Velocimetry is the measurement of the
velocityIn physics, velocity is the rate of change of position. It is a vector physical quantity; both speed and direction are required to define it. In the SI system, it is measured in meters per second: or ms-1. The scalar absolute value of velocity is speed...
of
fluidA fluid is a substance that continually deforms under an applied shear stress. All gases are fluids, but not all liquids are fluids. 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 dynamicsIn 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 controlProcess control is a statistics and engineering discipline that deals with architectures, mechanisms, and algorithms for controlling the output of a specific process. See also control theory....
applications, or in the creation of new kinds of fluid
flow sensorA 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 velocimetryParticle image velocimetry is an optical method of fluid visualization. It is used to obtain instantaneous velocity measurements and related properties in fluids. The fluid is seeded with tracer particles which, for the purposes of PIV, are generally assumed to faithfully follow the flow dynamics...
and
particle tracking velocimetryParticle tracking velocimetry is one of velocimetry methods, 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 velocimetryMolecular 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
interferometryInterferometry is the technique of diagnosing the properties of two or more waves by studying the pattern of interference created by their superposition. The instrument used to interfere the waves together is called an interferometer...
, ultrasonic Doppler methods,
Doppler-Doppler effect and its applications:* Doppler broadening* Doppler echocardiography* Doppler effect* Doppler fetal monitor* Doppler radar* Doppler sonography* Pulse-Doppler radar-Other:* Doppler, the mascot of the WNBA's Seattle Storm....
sensors, and new
signal processingSignal processing is an area of electrical engineering and applied mathematics that deals with operations on or analysis of signals, in either discrete or continuous time to perform useful operations on those signals...
methodologies. Temporal integration of velocimetric information can be used to totalize fluid flow.
In general, velocity measurements are made in the Lagrangian or Eulerian frames of reference (see
Lagrangian and Eulerian coordinatesIn 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...
).
Velocimetry is the measurement of the
velocityIn physics, velocity is the rate of change of position. It is a vector physical quantity; both speed and direction are required to define it. In the SI system, it is measured in meters per second: or ms-1. The scalar absolute value of velocity is speed...
of
fluidA fluid is a substance that continually deforms under an applied shear stress. All gases are fluids, but not all liquids are fluids. 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 dynamicsIn 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 controlProcess control is a statistics and engineering discipline that deals with architectures, mechanisms, and algorithms for controlling the output of a specific process. See also control theory....
applications, or in the creation of new kinds of fluid
flow sensorA 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 velocimetryParticle image velocimetry is an optical method of fluid visualization. It is used to obtain instantaneous velocity measurements and related properties in fluids. The fluid is seeded with tracer particles which, for the purposes of PIV, are generally assumed to faithfully follow the flow dynamics...
and
particle tracking velocimetryParticle tracking velocimetry is one of velocimetry methods, 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 velocimetryMolecular 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
interferometryInterferometry is the technique of diagnosing the properties of two or more waves by studying the pattern of interference created by their superposition. The instrument used to interfere the waves together is called an interferometer...
, ultrasonic Doppler methods,
Doppler-Doppler effect and its applications:* Doppler broadening* Doppler echocardiography* Doppler effect* Doppler fetal monitor* Doppler radar* Doppler sonography* Pulse-Doppler radar-Other:* Doppler, the mascot of the WNBA's Seattle Storm....
sensors, and new
signal processingSignal processing is an area of electrical engineering and applied mathematics that deals with operations on or analysis of signals, in either discrete or continuous time to perform useful operations on those signals...
methodologies. Temporal integration of velocimetric information can be used to totalize fluid flow.
In general, velocity measurements are made in the Lagrangian or Eulerian frames of reference (see
Lagrangian and Eulerian coordinatesIn 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 di ser Piero da Vinci was an Italian polymath, scientist, mathematician, engineer, inventor, anatomist, painter, sculptor, architect, botanist, musician and writer....
, 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 velocimetryLaser Doppler Velocimetry is a technique for measuring the direction and speed of fluids like air and water. In its simplest form, LDV crosses two beams of collimated, monochromatic, and coherent laser light in the flow of the fluid being measured...
, 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.