Fluid flow patterns

Fluid flow patterns

Many type of flow patterns can be visualized both physically (experimentally) and/or computationally. Flow visualization is very useful for both physical experiments as well as numerical methods. Visualization helps in understanding the whole picture. The flow pattern may be described by means of streamlines, stream-tubes, path lines and streak-lines.

Stream lines:

A stream line is an imaginary curve drawn through a flowing fluid in such a way that the tangent to it at any point gives the direction of the velocity of flow at that point. Streamlines are useful as indicators of the instantaneous direction of fluid motion throughout the flow field. For example, regions of recirculating flow and separation of a fluid off of a solid wall are easily identified by the streamline pattern. Since a fluid is composed of fluid particles, the pattern of flow of fluid may be represented by a series of stream-lines, obtained by drawing a series of curves through the flowing fluid such that the velocity vector at any point is tangential to the curve.

                                      

For a three-dimensional flow;

                                                   

Stream-tube:

A stream-tube is a tube imagined to be formed by a group of stream lines passing through a small closed curve, which may or may not be circular. Since the stream tube is bounded on all sides by streamlines and since the velocity has no component normal to a streamline, there can be no flow across the bounding surface of a stream tube. A fluid may enter or leave a stream tube only at its ends. A stream with a cross-sectional area small enough for the variation of velocity over it to be negligible is sometimes termed as stream filament.

Path line:

A path line may be defined as the line traced by a single fluid particle as it moves over a period. Thus, a path line will show the direction of velocity of the same fluid particles at successive instants of time.

Streak line:

A streak-line is the locus of fluid particles that have passed sequentially through a prescribed point in the flow. In experimental work often a color or dye or some other substance (such as smoke in case of gases) is injected into the flowing fluid, in order to trace the motion of the fluid particles. The resulting trail of color is known as a streak line or filament line.

For a steady flow, a streak line is same as a stream line and path line of a fluid particle.