Speckle Velocimetry Research Articles

Ultrafast two-dimensional ultrasonic speckle velocimetry: A tool in flow imaging

A technique for imaging fluid flows is proposed, based on the ultrafast analysis of the ultrasonic speckle signal backscattered by particles following the flow. Such an “ultrasonic speckle velocimetry” (USV) provides two-dimensional measurements of one component of the fluid velocity field at about 5000 frames per second. USV is applied to three different flows and future improvements of the technique are described.

Velocity and density field measurements by digital speckle method

Velocity and density field measurements based on image processing of laser speckle or pseudo-speckle pattern have been developed. Laser speckle velocimetry (LSV) or white-light speckle velocimetry (WSV), which corresponds to a high-image-density PIV, gives a local velocity vector map or whole field velocity contour map of a two-dimensional flow field seeded densely with fine particles. This technique has an advantage in high-speed flow measurement without limit of frame rate except for directional ambiguity. New techniques of laser speckle photography and laser speckle interferometry by means of digital image processing have been developed recently for density field measurements. In laser speckle photography, a local density gradient vector map is reconstructed by cross-correlation evaluation between the reference and the object speckle patterns. In laser speckle interferometry, an equi-density contour map is reconstructed by image subtraction between the reference and the object interferometric speckle patterns.

Whole field spatial frequency analysis of double-or multiple-exposed PIV images

Speckle Velocimetry or high-image density PIV gives a velocity vector map of a two-dimensional flow field by point-by-point spatial frequency analysis of local pattern at an interrogation spot in a double- or a multiple-exposed image of laser speckle or pseudo-speckle pattern generated by pulsed laser-light-sheet illuminations of a plane in the flow densely seeded with fine particles. The whole field spatial frequency analysis of the double- or multiple-exposed PIV image gives more useful information of the flow field. Optical Fourier transform is a conventional technique not only for the local spatial frequency analysis but for the whole field analysis. Filtering of spatial frequency is one of the typical techniques for the latter which can reconstruct a velocity contour or a component velocity contour map of the flow. Fundamentally this technique is a simple and efficient analogue method to get more information in the velocity field analysis of the flow than a digital image processing, but in practical applications the measurement is restricted to simple flows because of optical noise and low spatial resolution. In order to improve the technique, the fundamental characteristics of the filtering and the noise yielded in the filtering process must be investigated. Meanwhile, wavelet analysis can also be applied to the whole field spatial frequency analysis of PIV image. In this paper the filtering technique is examined by numerical convolution integral, and the results obtained are compared with ones obtained by the wavelet analysis.

Pulsed lasers in speckle photography: error owing to pulse width

The effect of the pulse width of a pulsed laser in the studies of speckle velocimetry and transient vibration analysis is discussed. Because of the motion of the object during an exposure, a sine function is obtained by using the pointwise filtering method. This function modulates the halo along with the Young's fringes. It is shown that for high object velocities the sinc function modifies the halo distribution; as a result, the error in calculating the fringe position increases. An aperture geometry for which the autocorrelation halo is made constant in certain regions is proposed in which the intensity variation in this region is the result of the modulating sinc function only. A closed-form solution for the shift in the position of the fringes in this region is obtained. Experimental results of the simulation are presented.

Automatic Young's fringe analysis in the source image plane

Typically Young's fringe pattern automatic analysis from a double-exposure image (e.g. a photograph) passes through an indirect processing stage on some intermediate parameter domain. Here, we propose a method based on a complicated image processing technique, operating directly with the source fringe image pixels, and providing remarkable accuracy and computational time. This method is intended for laser speckle velocimetry (LSV), particle image velovimetry (PIV), and digital image velocimetry (DIV) applications. Assuming a common fringe pattern model, we introduce a pre-processing stage to improve significantly the fringe discernment. A dynamic thresholding segmentation scheme, adjusted to the fringe spatial structure, follows to localize the fringes being quantitatively attributed with the corresponding eigenvectors. The algorithm has been tested on real patterns as well as on a set of artifically simulated images with pre-defined characteristics.

Analysis and design of an anamorphic optical processor for speckle metrology and velocimetry

Two-dimensional velocity or displacement fields that are recorded as double-exposure speckle patterns may be measured with an anamorphic optical processor. The anamorphic optical processor measures one component of the velocity or displacement throughout a linear region of the double-exposure speckle pattern. The signal in the output plane of such a processor is a fringe pattern. An analysis to predict fringe modulation based on Fourier optics and the statistics of speckle patterns is developed. The analysis specifies the effects of both the speckle-recording optics and the anamorphic optical processor on the fringe modulation. The observed loss of modulation when the velocity or displacement component perpendicular to the measured component is increased is also described. Measurements of fringe modulations that are produced by anamorphic optical processors with and without increased tolerance to the perpendicular velocity component are compared to results of the analysis. Fringe modulation versus relative exposure in the double-exposure speckle pattern are also measured and compared to the results of this analysis. An anamorphic processor that reduces the time for digital processing of the fringe patterns by a factor of N (typically 128, 256, or 512) relative to the two-dimensional Fourier transform optical processor (with identical spatial resolution) is demonstrated.

B210 車載用空調機ヒータユニット内流れの定量的可視化(第2報)

Recently, factors required for design automobile air conditioning system are amenity, noise control, automatic temperature control and so on. To improve these factors, basic analysis of flows in a heater unit or in ducts of air conditioning system is very important.In this report, velocity measurements and visualization of flows in a practical model of heater unit are carried out by means of the dual-beam-sweep laser speckle velocimetry, and flow patterns obtained by different operation modes are investigated.

38 車載用空調機ヒータユニット内流れの定量的可視化

Experimental analysis of flows in a heater unit of automo-bile air conditioning system is very important for fundamental design, improvement of efficiency, noise control and so on. In this report, velocity measurements and visualization of flows in a heater unit produced by different operation modes are carried out by means of the dual-beam-sweep laser speckle velocimetry and laser-light-sheet method, respectively.

Image processing for laser speckle velocimetry using the 2-D fast Fourier transform

An algorithm is described for the fringe analysis in laser speckle velocimetry. Based on the 2-D fast Fourier transform, the method relies on inherent features in the fringe pattern to remove efficiently the influence of the diffraction halo. A windowing operation is performed to enhance the reliability and reduce the influence of various noise contributions.

デュアルビームスイープ・レーザスペックル流速計の開発

The Laser Speckle Velocimetry (LSV) is one of the most useful method of quantitative visualization of flow fields. However, some techniques in this method are necessary to improve for a wide use. One of these is illumination technique.We propose a new illumination technique of a observation plane by sweeping of two laser beams scanned by the same face of a polygon mirror scanner. With this method seeding particles in the plane are illuminated by each sweeping beam with short time difference controlled by incident cross angle of two beams to the scanner and also angular velocity of the scanner.In this report, the characteristics of the dual-beam-sweep laser speckle velocimetry and application results are discussed.

Fringe positions in double-exposure speckle photography

Double-exposure records in speckle photography or particle image velocimetry are often evaluated by analysis of the system of Young's diffraction fringes. Fringe spacing, necessary to calculate the displacement, is determined from the positions of fringe maxima or minima. These, however, are influenced by the diffraction halo function and by fringe visibility. A generalized theory of the effects is presented, including position dependent visibility and fringe phase. Evaluations are given for disk-shaped particle images in particle image velocimetry, and for coherent and incoherent speckle photography. Fringe shifts are determined numerically for commonly encountered values of fringe density and visibility thus presenting a basis for rapid assessment of accuracy in metrological experiments.

The differential speckle velocimetry — A method to measure the velocity of random patterns

Random spatio-temporal intensity patterns occur especially in the field of laser light scattering. In many cases the spatial motion of the intensity field of the scattered light contains information on the scatterer (speckle anemometry). We propose a new technique for measuring this velocity, which is especially suited for fast changing patterns (fast speckle boiling), and show experimental results.

Space–time correlation properties of dynamic laser speckle in the near diffraction field of a longitudinally moving diffuse object

The space–time autocorrelation function of detected speckle-intensity fluctuation is derived for a quite general case in which both the diffuse object and the photodetector move along the axis of an illuminating Gaussian beam. Correlation properties of some particular cases of this general arrangement are then analyzed. The considerations presented here are limited to the near diffraction field in which the speckle complex amplitude is assumed to obey circular Gaussian statistics. New time-stationary processes are revealed, and the possibility of their application in speckle velocimetry is discussed.

Parallel optical evaluation of double-exposure records in optical metrology

The evaluation of double-exposure records in optical metrology (speckle photography or particle image velocimetry) is simplified by using two-step optical processing that is performed on many interrogation areas simultaneously by a 2-D array of narrow focused light beams. A first application of this procedure to the original record, if dimensioned properly, produces an array of small nonoverlapping Young's fringe systems. The photographic record of these patterns is subjected to the same operation once more, each beam illuminating precisely one pattern. The resulting output is an array of autocorrelation functions that are a direct representation of the displacement field since the spacing of respective side peaks gives the displacement. A single whole-field interrogation of the array of fringe systems produces an optical representation of accumulated displacement values thus rendering the statistics of the displacement field. The required matrix of light beams is generated by holographic optical elements.

Digital image velocimetry

A new technique, digital image velocimetry, is proposed for measurement of instantaneous velocity fields of time dependent flows. A time sequence of single-exposure images of seed particles are captured with a high speed camera, and the sampled images are then digitized on an image processor, enhanced, and superposed to construct a multiple-exposure image. Both the superposed image and a single-exposure image are digitally Fourier transformed for extraction of information on the velocity field. The new technique is designed to improve the flow measurement by eliminating some restrictions of existing optical methods such as laser speckle velocimetry and particle image velocimetry. These restrictions include the limited dynamic range of the velocity measurement, directional ambiguity of the velocity vector, and the difficulty of a real-time capability. The analysis and numerically simulated experiment show that improvement can be achieved with the new technique. Laboratory experiments are currently under investigation.

Airborne Ultrasonic Speckle Velocimetry

An ultrasonic speckle velocimetry has been proposed for blood flow measurement in the fields of clinical medicine. This method is capable of measuring the velocity of a target which moves along a direction perpendicular to the ultrasonic beam. The Doppler method cannot measure this velocity. A feasibility study of use of this method in the air is reviewed.

ビームスイープ・レーザスペックル流速計による非定常二次元流速分布計測および可視化 ぜん動流路内流れについての計測例

The Laser Speckle Velocimetry (LSV) is one of the most useful method of quantitative flow visualization, because it gives two-dimensional instantanious velocity distributions. But this method is not suitable for continuous measurement of unsteady flow, because normally the repetition rate of pulse laser is low.We have reported a new illumination technique by making use of CW laser, and named the Beam-Sweep Laser Speckle Velocimetry (BS-LSV). A plane of observation in a flow field seeded with tracer particles is illuminated by periodical one-way sweeps of a laser beam instead of pulsed light sheets. This method is suitable for measurements of an unsteady flow, because the sequential specklegrams can easily be recorded continuously.In this report, BS-LSV is applied to two-dimensional velocity measurements and visualization of pulsating flow in a peristaltic pump.

Electrooptical image shifting for particle image velocimetry

The direction of fluid motion can be determined in particle image velocimetry (and laser speckle velocimetry) if the image field is spatially shifted between the first and second exposures, so that the most negative fluid velocity still produces a positive particle image displacement. Two new techniques are described which accomplish very fast accurate image shifting using electrooptical hardware. These methods involve regulating the polarization of light scattered from the seeding particles by switching the polarization of the illuminating beam, and placing a birefringent unaxial plate in front of the photographing lens to displace the particle image between illuminating pulses.

Real-time photorefractive recording and optical processing for speckle velocimetry

A photorefractive crystal (Bi(12)GeO(20)) is used to record double-exposure speckle patterns produced by Q-switched laser pulses in real-time speckle metrology and velocimetry applications. The speckle pattern displacements are immediately measured with an optical processor of the type used for measuring speckle displacements recorded on photographic film. Operation of the optical processor is described and compared with the case of film recording. Observed spatial and temporal recording properties of the crystal in this imaging configuration are discussed and compared with published results for single-frequency grating recording in photorefractives. The system is applied to measuring velocities of a moving solid object.

Improving the accuracy and resolution of particle image or laser speckle velocimetry

Particle Image Displacement Velocimetry (PIDV) or Laser Speckle Velocimetry (LSV) is a non-intrusive 2-D method of obtaining instantaneous fluid velocities over an extended region. This method involves lighting a particle seeded flow with a sheet of light (usually from a laser) and photographing the particles at two instants of time on the same film. This film is then analyzed to determine the displacement of the particle over the known time interval. Applications of PIDV and LSV have been diverse, including flow past cylinders and airfoils by Lourenco et al. (1986) and Smith et al. (1986), transient Benard convection by Simpkins and Dudderar (1978), vortex pairing in jets by Meynart (1983) and internal gravity waves by Gartner et al. (1986). These studies however, have all been two dimensional. The velocities along the lighted plane were measured, from which the two dimensional vorticity fields and streamlines were calculated using finite differencing (Lourenco et al. 1986, and Smith et al. 1986).