Rubén Zorrilla

@miguelmaso @dcagritan My comment was only for letting you know. Non-core application changes must be done in a different PR.

Issues with Swimming DEM application are handled in #10179. About the fluid adjoint analysis stuff, we're (@sunethwarna) are discussing the best way to get rid of the `nsteps` in `problem_data`...

> I will get rid of the `nsteps` in the ajoints and will have usual `start_time` and `end_time` in `problem_data`. We don't have a problem with the `problem_data` section and...

> > I will get rid of the `nsteps` in the ajoints and will have usual `start_time` and `end_time` in `problem_data`. We don't have a problem with the `problem_data` section...

Indeed, now I remember that we designed this in such a way that you can internally override these methods to add your custom elements.

> > Indeed, now I remember that we designed this in such a way that you can internally override these methods to add your custom elements. > > I tried...

> > Not very nice but maybe we can add the `custom_` keyword in front of your custom element names. Then, if the element is not found and there is...

> @rubenzorrilla I added a test which shows the error as a user will see it, is that ok to you? Yes. This is great, we keep previous usability while...

Besides, I think it would be a good idea to rename the class to `VoxelUtilities` so we can wrap up all the future implementations in here (not only the volume...

Ping @maceligueta