A low frequency torsional rheometer

Summary An instrument has been developed for measuring the viscoelastic behaviour of polymer melts at low frequencies, in the range 10^–3 to 50 Hz. The sample is contained between a cone and a fixed plate, or between parallel plates. The moving member is driven in torsional oscillation through a torsion wire. The amplitude of the resulting oscillation is compared in amplitude and phase with the driven end of the torsion wire. The amplitudes are measured digitally using optical diffraction gratings, and either an oscilloscope or a high-speed ultra-violet recorder is used to determine the phase angle between the two signals. The moving member is supported on an air bearing, which provides a very low friction support with a high degree of positional control thus giving a well defined sample geometry. The torsion wire is driven using a vibrator with a d.c. drive amplifier fed from a very low frequency oscillator. The sample temperature is controlled to better than 0.01 °C, with temperature gradients across the sample of a similar order of magnitude. The temperature range of the instrument is from –50 °C to +200 °C.The angular resolution of the measuring system is 3 × 10^–5 radius, so that an accuracy of better than ±1% in the amplitude measurements can be obtained with the amplitude of shear in the sample kept sufficiently low that a linear stress-strain relation is maintained.With 3 figures     

Experimental and numerical study of flat momentumless wake

Flat momentumless wake behind wing profile has been studied both experimentally and with numerical simulation based on the second order Reynolds stress turbulence model. Some obtained regular trends of momentumless wake decay are mainly consistent with previous experiments by Dmitrenko et al. (Dmitrenko, Yu.M., Kovalev, I.I., Luchko, N.N., Cherepanov, P.Ya., 1987. J. Eng. Phys. 52, 743–751 (in Russian)), Cimbala and Park (Cimbala, J.M., Park, W.J., 1990. J. Fluid Mech. 213, 479–509; Park, W.J., Cimbala, J.M., 1991. J. Fluid Mech. 224, 29–47). Transverse distributions of statistical characteristics and its evolution along the flow axis depend on initial conditions and body shape. It was shown that characteristic turbulence parameters associated with the wake's width, mean velocity deficit, turbulence kinetic energy, and its dissipation rate are not constant as some of the theories predict.     

Heat Transport in Unsaturated Zone Thermal Energy Storage – Analysis with Two-Phase and Single-Phase Models

We analyze a field experiment where ambient air is injected into the soil during the summer and extracted again during the winter. A multiphase model accounting for the conductive transport as well as the convective transport with the moving liquid and gas phases is used along with a more simplified single-phase model where the convective transport is due to the gas alone. The latter model also accounts for subzero wintertime temperatures. The multiphase model captures well both the seasonal variations and the actual test sequence, the main calibration being in the adjustment of medium permeabilities based on the observed pressure responses. The effect of the injection pump on the temperature and humidity of the injection air needs to be known accurately. Taking into account the humidity of the injection air explicitly instead of using humidity-corrected enthalpy values also has an effect. The effect of various humidity and specific enthalpy assumptions is of the order of 1–1.5°C, while ignoring the wintertime subzero temperatures has an effect of 1–2°C. These differences are of the same order of magnitude as the heterogeneity-introduced differences in field data. Using the simplified single-phase model typically appears to cause a difference of 1–2°C, but can yield an even higher deviation of the order of 3–4°C.     

A cryostat for use with a thermal conductivity hot wire system

Summary A cryostat for use with a thermal conductivity hot wire system is described in which temperatures between –70°C and –130°C can be maintained constant to within (0.01°C). Control of temperature is made by adjusting the vacuum between a five component mixture and liquid oxygen. Extra control is made by adjusting the current through a heating coil immersed in the cryostat mixture. The temperature indicator is a platinum wire surrounding the tubes.     

Measurement of coatings' elastic properties by mechanical methods: Part 2. Application to thermal barrier coatings

Elastic properties of a thermal barrier ceramic coating composed of an NiCoCrAIY bond coat and a ZrO₂(Y₂O₃) top coat were measured by a four-point bending rig in the temperature range 20°C–900°C. Different types of specimens (i.e., with bond coat only or with bond coat and top coat, on one side or on both sides) were employed. Test procedures were based on the theory discussed in Part 1 to enhance accuracy and to estimate confidence intervals. In particular, the method employed at high temperature was calibrated at room temperature by comparing the results with those obtained by methods with low sensitivity to layer thicknesses. For the bond coat, Young's modulus was found to be temperature independent up to about 500°C; a decreasing trend was observed above this temperature. For the top coat, a slightly temperature range examined. A possible explanation is given on the basis of phase transformation and the microstructure of the two layers. At room temperature, Poisson's ratio for the bond coat was found to be near 0.3, whereas a near zero value was measured for the top coat.     

An automated high-pressure,high-temperature,low-frequency viscometer

A computerized apparatus for the measurement of low shear rate viscosity on polymer solutions at elevated pressure and temperature is described. The pressure is variable from 1 bar to 400 bar with an accuracy of ±5 bar. The temperature is variable from 0° to 100°C with an accuracy of ±0.3 K. The instrument is operated in a free-relaxation mode in which the decay of oscillation of the torsion pendulum after an initial displacement is recorded and used to compute the viscosity of the sample. The measurements are performed according to the contents of a user-specified control file, and the oscillation data are stored digitally and later analyzed for parameter estimation. The instrument operates in the very-low frequency range (0.03 to 0.25 Hz) and the accuracy of the measured viscosity is ±0.03 x 10^–3 Ns/m2.     

A torsional creep instrument and a torsional pendulum,both with accurate temperature control

Summary This paper describes a torsional pendulum and a torsional creep instrument. With the pendulum shear moduli between 10⁶ and 10¹⁰ N/m² can be measured at frequencies from 0.1 to 20 Hz. The creep instrument is suitable for measurement of shear compliances lower than 10^–7 m²/N in the time range from 1 to 10⁵ seconds. In both instruments, specimens are kept at the right temperature by blowing heated nitrogen gas through a surrounding thermostatic chamber. The signal of a platinum resistance thermometer, provided in each chamber, automatically controls the heating of the gas. Temperatures from –180 to +300 °C can be maintained with an absolute accuracy of ±1 °C and a long term stability of ±0.05 °C. It is shown that one cannot directly compare one and the same shear property, calculated from the shear modulus as measured with the pendulum as well as from the shear compliance as measured with the creep instrument. This is due to differences in the temperature of one thermostatic chamber over against the other. Finally, the paper presents a method to reduce these differences to ±0.1 °C, although the absolute accuracy of temperature control remains ±1 °C.

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Zusammenfassung Die Arbeit beschreibt ein Torsionspendel und eine Torsions-Kriechapparatur. Mit dem Pendel können Schermoduli zwischen 10⁶ und 10¹⁰ N/m² bei Frequenzen zwischen 0,1 und 20 °C gemessen werden. Die Kriechapparatur ist geeignet für die Messung von Scherkomplianzen kleiner als 10^–7 m²/N in Zeiten zwischen 1 bis 10⁵ sec. In beiden Geräten werden die Proben durch das Einblasen von erhitztem Stickstoff durch eine umgebende thermostatische Kammer bei der richtigen Temperatur gehalten. Die Anzeige eines Platin-Widerstandsthermometers, das in jeder Kammer angebracht ist, kontrolliert automatisch die Erwärmung des Gases. Es können Temperaturen zwischen –180 und +300 °C mit einer absoluten Genauigkeit von ±1 °C und einer Langzeitstabilität von ±0,05 °C eingestellt werden. Es wird gezeigt, daß ein direkter Vergleich der gleichen Schereigenschaft, die zum einen aus dem mit dem Pendel gemessenen Schermodul und zum anderen aus der mit der Kriechapparatur ermittelten Kriechkomplianz errechnet wird, nicht möglich ist. Das beruht auf Temperaturdifferenzen zwischen den thermostatisierten Kammern.Abschließend stellt die Arbeit eine Methode vor, um diese Differenzen auf ±0,1 °C zu senken. Die absolute Genauigkeit der Temperaturkontrolle bleibt bei ±1 °C.

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Paper presented at the Conference on Experimental Rheology, University of Bradford, April 17–19, 1968.     

The peculiar rheo-optical behavior of bisphenol A-polycarbonate and polymethylmethacrylate

The rheological and stress-optical behavior of the melts of several grades ob bisphenol-A-polycarbonate (PC) and polymethylmethacrylate (PMMA) is investigated. Pertinent flow birefringence measurements are carried out in a remodelled cone-plate apparatus [1]. The shear stress in the polymer melt is calculated from the dynamic moduli, which are determined separately. It is shown that the linear stress optical rule is obeyed. In this way, the stress-optical coefficient C of the melt can be determined. The low-Mw polycarbonates all behave as Maxwellian fluids. The main stress direction does not deviate significantly from 45°. In the temperature range from 160° to 260°C the stress-optical coefficients of the different grades lie between 3 and 4×10^–9 Pa^–1 and show a weak temperature dependence. The stress-optical coefficient of PMMA is about a factor of 100 lower and shows a peculiar temperature-dependence, changing its sign at 144°C. The results are discussed in terms of the anisotropy of the polarizability of the polymer chain.     

A note on the temperature-dependent hot-wire calibration method of Cimbala and Park

In an earlier note Cimbala and Park (1990) presented a method for calibrating constant temperature hot wires in incompressible gas flow which accounts for ambient temperature changes. Their method involves the retention of the temperature dependence in a form of King's Law. However, in practice the method proves difficult to implement due to a numerical difficulty in finding a best fit to the assumed functional form. This note presents a modified version of the method of Cimbala and Park which does not seem to have the numerical difficulty, as well as some further comments on the method.     

A linear viscoelastic model for solid polyethylene

Stress relaxation tests have been carried out on a blue, pipe grade PE 80 medium density polyethylene (BP Chemicals), to provide thermo-viscoelastic rheology for use in calculating thermal stresses in pipe production. Stresses up to 4 MPa were used, with strains up to about 2%, in tests at temperatures from 23° to 90°C. Within this range a linear viscoelastic model was applicable, provided the initial ramp strain rate was less than 7×10^–5 s^–1. The stress relaxation data was fitted directly by a model incorporating an elastic response to volumetric strains, and a generalised linear solid model, consisting of two Maxwell elements and a purely elastic element in parallel, for deviatoric strains. Arrhenius type temperature dependence of relaxation times and shear moduli is found, and within experimental accuracy the temperature dependence of all these model parameters is the same. As a consequence, and provided that the duration of the strain ramp is sufficiently short relative to relaxation times, the model leads to time-temperature superposition of the relaxation moduli, using the same shift factor on both the response magnitude and time axes.     

An experiment on the breakup of impinging droplets on a hot surface

Breakup characteristics of liquid droplets impinging on a hot surface are investigated experimentally with the wall temperatures in the Leidenfrost temperature range of 220–330°C for n-decane fuel. Factors influencing droplet breakup are wall temperature, impinging velocity, droplet diameter and impinging angle. The 50% breakup probability shows that the impinging velocity decreases linearly with the droplet diameter increase and there exists an optimum impinging angle near 80° having the minimum value in the impinging velocity for given wall temperature and droplet size. Near the wall temperature of 250°C corresponding to the Leidenfrost temperature, a peculiar nonlinear behavior in the breakup probability is observed.This work was supported by the Turbo and Power Machinery Research Center, Seoul National University.     

Analysis of behavior of graphite on the basis of nonlinear heredity theory

The behavior of graphite is described on the basis of the nonlinear heredity theory taking account of the temperature factor in the range 20–3000 °C. The necessary characteristics are obtained from, the data on creep and from the stress-strain diagrams. A physical interpretation of the obtained results is attempted.Translated from Zhurnal Prikladnoi Mechaniki i Tekhnicheskci Fiziki, No. 2, pp. 76–82, March–April, 1971.     

Development of temperature-compensated resistance strain gages for use to 800°C

This paper describes the development and evaluation of temperature-compensated resistance strain gages for use to 800°C. These gages included single-element gages and double-element half-bridge gages. The filament of single-element gages was fabricated from specially developed Fe–Cr–Al–V–Ti–Y alloy wire. When bonded to high-temperature Ni-based alloy GH39 after stabilization at 800°C for one hour, the apparent strain from room temperature to 800°C was less than 2000 m/m. Double-element gages were fabricated from Pt–W–Re–Ni–Cr–Y alloy wire (active grid) and Pt–Ir alloy wire (compensating grid). When bonded to different high-temperature alloy specimens and stabilized, and when ballast resistance in series with the compensating grid adjusted suitably, the gages' apparent strains from room temperature to 800°C were less than 2400 m/m.Effects of preoxidization of Fe–Cr–Al wire on the characteristics of the single-element gages are described.     

Computer-aided calibration and measurements with a quadruple hotwire probe

A method for calibration and measurement with a four-wire probe is described. For each of the wires a three dimensional calibration field is determined, thus no assumption like King's law or the cosine law need to be made. The velocity vector can then be detected in a fairly large angular range (± 40°) with a numerical search algorithm. First measurements in a free jet and a confined, strongly swirling flow are presented.A version of this paper was presented at the 11th Symposium on Turbulence, University of Missouri-Rolla, Oct. 17–19, 1988     

On the propagation of elastic waves through composite media

Summary With the aid of an ultrasonic pulse technique, the propagation of elastic waves (longitudinal as well as transverse) through polyurethane rubbers filled with different amounts of sodium chloride particles was studied. The velocity of both longitudinal and transverse waves was found to increase with filler content. From the measured wave velocities, the effective modulus for longitudinal waves,L, bulk modulus,K, and shear modulus,G, were calculated according to the relations for a homogeneous isotropic material. All three moduli appear to be monotonously increasing functions of the filler content over the whole experimentally accessible temperature range (–70 °C to + 70 °C forL andK;}-70 °C to about –20 °C forG) and they, moreover, reflect the glassrubber transition of the binder.Poisson's ratio,, was found to decrease with increasing filler content and show a rise at the high temperature side of the experimentally accessible temperature range (about –20 °C) as a result of the approach of the glass-rubber transition.In addition to the velocities, the attenuation of both longitudinal and transverse waves was measured in the temperature ranges mentioned. It was found that in the hard region tan _L as well as tan _G are independent of the filler content within the accuracy of the measurements. In the rubbery region, however, tan _L, increases with increasing filler content.Finally, the experimental data are compared with a simple macroscopic theory on the elastic properties of composite media.     

High-temperature pressure-shear plate impact experiments using pure tungsten carbide impactors

The pressure-shear plate impact experiment has been modified to test materials at high temperatures (up to 700°C). Together with the high strain rates characteristic of this experiment (10⁶ s^–1), the high-temperature capability allows the shearing resistance of materials to be measured under conditions unattainable with other testing equipment. The compressive and shear responses of pure tungsten carbide at different temperatures are presented, as well as the results of one test on OFHC copper at a temperature of 691°C and a shear strain rate of 1.4×10⁶ s^–1.     

Noncontacting strain measurements during tensile tests

The application of an innovative noncontracting Doppler laser extensometer is presented. True axial strain has been measured during tensile tests conducted on stainless-steel metal sheets over a range of strain rates (from 10^–4 to 10² 1/s) and temperatures (from –40°C to 400°C). The laser radiation scattered at the surface of the specimen is recorded during the duration of the experiment. The signals are then used to determine the evolution of the axial strain, which is subsequently combined with the load signal to construct the stress-strain curve for the material. Excellent agreement has been obtained between the total elongation predicted by the laser measurements and the actual values measured from the specimens. This technique offers several advantages over traditional strain-measuring technologies.     

Modelling the linear viscoelasticity of unfilled and carbon black loaded elastomers

Cyclic small strain deformation of unfilled and carbon black loaded vulcanised elastomers was investigated over a range of strain amplitudes, frequencies, and temperatures in order to determine and model the response of these materials. The elastomer used was a butadiene-acrylonitrile base polymer, KRYNAC 806. The carbon black filler was SRF N774 at a loading of 50 phr (parts per hundred by weight). Experiments were conducted in oscillatory shear using a Weissenberg rheogoniometer. Complex modulus data were obtained for a range of oscillatory shear strain amplitudes not exceeding 0.03 rads, for frequencies in the range 5 Hz–60 Hz and at temperatures between –20°C and 20°C. Time-temperature superposition was obtained for data above –10°C, with the same shifts applicable to both unfilled and carbon black loaded materials. Shifts were represented using the WLF equation. It was found that reduced data over a range of ten decades of log frequency were well represented by a Huet model. Varying the Huet model parameters thus, in principle, affords a means of modelling linear deformations of elastomers containing different loadings of carbon black filler.     

Effects of the anomalous temperature dependence of water density on surface gravity current

This paper reports experimental results on the propagation of a plane water jet at a temperature above the maximum-density temperature (4° C) along the free surface of initially quiescent water at a temperature of about 0°C. For comparison, experiments were performed in which the temperatures of the lower and higher layers were more than 4°C, other conditions being equal. The experiments revealed a number of new hydrodynamic effects, including peculiar flow instability and a fine structure of the density field at large times.Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 46, No. 1, pp. 63–69, January–February, 2005     

A calibrated multichannel analyzer of neutral charge-exchange particles for the study of pulsed processes in a plasma

The construction and calibration of a multichannel neutral particle-energy analyzer applicable for the study of the heating of the ion component of a plasma in processes of brief duration are described. The analyzer allows one to obtain nine points in the energy distribution under study in one cycle of operation of the experimental apparatus, which makes it possible to exclude the effect of non-duplication of the process on the results obtained. The instrument consists of a gas stripping chamber and a nine-channel electrostatic ion-energy analyzer based on a flat capacitor with the angle of entrance of the particles into the analyzing field = 45 °. Results are given on the calibration of the stripping chamber for hydrogen atoms H ° in the energy range = (0.3–10) keV and for He ° atoms in the range = (1–10) keV. Air, hydrogen, and helium were used as the conversion target gases.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 6, pp. 40–45, November–December, 1974.The authors thank V. S. Koidan and L. Chernysh for assistance in the work.