Tang, the random walk model takes into account the dispersion effect of instantaneous turbulent velocity fluctuations on the particle trajectory through the use of stochastic methods. So each time you run the simulation each particle takes a little different trajectory in turbulent flow. In order to know the tendency of the movement of the particles, you must analyze many particles to see their possible trajectories.

Hello Orlando,

Thank you so much for your response.

I also think random walk model will affect the particle trajectory， So when I discover the problem， I turn off the random walk. I want to see the single particle trajectory, so I don't select the option of interaction with continuous phase, only consider fluid acts on particle. when the flow field converged, I get the result. But I let the iteration continues iterating several steps(maybe about 10-50 steps), I will get another result, particle will enter different vortex regions, So I am confused.

Thanks a lot for your response and hope to hear from you ahead.

Thanks again,

Tang.

Hi Tang Ye,

Could you please check whether all or 90% of the particles injected at the inlet are coming out of the cfd domain or not ? If not you need to increase the maximum number of steps used to track the particle.

Could you please explain the various boundary conditions used and also the your problem set up in detail.

Also, FYI if you switch off the interaction with continuous phase, you are solving one way coupling and the results may not be accurate.

Thanks & Regards,

Rajesh.

Hello Rajesh Alayil,

Thank you so much for your response.

Almost all particles injected at the inlet come out of outlet, But a few particles

are incompleted， but the number of incompeted particles is always change with the iteration going on，I  also confused why the number always changes， as shown in the picture.

My settings:

1、fluid：steady,k-e turbulence model， pressure inlet ， presure outlet.

2、DPM：steady track, interaction with continuous phase，the maximum number of steps ：100000 ， step lenth factor：5，surface inject（inlet）， material： sand， diameter distribution：r-r， V：z-velocity0.1m/s， flow rate 7.65×10-7kg/s Diameter：min0.01，max0.125 mean：0.05， spread parameter 3.5 discrete random walk model , number of tries 3.

And I turn off the the interaction with continuous phase, only when the particles are injected by point, because I want to see the single particle trajectory. I think single particle may almost have no influence fluid.

But both surface inject and point inject the particle trajectory are alway changes with the iteration going on, particle will enter different vortex regions, and the particle residence time alway changes,So I am confused.

Do you have any problem？

Thanks a lot for your response and hope to hear from you .

Thanks again

Tang.

Fluent monitors each particle position, but there are rules for that, on DPM settings you will find track parameters, there you will define how long in time or distance Fluent will follow the particle. Each iteration Fluent verifies what particle does not meet this condition and them consider it incomplete, discarding it from tracking list.

Do not forget to check the wall behavior on dpm condition, you may have to consider it as wall film, escape condition, trapped (useful to register the amount of particles that reach a monitored surface. This is important to correct simulate the event, once reflecting particle may not be the correct condition (default)

Hello Julio Cesar Lelis Alves ,

Thank you so much for your response.

1、I think your topic is about unsteady track.

When using steady track, I think each iteration Fluent will calculate the movement

differential equations of particles in the whole calculation domain, and then get

the particles' trajectory. I'm not sure if what I say is correctly.

2、I set wall as reflect condition. It is set up according to the actual situation.

In fact, I want to simulated particle accumulation in the domain. Do you have any suggestions?

Thanks a lot for your response and hope to hear from you ahead.

Thanks again,

Tang.

Hi Tang Ye,

Settings looks good. Could you please try reducing the URF's, reduce the one for DPM source term to around 0.05 and see whether its having any influence. Also what are your solver setting, i mean flow,pressure and turbulence  discretizations schemes that you are using.

Thanks & Regards,

Rajesh.

Hello Rajesh Alayil,

Thank you so much for your response.

I set the URF for DPM source term to 0.3. This value which the default setting is 0.5, So 0.05 is it too small for this setting? Other solver settings are shown in the picture!

Do you have any other suggestions?

Thanks a lot for your response and hope to hear from you ahead.

Thanks again！

Tang.

Hi Tang,

Yes 0.05 is too small. But sometimes you need to lower the source term urf  fro smother convergence. I have used it and it worked.

Thanks & Regards,

Rajesh.

To see particle accumulation, if you want it by absolute numbers, I found a way but you have to work a little. First you have to access the iteration trace in a txt file. If you can not, this is the end. Otherwise, you must setup the region of accumulation as trapp, the trace file will show how many particles are hold there, in Fluent there is an output that gives to you the density of particles near this area, so... it is just a matter of multiplication. Total particles trapped x density