Flow visualization

Flow visualization depicts fluid motion through experimental images or computational representations. It reveals structures such as streamlines, vortices, and turbulent eddies.

Particle image velocimetry: An optical method that estimates fluid velocity by tracking tracer-particle displacement between images. It turns recorded tracer motion into quantitative velocity vectors.
Velocity field: A vector field assigning a fluid velocity to each point in space and time. Many flow visualizations encode this quantity directly or indirectly.
Wind tunnel: A facility that drives controlled airflow around models or objects for aerodynamic testing. Visualization shows how air moves around test articles and where separation occurs.
Eulerian description: A description of fluid motion that measures properties at fixed positions in space. It contrasts with particle-following views often suggested by tracer images.
Dye injection: A visualization technique that introduces colored dye into a fluid to reveal its motion and mixing. Dye streaks expose paths and mixing patterns in transparent flows.
Pathline: The trajectory traced by an individual fluid particle over time. Tracer-based images often approximate particle trajectories.
Flow separation: The detachment of a fluid boundary layer from a surface under an adverse pressure gradient. Visual patterns help locate separation and identify its changing behavior.
Lagrangian description: A description of fluid motion that follows individual fluid particles through time. It clarifies what particle tracking depicts, unlike measurements fixed in space.
Schlieren photography: An optical technique that images refractive-index variations, often revealing density gradients in transparent media. It makes shock waves and other density-driven structures visible without tracers.
Streakline: The curve formed by all fluid particles that previously passed through a fixed point. Continuous tracer injection produces streaklines, which can differ from particle paths.

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Flow measurementCompared with: Visualization reveals flow patterns, while flow measurement quantifies rates or totals.