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The capability of non-intrusively tagging the molecules in a flowing medium and observing their subsequent evolution offers exciting new possibilities for studying fluid dynamics. Molecular tagging methods have been used primarily for flow velocimetry, even though more recently they have also found novel uses in studying the Lagrangian evolution of passive scalar fields, entrainment and mixing in turbulent flows. Molecular Tagging Velocimetry (MTV) relies on molecules that can be turned into long lifetime tracers upon excitation by photons of an appropriate wavelength. These molecules are either naturally present in the flowing medium (i.e. unseeded applications) or need to be premixed. Typically a pulsed laser is used to `tag' the regions of interest, and those tagged regions are interrogated at two successive times within the lifetime of the tracer. The measured Lagrangian displacement vector provides the estimate of the velocity vector. This velocimetry approach offers particular advantages over particle-based techniques in flows in which the use of seed particles is not warranted or may lead to complications such as flow tracking problems and buoyancy effects.

While the earliest use of molecular tagging velocimetry can be traced back at least three decades, this technique has seen significant advances over the past ten years (sometimes under alternate titles such as laser-induced photochemical anemometry or flow tagging velocimetry). These advances have been driven by improvements in laser and imaging techniques, data analysis methods, and chemical design and synthesis of novel molecular structures. As a result, the use of this velocimetry technique has been increasing steadily. The MTV technique has been used in flows over a wide range of speeds, from liquid-phase flows with speeds below 1 mm s^-1 to gas-phase flows at supersonic speeds. The scope of the measurements covers a range from the instantaneous profile of one component of velocity vector along a tagged line to whole-field three-component velocity data over a plane which are obtained using stereo imaging. Some of the flows that have been investigated include pulsatile flow in tubes, internal circulation in droplets, unsteady boundary layer separation, convective flows in directional solidification, flows during intake and compression inside motored IC engines, free and wall-bounded turbulent flows, and highly three-dimensional vortex flows with strong out-of-plane motions. Summaries of various molecular tagging methods and an extensive reference list of papers and applications can be found in several review articles which have appeared over recent years [1-4].

The aim of this special feature is to provide the reader with an overview of the recent progress and current state-of-the-art in the instrumentation, application, and extensions of molecular tagging methods for velocimetry and mixing studies. Many thanks go to the authors and reviewers who made this possible. Thanks also go to IOP Publishing for supporting this initiative, and to the staff for their effort and hard work.

References

[1] Falco R E and Nocera D G 1993 Quantitative multipoint measurements and visualization of dense solid--liquid flows using laser induced photochemical anemometry (LIPA) Particulate Two-Phase Flow ed M C Rocco (London: Butterworth-Heinemann) pp 59-126

[2] Koochesfahani M M, Cohn R K, Gendrich C P and Nocera D G 1996 Molecular tagging diagnostics for the study of kinematics and mixing in liquid phase flows Proc. 8th Int. Symp. on Applications of Laser Techniques to Fluid Mechanics, 8-11 July, 1996, Lisbon vol I, 1.2.1-1.2.12; also in 1997 Developments in Laser Techniques and Fluid Mechanics ch 2, section 1, eds Adrian, Durao, Durst, Maeda and Whitelaw (Berlin: Springer)

[3] Koochesfahani M M 1999 Molecular tagging velocimetry (MTV): progress and applications AIAA Paper AIAA-99-3786

[4] Lempert W R 2000 Molecular Tagging Velocimetry Flow Visualization: Techniques and Examples ed A J Smits and T T Lim (London: Imperial College Press) in press

The effects of inlet conditions on downstream mixing in turbulent pipe flow have been studied with the use of photoactivatable fluorophores and standard laser-induced fluorescence techniques. The different inlet conditions included both geometry changes and changes in the manner in which the constituents were introduced into the flow. Results indicate that small changes in inlet geometry can greatly affect the downstream mixing rate. Changes in the geometry of the inlet had a greater influence on downstream mixing than did the manner in which constituents were introduced into the flow. Further experiments included a static mixer that was used in conjunction with two different inlet conditions. It was found that the inlet condition greatly affects the effectiveness of the static mixer. The static mixer is most effective when placed downstream of an inlet that produces scalar length scales that are similar in size to the elements in the mixer (i.e., the pipe diameter). Overall, the results of these experiments demonstrate that the method used to introduce two constituents to be mixed in pipe flow can profoundly affect the downstream mixing rate.

The use of caged dye photo-activated fluorophore velocimetry is described and representative examples are presented. After a brief survey of recently reported measurements, a more detailed example of the flow produced in a cylinder with a single rotating end wall is presented. Simultaneous stereoscopic image sets in the (r,z) and (r,θ) planes have been obtained over a Reynolds number range of roughly 10²-10⁵. At low Reynolds numbers (≃0-2000), the steady, axisymmetrical flow is found to quantitatively agree with predictions from a numerical flow solver. At higher Reynolds numbers (from 5×10³ to 10⁵), the flow develops considerable turbulent three-dimensional structure.

Two complementary unseeded molecular flow tagging techniques for gas-flow velocity field measurement at low and high temperature are demonstrated. Ozone tagging velocimetry (OTV) is applicable to low-temperature air flows whereas hydroxyl tagging velocimetry (HTV) is amenable to use in high-temperature reacting flows containing water vapour. In OTV, a grid of ozone lines is created by photodissociation of O₂ by a narrowband 193 nm ArF excimer laser. After a fixed time delay, the ozone grid is imaged with a narrowband KrF laser sheet that photodissociates the ozone and produces vibrationally excited O₂ that is subsequently made to fluoresce by the same KrF laser light sheet via the O₂ transition B ³Σ_u^-(v' = 0, 2) ← X ³Σ_g^-(v'' = 6, 7). In HTV, a molecular grid of hydroxyl (OH) radicals is written into a flame by single-photon photodissociation of vibrationally excited H₂O by a 193 nm ArF excimer laser. After displacement, the OH tag line position is revealed through fluorescence caused by OH A ²Σ⁺-X ²Π (3←0) excitation using a 248 nm tunable KrF excimer laser. OTV and HTV use the same lasers and can simultaneously measure velocities in low and high temperature regions. Instantaneous flow-tagging grids are measured in air flows and a flame. The velocity field is extracted from OTV images in an air jet using the image correlation velocimetry (ICV) method.

Raman excitation plus laser-induced electronic fluorescence (RELIEF) images the motion of oxygen molecules in air and other gas mixtures. This is accomplished by tagging oxygen molecules through vibrational excitation and imaging them after a short period of time by laser-induced electronic fluorescence. The vibrational lifetime of oxygen is sufficiently long and the signal sufficiently strong to allow this technique to be used over a wide range of flow conditions, from low subsonic to hypersonic, and in a variety of gas mixtures including high humidity environments. The utilization of a molecular tagging technique such as this is critical for environments in which seeding is impossible or unreliable and for measurements in which a wide range of scales needs to be observed simultaneously. Two experiments which have been conducted at national laboratories in medium- to large-scale facilities are reported. At the Arnold Engineering Development Center, RELIEF was used to examine velocity in a 1 m diameter tunnel for applications in the area of engine testing. At the NASA Langley Research Center, RELIEF is being used to examine supersonic mixing of helium in air in a coaxial jet in association with studies of fuel-air mixing in hypersonic engines. These applications are two examples of the wide range of practical uses for this new technology.

Molecular tagging velocimetry (MTV) involves intensive data reduction that extracts flow velocity information from the Lagrangian tracking of phosphorescing fluid material. A computationally efficient algorithm for the data reduction is thus of practical interest for processing large MTV data sets. We were motivated by this consideration into developing a simplified version of the spatial correlation technique, the decoupled spatial correlation technique, in an effort to seek a balance between accuracy and efficiency. By Taylor series analysis it is shown that, if the Lagrangian displacement vector can be roughly pre-determined, the two components in the displacement vectors that have to be solved simultaneously using the spatial correlation technique can now be determined independently in two orthogonal directions. This decoupling results in about an order of magnitude decrease in the CPU time required. An accuracy estimate based on artificial images that follow the motion of a line vortex indicates that the technique can determine displacements to within 0.08 pixel. This technique was also used to process MTV images acquired in a cross stream plane of the transverse jet. This flow is characterized by a large scale counter-rotating vortex pair (CVP). The velocity fields obtained clearly show the existence of this CVP, which provides further verification of this technique.

A new technique for the simultaneous measurement of velocity and concentration fields is described. We describe here applications in liquid-phase flows, but the methodology can be extended to gas-phase flows with appropriate tracers. In this single-laser, two-tracer approach, molecular tagging velocimetry (MTV) based on the use of a phosphorescent compound is combined with laser induced fluorescence (LIF) using fluorescein as a tracer. Results show that one can design experiments with minimal cross-talk between the LIF and MTV signals. Applications of the simultaneous MTV-LIF technique are demonstrated by performing simultaneous flow visualization and vorticity measurements in a low Reynolds number forced wake and simultaneous velocity-concentration measurements in a turbulent mixing layer. Preliminary data on the mean and RMS fluctuation of velocity and concentration are presented, together with the correlation between velocity and concentration fluctuations.

A method for the optimization of illumination in line-scan camera systems is proposed. By using this method it is possible to calculate the optimal positions and luminous intensities of the light sources to obtain the desired illuminance at a surface. The proposed method is especially suitable for producing uniform illumination on a line. The method can be used with several types of light source, because experimental general models can be used for the lamps. The method is based on the regularized least squares optimization. The performance of the method is evaluated by comparing the calculated and measured illuminances before and after the optimization.

This paper describes the development and application of a dual optical probe for local volume fraction, drop velocity and drop size measurements in a kerosene-water liquid-liquid two-phase flow. A sampling tube procedure and interface tests were used to calibrate the leading optical sensor and both optical sensors for measurements of the local volume fraction and drop velocity as described in this paper. Measurements were carried out in a large-scale vertical two-phase facility mainly at the pipe centre-line to demonstrate the advantages of using optical fibres with normal cut ends in a kerosene-water two-phase flow. Finally the applicability of this probe for measurements in gas-liquid two-phase flows and gas-kerosene-water three-phase flows has also been demonstrated.

An optical method is described to perform instantaneous thermometry along a line in low-density air seeded with I₂. Laser-induced fluorescence is generated by exciting the I₂ Cordes bands (D ¹Σ_u⁺ ← ¹Σ_g⁺) with a tunable ArF excimer laser near 193 nm. Temperatures from 295 to 648 K are measured by fitting the I₂ emission spectrum from 188 to 195 nm to a vibrational Boltzmann distribution. Uncertainties for average measurements are typically ±5% (±1σ).

A noise-immune method for phase retrieval of a single moiré interferometric fringe pattern is presented and discussed. The method is shown to provide accurate recovery of the phase information by a combined method based on modification of the local intensity histogram and use of a two-dimensional Fourier transform of the enhanced moiré fringe pattern. The principle of the method is described and the experimental results of moiré interferometric measurements with submicrometre sensitivity of the in-plane displacement fields of thick carbon fibre/PEEK composite laminates are presented as an example of the application of the technique.

Two Monte Carlo algorithms that calculate measurement uncertainties are presented. The algorithms are discussed in the context of the International Organisation for Standardization's (ISO's) published guidelines for the expression of uncertainty in measurement. They are intended for use in problems where more than one estimated input quantity has finite degrees of freedom. The Monte Carlo approach offers a calculation method that is intuitive, simple to implement, relatively insensitive to nonlinearity and does not require an expansion of the measurement function in a Taylor series.

Magnetic force imaging of soft magnetic materials is often a challenge due to perturbing effects of the probe's stray field on the magnetic structure of the sample if the probe and the sample are too close. Especially with one commonly employed imaging mode, the lift mode, which is used in some commercial instruments, this problem is very severe, since very small tip-sample distances frequently occur during the imaging of the sample topography. In this study the effect is demonstrated by the imaging of arrays of micro-structured Permalloy films. It is shown that perturbations can be diminished by depositing a mechanical spacer on top of the probe's magnetic layer, which ensures that a minimum distance between the magnetic part of the probe and the sample is always kept.

We present a simple and economical photoreflectance measurement methodology that can easily be adapted to conventional scanning optical microscopes. In this design, the laser source is sine-wave modulated so that the second-harmonic modulation distortion within the optical probe beam before and after reflection off the sample surface can be monitored. The desired photoreflectance measurement, which is taken as the change of reflectance as a result of varying the incident optical power, is then obtained from the change in the ratio of the fundamental and second-harmonic signals. We demonstrate our set-up and technique by measuring the implantation damage in nitrogen-implanted silicon samples.

This paper proposes a novel method for measuring nanometric displacement by detecting fringe movement of interferograms which is linearly proportional to the displacement of an object. Interferometers have been used for precision measurement of displacement, but they have such error sources as unequal gain of detectors, imbalance of beams and lack of quadrature. These error sources degrade the accuracy of the interferometer. However, the fringe movement of interferograms has little relation to these error sources. In order to investigate the performance of the proposed method, analyses and simulations of speckle noise, Gaussian noise and wavefront distortion were executed. Results of the simulations show that the proposed method is robust against these errors. Experiments were performed to verify this method.

The evaluation of the uncertainty in measurement in the presence of combined random errors and errors due to the analogue-to-digital conversion of measuring signals is studied. The following situation is investigated. Analogue measuring signals of a measurand under study are obtained. The signals are assumed to contain random errors with a Gaussian distribution and they are converted into the digital form. Only the resulting digital signals are to be available.

The Bayesian theory of measurement uncertainty is applied to the analysis. An estimator is assigned to the measurand and the distribution of this estimator is determined. This distribution can then be used to numerically calculate a best estimate of the measurand and its associated uncertainty.

The application of this approach to some examples is presented and compared with results obtained by treating random and analogue-to-digital conversion errors additively and independently, which is frequently done. In some cases, remarkable differences are found which can justify the increased time and effort involved in the approach proposed.

An image in an x-ray microscope is a projected image that includes all information throughout the thickness of the object being exposed. This results in a loss of level information from the different layers. In this work, a method has been developed to distinguish the level of metal layers in multilayer printed circuit boards (PCBs) without measurement coupons, and by the use of x-ray microscopy. To distinguish the levels of two metal layers, an indicator point was first defined on the edge of each layer and used for identifying the layer it belonged to. The sample was then moved parallel to the connecting line of the two indicator points. During the movement, the change trend in the projected distance between the indicator points was evaluated visually or by using a ruler or an image analyser. If the projected distance increases, the leading point is on the upper layer and the trailing point on the lower one. If the projected distance decreases, the leading point is on the lower layer, while the trailing point is on the upper one. For a layer structure with more than two layers, the level order can be determined by comparing each layer pair.

Spectral analysis and computer simulation are adopted to investigate the tracing of a two-dimensional fractal profile with a mechanical profiler with a spherical tipped stylus. It is shown that the critical wavenumber strongly depends on the fractal dimension D and the ratio of the non-scale parameter G to the tip radius r. By using the concept of the critical wavenumber, the spectral density of a measured profile can be estimated. Also, formulae for estimating the critical wavenumber and the error of the mean square height are developed.

This paper describes the design and fabrication of a six-component force/moment sensor testing machine and the evaluation of its uncertainty. This testing machine, which generates forces F_x (x-direction force), F_y and F_z from 50 to 500 N each and moments M_x (x-direction moment), M_y and M_z from 5 to 50 N m each, simultaneously or separately, consists of a body, a fixture, a force generating system, a moment generating system, weights and a control system. Forces and moments are automatically generated by the control system composed of a computer, three step motors and so on. Also, the relative expanded uncertainty of the testing machine is evaluated. The results show that the relative uncertainties for force components ±F_x and ±F_y and moment components ±M_x and ±M_y are less than 8.6×10^-4 and those for force components + F_z and -F_z and moment components ±M_z are less than 1.7×10^-3, 1.2×10^-5 and 1.7×10^-3 respectively.

The paper describes a novel automated separator, of which the key point is the determination of ash in coal by a technique based on the attenuation of dual energy γ-rays transmission. The technique is often used to determine the concentration of a higher atomic number (Z) component in a lower Z matrix. We designed a novel apparatus to realize the on-line non-contact measurement of ash in the virgin lump coal during the process of conveying. As a result, the gangues in the virgin lump coal are recognized by using a threshold ash method and removed by the pneumatic actuator. In this paper the principle of this measurement and the arrangement for separation are presented. Both the main factors affecting the dynamic determination and methods for reducing errors in analysis are also proposed. Furthermore, the experimental verification of the analysis has been conducted. The results are in good agreement with the theoretical analysis.

A shearography system that measures the deformation gradient using two orthogonal shear directions and performs phase stepping, using no moving mechanical components and a single CCD camera, is described. The light exiting from a highly linearly birefringent optical fibre is switched between two orthogonal linearly polarized states by tuning the optical wavelength of a laser diode via injection current modulation. A polarization sensitive Michelson interferometer is used to shear the image in orthogonal directions for p- and s-polarized light. The change in the optical wavelength is also used to provide a phase step in the pathlength imbalanced interferometer. In this way wavelength modulation of a laser diode source is used to accomplish simultaneously polarization multiplexing and phase stepping in the shearing interferometer. By carefully matching the optical wavelength shift, the optical fibre length and the pathlength imbalance in the interferometer, the π/2 polarization shift can be matched to the required phase step.

A new capacitance-based technique is described for detecting pinholes in electrically non-conducting flexible hermetically sealed packages with no headspace. The new technique is compared with the most suitable existing technique reported in the literature. It is demonstrated theoretically and through experimentation that the new technique provides more sensitive pinhole detection than the most suitable existing technique.

A novel method for measuring shear stress via a floating surface element beneath a turbulent boundary layer has been tested and implemented. The final design is used for measuring the surface shear stress in the boundary layer that forms over the salt playa of western Utah. Wind tunnel model studies were conducted to establish the characteristics of two competing sub-designs, and to determine their sensitivities and potential error sources. The wind tunnel based experiments included independent comparisons with the results of wall velocity gradient based measurements using hot-wire data. Overall, the results show that a floating element on a pool of liquid, here water, provides a more accurately measured surface shear stress than that determined utilizing an air bearing. In addition, the water pool technique is simpler to implement and produces a higher degree of repeatability. The wind tunnel results provided the design criteria needed to build a larger-scale device used to measure the surface shear stresses over the Great Salt Lake Desert in western Utah. This final design has been implemented and field results are presented and compared with sonic anemometer based estimates of the wall shear stress.

A pair of concentric cylinders, where the inner cylinder is moved axially through the outer fixed cylinder, is used to measure the dynamic viscoelastic properties of a fluid which occupies the annulus between them. A micro-Fourier rheometer is used to cause the inner cylinder to undergo small amplitude, band-limited pseudorandom oscillations in the axial direction, with simultaneous measurement of the instantaneous resistance force due to the fluid. The magnitude and phase of the force signal are used to calculate the storage and loss moduli G'(ω),G''(ω) as functions of oscillation frequency ω. Tests performed on two types of silicone oil show fair agreement with results conducted with the parallel plate squeezing flow geometry.