Direct interferometric measurement of streaming birefringence

The two refractive indices in the flow of a colloidal birefringent liquid are measured separately by means of a Mach-Zehnder interferometer. For a quantitative evaluation of the resulting interferograms it is not necessary to linearize the respective equations relating the refractive index distribution to the deformation velocity in the flow. Therefore it becomes possible to perform velocity measurements in the non-Newtonian flow range. An additional measurement of the mean flow rate enables one to determine the velocity field without the need of a calibration of the observed interference fringes.     

An improved method for the measurement of strong flow birefringence

An improved procedure is proposed to solve the problem of measuring strong birefringence on flowing samples which have a retardation larger than /2 The procedure can be applied to devices which are based on the modulation of the polarization vector with a high-speed rotating half-wave plate. It uses two optical configurations which have different responses to the retardation. By combining the information obtained from the two configurations, one can easily and accurately determine the actual retardation of the sample, irrespective of the quadrant in which the measured retardation is located, provided the sample is isotropic. This technique can also overcome the problem of large errors occurring in the vicinity of = (2m+1)/2 due to the limited sensitivity around these values of . The use of the technique is illustrated with measurements on an isotropic solution of poly(benzyl glutamate) in m-cresol, which is strongly birefringent during shear flow.     

Stress tensor measurement using birefringence in oblique transmission

A method for measuring the stress tensor of liquids obeying the stress-optical rule is presented. In particular, the procedure makes it possible to determine the shear stress, and the first and second normal stress differences for rheometric flows. This technique is an extension of the procedure recently described by Burghardt and coworkers (Brown et al., 1995) wherein light is sent obliquely through a sample sheared between transparent plates. However, in the present development, the light is transmitted in the plane containing the velocity gradient and neutral directions, thereby reducing the necessary optical measurements by one. A polystyrene-tricresyl phosphate (TCP) solution is used as the test sample. The first and second normal stress differences in steady shear flow measured by this method show good agreement with the mechanical results measured by Madga et al. (1993) using a modified cone and plate rheometer. The transient behavior of the first and second normal stress differences following the start-up of shear flow is also presented.     

Acoustical birefringence and the use of ultrasonic waves for experimental stress analysis

Stress-induced optical birefringence in transparent materials has long been a common technique of stress analysis. Although stress-induced acoustic birefringence was discovered more than 20 years ago, its development and actual applications are still limited. This paper will look at the similarities and differences between the propagation of light waves in photoelastic materials and the propagation of ultrasonic waves in deformed solids. Critical comparisons of the experimental methods employed in photoelasticity with those available in modern ultrasonic measuring technique show why previous studies on ultrasonic measurement of stresses were not very successful. A new experimental technique is devised for using ultrasonic waves for stress analysis. The technique employs a single rotatable 10-MHz shear transducer as the transmitter and receiver of ultrasonic pulses. The enlarged display of the 10-MHz modulated-pulse pattern of reflected echoes provides a convenient way to determine the directions of principal axis of the stress within ±3 deg. The pulse-echo-overlap method is used to measure the absolute velocities of the two principal shear waves. The difference in principal stresses is then calculated from the velocity measurements. Test results of common structural-aluminum and steel specimens under uniaxial compression show a linear relation between the velocity changes and the applied stress. Ultrasonic measurements of stress distribution in a 6.35-cm diameter, 1.9-cm-thick aluminum disk under diametric compression are also reported. Paper was presented at Third SESA International Congress on Experimental Mechanics held in Los Angeles, CA on May 13–18, 1973.     

New apparatus for the simultaneous measurement of stresses and flow birefringence in biaxial extension of polymer melts

Summary A new apparatus is described for the measurement of the viscosity of polymer melts in biaxial extension. Use is made of the method of two impinging fluid streams. The mentioned streams are guided by lubricated trumpet shaped metal walls. The force is measured which tends to separate the trumpets. At the same time, the flow birefringence is measured in the plane of symmetry, which contains the stagnation point. The linear stress-optical relation, which is well-known from shear experiments, turns out to be valid also in the new flow geometry, at least up to the deformation rates applied. Results of the steady state measurements carried out on a polystyrene melt are evaluated. Suggestions for a better performance of the mechanical measurements are given.

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Zusammenfassung Ein neuer Apparat zur Messung der Dehnviskosität von polymeren Schmelzen unter biaxialer Dehnung wird beschrieben. Hierfür wird die Methode des Aufeinanderprallens entgegengesetzt gerichteter Ströme verwendet. Die genannten Ströme werden durch geschmierte trompetenförmige Bohrungen herangeführt. Es wird die Kraft gemessen, welche die beiden Trompeten auseinanderzudrücken sucht. Gleichzeitig wird die Strömungsdoppelbrechung in der Symmetrieebene gemessen, die den Staupunkt enthält. Die lineare spannungsoptische Regel, die bei Scherexperimenten wohlbekannt ist, scheint auch für die neue Strömungsgeometrie zu gelten, zumindest im Bereich der angewandten Deformationsgeschwindigkeiten. Ergebnisse, wie sie im stationären Strömungszustand bei einer Polystyrol-Schmelze erhalten wurden, werden ausgewertet. Vorschläge zur besseren Ausführung der mechanischen Messungen werden gemacht.

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With 11 figures     

Automated measurement of birefringence: Development and experimental evaluation of the techniques

Traditional photoelasticity has started to lose its appeal since it requires a well-trained specialist to acquire and interpret results. A spectral-contents-analysis approach may help to revive this old, but still useful technique. Light intensity of the beam passed through the stressed specimen contains all the information necessary to automatically extract the value of retardation. This is done by using a photodiode array to investigate the spectral contents of the light beam. Three different techniques to extract the value of retardation from the spectral contents of the light are discussed and evaluated. An experimental system was built which demonstrates the ability to evaluate retardation values in real time.     

Improvement of cold-wire response for measurement of temperature dissipation

This work aimed at improving fine-scale measurements using cold-wire anemometry. The dissipation ɛ _θ of the temperature variance was measured on the axis of a heated turbulent round jet. The measurements were performed with a constant current anemometer (CCA) operating fine Pt–10%Rh wires at very low overheat. The CCA developed for this purpose allowed the use of the current injection method in order to estimate the time constant of the wire. In the first part of the paper, it is shown that the time constants obtained for two wire diameters −d=1.2 and d=0.58 μm – compare well with those measured at the same time using two other methods (laser excitation and pulsed wire). Moreover, for these two wires, the estimated time constants were in good agreement with those obtained from a semi-empirical relation. In the second part of the paper, a compensation procedure – post-processing filtering – was developed in order to improved the frequency response of the cold-wire probes. The measurements carried out on the axis of the jet (Re _D =16 500, Re _λ  ≃ 167) showed that the frequency response of the 1.2 μm wire was significantly improved. In fact, the spectral characteristics of the compensated signal obtained with the 1.2 μm wire compared fairly well with those from the 0.58 μm wire. Moreover, the results indicated that the compensation procedure must be applied when the cut-off frequency of the cold-wire f _c is lower than two times the Kolmogorov frequency f _K. In the case where f _c ≃ 0.6f _K, the compensation procedure can reduce the error in the estimate of ɛ _θ by more than 20%. When f _c ≃ 2f _K, the effect of the compensation is reduced to about 5%. Received: 3 November 2000/Accepted: 23 March 2001     

光学干涉计量中的位相测量方法

对光学干涉计量中常用的位相测量方法进行了综述,包括时间相移法、空间相移法、空间载波相移法和Fourier变换法以及位相展开问题。     

The use of sweep-frequency excitation for unsteady pressure measurement

The use of sweep-frequency excitation for rapid measurement of time-dependent pressures on wind-tunnel models is examined. Results obtained from two different wind-tunnels covering the Mach number range from 0.2 to 0.85, and a wide range of flow conditions, are compared with measurements made using the slower, traditional method of discrete-frequency excitation. It is concluded that the sweep-frequency excitation method can reduce testing time in certain flow conditions with no significant loss in accuracy.List of symbols M Mach number - p broadband rms local static pressure - q 12u ² (dynamic pressure) - R(Cp/) real (in-phase) part of oscillatory Cp/ - I(Cp/) imaginary (in-quadrature) part of oscillatory Cp/ - x/c chord station - wing incidence - canard or wing oscillatory amplitude (plotted in radians unless otherwise stated) - spanwise station - _c canard static incidence - _c canard effective incidence ( _c = 1.89 + _c –0.6) - (T) function of time - ² coherence function The coherence function between two signals x(f), y(f) is defined as - where - G _xy (f) = cross spectral density function between x and y - G _xx (f) = auto spectral density function of x - G _yy (f) = auto spectral density function of y - f = frequency     

Plate-deflection measurement by photogrammetric methods

A number of tests were performed to evaluate the feasibility of determining the deflections of a laterally loaded plate by means of stereo-photogrammetric methods. Mechanical measurements at several points were also made for each loading condition as well as performing an analytical solution for the centrally loaded case. A comparison of results indicated that this photogrammetric procedure used to determine deflections resulted in accuracies within ±0.020 in. Recommendations are made to improve greatly the accuracy potential of the stereo-photogrammetric method of deflection reading of a laterally loaded plate.     

Investigation of intermittency measurement methods for transitional boundary layer flows

Three turbulent intermittency methods, namely the , TERA (turbulent energy recognition algorithm), and M-TERA (modified turbulent energy recognition algorithm) methods, for identifying the intermittent flow characteristics associated with boundary layer transition from laminar to turbulent were considered and compared. The data used were obtained from hot-wire measurements in transitional boundary layer flows on a concave surface with a 2-m radius of curvature and on a flat plate. Comparisons show that the and TERA methods are more sensitive to the choice of threshold constants than the M-TERA method. In terms of the intermittency distribution across the boundary layer, the values obtained by the and TERA methods are unrealistically high in the near-wall region, while those obtained by the M-TERA method are more realistic. In the outer boundary layer region and outside the boundary layer, the and M-TERA methods give reasonable intermittency values, whereas the TERA method produces unrealistically high values in the region outside the boundary layer. In addition, the M-TERA method provides a sharper definition of theend of transition.     

Voronoï imaging methods for the measurement of granular flows

A set of digital imaging methods derived from the Voronoï diagram is proposed and tested on various liquid–granular flow applications. The methods include a novel pattern-based particle-tracking algorithm, as well as estimators of the three-dimensional granular concentration from two-dimensional images. The proposed algorithms are able to resolve individual grain motions even for rapid shear flows involving dense, fluctuating granular ensembles. Full automation is achieved, allowing the derivation of accurate statistics from large sets of individual measurements, as well as the construction of complete sets of grain trajectories. Results are presented for different applications: homogeneous fluidization, steady uniform debris flow, and unsteady debris surges.     

姿态检测地磁传感器误差分析与补偿方法

针对磁环境下的地磁传感器测量精度不足的问题,为了补偿地磁传感器存在的各种误差,在分析地磁传感器的自身误差和环境干扰误差的基础上,建立了地磁传感器误差校正模型,利用最大似然估计法将误差校正模型转化成多项式最小二乘拟合的基本方程形式,提出了基于QR因子分解法的补偿系数快速解算方法。分别求取仿真误差模型和实验误差模型的补偿系数,然后对地磁场矢量进行校正。结果表明,校正后的磁场强度和实际磁场强度基本一致,滚转角精度提高了近5倍,能够满足相关需求。     

用振动方法测量流体密度

介绍了用振动方法则量流体密度的实验原理、实验装置和实验方法, 这是振动力学的一个教学实验,具有实际应用价值。     

Improvement of measurement accuracy in micro PIV by image overlapping

Micro PIV uses volume illumination; therefore, the velocity measured at the focal plane is a weighted average of the velocities within the measurement volume. The contribution of out-of-focus particles to the PIV correlation can generate significant measurement errors particularly in near wall regions. We present a new application of image overlapping, which is shown to be very effective in improving the accuracy of time-averaged velocity measurements by effectively reducing the measurement depth. The performance of image overlapping and correlation averaging were studied using synthetic and experimental images of micro channel flow, both with and without image pre-processing. The results show that for flows without particle clumping, image overlapping provides the best measurement accuracy without any need for image pre-processing. For flows with particle clumping, image overlapping combined with band-pass filtering provides the best measurement accuracy. When overlapped images are saturated with particles due to a large number of image pairs, image overlapping measurement still does not show any visible pixel-locking effect. Image overlapping was found to have comparable or slightly reduced pixel-locking effects compared to correlation averaging. In addition, image overlapping utilizes significantly fewer computational resources than the other techniques.     

The use of phase-stepping for the measurement of characteristic parameters in integrated photoelasticity

A three-dimensional photoelastic body can be represented by an optically equivalent model, which consists of a linear retarder, δ, at a certain angle, θ, and a pure rotator, χ. These have been described as the characteristic retardation, δ, and the primary and secondary characteristic directions, θ and θ+χ. Until now these characteristic parameters have only been determined using manual, point-by-point collection methods which are involved and time consuming. Therefore an automated phase-stepping method has been developed to enable the determination of the three characteristic parameters for three-dimensional or integrated photoelasticity. Expressions have been derived to obtain δ, θ and θ+χ from six phase-stepped images. These images are collected using a CCD camera and the full-field data is processed using a standard personal computer. This novel method allows accurate, full-field maps of all three characteristic parameters to be obtained in a relatively short time, which makes full-field tomographic reconstruction of photoelastic data a real possibility. Both are SEM Members.     

Simultaneous measurement of transient stress and flow birefringence in one-sided compression (biaxial extension) of a polymer melt

Summary Investigation of time dependent behaviour of a polystyrene melt is carried out with the aid of a new apparatus for biaxial extension. Use is made of the method of two impinging fluid streams guided by lubricated trumpet shaped metal walls. The flow birefringence is measured in the plane of symmetry and, at the same time, the force is measured which tends to separate the trumpets. The linear stress-optical relation turns out to be valid in this new flow geometry. An accurate value for the stress-optical coefficient can be determined from the relaxation experiments. The stress build-up as calculated from the optical measurements, is compared with the pertinent result of the theory of linear viscoelasticity. For the desired interconversion of dynamic moduli use is made of the approximation by Schwarzl and Struik. The steady state measurements are checked by the results of the non-linear model of Acierno et al.With 16 figures and 2 tables     

Improvement of a power tiller cage wheel for use in swampy peat soils

This study was aimed at investigating traction performance of a cage wheel for use in swampy peat soils in Indonesia. The tests were conducted in a soil bin filled with peat soil taken from the swampy areas. A set up was developed to measure tractive performance of a single cage wheel. Deep sinkage and high wheel slip were identified as the major problems of using the existing cage wheel design in swampy peat soils. The results revealed that increasing the lug angle from 15 to 35° and the length of lug improved the tractive performance of the cage wheel significantly, while increasing the number of lugs from 14 to 18 and width of lug did not improve the tractive performance significantly. A cage wheel with lug size 325×80 mm, 35° lug angle, 14 lugs (26° lug spacing), with 2 circumferential flat rings installed on the inner side of the lugs, out performed the other settings for use with power tillers in swampy peat soils.