Hello,
I am long time Maya user. Also I have been using RealFlow a lot for fluid sims. So I am not exactly newbie Time has come to learn Houdini again.
H9 has lots of example links from the manual, but non of them seems to help to answer the question that I have.

How can I make volume-based fluid collide with deforming geometry ?

What I am trying to do is to use “river bed” example and add animated character imported as sequence of .obj files from Maya to make that fluid interact with that character. I reverse normals of my character and turn it into rigid body. That automatically links it to the existing DOP network. Fluid collides with the character, but uses only first frame of my .obj sequence as collision object, disregarding the rest of sequence.

If you go into the rbd object in dops there is a toggle for using deforming geometry try turning that on.

Just as a hint
Yes, Houdini can do this

But is it necessary?

It might be worth it for you to parent some non-deforming geometry (your proxy rig) and use that instead

Thanks for you help ! I am testing Apprentice version of H9 for potential use in our projects. Thus my attempts to understand what it can and what it can not do. And how to get stuff in and out of Houdini. And to show others that this app. can do something that we can not do already.
As to my problem - it is not solved yet. Turning “use deforming geometry” on imported RBD object in DOP network works for objects made and deformed inside Houdini, but not for sequence of .obj files. I realize that .objs might be not the most efficient way to bring stuff into Houdini. But this is something available for me right now.

From the help

Use Deforming Geometry
Causes the geometry for the object to be pulled from the chosen SOP at each timestep. If the SOP contains animated geometry, the RBD object’s geometry will also animate. If this option is used, the Use Point Velocity parameter of the RBD Solver should also be turned on to take into account the deformations when calculating collision responses.

So maybe try turning on Use point velocity parameter of the RBD solver

Thank you ! Works now. “Use point velocity for collision” option can be turned on in RBD solver and in Fluid solver as well. Best results in terms of accuracy of collision are (in my case) then this option is turned on for fluid solver only.