title: Comment on "Hydrodynamics of fractal continuum flow" and "Map of fluid flow in fractal porous medium into fractal continuum flow".
abstract: In two recent papers [Phys. Rev. E 85, 025302(R) (2012) and Phys. Rev. E 85, 056314 (2012)], the authors proposed fractal continuum hydrodynamics and its application to model fluid flows in fractally permeable reservoirs. While in general providing a certain advancement of continuum mechanics modeling of fractal media to fluid flows, some results and statements to previous works need clarification. We first show that the nonlocal character those authors alleged in our paper [Proc. R. Soc. A 465, 2521 (2009)] actually does not exist; instead, all those works are in the same general representation of derivative operators differing by specific forms of the line coefficient c(1). Next, the claimed generalization of the volumetric coefficient c(3) is, in fact, equivalent to previously proposed product measures when considering together the separate decomposition of c(3) on each coordinate. Furthermore, the modified Jacobian proposed in the two commented papers does not relate the volume element between the current and initial configurations, which henceforth leads to a correction of the Reynolds' transport theorem. Finally, we point out that the asymmetry of the Cauchy stress tensor resulting from the conservation of the angular momentum must not be ignored; this aspect motivates a more complete formulation of fractal continuum models within a micropolar framework.
authors: Division of Engineering and Applied Science, California Institute of Technology, Pasadena, California 91125, USA.
authors: Jun
authors: J
authors: Li
authors: Department of Mechanical Science and Engineering, Institute for Condensed Matter Theory, and Beckman Institute, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA.
authors: Martin
authors: M
authors: Ostoja-Starzewski