My v attribute of particle fluid is always {0,0,0}, I might use trail sop to get the v value each time.

How can I export the particle fluid's v attribute with dop?

I just setup a simple simulation via the shelf.
Create a sphere, move it up, make it a particle fluid object.
Create a groundplane, hit play to cache it a bit.

Inside your sphere_object1_fluid (If you start a fresh scene and leave all names by default), you'll have a dopimport1. If you middle click on it, you'll see that it will actually import the velocity attributes of your simulation. And if you open a Details View, and go frame by frame, you'll see that the values for v are changing.

If you just follow these steps, do you get different velocity values or also 0,0,0 an every frame?

It's a bit hard to guess what your problem is without providing a scene or further information of your setup. That's why you should test this simple setup first and then post back if it's still not working for you.


cheers,
Daniel

If the problem is occurring when you attach the particle fluid surfacer, you may need to go to the attributes tab and tell it to promote v

I found my problem is I used the dopimport2 which imports Geometry/visualization, so it has no v attribute.

Thanks for your help!