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March 13, 2020, 07:35 
High viscosity ratio in fire simulation

#1 
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Daniel Herranz
Join Date: Feb 2019
Posts: 5
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Hello, I am doing right now a transient simulation of an underground fire. The fire starts inside a train and the species and heat has to move arround the whole station.
I need to simulate like 1200(s) and my mesh has about 400k elements, which are polyhedra with maximum skewness 0.54 and minimum orthogonal quality 0.2. I have done an steady case without the fire, just with natural flow inside the station to start the transient case from this solution. For my transient simulation, I have set up a CFL=10 and an initial time step of 5e4 s. This time step grows continualy by a factor of 2 every time step. It reaches something like 0.15 (meaning a CFL=7) and then, suddenly, at second 5 or 6 of simulation, viscosity ratio exceeds 10^5 and time step starts to fall. My boundary conditions are ok as I checked them with my partners and the values are right. Which could be my problem? Mesh? Schemes?... Thanks 

March 13, 2020, 07:49 
Timestep

#2 
Senior Member

Fire spread rate is usually quite high. Hence, even with a very coarse mesh, 0.15 s is quite large value for timestep. You have to use a smaller timestep. You can certainly increase the timestep by a factor of 2 but doing that every timestep is not a good idea. Use an increase factor of 1.1 or maximum 1.2 if you want it to increase with each timestep. A reduction factor of 0.9 would do good. Fix the maximum value such that the CFL does not go beyond 10.
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March 13, 2020, 08:14 

#3  
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Daniel Herranz
Join Date: Feb 2019
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Quote:
So you are basically saying that my maximum time steps has to be in the order of 10^2? Or are you saying that a growth rate of 2 for time step is too high? I do not think the growth rate is the problem,as it goes to 0.15 and stay there for more than 6 or 7steps, I think it is more related to 0.15 being too high, which is a real problem for me. Thanks for your answer 

March 13, 2020, 08:22 
Growth Rate and Final TimeStep

#4 
Senior Member

If the objective is not to observe the initial spreading rate but the spreading extent over 1200 s, then the growth rate is not that important. However, if that is of concern, then a growth rate of 2 is rather high. Final timestep is certainly high and that's what causes the trouble with numerics.
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