Velocimetry
Encyclopedia
Velocimetry is the measurement of the velocity
of fluid
s, as often used to solve fluid dynamics
problems, or to study fluid networks, as well as in industrial and process control
applications, or in the creation of new kinds of fluid flow sensor
s. Methods of velocimetry include particle image velocimetry
and particle tracking velocimetry
, Molecular tagging velocimetry
, laser-based interferometry
, ultrasonic Doppler methods, Doppler
sensors, and new signal processing
methodologies. Temporal integration of velocimetric information can be used to totalize fluid flow. For measuring velocity and length on moving surfaces, laser surface velocimeter
s are used.
In general, velocity measurements are made in the Lagrangian or Eulerian frames of reference (see Lagrangian and Eulerian coordinates
). Lagrangian methods assign a velocity to a volume of fluid at a given time, whereas Eulerian methods assign a velocity to a volume of the measurement domain at a given time. A classic example of the distinction is particle tracking velocimetry, where the idea is to find the velocity of individual flow tracer particles (Lagrangian) and particle image velocimetry, where the objective is to find the average velocity within a sub-region of the field of view (Eulerian).
, who would float grass seeds on a flow and sketch the resulting trajectories of the seeds that he observed (a Lagrangian measurement). Today the basic idea is the same; the flow must be seeded with particles that can be observed by the method of choice. The seeding particles depend on many factors including the fluid, the sensing method, the size of the measurement domain, and sometimes the expected accelerations in the flow.
The fluid generally limits the particle selection according to its specific gravity; the particles should ideally be of the same density as the fluid. This is especially important in flows with a high acceleration (for example, high-speed flow through a 90-degree pipe elbow). Heavier fluids like water and oil are thus very attractive to velocimetry, whereas air ads a challenge in most techniques that it is rarely possible to find particles of the same density as air.
Still, even large-field measurement techniques like PIV have been performed successfully in air. Particles range from droplets of oil or water generated with a Laskin nozzle or atomizer (diameters in the micrometre range) to helium-filled soap bubbles (diameters in the millimetre range). For point measurements like laser Doppler velocimetry
, particles in the nanometre diameter range, such as those in cigarette smoke, are sufficient to perform a measurement.
In water and oil there are a variety of inexpensive industrial beads that can be used, such as silver-coated hollow glass spheres manufactured to be conductive powders (tens of micrometres diameter range) or other beads used as reflectors and texturing agents in paints and coatings. The particles need not be spherical; in many cases titanium dioxide particles can be used.
Velocity
In physics, velocity is speed in a given direction. Speed describes only how fast an object is moving, whereas velocity gives both the speed and direction of the object's motion. To have a constant velocity, an object must have a constant speed and motion in a constant direction. Constant ...
of fluid
Fluid
In physics, a fluid is a substance that continually deforms under an applied shear stress. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids....
s, as often used to solve fluid dynamics
Fluid dynamics
In physics, fluid dynamics is a sub-discipline of fluid mechanics that deals with fluid flow—the natural science of fluids in motion. It has several subdisciplines itself, including aerodynamics and hydrodynamics...
problems, or to study fluid networks, as well as in industrial and process control
Process control
Process control is a statistics and engineering discipline that deals with architectures, mechanisms and algorithms for maintaining the output of a specific process within a desired range...
applications, or in the creation of new kinds of fluid flow sensor
Flow sensor
A flow sensor is a device for sensing the rate of fluid flow. Typically a flow sensor is the sensing element used in a flow meter, or flow logger, to record the flow of fluids....
s. Methods of velocimetry include particle image velocimetry
Particle image velocimetry
Particle image velocimetry is an optical method of flow visualization used in education and research. It is used to obtain instantaneous velocity measurements and related properties in fluids...
and particle tracking velocimetry
Particle tracking velocimetry
Particle tracking velocimetry is a velocimetry method, i.e a technique to measure velocity of particles. The name suggests that the particles are tracked, and not only recorded as an image as it is suggested in another form, particle image velocimetry...
, Molecular tagging velocimetry
Molecular tagging velocimetry
Molecular Tagging Velocimetry is a specific form of flow velocimetry, a technique for determining the velocity of currents in fluids such as air and water. In its simplest form, a single "write" laser beam is shot once through the sample space...
, laser-based interferometry
Interferometry
Interferometry refers to a family of techniques in which electromagnetic waves are superimposed in order to extract information about the waves. An instrument used to interfere waves is called an interferometer. Interferometry is an important investigative technique in the fields of astronomy,...
, ultrasonic Doppler methods, Doppler
Photoacoustic Doppler effect
The photoacoustic Doppler effect, as its name implies, is one specific kind of Doppler effect, which occurs when an intensity modulated light wave induces a photoacoustic wave on moving particles with a specific frequency. The observed frequency shift is a good indicator of the velocity of the...
sensors, and new signal processing
Signal processing
Signal processing is an area of systems engineering, electrical engineering and applied mathematics that deals with operations on or analysis of signals, in either discrete or continuous time...
methodologies. Temporal integration of velocimetric information can be used to totalize fluid flow. For measuring velocity and length on moving surfaces, laser surface velocimeter
Laser surface velocimeter
A laser surface velocimeter is a non-contact optical sensor measuring velocity and length on moving surfaces. Laser surface velocimeters use the laser Doppler principle to evaluate the laser light scattered back from a moving object...
s are used.
In general, velocity measurements are made in the Lagrangian or Eulerian frames of reference (see Lagrangian and Eulerian coordinates
Lagrangian and Eulerian coordinates
In fluid dynamics and finite-deformation plasticity the Lagrangian specification of the flow field is a way of looking at fluid motion where the observer follows an individual fluid parcel as it moves through space and time. Plotting the position of an individual parcel through time gives the...
). Lagrangian methods assign a velocity to a volume of fluid at a given time, whereas Eulerian methods assign a velocity to a volume of the measurement domain at a given time. A classic example of the distinction is particle tracking velocimetry, where the idea is to find the velocity of individual flow tracer particles (Lagrangian) and particle image velocimetry, where the objective is to find the average velocity within a sub-region of the field of view (Eulerian).
Methods
Velocimetry can be traced back to the days of Leonardo Da VinciLeonardo da Vinci
Leonardo di ser Piero da Vinci was an Italian Renaissance polymath: painter, sculptor, architect, musician, scientist, mathematician, engineer, inventor, anatomist, geologist, cartographer, botanist and writer whose genius, perhaps more than that of any other figure, epitomized the Renaissance...
, who would float grass seeds on a flow and sketch the resulting trajectories of the seeds that he observed (a Lagrangian measurement). Today the basic idea is the same; the flow must be seeded with particles that can be observed by the method of choice. The seeding particles depend on many factors including the fluid, the sensing method, the size of the measurement domain, and sometimes the expected accelerations in the flow.
The fluid generally limits the particle selection according to its specific gravity; the particles should ideally be of the same density as the fluid. This is especially important in flows with a high acceleration (for example, high-speed flow through a 90-degree pipe elbow). Heavier fluids like water and oil are thus very attractive to velocimetry, whereas air ads a challenge in most techniques that it is rarely possible to find particles of the same density as air.
Still, even large-field measurement techniques like PIV have been performed successfully in air. Particles range from droplets of oil or water generated with a Laskin nozzle or atomizer (diameters in the micrometre range) to helium-filled soap bubbles (diameters in the millimetre range). For point measurements like laser Doppler velocimetry
Laser Doppler velocimetry
Laser Doppler Velocimetry , also known as Laser Doppler Anemometry , is the technique of using the Doppler shift in a laser beam to measure the velocity in transparent or semi-transparent fluid flows, or the linear or vibratory motion of opaque, reflecting, surfaces.-Technology origin:With the...
, particles in the nanometre diameter range, such as those in cigarette smoke, are sufficient to perform a measurement.
In water and oil there are a variety of inexpensive industrial beads that can be used, such as silver-coated hollow glass spheres manufactured to be conductive powders (tens of micrometres diameter range) or other beads used as reflectors and texturing agents in paints and coatings. The particles need not be spherical; in many cases titanium dioxide particles can be used.
External links
- Velocimetry portal is an online center for Laser Flow Diagnostic Techniques (PIV, StereoPIV, MicroPIV, NanoPIV, High speed PIV, PTV, LDV, PDPA, PLIF, ILIDS, PSP etc). This portal is being developed so as to provide as much information as possible about the Laser Flow Diagnostic Techniques in a consolidated manner. Services include Basic Principles, Applications, Discussion forums, Links to Links. A concentrated effort is taken to put together all the present and possible applications of PIV, StereoPIV, MicroPIV, NanoPIV, High speed PIV, PTV, LDV, PDPA, PLIF, ILIDS, PSP. Velocimetry portal aims to become as the reference point for all queries related to Laser Flow Diagnostic Techniques.