Strömungsachse

Strömungsachse (German: "flow axis") is a technical term used in fluid dynamics to denote an axis or direction that represents the predominant motion of a fluid relative to a body or in a conduit. It is commonly chosen to align with the freestream velocity vector in external aerodynamics, with the mean flow direction in pipelines and ducts, or with the mean-line through a blade row in turbomachinery. The Strömungsachse serves as a reference direction for defining flow-related angles, coordinate systems and components of velocity, such as angle of attack, incidence and pitch.

In practice the Strömungsachse may coincide with a geometric centerline (for straight pipes or symmetric bodies) or differ from the body axis when the body is yawed, rotated or operating in nonuniform flow. Analyses frequently adopt a streamline-aligned coordinate system (tangential, normal, binormal) in which one axis follows local flow direction to simplify the representation of convective terms and boundary-layer behaviour. Identification of the Strömungsachse can be based on freestream measurements, mean vector fields from computational fluid dynamics, or flow-visualization techniques.

The concept is important for design, testing and interpretation of experimental data because misalignment between body and flow axes affects forces, moments and performance. It is a pragmatic reference rather than a uniquely defined physical object, and its precise definition depends on the application and chosen averaging or measurement method.