# Lagrangian solver

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 July 26, 2022, 17:55 Lagrangian solver #1 New Member   Emad S Join Date: Dec 2021 Posts: 14 Rep Power: 4 Hi all. Since we define parcels as a number of particles, why do we have to define clouds? Thanks Emad

 July 26, 2022, 20:19 #2 New Member   Emad S Join Date: Dec 2021 Posts: 14 Rep Power: 4 Also - In discrete phase method (DPM) the forces are solved for parcels or particles or cloud in dpmfoam? - if nParticle=1 are dpm and dem exactly the same?

 July 27, 2022, 10:39 #3 New Member   Emad S Join Date: Dec 2021 Posts: 14 Rep Power: 4 And anothe question: I have defined stick parameter between the particles and the wall. When a particle sticks to a wall the whole cloud stickes or the parcel including that particle or just the individual particle?

July 29, 2022, 11:00
#4
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Josh Williams
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Quote:
 Originally Posted by emadsiadati Hi all. Since we define parcels as a number of particles, why do we have to define clouds? Thanks Emad
A cloud is a collection of parcels/particles. A parcel is a collection of particles. If number of particles in a parcel is one, then a cloud is simply a collection of particles.
Yes, if number of particles in parcel is one, then DPM = DEM. Otherwise, it is same as 'coarse DEM'.

July 31, 2022, 09:48
#5
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Quote:
 Originally Posted by joshwilliams A cloud is a collection of parcels/particles. A parcel is a collection of particles. If number of particles in a parcel is one, then a cloud is simply a collection of particles. Yes, if number of particles in parcel is one, then DPM = DEM. Otherwise, it is same as 'coarse DEM'.

How the diameter of the cloud is defined?
And do parcels leave a cloud?

August 11, 2022, 04:35
#6
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Josh Williams
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Quote:
A cloud does not have a diameter. A particle has a diameter. I am not sure if a parcel has a diameter.

Parcels do not leave a cloud. Particles do not leave a parcel (I think).

August 16, 2022, 12:37
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Quote:
 Originally Posted by joshwilliams A cloud does not have a diameter. A particle has a diameter. I am not sure if a parcel has a diameter. Parcels do not leave a cloud. Particles do not leave a parcel (I think).
Then what is the criterion for assigning the particles to a parcel and the parcels to a cloud?

If a cloud does not have a diameter then the forces like drag force which is a function of diameter is calculated for each parcel if it has a diameter, otherwise calculation is conducted over each particle!

What is the rationale behind defining clouds and parcels then?! We are just doing dem imo!

August 16, 2022, 13:18
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Josh Williams
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Quote:
 Originally Posted by emadsiadati If a cloud does not have a diameter then the forces like drag force which is a function of diameter is calculated for each parcel if it has a diameter, otherwise calculation is conducted over each particle!
You are not getting what a cloud is. A cloud is just the collective term for the particles or parcels in your simulation. The term cloud is pretty much meaningless. It is just openfoam terminology. Not sure how else to explain it.... Think of a cloud in the sky, made of, lets say, billions of water droplets. The motion of the cloud is not determined by calculating the drag force based on the diameter of a cloud (it would be ludicrious, the cloud to particle size ratio is probably like 1e20 metres, also at a macro-scale I guess you would consider it a highly porous medium, but that is not relevant). You calculate forces on each particle.

In OpenFOAM, you define the diameter of one particle. You define how many particles are in one parcel. You can calculate the forces using the particle diameter. Lets say you use parcel approach like MPPIC, you calculate drag based on particle diameter (p.d() in the source code). Then you apply some collision kernel (like Harris-Crighton in tutorials), which is based on the number of parcels in each cell, the number of particles in a parcel, and the individual particle diameter.

If you have further difficulty, I suggest you review the source code
Code:
`cd \$FOAM_SRC/lagrangian/intermediate/parcels/Templates/KinematicCloud`
and source code for calculating drag
Code:
` cd \$FOAM_SRC/lagrangian/intermediate/submodels/Kinematic/ParticleForces/Drag/SphereDrag`

Last edited by joshwilliams; August 17, 2022 at 11:50.

 August 17, 2022, 11:08 #9 New Member   Emad S Join Date: Dec 2021 Posts: 14 Rep Power: 4 Thanks! This was very illustrative.