Planar Doppler Velocimetry
Encyclopedia
Planar Doppler Velocimetry (PDV), also referred to as Doppler Global Velocimetry (DGV), determines flow velocity across a plane by measuring the Doppler shift
in frequency of light scattered by particles contained in the flow. The Doppler shift, Δfd, is related to the fluid velocity.
The relatively small frequency shift
(order 1 GHz) is discriminated using an atomic or molecular vapor filter. This approach is conceptually similar to what is now known as Filtered Rayleigh Scattering
(Miles and Lempert, 1990).
light. The Doppler shifted scattered light is then split into two paths using a beamsplitter and imaged onto the camera(s). In this manner the absolute absorption of scattered light, as it passes through an iodine cell placed in one of the beam paths, is measured at every spatial location within the object plane. For scattering by relatively large (i.e. Mie scattering
) particles, this absorption is a function of particle velocity
alone. Accurate calibration and image mapping algorithms have been developed with the result that velocity accuracies of ~1–2 m/s are possible. A sample image, taken from a sequence of images obtained using MHz rate PDV, is shown in Figure 2. More details concerning the history of PDV, the art of its application and recent advances can be found in comprehensive review articles by Elliott and Beutner (1999) and Samimy and Wernet (2000).
Unlike PIV, PDV requires only a single image of the flow field. This image may taken over a long period (relative to characteristics times scales within the flow) to produce time-averaged images or alternatively using a single laser pulse (approximately 10ns) to obtain a measurement of instantaneous flow velocities. The duration of a single laser pulse is at least an order of magnitude
shorter than pulse separations used within PIV. This feature of PDV enables improved resolution of sharp velocity discontinuites such as shock wave
s.
In addition, PDV has an inherently higher resolution than PIV (where small image subregions are used to determine the velocity typically 16 x 16 pixels) and a velocity measurement may be obtained for each pixel within the flow image. However, particularly in the case of instantaneous measurement using PDV, some pixel binning is used to attenuate the deleterious effects of laser speckle
and improve the Signal-to-Noise ratio
.
. Even with seeding, the laser frequency can fluctuate with time and must be monitored. These introduce additional complexity to the experimental set-up. PDV systems, although used in many laboratories, are not yet commercially available and can be quite expensive (equipment, data processing
, experience, labor, etc.) if built from scratch.
Doppler effect
The Doppler effect , named after Austrian physicist Christian Doppler who proposed it in 1842 in Prague, is the change in frequency of a wave for an observer moving relative to the source of the wave. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from...
in frequency of light scattered by particles contained in the flow. The Doppler shift, Δfd, is related to the fluid velocity.
The relatively small frequency shift
Frequency shift
In the physical sciences and in telecommunication, the term frequency shift has the following meanings:* Any change in frequency* A Doppler shift* Any change in the frequency of a radio transmitter or oscillator...
(order 1 GHz) is discriminated using an atomic or molecular vapor filter. This approach is conceptually similar to what is now known as Filtered Rayleigh Scattering
Rayleigh scattering
Rayleigh scattering, named after the British physicist Lord Rayleigh, is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light. The particles may be individual atoms or molecules. It can occur when light travels through...
(Miles and Lempert, 1990).
Equipment
Up to know, a typical one-component PDV instrument utilizes a pulsed injection-seeded Nd:YAG laser, one or two scientific grade CCD cameras and a molecular iodine filter as shown in Figure 1. The laser is used to illuminate a plane of the flow with narrow spectral linewidthSpectral linewidth
The spectral linewidth characterizes the width of a spectral line, such as in the electromagnetic emission spectrum of an atom, or the frequency spectrum of an acoustic or electronic system...
light. The Doppler shifted scattered light is then split into two paths using a beamsplitter and imaged onto the camera(s). In this manner the absolute absorption of scattered light, as it passes through an iodine cell placed in one of the beam paths, is measured at every spatial location within the object plane. For scattering by relatively large (i.e. Mie scattering
Mie theory
The Mie solution to Maxwell's equations describes the scattering of electromagnetic radiation by a sphere...
) particles, this absorption is a function of particle velocity
Particle velocity
Particle velocity is the velocity v of a particle in a medium as it transmits a wave. In many cases this is a longitudinal wave of pressure as with sound, but it can also be a transverse wave as with the vibration of a taut string....
alone. Accurate calibration and image mapping algorithms have been developed with the result that velocity accuracies of ~1–2 m/s are possible. A sample image, taken from a sequence of images obtained using MHz rate PDV, is shown in Figure 2. More details concerning the history of PDV, the art of its application and recent advances can be found in comprehensive review articles by Elliott and Beutner (1999) and Samimy and Wernet (2000).
Strengths
PDV is well suited for high-speed flow measurements where concerns about particle seeding make PIV impractical. Although PDV requires particles to scatter light, individual particles do not need to be imaged thus allowing the use of much smaller seed particles and making the measurements less sensitive to particle seed density. For example, in some unheated supersonic flow facilities it is possible to use condensation of a vapor, such as water, acetone or ethanol, to produce seed particles in the flow. Particles formed using this method, known as product formation, have been estimated to be ~50 micrometres in diameter.Unlike PIV, PDV requires only a single image of the flow field. This image may taken over a long period (relative to characteristics times scales within the flow) to produce time-averaged images or alternatively using a single laser pulse (approximately 10ns) to obtain a measurement of instantaneous flow velocities. The duration of a single laser pulse is at least an order of magnitude
Order of magnitude
An order of magnitude is the class of scale or magnitude of any amount, where each class contains values of a fixed ratio to the class preceding it. In its most common usage, the amount being scaled is 10 and the scale is the exponent being applied to this amount...
shorter than pulse separations used within PIV. This feature of PDV enables improved resolution of sharp velocity discontinuites such as shock wave
Shock wave
A shock wave is a type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium or in some cases in the absence of a material medium, through a field such as the electromagnetic field...
s.
In addition, PDV has an inherently higher resolution than PIV (where small image subregions are used to determine the velocity typically 16 x 16 pixels) and a velocity measurement may be obtained for each pixel within the flow image. However, particularly in the case of instantaneous measurement using PDV, some pixel binning is used to attenuate the deleterious effects of laser speckle
Speckle pattern
A speckle pattern is a random intensity pattern produced by the mutual interference of a set of wavefronts. This phenomenon has been investigated by scientists since the time of Newton, but speckles have come into prominence since the invention of the laser and have now found a variety of...
and improve the Signal-to-Noise ratio
Signal-to-noise ratio
Signal-to-noise ratio is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. It is defined as the ratio of signal power to the noise power. A ratio higher than 1:1 indicates more signal than noise...
.
Weaknesses
The main weakness of PDV is the complex optical set up required to get accurate measurements. For each component of velocity, two images (signal and reference) are required, which typically necessitates two cameras. To obtain all three components of velocity, therefore, requires the simultaneous use of up to six cameras, although recent work by Charret et al. (2006) and Hawkes et al. (2004) has progressively enabled the number of cameras required from six to a single camera. In addition, the laser used for the measurements must be narrow linewidth, which is typically performed by injection seeding of the laser cavityOptical cavity
An optical cavity or optical resonator is an arrangement of mirrors that forms a standing wave cavity resonator for light waves. Optical cavities are a major component of lasers, surrounding the gain medium and providing feedback of the laser light. They are also used in optical parametric...
. Even with seeding, the laser frequency can fluctuate with time and must be monitored. These introduce additional complexity to the experimental set-up. PDV systems, although used in many laboratories, are not yet commercially available and can be quite expensive (equipment, data processing
Data processing
Computer data processing is any process that a computer program does to enter data and summarise, analyse or otherwise convert data into usable information. The process may be automated and run on a computer. It involves recording, analysing, sorting, summarising, calculating, disseminating and...
, experience, labor, etc.) if built from scratch.
External links
- Planar Doppler Velocimetry by the Cranfield UniversityCranfield UniversityCranfield University is a British postgraduate university based on two campuses, with a research-oriented focus. The main campus is at Cranfield, Bedfordshire and the second is the Defence Academy of the United Kingdom based at Shrivenham, Oxfordshire. The main campus is unique in the United...
Engineering Photonics Group - http://www.ae.uiuc.edu/~elliottg/frs/frs.htm
- http://www.metrolaserinc.com/pdvtempl2.htm
- http://www.holomap.com/dgv.htm
- http://www.innssi.com/pdv01.htm
- http://www.dlr.de/at/en/desktopdefault.aspx/tabid-1657/2281_read-3723/