Use homogeneous results as the initial guess for an inhomogeneous simulation

JuPa · December 3, 2014, 05:05

Question

Hi, in an Eulerian-Eulerian multiphase simulation, will there be any issues using homogeneous results as the initial guess for an inhomogeneous simulation?

Background

I am performing a transient free surface simulation, where I have two continuous phases separated by a free surface. In the first instance there is nothing complex going on, i.e. no splashing, entrainment etc. So I'm modelling this as completely homogeneous (homogeneous momentum and turbulence). I want this to be my initial guess for the next part of the simulation.

In the next part of the simulation there will be gas entertainment, significant splashing etc. I will need to model the relative slip between the phases (water and vapour) to ensure adequate seperation, so will change to inhomogeneous for this portion of the simulation (I will provide an interfacial drag coefficient as required).

To reduce simulation time my idea is to run homogeneous, up to the point where I need to switch to inhomogeneous.

FYI I am using the mixture model to treat interfacial area density since the free surface model is too primitive in the inhomogeneous case (simply treats the IAD as the gradient of the volume fraction normal to the interface).

ghorrocks · December 3, 2014, 16:53

In concept using a homogeneous model as an initial condition for a inhomogeneous model sounds fine. Hopefully it is smart enough to map the variable fields to all phases - but I am not sure of this so you better check it.

JuPa · December 4, 2014, 05:29

How would one test to see if this is true before I commit to timely simulations?

I'm thinking of doing the free surface bump tutorial in homogeneous, and then using it as the initial guess for the inhomogeneous run.

But since the flow phenemena is the same in both homogeneous and inhomogeneous I'm not sure what to look for to see if there is a difference.

Opaque · December 4, 2014, 08:10

Setup the inhomogeneous case with backup results at the start of the run (see output control or expert parameters sections). You will get a backup file written before it solves the first iteration.

You can post-process such file in CFD-Post and see if the initial condition is what you expected given the initial values file.

Good luck.

JuPa · December 4, 2014, 08:57

Good idea! Thanks.

I'll reply with what I have found so people searching the forum can have an answer to this question.

Watch this space.

JuPa · December 26, 2014, 13:44

Hi,

Just an update: you can use homogeneous results as the initial guess for an inhomogeneous run.

Just ensure the material names are the same in both homogeneous and inhomogeneous so it can assign the variable fields to all phases correctly.

December 3, 2014, 05:05	Use homogeneous results as the initial guess for an inhomogeneous simulation	#1
JuPa Senior Member Mr CFD Join Date: Jun 2012 Location: Britain Posts: 361 Rep Power: 14	Question Hi, in an Eulerian-Eulerian multiphase simulation, will there be any issues using homogeneous results as the initial guess for an inhomogeneous simulation? Background I am performing a transient free surface simulation, where I have two continuous phases separated by a free surface. In the first instance there is nothing complex going on, i.e. no splashing, entrainment etc. So I'm modelling this as completely homogeneous (homogeneous momentum and turbulence). I want this to be my initial guess for the next part of the simulation. In the next part of the simulation there will be gas entertainment, significant splashing etc. I will need to model the relative slip between the phases (water and vapour) to ensure adequate seperation, so will change to inhomogeneous for this portion of the simulation (I will provide an interfacial drag coefficient as required). To reduce simulation time my idea is to run homogeneous, up to the point where I need to switch to inhomogeneous. FYI I am using the mixture model to treat interfacial area density since the free surface model is too primitive in the inhomogeneous case (simply treats the IAD as the gradient of the volume fraction normal to the interface).

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December 3, 2014, 16:53		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	In concept using a homogeneous model as an initial condition for a inhomogeneous model sounds fine. Hopefully it is smart enough to map the variable fields to all phases - but I am not sure of this so you better check it.

December 4, 2014, 05:29		#3
JuPa Senior Member Mr CFD Join Date: Jun 2012 Location: Britain Posts: 361 Rep Power: 14	How would one test to see if this is true before I commit to timely simulations? I'm thinking of doing the free surface bump tutorial in homogeneous, and then using it as the initial guess for the inhomogeneous run. But since the flow phenemena is the same in both homogeneous and inhomogeneous I'm not sure what to look for to see if there is a difference.

December 4, 2014, 08:10		#4
Opaque Senior Member Join Date: Jun 2009 Posts: 1,804 Rep Power: 32	Setup the inhomogeneous case with backup results at the start of the run (see output control or expert parameters sections). You will get a backup file written before it solves the first iteration. You can post-process such file in CFD-Post and see if the initial condition is what you expected given the initial values file. Good luck.

December 4, 2014, 08:57		#5
JuPa Senior Member Mr CFD Join Date: Jun 2012 Location: Britain Posts: 361 Rep Power: 14	Good idea! Thanks. I'll reply with what I have found so people searching the forum can have an answer to this question. Watch this space.

December 26, 2014, 13:44		#6
JuPa Senior Member Mr CFD Join Date: Jun 2012 Location: Britain Posts: 361 Rep Power: 14	Hi, Just an update: you can use homogeneous results as the initial guess for an inhomogeneous run. Just ensure the material names are the same in both homogeneous and inhomogeneous so it can assign the variable fields to all phases correctly.