Bulkfluidbevegelse

Bulkfluidbevegelse describes the macroscopic transport of a fluid as a continuous medium, driven by pressure differences, gravity, or external forces, and observed at length scales large compared with molecular spacing. It contrasts with molecular diffusion, which arises from random molecular motion and operates on much smaller scales. In engineering and geoscience, bulk movement is analyzed using the laws of continuum mechanics. In Norwegian-language sources, the term bulkfluidbevegelse is sometimes used to refer to this concept.

In Newtonian incompressible flows, the flow field is governed by the Navier–Stokes equations. For simple steady, laminar flow in a circular pipe, Hagen-Poiseuille’s law relates the volumetric flow rate to the pressure drop, Q = (π ΔP r^4) / (8 μ L). The Reynolds number Re = ρ U D / μ helps distinguish laminar from turbulent regimes and informs the appropriate modeling approach.

In porous media, bulk movement is often described by Darcy's law, q = - (k / μ) ∇P, where q is the Darcy flux, k the permeability, μ the viscosity, and ∇P the pressure gradient. These models are used for groundwater flow, oil recovery, filtration, and similar processes that involve flow through confined or structured environments.

Key quantities include the volumetric flow rate Q, mean velocity, and pressure distribution. Measurement techniques range from differential pressure sensors and flow meters to tracer tests and imaging methods such as particle image velocimetry.

Applications span water supply, wastewater treatment, chemical processing, heat exchangers, and environmental hydrology. Limitations arise in non-Newtonian, compressible, or highly turbulent flows, and in heterogeneous media where effective properties vary spatially and temporally. The study of bulkfluidbevegelse integrates fluid dynamics, transport phenomena, and porous media theory.