Learning Forum

[scabo]

Hi-

I have read the User guide. There are 2 things: In the main discrete dialogue box, there is an option for particle time step size. It is to inject particles in domain. Does it also control the particle trajectory integration size? There is another option to set length scale and step length factor in Tracking Parameter box. In user guide, it is mentioned that this controls the trajectory integration size. So is the first one only to inject particles in the domain? This is slightly confusing as the term particle time step size is used in both the cases. Any clarification is helpful, thanks.

[Rob]

Time step controls the injection but also how long to track each cycle of particle updates. The max steps, step length factor etc is then how many integration steps are done during the time step. So incomplete tracks can be due to too long a time step relative to the number of max steps or step length factor. 

[scabo]

So you are saying for each particle time step, there are these max no of steps and step length factor? But the particle integration time step size is controlled by step length factor right?(length/particle velocity)?

[Rob]

Probably.

[scabo]

Hi-There is a Define_dpm_timestep UDF. Is this to control the particle injection frequency or control the trajectory integration time step? I now realise that it may be the former. I have attached the screenshot from userguide. Can someone please clarify this.

[Rob]

Time step for injection and tracking updates. 

[scabo]

So it is not the particle trajectory integration step size? 

[Rob]

Doesn't look that way. 

[scabo]

Okay but in the Fluent UDF User Manual 2025 R2 page 267 this UDF is used to limit the particle time step size to one fifth of particle relaxation time. I think this indicates that this is the time step used in particle trajectory rather than injection settings? Shall I forward the picture of the page?

[Rob]

The time set in the DPM panel is used by the injection AND to track a parcel update. So if it's set as 1s new parcels are added every second and each parcel is tracked for the shortest of one second or until the max steps are reached.  

[scabo]

Yes but the udf define_dpm_timestep is for particle integration step. This is because fluent udf manual uses it like that as mentioned in my last message. Am i correct?

[Rob]

Try it and see.