2.2 FLOW VISUALISATION Flashcards
What are common methods of flow visualisation to see flow patterns?
Smoke injection in air
Dye injection in water & other fluids
Hydrogen bubbles in water
Particle image velocimetry (PIV)
What are streamlines?
A line that is tangential to the instantaneous velocity direction.
Fluid does not cross this line and streamlines cannot cross.
Variety of methods are used to visualise the flow and there are 2 distinct types of particle trajectories, both sometimes loosely referred to as “streamlines”.
What are pathlines?
The trajectory or path of a given fluid particle.
If we marked a drop of water with a dye marker & followed that drop using stroboscopic photography then this would produce a pathline.
Long exposure photography would produce the same line.
May be thought of as a record of the path a fluid particle has taken.
Pathlines may intersect since different particles may follow different paths at different times.
What are streaklines?
The instantaneous locus or trace of all the fluid particles that pass through a given fixed point in space.
If we injected dye continuously from a fixed point, the dye would form a streakline as it passed downstream.
Streaklines cannot intersect because two particles cannot occupy the same space at the same instant.
In steady flow (where velocity doesn’t change with time), streamlines, pathlines and streaklines are identical.
What is a time line?
The instantaneous location of a line of fluid particles in the flow.
Usually the initial formed line is perpendicular to the flow boundary.
Looking at the changes in shape of the line tells us about the relative changes in flow velocity e.g. away from a boundary.
What is a streamtube?
An imaginary tube whose walls are streamlines.
It may be envisaged as a bundle of streamlines.
Just as streamlines cannot cross, the boundary of the streamtube cannot be crossed.
Pipes or channels are essentially stream tubes.
