CE 319F LAB 3
3) FLOW VISUALIZATION

3.1) OBJECTIVES

The objectives of this lab are to understand some flow visualization techniques and, in the process of learning about the visualization techniques, to also learn about some simple types of flows.

3.2) BACKGROUND

Flow visualization incorporates many of the basic principles involved in fluid mechanics. Observation of fluid flow can give insight to the concepts of pressure, velocity and force.

There are four basic line patterns used to describe flows:

  1. A streamline is a line drawn such that the velocity vector at every point along the line is tangent to the line.  A streamline is an Eulerian description.  Examples of streamlines are shown in Fig. 13.10 on page 528 of the textbook.
  2. A pathline is the line which a given fluid particle follows.  A pathline is a Lagrangian description.
  3. A streakline is a line of particles which have all passed through the same point in a flow.  A streakline is a Lagrangian description.  Examples of streaklines are a line of smoke from a chimney and a dye streak in a river where dye has been injected through a tube.  The smoke identifies the air that has flowed past the top of the chimney and the dye identifies the water particles that have flowed past the end of the dye injection tube.  
  4. A time line is the line formed by a set of fluid particles that were tagged along a line or curve at a previous instant in time.
The streamline is an instantaneous line, while the streakline is generated by the passage of time.  If the flow is steady, pathlines and streaklines are the same as streamlines because the velocity does not change magnitude or direction at each point over time.  Streamlines are difficult to generate experimentally in unsteady flow unless one marks a great many particles and notes their direction of motion during a very short time interval.  A streakline can be produced experimentally by the continuous release of marked particles from a given point.

Flow visualization can be done by different methods:

  1. dye, smoke, or bubble discharges
  2. surface powder or flakes on liquid flow
  3. tufts of yarn attached to boundary surfaces
  4. luminescent additives
  5. evaporative coatings on boundary surfaces
  6. optical techniques
Two-dimensional flow patterns around various shapes can be demonstrated by injecting dye into the flow of water between two glass plates.  The lines created in this type of demonstration are streaklines.

3.3) PROCEDURES

A video will be shown and discussed.  The video demonstrates some flow visualization techniques and illustrates streamlines, streaklines, pathlines, and time lines.  There are some topics in the video that go beyond what has been discussed in class, but even these topics should help give an improved appreciation for fluid flow problems.