In the previous lesson, we understood the difference between rotational and irrotational
flows. We also learnt about vortex motion and the importance of vorticity. Naturally, the
next questions that comes to mind are - How can we visualize the flow? How do we visualize a vortex?
Can we experience what happens to flow around an aircraft wing? In this lesson, we are going to
discuss some flow visualization techniques used in the engineering design cycle to develop products.
In most scenarios, flow visualization provides a qualitative visual understanding of what the
flow looks like. In some cases, like in the case of modern computational flow visualizations,
the resultant information is useful to quantify these flows for various engineering applications.
In this lesson, we will limit ourselves to the qualitative aspects of flow visualization.
Broadly speaking, visualization techniques can be classified as experimental and computational.
Most visualizations analyze the flow behavior through surface flow patterns. Experiments
conducted in wind-tunnel facilities use the concept of streamlines to understand
the aerodynamic shape of an aircraft or an automobile. Wind-tunnels mimic the motion
of these means of transportation. Generally, a scaled model of an automobile or an aircraft is
used in these wind-tunnel tests. Sometimes, the object of interest is only a small portion - such
as an aircraft wing or an automobile spoiler. The scaled model is fastened in a wind-tunnel
facility and is maintained stationary. High-speed air is blown on these models
using a large rotating fan. The air moving around this stationary object gives us an
understanding on what would happen if the object were moving at a certain speed.
White smoke jets are released at specific locations to create streamlines around these
scaled models. These smoke jets are released from either one or a rake of multiple smoke
generators in the wind-tunnel. The generated smoke blows around the body being tested. The
interaction of these smoke lines with the body of the aircraft or automobile
helps the engineer understand the impact of its shape. These smoke lines provide a
macroscopic understanding of the overall flow around the body. A general sense of where the
air is flowing and where the vortices are being generated can obtained from these smoke lines.
In addition to white smoke, fluid engineers make use of flow tufts to understand and
visualize the flow close to the body. Tufts are small strips of yarn or strings about 15 cm long.
One end of the tuft is securely glued to the surface of the model to be tested before it is
placed inside the wind-tunnel. As air blows over these flow tufts, the loose end of these tufts
is free to move with the flow. Because the length and weight of these tufts are carefully selected,
they provide a good understanding of the flow very close to the body without actually altering
the flow. These are particularly crucial in obtaining qualitative information on crossflow,
reversed flow, and flow-separation close to the solid [body of the aircraft]. These
flow behaviors influence the overall resistance offered by the body to the flow.
Another popular flow visualization technique employed in high-speed wind tunnels is to observe
oil-flow steaks over surfaces. The surface of the test object such as wing of an aircraft are coated
with viscous oil. The oil is transported by the flowing high-speed air, leaving oil streaks on the
surface of the test object. These oil streaks are good indicators of flow separation over
the body. After a considerable amount of testing, the wind-tunnel is stopped and photographs of the
oil streak patterns on the surface are taken. The initial thickness of the oil
film is an important factor to obtain meaningful results from this flow visualization technique.
Wind-tunnels are a great way of visualizing external air flow
over bodies. In a similar manner, water tunnels are used to visualize and test
hydrodynamic behavior of submerged bodies under water. For these submerged bodies,
it is important to understand the lift or drag forces acting on them. In these water tunnels,
the model to be tested in kept stationary in flowing water and the flow over these submerged
bodies is visualized by injecting colored dyes into the flow. Similar to the white smoke lines
in a wind-tunnel, the lines obtained by injecting these colored dyes provide information about the
macroscopic flow around the submerged body. In addition, it also provides information
related to the whereabouts of vortices. Certain water tunnels have the ability
to increase or reduce the internal static pressure to perform cavitation studies. These
studies are extremely important for designing propeller blades of a submarine or a ship.
Schlieren photography is an optical visualization technique which distinguishes fluids with varying
densities. It uses the principle that light rays bend differently when passing
from a denser medium to a rarer one or vice versa. It uses a bright source of light,
a knife-edge - for focusing the light better, and two concave mirrors on either side of the
flow being visualized. It was primarily developed for studying high-speed supersonic flows. Shock
waves are common in supersonic flows and this visualization technique was developed to capture
these shock waves. A shock wave is a thin narrow region with high gradients of pressure, density,
and temperature. The rays of light passing through this shock waves are bent and are
stopped by the knife edge and do not reach the recording device. They are therefore captured as
dark lines on the image. Schlieren photography can also be used visualize the flow around a hot body.
All these experimental visualization techniques require careful calibration and instrumentation.
In fact, most of these experimental set-ups are expensive and can only be operated by
trained staff. Moreover, not all aspects of flow physics are brought out through
these experimental techniques. Therefore, in this modern age, computer models are capable
of providing insights into flow physics that cannot be captured experimentally. For fluids,
these models are generally referred to as computational fluid dynamics, (CFD, in short). The
results obtained from these models are capable of analyzing flow data and finally display the flow
as surface plots such as contours, vector plots, streamlines, pathlines, and particle tracks plot.
With the invention of virtual reality, users can now get an immersive experience of the flow field.
We are now able to travel along a streamline or visualize the vortex in the flow-field from its
center. The beauty of computational visualization lies in the fact that the engineer can now
understand aspects of the flow which might not be possible with experimental techniques.
In fluid dynamics, it is important to use both these sets of flow visualizations. In fact,
in most industries, experiments and computations go hand in hand during the product design cycle.