Anyone who has ever turned on a tap knows something about
fluid dynamics. Whether a fluid is flowing through household plumbing or
industrial oil and gas pipelines, when it runs slowly its flow is smooth, but
when it runs quickly its flow is more chaotic.
More than 130 years ago, British physicist and engineer
Osborne Reynolds described fluid flowing at low speeds as 'laminar,' meaning it
flows smoothly in a single direction, and fluid flowing at high speeds as
'turbulent,' meaning it experiences chaotic changes in pressure and energy.
Reynolds developed a set of equations to describe the relationship between the
speed at which a fluid flows and the friction that is created between it and
the pipe.
Engineers
still use Reynolds's "laws of resistance" today to calculate how much
energy is lost to friction as liquids and gases flow through a pipe. However,
one mystery has remained unsolved: what happens when a flow transitions from
laminar to turbulent?
"In
transitional flow, friction varies with no discernible patterns," says Dr.
Rory Cerbus, a postdoctoral researcher at the Okinawa Institute of Science and
Technology Graduate University (OIST). Until now, the laws of resistance for
transitional flow were unknown, making it difficult to calculate friction and energy
loss during this type of flow.
Cerbus
and other researchers in the Fluid Mechanics Unit and the Continuum Physics
Unit at OIST have found a surprisingly simple solution to this 130-year old
conundrum. "We have shown that, although the transitional state appears to
be a menagerie of flow states, these can all be characterized by laws we
already know," says Professor Pinaki Chakraborty, leader of the Fluid
Mechanics Unit. "This simplifies a fundamental problem."
Transitional
flow is known to consist of intermittent patches of different types of flow,
which alternate along the pipeline. In the standard approach to measuring
friction in transitional flow, they are simply lumped together.
The
OIST researchers instead analyzed the patches of smooth and chaotic flow
separately. They ran water through a 20-meter glass pipe. By adding small
particles to the water and illuminating it with a laser, they could measure the
speed of the flow. This allowed them to cleanly identify the alternating
patches of smooth and chaotic flow in the transitional flow. They then measured
the friction inside the individual patches using pressure sensors.
"We
repeated a textbook experiment that is routinely done by thousands of
engineering undergraduates every year all around the world," says Cerbus,
lead author of the paper, which was recently published in Physical Review
Letters. "We used essentially the same tools, but with the crucial
distinction of analyzing the patches separately," he says.
The
researchers showed that despite the outward complexities, the law of resistance
for the smooth patches is consistent with laminar flow, while the law of
resistance for the chaotic patches is consistent with turbulent flow.
Therefore, transitional flow can be studied using Reynolds's original laws of
resistance.
Understanding
how much energy is required to pump fluid through a pipeline when it is flowing
in the transitional state could help industries, such as oil refineries,
minimize energy waste and improve efficiency.
"If
you look carefully, you find that often there is simplicity beneath
complexity," says Chakraborty.
https://www.sciencedaily.com/releases/2018/02/180201092104.htm
