Flow visualization and flow measurement are different. Most flow visualization
techniques provide a general macroscopic understanding of the flow field. However,
flow measurement techniques provide ways of precisely measuring the flow variables
such as pressure, velocity, or temperature. These techniques allow engineers and product
designers to quantify the flow. Model results obtained by solving the flow on computers using
CFD are carefully calibrated against these measurements obtained by experimentalists.
Most flows we see in nature are unsteady. The flow field variables, whether it is velocity,
pressure, or temperature, vary continuously with time. However, under certain flow conditions,
these variables can be assumed steady. Whether the flow is steady or unsteady, we are
generally interested in the mean values of these parameters. We want to measure either the spatial
or temporal (which is time-average) average values of various flow variables during our experiments.
In this lesson, we'll talk about steady ways of measuring pressure, velocity, and temperature.
When it comes to measuring the lift force generated by an aircraft wing or the downforce
generated on a Formula One racing car, or the torque and power generated by a turbine blade,
it all comes down to the pressure force exerted by the fluid on the surface of
these individual parts. It is because of this that measuring pressure becomes
important in understanding and designing various products. The simplest way of
measuring pressure is to use a barometer or a manometer. In addition to these,
static pressure taps measure the pressure at a point in the flow field. The recorded value
of pressure is processed through an electronic data logger. Another way of measuring pressure
is through a Bourdon pressure gauge. These are primarily used for measuring pressures
in closed systems such as pressure vessels and automobile tires. They are a mechanical way of
measuring pressure and rely on the deformation of a circular tube. This straightening and deforming
tube causes a deflection in a spring-loaded needle which provides the necessary pressure measurement.
Velocity measurements in a flow field gives us an idea of what the flow field looks like. In fact,
knowledge of velocity helps us estimate the overall resistance offered by a body as it
is moving through a fluid. This is extremely important for making the body of an aircraft or
a formula 1 car. The entire field of aerodynamic shapes stems from this requirement to reduce the
overall resistance offered by the flow. Velocity can be measured directly or indirectly. An
indirect way of measuring velocity is to obtain the overall flow rate. Flow rate provides an
indication of the rate at which a certain fluid volume is being transported across
the system. Knowledge of this flow rate can help us understand the average velocity of the fluid.
Most velocity measurements are based on an important principle in fluid dynamics called
the Bernoulli's principle. According to this, velocity in the system can be calculated based
on the total pressure and difference between the static pressures measured at 2 specific points.
It is important to note that this principle can be applied to flow fields under certain
conditions. A more elaborate explanation of this principle as well as the conditions under
this can be applied are discussed in the later sections of this MOOC. For now, it is sufficient
to know that we can calculate the velocity of the fluid using the following equation.
The simplest way of measuring the local velocity of the fluid is to use a pitot
tube. A pitot tube is an L-shaped tube where measurements are taken both at the
tip and on its sides. The total pressure or the stagnation pressure of the fluid is measured at
its tip and the static pressure measurements at taken on the side. The local fluid velocity is
calculated using these measurements based on the Bernoulli's principle.
Venturi tube is another common way of measuring the fluid velocity. The venturi tube has a narrow
section where the flow gets restricted. This section is commonly referred to as the throat
of the venturi. Velocity is calculated based on pressure drop measurements across the throat and
by taking the area ratios between the upstream section and the throat. A similar principle
for measuring the mean velocity of the flow is provided by devices such as orifice plate and
flow nozzles. Turbine flow meters and rotameters are other common velocity measuring instruments.
Hot wire anemometer is another intrusive way of measuring the local velocity in the flow field.
In this method, we insert a thin long wire into the flow. This wire is electrically heated and
attempts to measure the local flow temperature. Based on the overall heat flux and the temperature
difference between the wire and the fluid , a local velocity is calculated. This method,
however, makes use of carefully calibrated coefficients to estimate the velocity
of the flow. The final velocity measurements are noisy and requires time-averaging.
Until now all the velocity measurement devices that we discussed are intrusive. That is,
they cause minor disturbances to the flow field. To avoid this and to get a precise measurement,
we use optical techniques called the Laser Doppler Velocimetry and particle image velocimetry.
A laser Doppler velocimeter focuses a beam of laser on a point in the flow field which
is seeded by a certain concentration of seed particles. These particles scatter the light
from a laser and the scattered light gives us a sense of the local velocity at that point. Unlike
LDV which is a point measurement, we are able to capture 2D and 3D flow fields using particle
image velocimetry. In PIV, we use two light pulses from the laser through the trace particles. These
light pulses are captured by a recorder and the 2D or 3D flow fields are reconstructed.
Temperature measurements are important when it comes to solving either a compressible
flow or a heat transfer problem. Knowledge of temperature gives us an idea of how hot or cold
a certain flow is. This is extremely important in applications such as electronics cooling or
thermal management of battery packs in electric vehicles. In these applications, the knowledge
of fluid temperature helps engineers provide better heat transfer solutions.
There are different ways of measuring temperature and they can be broadly classified as mechanical,
electrical, and radiation based. A common thermometer, which works on expansion or
contraction of a liquid in a glass tube, is one way of measuring temperature. A bimetallic strip,
which is commonly used in temperature regulators in homes and offices, relies on the principle that
different metals have different coefficients of thermal expansion. A bimetallic strip has 2 metal
strips and they bend in different directions depending on the temperatures they sense.
Thermistors and thermocouples are common examples of electrical means of measuring temperature.
Thermistors rely on the fact that electrical resistance of metals and semiconductors change
with temperature. An increase or decrease in the value of electrical resistance of these
metals or semiconductors help us estimate the temperature. Thermocouple, on the other hand,
operates on the principle of Seebeck effect. Two different metal strips produce an electromotive
force or voltage when there is a temperature gradient between them. The thermocouple senses
this voltage and passes it to a data logger which provides the final temperature reading. It is
important to note that these temperature measuring devices only provide a point measurement.
An infrared Camera senses the infrared radiation emitted by a body and converts it to a temperature
field. This is a non-intrusive way of measuring temperature. In contrast to
all the other devices discussed earlier, an infrared Camera can help us provide the
entire temperature field map. Usually hot spots are identified by red and cold spots by blue.
There are many other ways of measuring flow variables to calibrate the flow
field. These measurement techniques are used by engineers across all fields to
test and come up with design modifications to develop cheaper, efficient, and safer products.