E1C09 09/14/2010
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interpretation in strongly dynamic flows can be complicated (17,25). Hot-film sensors are less
fragile and less susceptible to contamination than hot-wire sensors. Probe blockage is not significant
in large ducts and away from walls. Thermal anemometers are 180 degrees directionally ambiguous
(i.e., flows from the left or right directions give the same output signal), an important factor in flows
that may contain flow reversal regions. An industrial-grade system can be built rather inexpensively.
The thermal anemometer is usually calibrated against either pressure probes or an LDA.
Laser Doppler Anemometer
The laser Doppler anemometer (LDA) is a relatively expensive and technically advanced point
velocity measuring technique that can be used for most types of flows but is also well suited to hostile,
combusting, or highly dynamic (unsteady, pulsatile, or highly turbulent) flow environments. It offers
good frequency response, small spatial resolution, no probe blockage, and simple signal interpretation,
but requires optical access and the presence of scattering particles. This method provides very good
temporal resolution for time-accurate measurements in turbulent flows. The method measures the
velocity of particles suspended in the moving fluid, not the fluid velocity, so careful planning is
required in particle selection to ensure that the particle velocities represent the fluid velocity exactly.
The size and concentration of the particles govern the system frequency response (26,27).
Particle Image Velocimetry
Particle image velocimetry (PIV) is a relatively expensive and technically advanced full-field
velocity measuring technique that can be used for most types of flows, including hostile and
combusting flows. There is no probe blockage of the flow, but it requires optical access and the
presence of scattering particles. The method provides an instantaneous snapshot of the flow,
providing excellent views of flow structures. Time-dependent quantification of such dynamic flows
is possible, but frequency bandwidth is limited to camera frame rate and spatial resolution. As with
LDA, this method measures the velocity of particles suspended in the moving fluid, not the fluid
velocity, so careful planning is required in particle selection to ensure that the particle velocities
represent the fluid velocity exactly.
9.10 SUMMARY
Several reference pressure instruments have been presented that form the working standards for
pressure transducer calibration. Pressure transducers convert sensed pressure into an output form
that is readily quantifiable. These transducers come in many forms but tend to operate on hydrostatic
principles, expansion techniques, or force-displacement methods.
In moving fluids, special care must be taken when measuring pressure to delineate between
static and total pressure. Methods for the separate measurement of static and total pressure or for the
measurement of the dynamic pressure are readily available and well documented. But improper
measuring technique causes errors, lowering the total pressure or increasing the static pressure.
Measuring the local velocity within a moving fluid can be accomplished in a number of ways.
Selecting the proper tool requires assessment of the need: mean or fluctuating velocity, point or full
field measurement, optical access or opaque boundaries or fluid. Specifically, dynamic pressure,
thermal anemometry, Doppler anemometry, and particle velocimetry methods have been presented.
As discussed, each method offers advantages over the others, and the best technique must be
carefully weighed against the needs and constraints of a particular application.
9.10 Summary 415
15:4:56 Page 415
interpretation in strongly dynamic flows can be complicated (17,25). Hot-film sensors are less
fragile and less susceptible to contamination than hot-wire sensors. Probe blockage is not significant
in large ducts and away from walls. Thermal anemometers are 180 degrees directionally ambiguous
(i.e., flows from the left or right directions give the same output signal), an important factor in flows
that may contain flow reversal regions. An industrial-grade system can be built rather inexpensively.
The thermal anemometer is usually calibrated against either pressure probes or an LDA.
Laser Doppler Anemometer
The laser Doppler anemometer (LDA) is a relatively expensive and technically advanced point
velocity measuring technique that can be used for most types of flows but is also well suited to hostile,
combusting, or highly dynamic (unsteady, pulsatile, or highly turbulent) flow environments. It offers
good frequency response, small spatial resolution, no probe blockage, and simple signal interpretation,
but requires optical access and the presence of scattering particles. This method provides very good
temporal resolution for time-accurate measurements in turbulent flows. The method measures the
velocity of particles suspended in the moving fluid, not the fluid velocity, so careful planning is
required in particle selection to ensure that the particle velocities represent the fluid velocity exactly.
The size and concentration of the particles govern the system frequency response (26,27).
Particle Image Velocimetry
Particle image velocimetry (PIV) is a relatively expensive and technically advanced full-field
velocity measuring technique that can be used for most types of flows, including hostile and
combusting flows. There is no probe blockage of the flow, but it requires optical access and the
presence of scattering particles. The method provides an instantaneous snapshot of the flow,
providing excellent views of flow structures. Time-dependent quantification of such dynamic flows
is possible, but frequency bandwidth is limited to camera frame rate and spatial resolution. As with
LDA, this method measures the velocity of particles suspended in the moving fluid, not the fluid
velocity, so careful planning is required in particle selection to ensure that the particle velocities
represent the fluid velocity exactly.
9.10 SUMMARY
Several reference pressure instruments have been presented that form the working standards for
pressure transducer calibration. Pressure transducers convert sensed pressure into an output form
that is readily quantifiable. These transducers come in many forms but tend to operate on hydrostatic
principles, expansion techniques, or force-displacement methods.
In moving fluids, special care must be taken when measuring pressure to delineate between
static and total pressure. Methods for the separate measurement of static and total pressure or for the
measurement of the dynamic pressure are readily available and well documented. But improper
measuring technique causes errors, lowering the total pressure or increasing the static pressure.
Measuring the local velocity within a moving fluid can be accomplished in a number of ways.
Selecting the proper tool requires assessment of the need: mean or fluctuating velocity, point or full
field measurement, optical access or opaque boundaries or fluid. Specifically, dynamic pressure,
thermal anemometry, Doppler anemometry, and particle velocimetry methods have been presented.
As discussed, each method offers advantages over the others, and the best technique must be
carefully weighed against the needs and constraints of a particular application.
9.10 Summary 415