Schmierlinien

Schmierlinien, also known as flow lines or streaklines, are a concept used in fluid dynamics to visualize the motion of a fluid. They represent the path traced by all the fluid particles that have passed through a particular fixed point in space over a period of time. Imagine a continuous injection of dye into a flowing river at a single point. The trail of dye that forms downstream would be a schlierlinie. This is distinct from a pathline, which is the trajectory of a single fluid particle over time, and a streamline, which is instantaneous and shows the direction of fluid velocity at a given instant.

The generation of schlierlinien requires knowledge of the fluid's velocity field over time. For a stationary flow, schlierlinien are identical to streamlines. However, in unsteady flows, schlierlinien and streamlines diverge. Observing schlierlinien can provide insights into the behavior of complex fluid flows, particularly in situations where the flow is not constant. They are often used in engineering and scientific contexts to analyze phenomena such as turbulence, wakes behind objects, and the dispersion of pollutants. Experimental visualization techniques, such as injecting smoke or dye into a fluid and capturing images over time, are commonly used to reveal schlierlinien.