fluidstrømning

Fluidstrømning refers to the movement of fluids, including liquids and gases, through or within a system. It arises from pressure differences, gravity, buoyancy, and other forces, and is a central topic in engineering, meteorology, physiology, and many industrial processes. The study falls under the broader field of fluid dynamics.

Governing equations: The motion of fluids is described by conservation laws. The continuity equation expresses mass conservation, and the Navier-Stokes equations express momentum conservation. For a Newtonian, incompressible fluid with density ρ and dynamic viscosity μ, the equations are: ∇·u = 0 and ρ(∂u/∂t + u·∇u) = -∇p + μ∇^2u + f. Compressible flows involve density changes and often energy equations along with momentum and mass conservation.

Flow regimes: Flows can be laminar or turbulent. Laminar flow is smooth and orderly; turbulent flow is chaotic and characterized by fluctuations and enhanced mixing. The Reynolds number, Re = ρUL/μ, helps distinguish regimes, with higher Re more prone to turbulence in many geometries. In engineering, turbulence models and computational fluid dynamics (CFD) are commonly used to analyze complex flows.

Boundaries and profiles: At solid walls, the no-slip condition applies, meaning fluid velocity matches the wall velocity. Far from boundaries, flow tends toward ambient conditions. In long, straight pipes, fully developed laminar flow has a parabolic velocity profile (Poiseuille flow). Turbulent boundary layers exhibit more complex momentum transport.

Applications and methods: Fluidstrømning modeling informs piping design, aircraft and vehicle aerodynamics, weather and ocean circulation, and physiological flows. Measurements include flow rate, velocity profiles, and pressure drops, using tools such as pitot tubes, tracers, and CFD simulations.