My guess is that it probably is referring to Fluid Dynamics, as that is a branch of study in the chemical engineering field.
As an explanation (if that is what was intended), it is the study and mathematical understanding of the flow of materials (not only liquid but also other phases of matter, although liquid is the most commonly studied), as well as an understanding of the physics and theory that underlies such dynamic systems. As stated, this is something that I deal with sometimes in my job, but if anyone has any questions regarding this that I can not answer, a coworker of mine and a member of the RMU playtest group that I GM has a PhD in this area (from MIT), so if I can't answer then I can try to get him involved in the discussion as he is extremely knowledgeable in this area. While I work in the "Process Separations" discipline of Core R&D for my employer (thus we deal with chemical separations technologies and have our companies experts in that area (Distillation, Crystallization, Absorption, Adsorption, etc.) so we are familiar with fluid flow), he works in a "sister" group to the one I work in the "Fluid Mechanics and Mixing" discipline of core R&D, where such things are their area of expertise (the flow and mixing of chemicals) so he is one of our companies SME's ("Subject Matter Expert") in this field.