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DPM time step

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Old   September 17, 2015, 03:33
Red face DPM time step
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Unnikrishnan T P
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I am doing a spray steady state simulation. Since it it steady state injection timing was given to be 0-1000seconds. And for unsteady particle tracking, particle time step was 0.001 and no of steps was 10. How can I provide these parameters in the right way? What do they mean exactly, I need an explained answer in a very basic way... pls help me out...
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Old   September 17, 2015, 15:01
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Jim
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Discrete particle Modelling is a Eulerian-Lagrangian model.
The fluid is Eulerian - the fluid's flow is discretised in the form of the mesh.
The particle is Lagrangian - its movement is discretised into time steps.
After it is injected, the particle is affected by the velocity of the fluid in the mesh element it is currently in. If the fluid is moving faster than the particle, it accelerates, if slower, it decelerates, and this applies all 3 directions, so the fluid also causes the particle to change direction.
For each time step, the particles next position is calculated based on the previous conditions of velocity, position etc. This continues till the particle makes the end of its particle track and evaporates, reaches the outlet, etc.

In Fluent, in the default settings particles take 5 time steps to traverse 1 mesh cell. You can increase this by increasing the "step length factor" (under models->DPM). And I think there's other ways of doing it in addition to this.
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Old   September 18, 2015, 02:37
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Unnikrishnan T P
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Thanks for the reply... But how can we know the correct step length factor? If I provide a value of 5 or 10 the solution will be different ri8? And how much this step length factor will affect the solution??
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Old   October 13, 2015, 07:34
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this is very important to the solution because you are changing something very fundamental - you're changing the size of the discretisation step. As you make this step smaller you get closer and closer to the true solution, the same is true of any numerical integration.

Also bear in mind that in fluent the "accuracy control" option can take priority over the "step length factor" option.

you could try the following:
turn off accuracy control. double the step length factor and re-run the simulation, and keep doing this until it has little effect on the final answer (similar to Grid independence study) ?
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Old   October 13, 2015, 14:25
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Unnikrishnan T P
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Thank you... I will try it...
I have another doubt...
Even though my problem is steady state, it is never becoming very accurately steady..
In the command window, it shows injecting 50 parcels but the no of parcels escaping at the outlet is varying between 45 and 55. I chose a surface monitor outlet which indicates the mass weighted average of the discrete phase at the outlet. After about 1500 iterations, the graph become constant, but it is still fluctuating little bit. that is, the mass weighted average is vs iteration graph is not a perfectly horizontal line even after 3000 iterations
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no of steps, particle time step, steady state dpm, unsteady tracking

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