Modelling Fluid Regimes at Nano/Meso Scales - Fluid Properties at Nano/Meso Scale: A Numerical Treatment - page 126

Direction of flow
Dye
Figure 5.2 Parallel motion of a filament of dye within a laminar flow.
As the valve is opened further, the velocity of the fluid in the tube increases,
and at some point the stream of dye begins to oscillate. If the valve were to be
opened further, there comes a point at which the stream begins to diffuse at a
distance away from the inlet. Further opening of the valve gives rise to a point at
which a sudden breakdown of the dye stream at a distance from the inlet occurs,
where the dye mixes almost completely with the water. Reynolds noticed that
these disturbances only occurred at high flow speeds at a distance away from the
inlet and that the mixing commenced suddenly.
The mixing of flow that occurs at these high flow rates is known as the turbu-
lent flow regime. At this point, the fluid cannot be described in terms of layers of
fluid at constant velocity across the channel, but particles of fluid (in terms of the
continuum description of a fluid particle) mix across the width of the tube. The
fluid particles in this flow regime have components of velocity that are not just
in the direction of flow, and their paths criss-cross over each other in a seemingly
unpredictable and chaotic way (Figure 5.3). In turbulent flow, the motion is irreg-
ular and conforms to no pattern in terms of frequency or formation of eddies, as
the mixing occurs on a wide range of scales. However, there still remains a bulk
Direction of flow
Dye
Figure 5.3 Chaotic mixing of filaments of dye within a turbulent flow.