Table of contents

Similarity laws have opened the door for fluid dynamic experiments using subscale models. These laws enable the study of dynamically similar flows with geometrically similar models that are a fraction of the full-scale size with a concomitant reduction in required power. In some instances these similarity laws require experimental facilities that operate with fluids under very high temperatures and pressures in order to push the similarity variables to full-scale values or at least to useful values.

High-enthalpy flow conditions encountered in hypersonic flight require testing facilities that produce chemically reacting flows and plasmas. Ignition processes and chemical kinetics generally require high temperature and pressure facilities. Shock tubes provide a reliable and repeatable means to produce conditions for high enthalpy flows and chemical kinetics. The short duration of any flow state in a shock tube, usually of the order of milliseconds, presents measurement challenges for fluid properties and reaction rates. Similarly, extremely fast response times are required to measure transient flow quantities in turbomachinery. The exploitation of material properties to create useful test conditions often introduces 'side effects' that must be accounted for in models and measurements. Paradoxically, the most difficult part of some experiments can be the understanding and management of these effects. For example, the thermodynamic state of a working fluid might be far from that encountered by a full-scale system, or the mechanical properties and dynamical behaviour of structural materials might determine the feasibility of meaningful measurements. For modelling and risk management, the behaviour of candidate materials and working fluids must be thoroughly understood.

In this special feature, we survey some of the facilities and methods that have been developed for the measurement of fluid properties and processes under a wide range of conditions. Our focus is on facilities developed for aero/fluid dynamic systems that range from high-Reynolds number facilities that employ high-pressure water and dense gases, to shock tubes for chemical kinetics and high-enthalpy flows, to a low-pressure shock tube for instrument calibration. The authors in this issue were asked to present new results that highlight some of the unique features of their test facilities that both explore and exploit the physical and thermodynamic properties of fluids.

I thank the authors for their response to this call, and on their behalf I offer my sincere thanks to the editorial staff of Measurement Science and Technology for their extremely efficient and professional support in assembling this special feature. I also extend a personal thanks to Professor John Foss for first suggesting this special feature and for his support while it was assembled.

The William B Morgan Large Cavitation Channel (LCC) is a large variable-pressure closed-loop water tunnel that has been operated by the US Navy in Memphis, TN, USA, since 1991. This facility is well designed for a wide variety of hydrodynamic and hydroacoustic tests. Its overall size and capabilities allow test-model Reynolds numbers to approach, or even achieve, those of full-scale air- or water-borne transportation systems. This paper describes the facility along with some novel implementations of measurement techniques that have been successfully utilized there. In addition, highlights are presented from past test programmes involving (i) cavitation, (ii) near-zero pressure-gradient turbulent boundary layers, (iii) the near-wake flow characteristics of a two-dimensional hydrofoil and (iv) a full-scale research torpedo.

Here we review the advantages and disadvantages of using highly compressed air and other working fluids to test submarines and aircraft at full-scale Reynolds numbers. The conclusions are based on the design and implementation of two facilities at Princeton: the DARPA/ONR Superpipe apparatus built at Princeton to enable accurate pipe flow measurements over three orders of magnitude in Reynolds numbers, and the ONR High Reynolds Number Testing Facility (HRTF), designed to test submarine models at 1/5th full-scale Reynolds numbers.

A shock-tube facility consisting of two, single-pulse shock tubes for the study of fundamental processes related to gas-phase chemical kinetics and the formation and reaction of solid and liquid aerosols at elevated temperatures is described. Recent upgrades and additions include a new high-vacuum system, a new gas-handling system, a new control system and electronics, an optimized velocity-detection scheme, a computer-based data acquisition system, several optical diagnostics, and new techniques and procedures for handling experiments involving gas/powder mixtures. Test times on the order of 3 ms are possible with reflected-shock pressures up to 100 atm and temperatures greater than 4000 K. Applications for the shock-tube facility include the study of ignition delay times of fuel/oxidizer mixtures, the measurement of chemical kinetic reaction rates, the study of fundamental particle formation from the gas phase, and solid-particle vaporization, among others. The diagnostic techniques include standard differential laser absorption, FM laser absorption spectroscopy, laser extinction for particle volume fraction and size, temporally and spectrally resolved emission from gas-phase species, and a scanning mobility particle sizer for particle size distributions. Details on the set-up and operation of the shock tube and diagnostics are given, the results of a detailed uncertainty analysis on the accuracy of the test temperature inferred from the incident-shock velocity are provided, and some recent results are presented.

The design of an electrical conductivity measurement channel for a shock tube is described. This measurement channel is used for the study of weakly ionized, high-enthalpy flows of gases seeded with alkali salts. The theory for determining the dimensions of the measurement channel and the electrical power supply for the channel is based on Ohm's law. Data are shown which demonstrate that the channel performs well. However, the measured electrical conductivity was one or two orders less than theoretical values. The current traces for each case show that the peak current occurred behind the contact surface, which indicates that some of the seed was entrained behind the test gas originally in the driven tube. An analysis of the effect of Joule heating on the measured conductivity was conducted. The result of increased temperature due to Joule heating in the measurement channel is believed to be minimal. Reasons for the discrepancy are given.

Two cylindrical fast-response pressure probes for unsteady flow measurements in turbomachinery have been designed and tested. Commercially available miniaturized pressure sensors have been used to ensure high reliability and low manufacturing costs. Analytical and numerical models have been applied to the design of the line–cavity system connecting the pressure tap and the (encapsulated) sensor, to improve the dynamic behaviour of the probes. Dynamic calibrations in a low-pressure shock tube have also been carried out to determine the probe transfer function and to extend the probe operating range beyond the resonance frequency of the line–cavity system. One of the new probes exhibits a frequency response of about 80 kHz in the range 0–35 kPa, a suitable value for most turbomachinery applications.

Shock tube flows resulting from the incomplete burst of the diaphragm are investigated in connection with the dynamic calibration of fast-response pressure probes. As a result of the partial opening of the diaphragm, pressure disturbances are observed past the shock wave and the measured total pressure profile deviates from the envisaged step signal required by the calibration process. Pressure oscillations are generated as the initially normal shock wave diffracts from the diaphragm's orifice and reflects on the shock tube walls, with the lowest local frequency roughly equal to the ratio of the sound speed in the perturbed region to the shock tube diameter. The energy integral of the perturbations decreases with increasing distance from the diaphragm, as the diffracted leading shock and downwind reflections coalesce into a single normal shock. A procedure is proposed to calibrate fast-response pressure probes downwind of a partially opened shock tube diaphragm.

This study describes a robust bubble image recognition algorithm that detects the in-focus, ellipse-like bubble images from experimental images with heavily overlapping bubbles. The principle of the overlapping object recognition (OOR) algorithm is that it calculates the overall perimeter of a segment, finds the points at the perimeter that represent the connecting points of overlapping objects, clusters the perimeter arcs that belong to the same object and fits ellipses on the clustered arcs of the perimeter. The accuracy of the algorithm is studied with simulated images of overlapping ellipses, providing an RMS error of 0.9 pixels in size measurement. The algorithm is utilized in measurements of bubble size distributions with a direct imaging (DI) technique in which a digital camera and a pulsed back light are used to detect bubble outlines. The measurement system is calibrated with stagnant bubbles in a gel in order to define the bubble size dependent effective thickness of the measurement volume and the grey scale gradient threshold as a focus criterion. The described concept with a novel bubble recognition algorithm enables DI measurements in denser bubbly flows with increased reliability and accuracy of the measurement results. The measurement technique is applied to the study of the turbulent bubbly flow in a papermaking machine, in the outlet pipe of a centrifugal pump.

We propose a sub-nanometric positioning control method for piezoelectric actuators based on a home-made high frequency phase shifting electronic circuit and a heterodyne interferometer. The method has been set up. Using a readily available piezoelectric actuator, we demonstrate that our method allows back and forth displacement without nonlinearities. Repeatability of 0.053 nm has been obtained over 1 µm displacement range. Displacement steps as low as 260 pm are presented. Such a method could be very useful for the nanotechnology and nanometrology communities for nano-scale manipulations or nano-assembly applications.

Use of spatial phase measurement techniques in a displacement-measuring grating interferometer using detector width compensation becomes impractical due to the highly sensitive nature of the phase spectrum at the wrapped edges (−π and π) to the noise in the system. A usual unwrapping mechanism to accumulate phase fails to avoid any false jump of phase cycle near these edges and results in unwanted errors in displacement data. A new way of using such algorithms is proposed, which avoids these edges during accumulation of phase and hence reduces errors in displacement measurements. A conventional 90° phase step, four-point algorithm can be used simultaneously with a modified three-point phase algorithm, which gives a similar phase incremental value except the difference in instantaneous phase value. We have implemented this shift in origin to avoid the wrapped edges and have used a sequence of incremental phase values to generate cumulative phase and therefore displacement data by simultaneously using both algorithms in our detector-width-compensated displacement-measuring grating interferometer. Simulation results are introduced to show the usefulness and accuracy of this approach compared to a conventional phase unwrapping mechanism.

This paper describes an experimental apparatus and procedure for the calibration of the ultrasonic lubricant-film thickness measurement technique. It also presents a study of the accuracy of the technique. The calibration apparatus is demonstrated on a three layer steel–mineral oil–steel system. This was chosen to be representative of a typical bearing system which is the industrial application of the technique. In such bearing systems the lubricant-film thickness typically ranges from 0.1 to 100 µm. The calibration apparatus uses a high precision piezoelectric displacement translator to controllably displace one of the steel surfaces relative to the other and hence alter the lubricant-film thickness by a known amount. Through-thickness resonant frequency measurements are then used to accurately measure a thick lubricant film (h > 10 µm). These resonant frequency measurements form the starting point of the calibration. The displacement translator is then used to reduce the lubricant-film thickness into the, more practically interesting, low micron range. In this range the amplitude of the measured reflection coefficient is used via a spring interface model to calculate the lubricant-film thickness. Issues of ultrasonic beam alignment and frequency of operation are discussed. A detailed study of the effect of reflection-coefficient errors on the resultant thickness measurement is presented. Practical guidelines for use of the calibration are then defined and calibration is demonstrated experimentally over the range 0.5–1.3 µm.

A procedure is described for measuring the complex permittivity of dielectric thin films with relatively large dielectric constants (ε: about 10²–10⁴) and considerable losses (tan δ: about 10⁻³–1). The ferroelectric films studied here are deposited on a relatively low-ε dielectric substrate and placed in the centre of a rectangular waveguide parallel to the direction of wave propagation. The dielectric constants and losses of thin films are determined not by the shift of the resonant peak, but by numerical analysis of measured scattering parameters using a vector network analyser. The proposed method does not require electrode deposition on the film surface, and provides the natural properties of the film without complications due to conductive electrodes and their interfaces.

A new class of electrostatic energy analysers with a bounded cylindrical focusing field is presented. The focusing field used is a solution of a Laplace equation ∇²U(R, Z) = 0 with boundary conditions U(R₁, Z) = U(R, 0) = U(R, L) = 0 and U(R₂, Z) = V, and restricted by concentric cylindrical surfaces and two flat surfaces perpendicular to the axis of symmetry. A charged particle beam enters the field through a face window in a flat boundary electrode. Regimes of second-order focusing have been found for configurations with a point source, located on the axis of symmetry and for an extended source of large angular size. In particular, the new analysers can be used for electron spectroscopy of distant surfaces or surfaces with large roughness or even with deep dimples. These instruments in principle have no perceptible end-fields, which usually distort focusing in most of the analysers known. The design of a compact analyser for remote Auger electron spectroscopy and some experimental results, showing the capability of using these instruments for scientific and technological applications, are given.

In this paper, we propose a comparative study between a gravimetric apparatus operating under dynamic conditions and a pulse chromatographic device developed for the determination of Henry constants of adsorption for VOC–zeolite systems. In both cases, we provide a description of the experimental set-up and procedure, as well as a complete report on the treatment of the rough experimental data. The experimental errors are also discussed. The comparison work is based on the study of the adsorption of toluene on a NaY zeolite (Si/Al 2.43) for temperatures ranging from 503 to 623 K. The maximal discrepancy found between the experimental Henry constants was 15.0%. The pulse chromatographic method is only dedicated to high-temperature measurements. For low-temperature experiments, the rough data cannot be treated in an efficient way, and it is not possible to obtain reliable Henry constant values. The dynamic gravimetric method is not temperature limited. It is however time-consuming, especially when low-temperature measurements (not presented in this paper) are concerned. Both methods are complementary if the determination of Henry constants is required in a wide temperature range.

The paper proposes the criteria for selecting the sampling condition in three-dimensional surface measurement. The criteria are based on the fact that there is a relationship between the correlation length of the measured surface and its high-frequency cut-off. Through the use of the sampled surfaces and the analytical models, it is shown that the criteria are equivalent to the power spectral analysis.

Artificially engineered multiphase heterostructures with high permittivity, high permeability and low dielectric and magnetic losses are desired for microwave applications. In addition, the direct conversion of electrical (and/or magnetic) energy to mechanical work through a material response is important for many practical applications. Thus, there is a need for sensitive and quantifiable techniques to probe how uniaxial strain affects the complex effective permittivity or magnetic permeability of particulate-filled polymers. We describe an apparatus for in situ studies of the effective electromagnetic properties of filled polymers under elongation. As currently configured, our new system will already be of significance to a wide variety of research, and in particular in the materials, automotive as well as aeronautical science. In this paper, we describe the design and operation of the measurement system. Two examples of preliminary observations of electromagnetic properties of filler reinforced polymeric materials under axial strain have been obtained and are presented to illustrate the utility of this instrumentation. On one hand, the effective permittivity of carbon-black-filled SBR (styrene-butadiene rubber) is discussed as a function of the carbon black volume fraction, frequency and extension ratio. On the other hand, we also show how the effective permeability of plasto-ferrite at microwave frequency changes due to external stress. This paper concludes with suggestions for possible research topics of current interest where the knowledge of material parameters under stress would be beneficial to the basic understanding of physical processes.

A low-cost, non-destructive and flexible technique based on thermoreflectometry is presented in this paper to map the temperature of running devices integrated on a silicon chip. The analysed device is a micromachined gas sensor using silicon-on-insulator technology (SOI). The interests and limitations of the proposed optical technique are described and compared with commonly used methods in microelectronics. Moreover, experimental results of thermal mapping for micromachined gas sensors are also compared with numerical simulations. From the measured temperature distribution over the entire active area of micromachined gas sensors, optimized designs can be proposed for avoiding hot spots that degrade the performance of the integrated gas sensors.

Previously, the authors have developed a subnanometre heterodyne interferometric system known as a three-mode heterodyne interferometer (TMHI). By converting the target displacement into twice the phase variation of a conventional interferometer, the TMHI has achieved position resolution of 0.044 nm even with a single-pass interferometer. However, the heterodyne beat frequency in TMHI is quite low, of the order of several hundred kilohertz, which limits the measurable target speed to several tens of millimetres per second. In this paper, we present a method to extend the measurable target speed of the TMHI without increasing the heterodyne beat frequency of a light source. Keeping the excellent position resolution of the TMHI, the proposed method provides sufficient response to high speed movement of the target. The system comprises a pair of TMHI and utilizes two conjugate signals generated by them selectively according to the direction of movement of the target. Possible errors in switching the two signals are analysed by introducing an error zone model. The switching function was demonstrated using the mechanical vibration of the interferometer and proved to be free from erroneous counts. It was also confirmed that the system can respond at a speed that exceeds that determined by the beat frequency.

A novel signal conditioning circuit suitable for push–pull-type resistive transducers is proposed. The circuit developed is capable of providing a linear output over a wide range of values of the measurand. Even when the transducer has an inverse relationship with the measurand, the circuit provides a linear output. The push–pull-type resistive transducer becomes an integral part of a relaxation oscillator, the duty cycle ratio of the output of which becomes proportional to the measurand. Since the output depends only on the relative sensitivity of the transducer and a pair of dc excitation voltages, it is possible to obtain very low errors. The various sources of error in the circuit are analysed and quantitative expressions to estimate such errors are derived. The circuit was set up in the laboratory and the results obtained from the prototype are presented. The prototype possessed an error of ±0.02% of the reading. It is seen that this circuit is not only simple but also produces errors which are much less than those from circuits currently mentioned in the literature.

In digital holography, a challenge inherited from speckle interferometry is the quantitative data processing of time-averaged holograms in view of estimating the full field of vibration amplitudes. Fundamentally, the greatest obstacle comes from the orthogonal components of time-averaged digital holograms, corrupted by multiplicative, high-frequency phase noise covering the deterministic vibration-related phase. The paper describes a novel method of obtaining through digital holography high-resolution time-averaged fringe patterns. The three-level approach includes the elimination of the multiplicative high-frequency phase noise by synchronous detection followed by low-pass filtering. The high-frequency information needed by this procedure is taken from the digital hologram of the object at rest. The complete procedure includes an automatic correction of the environmental phase drift introduced by the synchronous detection. The different stages of data processing are illustrated by experimental results.

Viewpoint planning plays an important role in automatic 3D model construction. The planning process desires not only the ability to automatically determine the next sensing position, but also the ability for self-termination. In this paper, we use the trend surface, which is the regional feature of a surface for describing the global tendency of change, to compute the next best view in two steps. The exploration direction is determined by analysing the surface curvature of the trend surface. The next view pose is obtained by imposing such constraints as resolution, focus and field of view. A self-termination criterion is proposed for judging the completion condition in the measurement and reconstruction process. The termination condition is derived based on changes in the volume obtained from the last two successive viewpoints. Experimental results show that the method is effective in practical implementation.

The surface profile of a rough object is obtained by white light interferometry. The real and noisy simulated correlograms are analysed by using sliding average, continuous wavelet transform and a new algorithm that is called the continuous wavelet transform phase method. Measurement repeatability is calculated for each algorithm and it has been shown that the continuous wavelet transform phase method gives a smaller peak to valley value and standard deviation than other methods. Hence, this algorithm can be used to obtain a good repeatability or standard deviation in white light interferometry.

Automated sample and reagent delivery to planar microarrays requires the attachment of flow channels to the sensing surface. When evanescent measurements are required, the attachment of the flow chamber causes particular problems. The flow chamber scatters the excitation light, increasing the background signal above which the fluorescent signal must be detected. Introducing a light-absorbing material (carbon black) into the PDMS flow channels has been shown to decrease the measured background signal by 66%. The benefits of background image subtraction are also discussed.