow then do not worry about it and what you have described is fine. But if you want to form a bed or a bed which then can be scoured then it is more complex. The issue is that there is no limit to the volume fraction of particles by default. The particles will keep building up assuming they take no space until the volume fraction reaches 1. But in reality there is a maximum packing fraction where all the particles are packed together, but the fluid exists in the gaps between the particles. This maximum packing fraction is a volume fraction less than 1.