A spatial correlation technique for estimating velocity fields using molecular tagging velocimetry (MTV)

A direct spatial image correlation technique is presented for estimating the Lagrangian displacement vector from image pairs based on molecular tagging diagnostics. The procedure provides significant improvement in measurement accuracy compared to existing approaches for molecular tagging velocimetry (MTV) analysis. Furthermore, this technique is of more general utility in that it is able to accommodate other laser tagging patterns besides the usual grid arrangement. Simulations are performed to determine the effects of many experimental and processing issues on the sub-pixel accuracy of displacement estimates. The results provide guidelines for optimizing the implementation of MTV. Experimental data in support of this processing technique are provided.     

Laser velocimetry measurements in a laboratory vaned diffuser

Laser velocimetry measurements were made within a laboratory radial vaned diffuser with three different blade configurations. Measurements were made through passages with four, six and eight blades installed at off design conditions. Also, in the eight blade diffuser measurements were made between the blade passage exit and diffuser exit so that the complete secondary flow could be defined. The flow was found to separate from the blades and form large separation zones. The separation zones consisted primarily of two vortices rotating in opposite directions. At the passage exit the separation region encompassed 23% of the circumferential area for the four blade diffuser, 45% for the six blade and 40% in the eight blade diffuser. Separation occurred at 23%, 27% and 50% from the leading edge of the blades for the 4, 6 and 8 bladed diffusers, indicating that more blades better controlled the separation. Turbulence intensities ranged from approximately 5% to 15% in the primary flow and reached a few hundred percent in the secondary flow within the separation regions.     

Laser velocimetry measurements in a horizontal gas-solid pipe flow

This paper presents laser measurements of particle velocities in a horizontal turbulent two-phase pipe flow. A phase Doppler particle analyzer, (PDPA), was used to obtain particle size, velocity, and rms values of velocity fluctuations. The particulate phase consisted of glass spheres 50 m in diameter with the volume fraction of the suspension in the range _p=10^-4 to _p=10^-3. The results show that the turbulence increases with particle loading.List of symbols a particle diameter - C _va velocity diameter cross-correlation - d pipe diameter - Fr ² Froude number - g gravitational constant - p(a) Probability density of the particle diameter - Re pipe Reynolds number based on the friction velocity - T characteristic time scale of the energy containing eddies - T _L integral scale of the turbulence sampled along the particle path - u, U, u characteristic fluid velocities: fluctuating, mean and friction - v characteristic velocity of the paricle fluctuations - f expected value of any random variable f - f¦g expected value of f given a value of the random variable g - _p particle volume fraction - _p particle response time - absolute fluid viscosity - v kinematic fluid viscosity - _p, _f densities, particle and fluid - _a ² particle diameter variance - _va ² velocity variance due to the particle diameter variance - _vT ² total particle velocity variance - _vt ² particle velocity variance due to the response to the turbulent field     

Time and spatial correlation measurements in the turbulent wake of a circular cylinder using Laser Doppler Anemometry

The present paper reports Laser Doppler Anemometry (LDA) measurements in the turbulent (R _d =1300) wake of a circular cylinder (d=2 mm). On several typical locations in the wake, at cross-section x/d=125, temporal and lateral space velocity correlations have been measured. Temporal autocorrelation functions were determined from the randomly sampled LDA data by means of the Slotting Technique. The autocorrelation functions which were obtained were too low, the discrepancy giving an indication of the influence of noise in the measurements. The influence of noise generated by the photomultiplier tubes, which appeared to be the most important noise source, could be reduced by a cross-correlation technique. A partly new LDA system for two-point velocity correlation measurements was developed, consisting of an elongated measuring volume and a two-point detection system including a semireflecting mirror to divide the scattered light. The results of this investigation clearly demonstrate that LDA is a powerful tool for measuring time and spatial correlations and related properties of turbulence.     

Analysis and treatment of errors due to high velocity gradients in particle image velocimetry

This paper deals with errors occurring in two-dimensional cross-correlation particle image velocimetry (PIV) algorithms (with window shifting), when high velocity gradients are present. A first bias error is due to the difference between the Lagrangian displacement of a particle and the real velocity. This error is calculated theoretically as a function of the velocity gradients, and is shown to reach values up to 1 pixel if only one window is translated. However, it becomes negligible when both windows are shifted in a symmetric way. A second error source is linked to the image pattern deformation, which decreases the height of the correlation peaks. In order to reduce this effect, the windows are deformed according to the velocity gradients in an iterative process. The problem of finding a sufficiently reliable starting point for the iteration is solved by applying a Gaussian filter to the images for the first correlation. Tests of a PIV algorithm based on these techniques are performed, showing their efficiency, and allowing the determination of an optimum time separation between images for a given velocity field. An application of the new algorithm to experimental particle images containing concentrated vortices is shown.     

4D Magnetic resonance velocimetry for mean velocity measurements in complex turbulent flows

An adaptation of a medical magnetic resonance imaging system to the noninvasive measurement of three-component mean velocity fields in complex turbulent engineering flows is described. The aim of this paper is to evaluate the capabilities of the technique with respect to its accuracy, time efficiency and applicability as a design tool for complex turbulent internal geometries. The technique, called 4D magnetic resonance velocimetry (4D-MRV), is used to measure the mean flow in fully developed low-Reynolds number turbulent pipe flow, Re=6400 based on bulk mean velocity and diameter, and in a model of a gas turbine blade internal cooling geometry with four serpentine passages, Re=10,000 and 15,000 based on bulk mean velocity and hydraulic diameter. 4D-MRV is capable of completing full-field measurements in three-dimensional volumes with sizes on the order of the magnet bore diameter in less than one hour. Such measurements can include over 2 million independent mean velocity vectors. Velocities measured in round pipe flow agreed with previous experimental results to within 10%. In the turbulent cooling passage flow, the average flow rates calculated from the 4D-MRV velocity profiles agreed with ultrasonic flowmeter measurements to within 7%. The measurements lend excellent qualitative insight into flow structures even in the highly complex 180° bends. Accurate quantitative measurements were obtained throughout the Re=10,000 flow and in the Re=15,000 flow except in the most complex regions, areas just downstream of high-speed bends, where velocities and velocity fluctuations exceeded MRV capabilities for the chosen set of scan parameters. General guidelines for choosing scanning parameters and suggestions for future development are presented.     

Measurement of two-dimensional velocity fields in porous media by particle image displacement velocimetry

Local measurements of the phase function and of two components of the velocity can be performed in transparent porous media by means of particle image displacement velocimetry (P.I.D.V.). Some preliminary results are presented and discussed.     

Image correlation velocimetry applied to discrete smoke-wire streaklines in turbulent pipe flow

Two-colour image correlation velocimetry is applied to distinct smoke-wire streaklines in turbulent pipe flow. A single frame from a 35-mm film camera is exposed to two consecutive flashes, one from a red and one from a blue stroboscope. The resulting composite image is digitized, and the color components are extracted to obtain two images. Image correlation is then applied to estimate the velocity which is then compared with hot-wire velocity measurements.     

Laser Doppler velocimetry measurements in an interior flow test facility: A database for CFD-code evaluation

Measurements in a test facility for a complex interior flow are provided as a database for CFD code evaluation. For pure forced convection as well as to a minor extent also for mixed convection flows, the three time-averaged components of the velocity vector and all components of the Reynolds stress tensor are measured in selected cross sections. Special attention is given to the inflow into the main flow section, since it is the important part of the boundary condition for a numerical solution.     

Simultaneous measurements of velocity and stress distributions in polyisobutylenes using laser-Doppler velocimetry and flow induced birefringence

An experimental device was set up for the synchronous measurement of velocities and stresses in polyisobutylenes using laser-Doppler velocimetry (LDV) and the two-colour flow-induced birefringence method (FIB). The materials investigated are three low molecular polyisobutylenes. Velocity (LDV) and stress (FIB) measurements are performed in the flow entrance region and inside a slit die with a contraction ratio of 1:10. The behaviour of the polyisobutylenes is Newtonian under the flow conditions applied. Therefore, the stresses inside the fluids can be calculated and compared to the stresses experimentally determined. A good agreement in shear and elongational flows was found between the calculated (LDV) and directly measured stresses (FIB). This result demonstrates the applicability of the experimental setup as an optical rheometer that can preferentially be used to measure elongational properties of low viscous fluids.

Simultaneous concentration and velocity measurements using combined laser-induced fluorescence and laser Doppler velocimetry: Application to turbulent transport

This paper describes the implementation of an optical technique, allowing to perform concentration and velocity measurements simultaneously and at the same point. This method is based on the coupling of laser-induced fluorescence of rhodamine B, applied to the determination of local concentration, and laser Doppler velocimetry. The method developed provides an accurate measurement of the concentration-velocity cross-correlation. The latter is a parameter linked to the eddy diffusivity tensor of a passive contaminant. This method was tested with a turbulent submerged free jet and it allowed the determination of the mean field of concentration and velocity, the concentration-velocity cross-correlation, and the local eddy diffusivity.List of symbols C molar concentration - c fluctuating part of the concentration - mean value of the concentration - concentration-velocity cross-correlation - D molecular diffusivity - (D _eddy) _ij eddy diffusivity tensor - I _abs absorbed intensity - I _e local incident intensity - K _opt optical constant - N number of samples - r _c half-width radius for the concentration profile - r _v half-width radius for the velocity profile - S _f fluorescence signal - Sc Schmidt number - V _c collection volume - U velocity - U _e flow velocity in the channel - U _i injection velocity Greek symbols kinetic energy dissipation rate - ₁ molar extinction coefficient for the laser radiation (in m²mol^–1l^–1) - ₂ molar extinction coefficient for the fluorescence signal (in m²mol^–1l^–1) - quantum yield - _c Batchelor scale - _k Kolmogorov scale - v kinematic viscosity - normalized values     

Molecular tagging velocimetry and other novel applications of a new phosphorescent supramolecule

 The development and applications of a new class of water-soluble compounds suitable for molecular tagging diagnostics are described. These molecular complexes are formed by mixing a lumophore, an appropriate alcohol, and cyclodextrin. Using 1-BrNp as the lumophore, cyclohexanol is determined to be the most effective overall among the alcohols for which data are currently available. Information is provided for the design of experiments based on these complexes along with a less complex method for generating the grid patterns typically used for velocimetry. Implementation of a two-detector system is described which, in combination with a spatial correlation technique for determining velocities, relaxes the requirement that the initial tagging pattern be known a priori, eliminates errors in velocity estimates caused by variations in the grid pattern during an experiment, and makes it possible to study flows with non-uniform mixtures. This detection and analysis combination also solves one of the problems associated with using caged fluorescein to study high-speed flows. In addition to the traditional implementation for velocimetry, novel applications for studying the Lagrangian evolution of both reacting and non-reacting interfaces and obtaining combined passive scalar/velocity measurements are demonstrated. Received: 26 August 1996/Accepted: 13 March 1997     

Laser Doppler velocity bias in separated turbulent flows

Velocity bias effects on data obtained with a coincident two channel laser Doppler velocimeter in a highly turbulent separated supersonic flow are presented. Probability distributions of the fluctuating velocities were distorted by velocity bias in a manner consistent with theory and a two-dimensional velocity inverse weighting function bias correction produced reasonable appearing velocity probability distributions. The addition of an approximate correction term to account for the effects of the unmeasured third velocity component improved these results but had little effect on the velocity statistics. Experimental factors that could partially compensate or falsely add to the velocity bias, conditions for the bias to occur, and conditions for which the bias may also be observed and corrected for are discussed.     

Computed tomographic X-ray velocimetry for simultaneous 3D measurement of velocity and geometry in opaque vessels

Computed tomographic X-ray velocimetry has been developed for simultaneous three-dimensional measurement of flow and vessel geometry. The technique uses cross-correlation functions calculated from X-ray projection image pairs acquired at multiple viewing angles to tomographically reconstruct the flow through opaque objects with high resolution. The reconstruction is performed using an iterative, least squares approach. The simultaneous measurement of the object’s structure is performed with a limited projection tomography method. An extensive parametric study using Monte Carlo simulation reveals accurate measurements with as few as 3 projection angles, and a minimum required scan angle of only 30°. When using a single/source detector system, the technique is limited to measurement of periodic or steady flow fields; however, with the use of a multiple source/detector system, instantaneous measurement will be possible. Synchrotron experiments are conducted to demonstrate the simultaneous measurement of structure and flow in a complex geometry with strong three-dimensionality. The technique will find applications in biological flow measurement, and also in engineering applications where optical access is limited, such as in mineral processing.     

Full-field optical strain measurement having postrecording sensitivity and direction selectivity

An optical method of strain measurement is described which possesses numerous advantages over conventional and holographic interferometry. The method consists of imaging the coherently illuminated object by a lens, and double-exposing a photographic plate in a misfocused image plane before and after deformation of the object. The processed photographic plate or ‘specklegram’ is subsequently Fourier-filtered to provide fringe patterns representing derivatives of surface displacements with respect to any desired direction and with variable and controllable sensitivity.     

Measurement of acoustic velocity in the stack of a thermoacoustic refrigerator using particle image velocimetry

Thermoacoustic refrigeration systems generate cooling power from a high-amplitude acoustic standing wave. There has recently been a growing interest in this technology because of its simple and robust architecture and its use of environmentally safe gases. With the prospect of commercialization, it is necessary to enhance the efficiency of thermoacoustic cooling systems and more particularly of some of their components such as the heat exchangers. The characterization of the flow field at the end of the stack plates is a crucial step for the understanding and optimization of heat transfer between the stack and the heat exchangers. In this study, a specific particle image velocimetry measurement is performed inside a thermoacoustic refrigerator. Acoustic velocity is measured using synchronization and phase-averaging. The measurement method is validated inside a void resonator by successfully comparing experimental data with an acoustic plane wave model. Velocity is measured inside the oscillating boundary layers, between the plates of the stack, and compared to a linear model. The flow behind the stack is characterized, and it shows the generation of symmetric pairs of counter-rotating vortices at the end of the stack plates at low acoustic pressure level. As the acoustic pressure level increases, detachment of the vortices and symmetry breaking are observed.