Flow visualization
Flow visualization is the process of making the motion of fluids and the physics of this motion visible. In experiments this is achieved mainly by injection of visible material (e.g. dye) into the flow. With the increasing importance of flow simulations using computers (computational fluid dynamics), illustrating the flow with computer graphics has become a main branch of flow visualization. Flow visualization aims at helping engineers to understand the physics an characteristics of flows.
Experimental Flow Visualization
As mentioned, the most important technique for visualizing flow in real experiments is injecting material into the fluid and observing how the particles of this material are advected by the flow. The material can be of very different kind. Smoke is often used to visualize gas flow, air bubbles can be injected into water to show vortical motion, and dye can be used to illustrate the behavior of most liquids.
Computer Graphics
The illustration of flow by computer graphics can be applied for any flow which is available as measured or simulated data. The data can consist of different quantities like velocity, pressure and density. Each of these quantities can be visualized by certain types of graphical representation. Most of the experimental techniques can be imitated for simulation data. Traces of particles, for example, can be computed mathematically and the resulting curves can be drawn as simple lines on the display. In addition special features like lines representing centers of vortices ("vortex core lines") can be derived mathematically from the give quantities. Drawing these features can further aid the understanding of the flow characteristics.
Flow Visualization Techniques
- Streamlines: particle traces in steady flow.
- Pathlines: particle traces in unsteady flow.
- Streaklines: lines consisting of particles released continuously into the flow.
- Time Lines: lines consisting of particles released simultaneously into the flow.
- Streamsurface: continuum of traces of particles released into a steady flow from a line (continuum of streamlines).
- Pathsurface: continuum of trace of particles release into a unsteady flow from a line (continuum of pathlines).
- Surface oil flow: the body of an object immersed in a flow is covered with oil. The flow leaves streaks in the oil which depict the flow structure on the object's surface. This technique can be imitated by Line Integral Convolution on computers.