Fluid Dynamics
What Is Fluid Dynamics?
By Jim
Lucas, Live Science Contributor
Fluid dynamics is "the branch of applied science that is concerned with the movement of liquids and gases," according to the American Heritage Dictionary.
Fluid dynamics is
one of two branches of fluid mechanics, which is the study of fluids and how
forces affect them.
(The other branch
is fluid statics, which deals with fluids at rest.)
Scientists across
several fields study fluid dynamics.
Fluid dynamics
provides methods for studying the evolution of stars, ocean currents, , weather
patterns, plate tectonics and even blood circulation.
Some important
technological applications of fluid dynamics include rocket engines, wind
turbines, oil pipelines and air conditioning systems systems.
What is flow?
The movement of liquids and gases is
generally referred to as "flow," a concept that describes how fluids
behave and how they interact with their surrounding environment — for example,
water moving through a channel or pipe, or over a surface.
Flow can be either steady or unsteady.
In his lecture notes, "Lectures in Elementary Fluid Dynamics"
(University of Kentucky, 2009) J. M. McDonough, a professor of engineering at the
University of Kentucky, writes, "If
all properties of a flow are independent of time, then the flow is steady;
otherwise, it is unsteady."
That is, steady flows do not change over time.
An example of steady flow would be water flowing through a
pipe at a constant rate.
On the other hand, a flood or water pouring from an
old-fashioned hand pump are examples of unsteady flow.
Flow can also be either laminar or turbulent.
Laminar flows are smoother, while turbulent flows are more
chaotic.
One important factor in determining the state of a fluid’s
flow is its viscosity, or thickness, where higher viscosity increases the
tendency of the flow to be laminar.
Patrick McMurtry, an engineering
professor at the University of Utah, describes the difference in his online
class notes, "Observations About
Turbulent Flows" (University of Utah, 2000), stating, "By
laminar flow we are generally referring to a smooth, steady fluid motion, in
which any induced perturbations are damped out due to the relatively strong
viscous forces. In turbulent flows, other forces may be acting the counteract
the action of viscosity."
Laminar flow is desirable in many situations, such as in
drainage systems or airplane wings, because it is more efficient and less
energy is lost.
Turbulent flow can be useful for causing different fluids
to mix together or for equalizing temperature.
According to McDonough, most flows of interest are
turbulent; however, such flows can be very difficult to predict in detail, and
distinguishing between these two types of flow is largely intuitive.
An important factor in fluid flow is the fluid's Reynolds number
(Re),
which is named after 19th century scientist Osborne Reynolds, although it
was first described in 1851 by physicist George Gabriel Stokes.
McDonough gives the definition of Re as,
"the ratio of inertial to viscous
forces."
The inertial force is the fluid's resistance to change of
motion, and the viscous force is the amount of friction due to the viscosity or
thickness of the fluid.
Note that Re is not only a property of the fluid; it
also includes the conditions of its flow such as its speed and the size and
shape of the conduit or any obstructions.
At low Re,
the flow tends to be smooth, or laminar, while at high Re,
the flow tends to be turbulent, forming eddies and vortices.
Re can be used to predict how a gas or liquid will flow
around an obstacle in a stream, such as water around a bridge piling or wind
over an aircraft wing.
The number can also be used to predict the speed at which
flow transitions from laminar to turbulent.
Liquid
flow
The study of liquid flow is called hydrodynamics.
While liquids include all sorts of substances, such
as oil and chemical solutions, by far the most common liquid is water, and most
applications for hydrodynamics involve managing the flow of this liquid.
That includes flood control, operation of city water
and sewer systems, and management of navigable waterways.
Hydrodynamics deals primarily with the flow of water in
pipes or open channels.
Geology professor John Southard's
lecture notes from an online course, "Introduction
to Fluid Motions " (Massachusetts
Institute of Technology, 2006), outline the
main difference between pipe flow and open-channel flow: "flows in closed conduits or channels, like pipes or air ducts,
are entirely in contact with rigid boundaries," while "open-channel flows, on the other hand,
are those whose boundaries are not entirely a solid and rigid material."
He states, "important open-channel flows are rivers, tidal currents,
irrigation canals, or sheets of water running across the ground surface after a
rain."
Due to the differences in those boundaries, different forces
affect the two types of flows.
According to Scott Post in his book, "Applied and Computationsl Fluid
Mechanics," (Jones & Bartlett, 2009), "While
flows in a closed pipe may be driven either by pressure or gravity, flows in
open channels are driven by gravity alone."
The pressure is determined primarily by the height of the
fluid above the point of measurement.
For instance, most city water systems use water towers to
maintain constant pressure in the system.
This difference in elevation is called the hydrodynamic
head.
Liquid in a pipe can also be made to flow faster or with
greater pressure using mechanical pumps.
Gas flow
The flow of gas has many similarities
to the flow of liquid, but it also has some important differences.
First, gas is compressible, whereas liquids are generally
considered to be incompressible.
In "Fundamentals of Compressible Fluid Dynamics" (Prentice-Hall, 2006),
author P. Balachandran describes compressible fluid, stating, "If the density of the fluid changes
appreciably throughout the flow field, the flow may be treated as a
compressible flow."
Otherwise, the fluid is considered to be incompressible.
Second, gas flow is hardly affected by gravity.
The
gas most commonly encountered in everyday life is air; therefore, scientists
have paid much attention to its flow conditions.
Wind causes air to move around buildings and other
structures, and it can also be made to move by pumps and fans.
One
area of particular interest is the movement of objects through the atmosphere.
This branch of fluid dynamics is
called aerodynamics, which is "the
dynamics of bodies moving relative to gases, especially the interaction of
moving objects with the atmosphere," according to the American
Heritage Dictionary.
Problems in this field involve reducing drag on automobile
bodies, designing more efficient aircraft and wind turbines, and studying how
birds and insects fly.
Bernoulli's principle
Generally, fluid moving at a higher
speed has lower pressure than fluid moving at a lower speed.
This phenomenon was first described by Daniel Bernoulli in
1738 in his book "Hydrodynamics,"
and is commonly known as Bernoulli's principle.
It can be applied to measure the speed of a liquid or gas
moving in a pipe or channel or over a surface.
This
principle is also responsible for lift in an aircraft wing, which is why
airplanes can fly.
Because the wing is flat on the bottom and curved on the
top, the air has to travel a greater distance along the top surface than along
the bottom.
To do this, it must go faster over the top, causing its
pressure to decrease. This makes the higher-pressure air on the bottom lift up on the wing.
Problems in fluid dynamics
Scientists often try to visualize
flow using figures called streamlines, streaklines and pathlines.
McDonough defines a streamline as "a continuous line within a fluid such
that the tangent at each point is the direction of the velocity vector at that
point."
In other words, a streamline shows the direction of the
flow at any particular point in the flow.
A streakline, according to McDonough,
is "the locus [location] of all
fluid elements that have previously passed through a given point."
A pathline (or particle path), he
writes, is "the trajectory of an
individual element of fluid."
If the flow does not change over time, the pathline will
be the same as the streamline.
However, in the case of turbulent or unsteady flow, these
lines can be quite different.
Most
problems in fluid dynamics are too complex to be solved by direct calculation.
In these cases, problems must be solved by numeric methods
using computer simulations.
This area of study is called
numerical or computational fluid dynamics (CFD), which Southard defines as "a branch of computer-based science
that provides numerical predictions of fluid flows."
However, because turbulent flow tends to be nonlinear and chaotic,
particular care must be taken in setting up the rules and initial conditions
for these simulations.
Small changes at the beginning can result in large
differences in the results.
The
accuracy of simulations can be improved by dividing the volume into smaller
regions and using smaller time steps, but this increases computing time.
For this reason, CFD should advance as computing power
increases.
Jim Lucas is a freelance writer
and editor specializing in physics, astronomy and engineering. He is general
manager of Lucas Technologies.
https://www.livescience.com/47446-fluid-dynamics.html
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