linjedrag

Linjedrag is a term used in fluid dynamics and engineering to denote the drag force acting on slender line-like bodies moving through a fluid, such as cables, hoses, or filaments. The concept is particularly relevant for long, flexible lines interacting with flow, where the line’s length, curvature, material properties, and tension influence the overall drag.

Etymology: The word combines linje, meaning line in several Scandinavian languages, with drag, meaning resistance or drag.

Description: Drag on a line depends on its orientation to the flow, the line’s length L, diameter d, and the local velocity v. Flexible lines deform under flow, changing projected area and flow regime; thus the effective drag coefficient Cd is not constant. Drag can be decomposed into form drag and skin-friction drag, and is often modeled with adaptations of the Morison equation or slender-body theory for line-like bodies.

Measurement and modeling: In practice, engineers use analytical models, empirical coefficients, and numerical simulations (CFD) to estimate line drag. For very long lines, interactions with supports, flow separation, and vortex shedding may cause oscillations such as flutter or vortex-induced vibration, which are accounted for in design.

Applications: Linjedrag considerations appear in offshore mooring lines, underwater power or fiber-optic cables, tethered underwater or aerial platforms, and marine energy devices. It informs routing, buoyancy planning, and installation.

See also: Drag, Morison equation, Vortex-induced vibration, Slender-body theory, Tethered systems.