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May 5, 2012, 04:55 
particle time step size, number of time steps in DPMPM

#1 
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leila
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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. 

May 5, 2012, 08:31 

#2 
Senior Member

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,
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Amir 

May 6, 2012, 03:05 

#3 
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leila
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Dear Amir
As I understand from the help the "length scale" is the distance that the particle will travel before its motion equations are solved again and its trajectory is updated and the "step length factor" is the number of time steps required to traverse the current continuous phase control volume. So why you say "cell"? 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. 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"??? 

May 6, 2012, 07:07 

#4  
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Quote:
yes, that's right! the current cell (computational cell) is the current control volume the particle is there. Quote:
Quote:
Bests,
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Amir 

May 8, 2012, 02:10 

#5 
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leila
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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? 

May 8, 2012, 15:06 

#6  
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Quote:
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Bests,
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Amir 

May 9, 2012, 01:03 

#7 
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leila
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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 

May 9, 2012, 10:01 

#8  
Senior Member

Dear Leila,
Quote:
Quote:
Bests,
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Amir 

November 21, 2012, 14:27 

#9 
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Mina
Join Date: May 2012
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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 1.5E3 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 submodel Thanks in advance 

January 24, 2014, 08:42 

#10 
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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?


January 24, 2014, 09:51 

#11  
Senior Member

Quote:
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,
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Amir 

January 24, 2014, 13:34 

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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 

January 24, 2014, 15:51 

#13  
Senior Member

Quote:
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,
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Amir 

February 27, 2014, 07:43 

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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. 

February 27, 2014, 11:39 

#15  
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I guess it would be better to take a look at the manual sec. 22. Bests,
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Amir 

March 1, 2014, 02:41 

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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 

March 1, 2014, 04:34 

#17  
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Bests,
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Amir 

March 17, 2014, 03:53 

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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 

March 17, 2014, 06:00 

#19  
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You can take a look at any text book in this field; aerosols. For instance, this one: "Aerosol Technology, William C. Hinds" Bests,
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Amir 

April 14, 2014, 09:31 

#20 
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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 4.22e7 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 

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