One characteristic of fluid flow that makes it challenging to study is that in most cases, unlike solid mechanics, the medium is invisible. Experimental fluid mechanics makes use of smoke or dye to obtain visual insight into the physics of a flow. Likewise, it is helpful in analysis to obtain an analogous mathematical “visual” representation of a flow field whenever possible. The following three types of flow lines are of great value in flow visualization:
1. Streamlines. These lines are everywhere tangent to the velocity vector. They are most useful in describing a steady flow because in that case, each streamline is the path that a fluid particle traces as it traverses the flow field. In an unsteady flow, the velocity vector changes in both direction and magnitude with time at any given point; therefore, only instantaneous streamlines are useful in visual interpretation of the field. That is, they describe how fast and in what direction the fluid particles are moving at a given instant. Because streamlines are always parallel to the velocity vector, flow cannot pass across a streamline.
2. Streaklines. These lines would be traced out by a marker fluid that is con – tinuosly injected into the flow stream at a given point. This is a commonly used experimental flow-visualization technique. For example, smoke often is injected into a flow to make the details visible. If the flow is steady, the filaments of smoke particles trace out the streamlines that pass through the injection points.
3. Pathlines. These are lines traced out in time by a given particle as its motion would be explained in a Lagrangian mathematical description.