[Lapidus]

Hello,

Today I come to you with a question regarding Lagrangian particles: I would like to create a scene showing the trajectories of the particles. Usually this is done with the information contained in a track file, but this only works for steady sims. A simulation history file can be used to see the movement of the different particles in time, which is both useful and insightful. Yet, I am after the streamlines rather than the particles themselves.

I was thinking maybe a massless particle can be injected as kind of a seed for the streamlines, nonetheless I believe this would grant me the paths followed by the gas rather than the particles.

Anybody has experience creating the described scene? Thank you beforehand!

[FMDenaro]

Originally Posted by Lapidus

Hello,

Today I come to you with a question regarding Lagrangian particles: I would like to create a scene showing the trajectories of the particles. Usually this is done with the information contained in a track file, but this only works for steady sims. A simulation history file can be used to see the movement of the different particles in time, which is both useful and insightful. Yet, I am after the streamlines rather than the particles themselves.

I was thinking maybe a massless particle can be injected as kind of a seed for the streamlines, nonetheless I believe this would grant me the paths followed by the gas rather than the particles.

Anybody has experience creating the described scene? Thank you beforehand!

You are wrong in the theory, in the unsteady scenario the streamlines are different from the path-lines. You need to compute the streamlines at each time by definition of envelop of the velocity vector field. Then you can create a movie of the time-evolution of the streamlines from the collected data-base.

[Lapidus]

Originally Posted by FMDenaro

You are wrong in the theory, in the unsteady scenario the streamlines are different from the path-lines. You need to compute the streamlines at each time by definition of envelop of the velocity vector field. Then you can create a movie of the time-evolution of the streamlines from the collected data-base.

Thank you for the answer!
Can you dwell on in what way streamlines and path-lines differ for the transient simulation?
In order to gather the data of the paths followed by particles in time; can you elaborate on the way of doing so (envelope of the velocity vector field)?

Regards

[FMDenaro]

Originally Posted by Lapidus

Thank you for the answer!
Can you dwell on in what way streamlines and path-lines differ for the transient simulation?
In order to gather the data of the paths followed by particles in time; can you elaborate on the way of doing so (envelope of the velocity vector field)?

Regards

This is a topic for any student in its first-course of fluid mechanics. Read and study that in any fundamental textbook.

[Lapidus]

Originally Posted by FMDenaro

You are wrong in the theory, in the unsteady scenario the streamlines are different from the path-lines. You need to compute the streamlines at each time by definition of envelop of the velocity vector field. Then you can create a movie of the time-evolution of the streamlines from the collected data-base.

I have indeed had several fluid-mechanics courses over the years, yet, that is beyond the point.

Relevant to the thread is how to plot the path-lines followed by the Lagrangian particles. Since the calculation is Lagrangian, that information should be stored somewhere. In case of a steady sims, it is contained in the track file (in Star CCM+).

What exactly do you mean by the "definition of the envelope of the velocity vector field"? You seem knowledgeable about simulation and have probably encountered a similar situation in the past. Thanks!

[FMDenaro]

Originally Posted by Lapidus

I have indeed had several fluid-mechanics courses over the years, yet, that is beyond the point.

Relevant to the thread is how to plot the path-lines followed by the Lagrangian particles. Since the calculation is Lagrangian, that information should be stored somewhere. In case of a steady sims, it is contained in the track file (in Star CCM+).

What exactly do you mean by the "definition of the envelope of the velocity vector field"? You seem knowledgeable about simulation and have probably encountered a similar situation in the past. Thanks!

Again, such definitions are very basic.
The streamlines is a curve in the space that obeys the constraint


dr(t) x v(x,t) = 0 at each time t
+ initial condition


that is the vector field is always tangent along the whole curve. This is an Eulerian representation, you compute the streamline in a fixed volume.


The pathline is a curve in the space-time that obeys the governing equation



dx/dt = v[x(t),t]

+ initial condition


that is the locus of all the positions of a particle that starts from an initial position at t=0. This is a Lagrangian representation.


The numerical computation is quite simple, you can use Tecplot to create the movies.

[adrin]

Filippo has done an excellent job of explaining the difference! From a practical point of view, I don't use CCM+ or other commercial codes, so I don't know which buttons to push, so to speak, but plotting path lines seems fairly straightforward -> just save (and/or plot) the positions of (massless) particles that you would (continuously?) inject from a set of stations in the fluid. These particle positions, when displayed in an animation, would show the path lines. To repeat Filippo, there is no such thing as streamline in an unsteady flow (creating the streamlines for each fixed time and then displaying them as a time evolution doesn't really make sense either, though many researchers do this probably out of a habit of having worked on steady-state problems for such a long time)

adrin