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Drug release (add. var transport) for months, problem with time step selection 

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September 12, 2016, 02:21 
Drug release (add. var transport) for months, problem with time step selection

#1 
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Ftab
Join Date: Sep 2011
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Hi all,
I am modeling drug release (additional variable in kg /m^3) from multiple micronsized (80 um) humps into a cylinder of 4mm diametr, with no symmetry. The 3D mesh becomes gigantic (25 millions). I have fluid domain (max velocity 0.5 m/s) and porous domain (max vel= 0.0005 mm/s), both having humps and mass transport. Drug release BC is variable in time (days) and should be modeled for two month period. The flow is unsteady with period of 2 seconds. I now have these questions:  As drug release is having scales of days, can I model the high frequency flow as Steady state? If yes, is it correct to solve flow steadily, converge and then turn flow simulation off and only solve AV?  Courant No. is irrelevant for CFX. What should be the time step then for drug release (AV) which should be modeled for 60 days? Small time steps (seconds) will take for ever for this huge mesh to finish. 

September 12, 2016, 02:43 

#2 
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Ftab
Join Date: Sep 2011
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To update:
Running it with Steady Flow, and then unsteady AV (still do not know if correct)  Convergence criteria is set to 1e5 (tight?)  Time step: fixed to 1s >> Corant 999 > not converging in 20 iterations  Time step: fixed to 0.2s >> Corant 135> Converges in 7 iterations  Time step: fixed to 0.1s >> Corant 65> Converges in 4 iterations Freaks me out if I will need to run it with time step bellow 1 sec and simulate 60 days!!! Adaptive time stepping??? good idea? 

September 12, 2016, 03:04 

#3  
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Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
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This simulation has a massive range in time scales (flow = milliseconds, drug accumulation = days) which means simply modelling it as a transient simulation is not a very effective approach. You should look at a separation of variables approach so you can model fluid and drug time scales as separate simulations. For instance use the fluid simulation to get the drug mass flow rate, then use a drug only model to model the accumulation of drug. Your comment about a steady flow then transient AV is an example of this approach  just be careful to check the steady result is equivalent to the transient case, at least for a short period of time. Quote:
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September 12, 2016, 23:00 

#4  
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Ftab
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Thanks a lot Glenn for your perfect answer as always.
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September 12, 2016, 23:32 

#5  
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Glenn Horrocks
Join Date: Mar 2009
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My experience with CFX is that: * Convergence tolerance is pretty independent of time step or mesh. Once you have found a good convergence tolerance you can change the mesh and time step and your convergence tolerance should still be good. * Mesh size and time step are closely linked. So if you change the mesh you definitely need to change the time step. * Initially it will be just a simple linear scaling between mesh size and time step size but as the finer mesh starts to resolve finer features it departs linearity. * A big reason I recommend adaptive time stepping to 35 coeff loops per iteration is because it means you do not need to do a sensitivity analysis on time step size. Removing one variable to do the sensitivity study on dramatically simplifies it. Quote:


September 15, 2016, 15:51 

#6 
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Ftab
Join Date: Sep 2011
Posts: 72
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Perfect as always Glenn!
You are a blessing to this community. I think I have enough material to get busy for now. Thanks, Ftab 

Tags 
additional variable, cfx 15, time step size 
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