490 CHAPTER 8. LABORATORY MEASUREMENT AND ANALYSIS
8.2.4 Flow Visualization
Flow visualization encompasses a variety of qualitative and quantitative
techniques that provide information about the structure of moving fluids.
Visualization techniques range from simple to sophisticated; but in all cases,
visual observation of flow features brings the human mind into the analysis
processes, and this is one of the most important benefits of flow visualization. In some cases, relatively simple visual observations may reveal
significant characteristics of a complex flow situation that would not be revealed adequately by instruments alone. For this reason flow visualization
is often used to compliment quantitative point measurements.
All flow visualization techniques involve the placement of some type of
neutrally buoyant, visual tracer in the flow that can be followed with the eye
or be photographed. For example, confetti can be floated on the surface of
the water, its progress photographed, and its velocities quantified. Beneath
the surface, dyes7 can be injected into the flow and monitored as they move
with the flow.
Potassium permanganate or fluorescein are the most common dyes (Sharp 1981).
This is why hot-wire anemometers axe not used in water unless the wire has been
coated to prevent electrolysis.
The simplest visualization methods consist of a small number of visual tracers that can be individually followed as they move along their
Lagrangian paths. The classic example is a vertical array of several neutrally buoyant balls that move in circular orbits with the passage of a wave.
Through multi-exposure photographs, the orbital paths can be determined
with a fair degree of accuracy.
Another visualization technique is the generation of a line of hydrogen
bubbles by an electrically charged wire immersed in the flow (Sharp 1981).
When the circuit is activated, bubbles form on the wire by electrolysis and
are swept away by the flow8. High-speed cameras or video can capture the
velocity profiles as indicated by the line of bubbles. Further details of this
technique were given by Sharp (1981).
Image processing techniques can be used to analyze frame-by-frame digital images obtained during flow visualization experiments. Depending on
the method of visualization and processing techniques employed, various
features of the flow field, such as velocities and particle paths, can be quantified. Wessels, et al. (1990) discussed some of the more sophisticated flow
visualization techniques and instrument systems such as the laser fluorometer, shadowgraphs, and optical interferometry.
High-speed and time-lapse photography are often an integral part of
some flow visualization techniques, but these types of photography also are
useful for quantifying other model features as well. For example, high-speed
8.2.4 Flow Visualization
Flow visualization encompasses a variety of qualitative and quantitative
techniques that provide information about the structure of moving fluids.
Visualization techniques range from simple to sophisticated; but in all cases,
visual observation of flow features brings the human mind into the analysis
processes, and this is one of the most important benefits of flow visualization. In some cases, relatively simple visual observations may reveal
significant characteristics of a complex flow situation that would not be revealed adequately by instruments alone. For this reason flow visualization
is often used to compliment quantitative point measurements.
All flow visualization techniques involve the placement of some type of
neutrally buoyant, visual tracer in the flow that can be followed with the eye
or be photographed. For example, confetti can be floated on the surface of
the water, its progress photographed, and its velocities quantified. Beneath
the surface, dyes7 can be injected into the flow and monitored as they move
with the flow.
Potassium permanganate or fluorescein are the most common dyes (Sharp 1981).
This is why hot-wire anemometers axe not used in water unless the wire has been
coated to prevent electrolysis.
The simplest visualization methods consist of a small number of visual tracers that can be individually followed as they move along their
Lagrangian paths. The classic example is a vertical array of several neutrally buoyant balls that move in circular orbits with the passage of a wave.
Through multi-exposure photographs, the orbital paths can be determined
with a fair degree of accuracy.
Another visualization technique is the generation of a line of hydrogen
bubbles by an electrically charged wire immersed in the flow (Sharp 1981).
When the circuit is activated, bubbles form on the wire by electrolysis and
are swept away by the flow8. High-speed cameras or video can capture the
velocity profiles as indicated by the line of bubbles. Further details of this
technique were given by Sharp (1981).
Image processing techniques can be used to analyze frame-by-frame digital images obtained during flow visualization experiments. Depending on
the method of visualization and processing techniques employed, various
features of the flow field, such as velocities and particle paths, can be quantified. Wessels, et al. (1990) discussed some of the more sophisticated flow
visualization techniques and instrument systems such as the laser fluorometer, shadowgraphs, and optical interferometry.
High-speed and time-lapse photography are often an integral part of
some flow visualization techniques, but these types of photography also are
useful for quantifying other model features as well. For example, high-speed