particle time step size, number of time steps in DPMPM
 

hi
i want to solve a DPM problem with unsteady particle tracking. in discrete phase model panel what is the "particle time step size (s)" and "number of time steps"?
is "particle time step size" related to length scale/step length factor? I have been confused because I dont know how fluent track particles unsteady in an steady contineous flow field:(
please help.

Dear Leila,

The particle trajectories are computed with lagrangian approach; i.e., particle paths are computed via unsteady integration here which obviously needs time step size and number of them but there is a restriction here that a particle cannot jump over a cell in its path. you can implicitly set time step with specifying length scale or step length factor which is a successive number of iteration that particles remain in one cell; you can also deactivate automation and provide time step size explicitly which suggested in specific goals.

Bests,

yes, that's right! the current cell (computational cell) is the current control volume the particle is there.

Quote:

Originally Posted by lili (Post 359527) And I want to know in discrete phase model panel what is the difference between length scale/step length factor (bellow the tracking parameters) which control time step size and "particle time step size" (bellow particle treatment) which appears when unsteady particle tracking is ticked.

These are 3 different ways for setting particle time step in steady/unsteady tracking. (refer to the unsteady particle tracking section of manual)

Quote:

Originally Posted by lili (Post 359527) Also in particle treatment field what does "number of time steps" means? In help said "When you increase the Number of Time Steps, the droplets penetrate the domain faster"???

yep, that's the number of particle advancement in the domain. (number of integration)

Bests,

Dear Amir
Thanks for your helpful guide. but I cant fully understand the procedure fluent uses to solve a coupled problem in which contineous phase is steady and particle tracking is unsteady. I think fluent first solves the contineous flow field and then in t=0 injection and particle tracking occurs then depend on "Number of contineous phase iteration per DPM iteration" some iteration for contineous phase are done and then again injection. but how long is each injection? I think it is "particle time step size"*"number of time steps" am I right?and another question: what happens if "update DPM sources every flow iteration" is ticked?

Exactly, but note that these are not necessarily new injections! these can be advancements of previously injected particles.
The effects of particles on flow field are computed more realistically in each time step but in steady flow fields it can just affect the required time for convergency.

Bests,

hi
yani age "update DPM sources every flow iteration" tick bekhore convegence behtar mishe? ye soale dige: shoma az openfoam chizi midunid? nesbate be fluent che mazaya va eibhaee dare?
thanks

Dear Leila,
Generally, It can reduce convergence time but it may cause other issues.
yes, I've done few projects with it specially in viscoelastic field. it's not very user friendly but you'll like it at last. It's not fair to compare openFOAM with FLUENT :D. OpenFOAM is much more powerful that FLUENT in all aspects and it's also open source.

Bests,

Dear Amir and Leila ,
The above mentioned clarifications helped me a lot in understanding lots of issues related to the DPM , however i still have some concerns , kindly bear with the following :
- If my injection flow rate is 1.5 g/s and particle time size is 0.001 (unsteady tracking ) No. of time size =1 i.e. the pulse of the injection will be short (1*0.001 ) the start and stop time of the injection was set to 0 -300 sec. and the no. of particles is 60 droplets

1) Fluent at each DPM injection is injecting the 1.5E-3 which is 1.5 *0.001 and this no. is not increasing , why ???

2) As you said (Amir ) the Update DPM option is really affecting the convergence time , but what i don't understand is the relation btw that option and the famous graph of URF interphase exchange terms

3) in the best practice guide : they are saying to reduce the URF of DPM and increase the no of continous iterations per DPM iterations , which is totally contradictory with the 2 way couple strategy and to the above mentioned graph . i.e for the results to take effects URF of DPM should be increased and the no. of iterations to be reduced

Can you please advise , since i can tell that you are seniors in the DPM sub-model
Thanks in advance

aamir sir can you please tell me what is exact meaning of stop time in the set injection properties in the fluent? for micron size water droplet what should be its value? is it really affecting the simulation results?

Hi,

Actually these variables are described in the manual if you can take a look. You can set how the particles are injected in your domain. This happens between "start time" and "stop time"; this is a physical BC regardless of the particle type!

Bests,

Thank you sir for your quick reply.
Sir my problem is evaporation of 10 micrometer droplet in air (continuous phase). So i did "injection type" as a 'surface' with injection from velocity inlet BC. "the particle type" I have taken as 'inert' and "material" as 'water liquid' then i set point properties. Have set problem correctly? In theory guide of fluent about 'start' and 'stop times' they gven Injections with start and stop times set to
zero will be injected only at the start of the calculation ( t=0).' still not clear the meaning.
Thank you sir

I don't have any idea regarding the evaporating particles. I guess you need to change the type to droplet not inert.
In an unsteady flow, the particles can inject over time with a specified time step. For instance, if you set "start time=5" and "stop time=10", injection of particles starts at t=5 s and continues until t=10 s with specified particle time step.

Bests,

Amir sir, from your previous posts I understood the exact meaning of 'particle time step size' and 'number of time steps'. About maximum number of steps under tracking parameters I know is 'step length factor * number of control volumes (elements)=maximum number of time steps. Which are these steps? Is there any relationship between particle time step size and maximum number of time steps?
Also suppose in set injection properties we set 'start time=0 sec' and 'stop time =10sec' and if 'particle time step size is 0.1 s' then does it mean 100 iterations per control volume.
In fluent DPM, (for 10 micron droplet case) during iterations , after certain number of continuous phase iterations, there will be 'advancing DPM injections' , so injecting suppose 570 particles with certain mass, is it mean by mass of 570 droplets?

Thank you sir.

Actually, what we need in tracking particles are step size and number of steps. When you're setting the particle time step, the step size is specified and the number of steps would be set by considering the current flow field time. However, the time step can be changed if you have decreased the max number of steps as a controlling criterion.
Of course not! Injecting and tracking are completely 2 different concepts!
Depends on the particle properties. If they are droplets, the mass of 570 droplets will be added.

I guess it would be better to take a look at the manual sec. 22.

Bests,

Thank you sir,
Can you please tell me which manual ? I have 3 guides, user, theory and tutorial of ansys fluent 14.0 but any of these sec.22 is not related with DPM.

Regards

I mean the user guide, DPM section. In the manual of FLUENT 6.3, it's sec. 22.

Bests,

Hi Amir Sir

Sir, if I treat the particles in steady fashion, then does it mean that discrete phase equations which are ODEs with time as independent variable will not be solved?

Also in my cooling of heat sink model with water droplets of 10 micrometer, if I reduce the number of continuous phase iterations then domain is overcooled if I increase those it will not happen, so can say that this particular number will directly proportional to certain region of domian (CV) where evaporation will happen because of particle tracking.....
Sir can you suggest me some books or material regarding DPM as from manual I could not clear totally the processes happening in DPM

Regards

No! The DPM equations are solved in unsteady fashion, but they can be considered in steady or unsteady flow fields. These two are different.
Nope. In each numerical analysis, you should assure the iterations are quite enough and increasing them any further wouldn't change the results. I don't see any proportion ....
You can take a look at any text book in this field; aerosols. For instance, this one:
"Aerosol Technology, William C. Hinds"

Bests,

Hi sir

In my heat sink cooling problem actually I want relative humidity of 15% at the exit of the sink but I am getting it 7.42% , I tried with different combinitions of particle time step size and number of time steps. In this particular process I have given mass flow rate of 4.22e-7 and stop time of injection as 100s for every combinition, But still my rate of evaporation and hence RH is not increasing. (In my problem the relative reynolds number is 0, air velocity is 1 m/s). Can you suggest me a way to resolve the problem?

Regards
Ruturaj

Hi Amir sir

In my heat sink cooling problem actually I want relative humidity of 15% at the exit of the sink but I am getting it 7.42% , I tried with different combinitions of particle time step size and number of time steps. In this particular process I have given mass flow rate of 4.22e-7 and stop time of injection as 100s for every combinition, But still my rate of evaporation and hence RH is not increasing. (In my problem the relative reynolds number is 0, air velocity is 1 m/s). Can you suggest me a way to resolve the problem?

Regards
Ruturaj

Hi amir sir

Actually I am able to validate results of one of paper by using DPM in FLUENT. I drawn geometry with symmetry boundary condition, but I was giving total mass flow rate through duct. But that mass flow rate must be half. So with excess mass my sink was getting overcooled. Because of your previous quick replies, I was able to understand various terms in fluent physically. Thanks a lot.

Particle status for DPM iteration
 

Hello

I have read the Fluent User guide and also have read the posts regarding this but I am not able to comprehend the unsteady particle tracking of Fluent. I am running unsteady simulations for continuous phase and also want to have unsteady particle tracking. In the particle treatment I choose unsteady and I also choose to inject particles with fluid flow time step. Till here its good but now it starts to get messy.
Firstly, if I want to inject particles at fluid flow time step why does it still asks about particle time step size and what is this parameter exactly?
Secondly, the time step size of my continuous phase is 1e-5 and I keep my particle time step size as 0.0001. When I run the simulation, Fluent recognizes the number of parcels but the status for the parcels it prints on the console is wierd.
Updating solution at time level N... done.
Injecting 31360 particle parcels with mass 5.375e-09 at t = 0

DPM Iteration ....
number tracked = 31360, escaped = 0, aborted = 0, trapped = 0, evaporated = 0, incomplete = 0, incomplete_parallel = 0

My question why the status of the particle is zero?

When I try to reduce the time step size of the continuous phase to 1e-4, the same 31360 particle parcels are tracked and gives proper status (in my case they are droplets and should evaporate).
The mass flow rate of the DPM phase is 0.5375 g/s and I inject 31360 particle parcels from injection file.

Dear Amir,

I want to inject a fluid in my flow inside a nozzle that has gas in it, I used plain atomizer model. I am not sure about the diameter. What diameter should I choose?

particle time step size
 

hi every body, I have benefited from these fruitful discussions, and I reviewed the common resources those are explaining the DPM, and my convictions as follow
1- either you used unsteady or steady particle tracking, there will be a time advancing for the particle to find the trajectory. don't forget that the particle ODE's independent variable is the time(t).therefore, the 'steady' term does not mean the cancelling out of the time derivative.
2- the main difference between the unsteady or steady treatment of the particles is the timing of DPM source update to the continuous phase governing equations.
3- I think that the particle time step size and the number of time steps below the particle treatment are related to when should the DPM sources be updated to the continuous phase equations.

hi every body, I have benefited from these fruitful discussions, and I reviewed the common resources those are explaining the DPM, and my convictions as follow
1- either you used unsteady or steady particle tracking, there will be a time advancing for the particle to find the trajectory. don't forget that the particle ODE's independent variable is the time(t).therefore, the 'steady' term does not mean the cancelling out of the time derivative.
2- the main difference between the unsteady or steady treatment of the particles is the timing of DPM source update to the continuous phase governing equations.
3- I think that the particle time step size and the number of time steps below the particle treatment are related to when should the DPM sources be updated to the continuous phase equations.