Learning Forum

[Andrew Roberts]

Hi,

 

I am looking at this:

 

• I wondered if you could tell me the equivalent timestep and number of timesteps (unsteady settings) for the tracking parameters, if I set max. number of steps = 50,000 and step length factor = 5 (steady settings)?

Alternatively:

• What is the meaning of the step length factor = 5? Is it the same as a maximum cell-based Courant number = 0.2?
• What is the meaning of max number of steps - is this like the number of timesteps where the timestep is the one limited by the maximum cell-based Courant number?

[Rob]

It's all covered in the User's Guide and/or Theory Manual. Basically, those settings are to govern how many times (maximum) the particle trajectory & position are checked/updated per cell and in total. The Numerics are also responsible for the number of times it's checked per cell to make the calculation more efficient: I suggest not messing with most of those without a good understanding of what they mean. 

[Andrew Roberts]

Thanks for your reply. As far as I understand it, this is assuming a steady state condition (because unsteady is not checked).

If it is checking like Courant Number: what is the timestep that would be used? (is this set to unity) - I couldn't find the answer in the guides. So the particle is checked N times (step length factor) according to the ratio of the cell velocity and cell size only?

 

[Rob]

More or less, the trajectory (using the above panel) would be checked up to 5 times in a cell, probably less if accuracy control determines not much is happening.  https://ansyshelp.ansys.com/account/Secured?returnurl=/Views/Secured/corp/v231/en/flu_ug/flu_ug_sec_discrete_use_oview.html

After 50,000 checks the parcel would be shown as incomplete if still in the domain. 

[Andrew Roberts]

So, it doesnt have synchronous physical time, in other words it is possible for the particles to exist at different points in time within the same domain?

Step length factor = Number of Lagrangian steps per Eulerian step (cell size)

Physical Lagrangian time (seconds) could be anything between 1 and 50,000 x (velocity/cell size) x step length factor

[Rob]

With steady the total trajectory is what is seen. The particle has a flight time, and can exchange mass/energy/momentum with the cells. Hence the volume fraction limitations.