screening distance of The Electrostatics Model

Alice Sun

I have a case containing 40000 particles (radius: 0.3mm) and I use the Electrostatics Model.

When I set the screening distance to 5Rmin and 10Rmin respectively, the motion of the particles is significantly different, and the average electrostatic force of the particles is very different (about 3 times). 

Why is there such a big difference? Shouldn't particles that are far away have little effect on the electrostatic force received by this particular particle? Isn’t the screening distance of 5Rmin large enough? According to my understanding, the results of 5Rmin and 10Rmin should be very close.

I have read the introduction of EDEM2023 and the reference of the eletrostatics model, but I am still confused.

 

Stephen Cole

Hi Alice,

 

It might be worth checking the charge of the particles and expected force vs time with the settings you have.  There should be a big drop of in force with distance but we also don't want the screening distance to influence the results, so it could be that just with the high charge you need a larger distance.

Regards

Stephen

Stephen Cole

Hi Alice,

 

It might be worth checking the charge of the particles and expected force vs time with the settings you have.  There should be a big drop of in force with distance but we also don't want the screening distance to influence the results, so it could be that just with the high charge you need a larger distance.

Regards

Stephen

Alice Sun

Stephen Cole_21117 said:

Hi Alice,

 

It might be worth checking the charge of the particles and expected force vs time with the settings you have.  There should be a big drop of in force with distance but we also don't want the screening distance to influence the results, so it could be that just with the high charge you need a larger distance.

Regards

Stephen

I set the charge of the particles to always be 0.01nC per particle

How do I set the screened distance? Should I keep increasing the screened distance until the electrostatic force on the particles is not affected by the screened distance?

 

Stephen Cole

Alice Sun said:

I set the charge of the particles to always be 0.01nC per particle

How do I set the screened distance? Should I keep increasing the screened distance until the electrostatic force on the particles is not affected by the screened distance?

 

Hi Alice,

 

If the charge value is within the range you would expect then yes, I would run a test to check the screening distance isn't influencing the results.  Or potentially check the same via a hand calculation to check the expected force vs distance.

Regards

Stephen

Alice Sun

Stephen Cole_21117 said:

Hi Alice,

 

If the charge value is within the range you would expect then yes, I would run a test to check the screening distance isn't influencing the results.  Or potentially check the same via a hand calculation to check the expected force vs distance.

Regards

Stephen

Does this screened distance have anything to do with the cutoff distance usually used in calculating electrostatic forces? Because the cutoff distance usually used for calculating electrostatic force is set to 3-4 particle radii, not more than 5. So is it possible that the screened distance should not be set too large?

Stephen Cole

Alice Sun said:

Does this screened distance have anything to do with the cutoff distance usually used in calculating electrostatic forces? Because the cutoff distance usually used for calculating electrostatic force is set to 3-4 particle radii, not more than 5. So is it possible that the screened distance should not be set too large?

Hi, my understanding is that this would be the same, the screening distance is the distance at which no force is calculated.  This is purely to reduce computational effort so reducing this does reduce the accuracy of the model.

Regards

Stephen

Alice Sun

Stephen Cole_21117 said:

Hi, my understanding is that this would be the same, the screening distance is the distance at which no force is calculated.  This is purely to reduce computational effort so reducing this does reduce the accuracy of the model.

Regards

Stephen

Thanks for your patience.

But how to check the screened electrostatic forces via a hand calculation? Hand calculations only give an approximate value of the electrostatic force between two particles, how does this determine the value of the screening distance I should use?

 

Stephen Cole

Alice Sun said:

Thanks for your patience.

But how to check the screened electrostatic forces via a hand calculation? Hand calculations only give an approximate value of the electrostatic force between two particles, how does this determine the value of the screening distance I should use?

 

Hi Alice,

 

It wouldn't give you a direct value for screening distance but it should give you an indication.  If after a distance of X the force screened out is only a very low percentage of the overall it allows you to determine the accuracy you would like in your case.

Regards

Stephen

Alice Sun

Stephen Cole_21117 said:

Hi Alice,

 

It wouldn't give you a direct value for screening distance but it should give you an indication.  If after a distance of X the force screened out is only a very low percentage of the overall it allows you to determine the accuracy you would like in your case.

Regards

Stephen

However, when I use 5R, 10R, 15R, 20R screening distances respectively, the average electrostatic force received by the particles (especially those near the container wall) varies greatly. My particles have a radius of 0.4mm and the container is a cylinder with a diameter of 25mm at the bottom and a height of 40mm. The range included in calculating the electrostatic force of the particles from 10R is already large.  Do I need to increase the screening distance to include the entire powder bed to make it work? This is too time consuming to calculate. But as soon as I change this screening distance, the electrostatic force calculation is very different again!