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Streaklines, Streamlines, and Particle Paths


This 10-minute video tutorial will show you how to calculate streaklines, streamlines and particle paths using Tecplot 360.

What follows is a lightly edited transcription of the video.

Let’s first go ahead and load a solution. The solution we’re going to load is an unsteady solution of vortex shedding. We’ll look at first a few time steps and since this is in a solution that has a mesh that doesn’t change, we’ll just pull down, grab the mesh as well, add it to the list, and maybe we’ll just take the last couple of time steps. We’ll add these to the list and we’ll go ahead and open them.

You can see the progress bar at the bottom of the Tecplot 360 window. It’s reading through the additional solution times. Notice that it is setting up zones. It’s not loading any data. We’re merely going through and looking at all the time steps. We are trying to figure out if the mesh is moving, which in this case it is not. But if it were moving, it would start to assign zones. It’s looking for time step information that will be used to assign a time strand, which is important when we actually do the streakline calculation. It is not, repeat, not loading the data, which is good because we’re talking about a solution that is close to about 7 or 8 gigabytes, and it just would take a long time to load.

Once it’s done, you’ll see the image open. What you are seeing is the extent of the domain in the volume for each of the blocks in the solution. I’ll go to the Zone Style dialog and just select one. I press CNTL A, which selects all the zones.

The surfaces that we’re going to plot are the J surfaces. If you right click and go to the J-planes, you’ll see that it will identify the cylinder. These particular J-planes aren’t necessary for the solution. And we’re going to select these surfaces and hide them. Hiding them turns off the surfaces you don’t want to use or see. I’m also going to turn off the bounding box because I don’t want to see the extent of the domain.

What we’re interested in is a simple cylinder. Let’s drop in a slice. The slice is going the X direction. If I want to change it to the Y direction, I can just type “Y”. And that’s actually perfect, because what we want to do is look at streamlines and streaklines on either side of the cylinder.

To turn on streamtraces, select Streamtraces, and go to the Streamtrace tool.

At this point, I’m going to create a rake. I’m not going to put in individual positions. I’m going to get two rakes, a rake on the upwind side and a rake on the downwind side. And I’ll show you why that’s important here in a second.

You can see these are 3D, and if I animate, you’ll see that these are instantaneous streamtraces. Effectively every time we go from time step to time step it’s going to recalculate or reintegrate through the vector field based on the seed position for each of the lines.

For a steady state solution, streamtraces are fine. But we’re interested in looking at streakline analysis and particles with mass.

I’ll go ahead and press Pause. There’s a lot of solution time steps I’ve brought in and we don’t need to see them all.

We’re going to move first into the analyze capabilities, and then we’re going to go to particle paths and streaklines.

There are two types of calculations one can do, a particle path or a streakline. Particle paths will effectively follow the streamline, or the original streamline from time 0. The thing that would make them interesting is that you can use mass, so we can use a ballistic coefficient. We can use ablation. And the idea here is that in principle, you can start those particles with 0 velocity or a specified velocity. You can start them with gravity in the minus X, Y, Z, or plus X, Y, Z position. So that’s one type of calculation.

There is also an integration time step. This is used when doing streakline calculations. Now a couple of things on streaklines, you can release more than one particle each time step. So a streakline calculation is different than the streamline integration in that the streakline is going to release those particles in time and when it goes to the next time step, it will release particles based on where those particles have migrated in the vector field from the first time step. And I’ll show you what that looks like here in a second.

Let’s release two particles per time step. We’re going to do this per solution time. We’ll leave the default on the integration time step. And we’ll hit Calculate.

Depending on the size of your solution, the number of time steps, the calculation for particle paths and streaklines, it can take a while because a fair amount of computational resources are needed.

Once it’s done, you’ll get a message telling you that the particle velocity has been stored in the current vector variables, and the particle calculation was successful.

We’re going to turn on the scatter symbol. If we go to the Zone Style dialog, you’ll see that we have a series of streamlines. And we’ll go to scatter, and we’ll just confirm that, in fact for the majority of the zones here, we don’t want to show scatter. Streaklines we do want to show. We’ll start here, go down to the bottom, there are 20 of them, and we’ll select them.

Outline color, I’m going to change that to be RHO. And in terms of scatter size, I’ll make it a little smaller.

Now we’re at the end time step here. Let’s go back to the beginning, and if I animate these streamtraces or animate these streaklines, you can see there are the particle paths or the streaklines in this case. So they’re evolving in time, and they’re going to go through, and as we go through the solution, they’ll actually develop.

This is a simple way to look at streamlines. The dialogue doesn’t dismiss, so I’ll just move it off the screen for the animation effect. You’ll notice that if I hit play that the streaklines themselves aren’t really animating. That’s because they’re stored slightly differently.

I wanted you to be aware that you have to animate these streaklines from the dialogue. Let’s animate this again so you can see those streaklines developing. It is a relatively easy operation to generate streaklines.

In this case you’ll see that we’re actually obscured a little bit because of the slice. I’m going to turn the slice off, so I will stop this for a moment. If you want to turn off the slice, you can just double click on the slice here, that’s a streamtrace. And you can turn it off. Or just undo slices. Undo the streamtraces. Let’s rotate this around a tad. We’ll see the scatter here is actually shown. For the zones, we’re going to turn on translucency so you can see it a little better.

And now we’ll go ahead and animate again on screen, and so you can see where those streaklines are going as they develop in time.

It’s a pretty nice way to look at turbulent flows. And you’ll see that it continues to release two streams per time step. It looks like these streaklines on the upwind side seem to be kind of slowly approaching the cylinder.

And that’s how you do the calculation. Again, you can also use mass. So if you wanted to use mass, you can use ballistic coefficients as well. So there’s lots of different options. You can change the integration time step. You don’t have to release it just with the solution time level. You can make it arbitrary. And that’s how you do a streakline animation.

Thanks for watching!

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