I don't exactly understand what you mean, but instead of trajectories you can use cut plots to show flow vectors. You can set a base plane and then choose whatever offset you want for the cut plot to get any slice of the domain that you like, using cut plots.
Thanks for the reply, i will try the cut plot.
However I am following this tutorial
And it show the flow trajectories in vertical align in the center of the z axis.(without cut plots)
See the tutorial picture.
But I have the flow trajectories in horizontal arrange, and all passing under the foil(see my first picture).
And don´t find how to configure it.
That tutorial is not very good, in my opinion. The presenter makes a lot of mistakes. His flow domain is tiny, he never even checks his mesh before running the simulation - it's a case of somebody who knows "just enough to be dangerous". I do not recommend following that tutorial.
As for your original question, there are a few ways of setting the origin of your trajectories:
1. Create a 'dummy volume' that is not included in the simulation geometry, and pick one of its faces to create equally spaced trajectory origins.
2. Manually set your origin by clicking on the screen
Check the options in the flow trajectory tools to get more info.