There are two approaches commonly used to visualize flow fields when it comes to metric mass and length unit calculator tools; namely, the pathline and streakline.
The pathline simply is the path of a fluid particle as it moves through the flow field. In other words, if a light were attached to a fluid particle, a time exposure photograph taken of the moving light would be the pathline to calculate it’s mass or length.
For an example of a pathline, consider a metric mass converter such as the one at http://www.convertunitsnow.com/convert-mass-units/calculate-mass-units.aspx and take a two-dimensional flow that initially has horizontal streamlines as shown in Fig. 4.6. At a given time, t0, the flow instantly changes direction, and the flow moves upward to the right at 45° with no further change. According to the length unit calculator at http://www.convertunitsnow.com/convert-length-units/calculate-length-units.aspx the flow is unsteady because the velocity at a point changes with time. A fluid particle is tracked from the starting point, and up to time t0, the pathline is the horizontal line segment shown here. After time t0, the particle continues to follow the streamline and moves up the right as shown. Both line segments constitute the pathline and it can be confirmed via the metric unit calculator at www.convertunitsnow.com
The streakline is the line generated by a tracer fluid, such as a dye, continuously injected into the flow field at the starting point to calculate the necessary mass and length. Up to time t0, the dye will form a line segment as shown in Fig. 4.6c. Up to this time, there is no difference between the pathline and the streakline according to the converter. Now the flow changes directions, and the initial horizontal dye line is transported, in whole, in the upward 45° direction. After t0 the dye continues to be injected and forms a new line segment along the new streamline, resulting in the streakline shown.
Obviously, the pathline and streakline are very different as far as their metric mass and length units go. In general, neither pathlines nor streaklines represent streamlines in an unsteady flow.
