how to define injection timing in DPM injection with unsteady particle tracking ?

pkp · March 15, 2016, 17:27

Hello there,
I am using unsteady particle tracking in the DPM injection model. The spray is injected into the continuous fluid (Air). How can I switch on spray for some specific time (eg. for 5 seconds) ?
What is the physical significance of "Particle time step size (s)" and "Number of time steps" in unsteady particle tracking ? How can we calculate these parameters ?

Thanks
Presteina

CeesH · March 16, 2016, 06:01

Hi pkp,

If you set up an unsteady particle track simulation, you need to set a start time and stop time. These determine how long your particle injection runs; if you want to inject for 5 seconds, set start to 0 and stop to 5 seconds.

Regarding particle timestep size; you can move particles at a different timescale than the fluid (which you set at Run calculation). For example, you can do 10 particle timesteps per fluid timestep, or vice versa. You can also select how often you want to inject particles; so if your fluid timestep is 0.1s and particle timestep 0.01s, then choosing particle timestep injects 10 times more particles.

pkp · March 20, 2016, 19:50

Thank you sir for your reply.

I got the point about the injection start time and stop time.
About the time step problem : I have continuous fluid as air (Steady state) and particle tracking is unsteady. I am giving a mass flow rate for the injection (2.33e-3 kg/s). But when injection starts in fluent, It keeps on injecting for so many times. It keeps on saying that injecting 39 particle parcels of mass 2.33e-6.
I dont know how to control this factor ? How does this injection mass is calculated ?
Please help me with the problem. Thank you so much in advance.

`e` · March 20, 2016, 23:47

From another thread:

Quote:

Originally Posted by `e`

Particles are injected at each time step (within a specified range) for unsteady DPM cases. If you only want to track a single particle then set the start and stop times at 0 s. The mass flow rate is arbitrary (assuming one-way coupling); although I believe it's inversely related to the fluid time step (for example, dt = 0.1 s would need a mass flow rate 10 times the mass of a single particle to give one particle in the parcel).

For your case, the mass flow rate sounds like an important parameter. What fluid time and particle time steps are you using and what are the start/stop times?

pkp · March 21, 2016, 13:17

Hello Sir,
I am working on a conceptual design. So I don't have specified data for that. But for initially, I want to inject the fluid in to the chamber with mass flow rate of 2.33e-3 kg/s for 5 seconds. I am not clear with the time step method ( as I am new to the unsteady problem), so I am using time step size as 0.01 (s) and number of time steps as 500. That's totally arbitrarily.
Can you please explain me, how to set the time step size and number of time steps for injection of 5 s at the mentioned flow rate ?

Thank you so much in advance. I really appreciate your help.

Saptarshi · April 26, 2018, 18:21

Hi everyone, I want to solve steady flow for continuous phase and unsteady flow for discrete phase. I am not able to understand the physical significance of 'particle time step size'. Whatever I understood that if I set length scale or step length factor, then fluent will calculate integration time step size. Then what is "particle time step size" and "number of time steps". I already read some earlier threads regarding this, but couldn't understand properly. Please help me. Thanks in advance

ali.faghani · December 26, 2020, 18:40

Hi Is it possible to simulate a nanofluid (zero velocity) using the Discrete Phase Model(DPM) at Ansys Fluent? Could you please help me? When the Nanofluid enteres the tube at a certain velocity, I can inject the particle into the tube as <surface> and take its velocity equal to the velocity of the base fluid, and simulation will occur. But at zero Velocity i dont know how to inject the particles Regards

pakk · December 27, 2020, 05:11

I did not do this for a while, but I thought you can just choose velocity to be 0, does that not work for you?

ali.faghani · December 27, 2020, 06:08

I haven't tried it. and i don't know how to make a <file> to inject particle.
Do you have any sample file?
As you know in this problem we don't specify mass flow rate because it doesn't have velocity. But i have a certain Volume fraction of particles
Can we define Volume fraction in file ? or something similar Volume fraction

fatma123 · December 16, 2021, 05:34

I am using unsteady particle tracking in the DPM injection model. The spray is injected into the continuous fluid (Air). How can I switch on spray for some specific time (eg. for 5 seconds) ?
What is the physical significance of "Particle time step size (s)" and "Number of time steps" in unsteady particle tracking ? How can we calculate these parameters ?

March 15, 2016, 17:27	how to define injection timing in DPM injection with unsteady particle tracking ?	#1
pkp New Member Presteina Join Date: Feb 2016 Location: Canada Posts: 16 Rep Power: 10	Hello there, I am using unsteady particle tracking in the DPM injection model. The spray is injected into the continuous fluid (Air). How can I switch on spray for some specific time (eg. for 5 seconds) ? What is the physical significance of "Particle time step size (s)" and "Number of time steps" in unsteady particle tracking ? How can we calculate these parameters ? Thanks Presteina

March 16, 2016, 06:01		#2
CeesH Senior Member Cees Haringa Join Date: May 2013 Location: Delft Posts: 607 Rep Power: 0	Hi pkp, If you set up an unsteady particle track simulation, you need to set a start time and stop time. These determine how long your particle injection runs; if you want to inject for 5 seconds, set start to 0 and stop to 5 seconds. Regarding particle timestep size; you can move particles at a different timescale than the fluid (which you set at Run calculation). For example, you can do 10 particle timesteps per fluid timestep, or vice versa. You can also select how often you want to inject particles; so if your fluid timestep is 0.1s and particle timestep 0.01s, then choosing particle timestep injects 10 times more particles. fruitkiwi, pkp and fatihgulec like this.

April 26, 2018, 18:21	DPM time step	#6
Saptarshi New Member Join Date: Sep 2015 Posts: 4 Rep Power: 11	Hi everyone, I want to solve steady flow for continuous phase and unsteady flow for discrete phase. I am not able to understand the physical significance of 'particle time step size'. Whatever I understood that if I set length scale or step length factor, then fluent will calculate integration time step size. Then what is "particle time step size" and "number of time steps". I already read some earlier threads regarding this, but couldn't understand properly. Please help me. Thanks in advance

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March 20, 2016, 19:50		#3
pkp New Member Presteina Join Date: Feb 2016 Location: Canada Posts: 16 Rep Power: 10	Thank you sir for your reply. I got the point about the injection start time and stop time. About the time step problem : I have continuous fluid as air (Steady state) and particle tracking is unsteady. I am giving a mass flow rate for the injection (2.33e-3 kg/s). But when injection starts in fluent, It keeps on injecting for so many times. It keeps on saying that injecting 39 particle parcels of mass 2.33e-6. I dont know how to control this factor ? How does this injection mass is calculated ? Please help me with the problem. Thank you so much in advance.

March 21, 2016, 13:17		#5
pkp New Member Presteina Join Date: Feb 2016 Location: Canada Posts: 16 Rep Power: 10	Hello Sir, I am working on a conceptual design. So I don't have specified data for that. But for initially, I want to inject the fluid in to the chamber with mass flow rate of 2.33e-3 kg/s for 5 seconds. I am not clear with the time step method ( as I am new to the unsteady problem), so I am using time step size as 0.01 (s) and number of time steps as 500. That's totally arbitrarily. Can you please explain me, how to set the time step size and number of time steps for injection of 5 s at the mentioned flow rate ? Thank you so much in advance. I really appreciate your help.

December 26, 2020, 18:40		#7
ali.faghani New Member alifaghani Join Date: Dec 2020 Posts: 4 Rep Power: 6	Hi Is it possible to simulate a nanofluid (zero velocity) using the Discrete Phase Model(DPM) at Ansys Fluent? Could you please help me? When the Nanofluid enteres the tube at a certain velocity, I can inject the particle into the tube as <surface> and take its velocity equal to the velocity of the base fluid, and simulation will occur. But at zero Velocity i dont know how to inject the particles Regards

December 27, 2020, 05:11		#8
pakk Senior Member Join Date: Nov 2013 Posts: 1,965 Rep Power: 27	I did not do this for a while, but I thought you can just choose velocity to be 0, does that not work for you?

December 27, 2020, 06:08		#9
ali.faghani New Member alifaghani Join Date: Dec 2020 Posts: 4 Rep Power: 6	I haven't tried it. and i don't know how to make a <file> to inject particle. Do you have any sample file? As you know in this problem we don't specify mass flow rate because it doesn't have velocity. But i have a certain Volume fraction of particles Can we define Volume fraction in file ? or something similar Volume fraction

December 16, 2021, 05:34		#10
fatma123 New Member fatma Join Date: Dec 2021 Posts: 2 Rep Power: 0	I am using unsteady particle tracking in the DPM injection model. The spray is injected into the continuous fluid (Air). How can I switch on spray for some specific time (eg. for 5 seconds) ? What is the physical significance of "Particle time step size (s)" and "Number of time steps" in unsteady particle tracking ? How can we calculate these parameters ?