Local Measurement Techniques Research Articles

Vorticity balance and time scales of a two-dimensional airfoil in an unsteady free stream

In unsteady aerodynamic research, there exists a long list of operational variables due to the complexity of the flow. For engineering design purposes, parametric study is the conventional approach. In this investigation, an alternative approach is used. It is demonstrated here that the physical insights of the intricate unsteady aerodynamic flow can be understood from a simple fundamental vorticity balance concept. The test configuration is a stationary two-dimensional NACA 0012 airfoil placed at the static stall angle of 12° in an unsteady free stream. A local circulation measurement technique is introduced to survey the evolving vorticity field on the airfoil. The information about the vorticity convection and the surface vorticity flux over the wing provides a basic understanding regarding the variation of the unsteady lift.

Fabrication of metal matrix composites using a helical induction stirrer

The working principle and the peculiarities of a new electromagnetic rheocaster, which is based on the use of rotating permanent magnets, and which allows the production of intense three-dimesional multiphase flows in solidifying semisolid alloy slurries and metal matrix composites, are described. This process can be applied to the direct continuous casting of billets, tubes and slabs and is characterized by very low electric power consumption. Local measurement techniques are applied to the study of the evolution of non-newtonian magnetohydrodynamic multiphase flow phenomena with the rotational speed of the inductor, the solid fraction of the aluminium alloy matrix and the size and volume percentage of SiC particles. Satisfactory results concerning the microstructure of solidified aluminium slurries and aluminium matrix composites (homogeneity, crystal shape, grain size, fraction of primary solid, distribution of SiC particles) were obtained. A discussion is presented relating the metallurgical findings to the heat and three-phase flow measurements.

Elaboration of metal matrix composites from thixotropic alloy slurries using a new magnetohydrodynamic caster

The working principle and the peculiarities of a new electromagnetic rheocaster, which is based on the use of rotating permanent magnets and which allows the production of intense three-dimensional (3-D) multiphase flows in solidifying semisolid alloy slurries and metal matrix composites, are described. This process can be applied to the direct continuous casting of billets, tubes, and slabs and is characterized by very low electric power consumption. Local measurement techniques are applied to the study of the evolution of non-Newtonian magnetohydrodynamic multiphase flow phenomena with the rotational speed of the inductor, the solid fraction of the aluminum alloy matrix, and the size and volume percent of SiC particles. An order of magnitude analysis of the various forces acting on the suspended crystals and SiC particles is given. The Theological behavior of electromagnetically rheocast ferrous metals, simulated by a lead-tin alloy, is also investigated. Satisfactory results concerning the microstructure of solidified aluminum slurries and aluminum matrix composites (homogeneity, crystal shape, grain size, fraction of primary solid, and distribution of SiC particles) were obtained. A discussion is presented relating the metallurgical findings to the heat and three-phase flow measurements.

Photovisco-elastoplastic behavior of polycarbonate material under creep and tension tests

Polymers are widely used as photomechanical models of a prototype material (often a metal). Photoplasticity is one of the methods used in order to show the behavior of plastic materials stressed beyond the linear elastic limit. To illustrate this process we have analyzed the photovisco-elastoplastic behavior of polycarbonate as a photoplastic material. In this paper a technique for local and simultaneous measurement of birefringence and principal strains is presented. The mechanical and optical properties, at room temperature, have been evaluated by means of uniaxial tension tests. A series of creep tests has been carried out in order to study the photovisco-elastoplastic behavior of polycarbonate. In two different experiments we analyzed nonlinear birefringence and the amplitude of the corresponding strains. We could thus evaluate the distribution of strains and the distribution of uniaxial stress for each birefringence state and vice versa.

Production of metal matrix composites ingots from semisolid alloys

The working principle and the peculiarities of new electromagnetic rheocasters, which are based on the use of rotating permanent magnets and which allow the production of intense three-dimensional multiphase flows in solidifying semi-solid alloy slurries and metal matrix composites, are described. These processes are likely to be applied to the direct continuous casting of billets, tubes and slabs. They are also characterized by a very low electric power consumption. Local measurement techniques are applied to the study of the evolution of the fluid flow phenoma with the rotation speed of the inductor, the solid fraction, and the size and volume percent of SiC particles. A discussion is presented relating the metallurgical findings to the heat and fluid flow measurements. Satisfactory performances concerning the microstructure of solidified aluminum slurries and aluminum matrix composites (homogeneity, cyrstal shape, grain size, fraction of primary solid, distribution of SiC particles) were obtained

Elaboration of semisolid alloys by means of new electromagnetic rheocasting processes

The working principle and the peculiarities of new electromagnetic rheocasters, which are based on the use of rotating permanent magnets and which allow the production of intense stirring in solidifying semisolid alloy slurries, are described. Local measurement techniques are applied to the study of the evolution of the electromagnetic, hydrodynamic, and thermal phenomena with time inside vigorously agitated melt-solid mixtures. Satisfactory performances concerning the microstructure of solidified aluminum thixotropic slurries (homogeneity, crystal shape, grain size, and fraction of primary solid) were obtained.

Transient local magnetic field measurement in a bumpy torus by rapid-frequency-scan laser spectroscopy

A new technique for local magnetic field measurement in a bumpy torus NBT-1M (which is a magnetic plasma confinement device) has been developed using a rapid-frequency-scan (RAFS) laser system combined with sodium atomic beam probing. The RAFS laser beam excites the sodium D1 transition in the plasma cavity, and the Zeeman pattern is determined with laser fluorescence spectroscopy in a single laser shot. From the Zeeman splitting, the transient magnetic field distribution in the plasma, produced by electron cyclotron resonance heating (ECH), is measured with an accuracy of ±3.2%, a spatial resolution of 5 mm, and a temporal resolution of 5 μs.

Experimental study of continuous electromagnetic casting of aluminum alloys

This work begins with a summary of the characteristics of electromagnetic casting and the present knowledge of the subject. Also described is the use of new local measurement techniques for velocity, magnetic field, current density, and phase difference, which allow experimental investigation of the flow of molten metal in industrial equipment (up to 700 °C) in the presence or absence of an induction magnetic field. Next, these methods are applied to the study of electromagnetic and hydrodynamic phenomena, inside the sumps of both rectangular and circular cross-section ingots of aluminum alloys cast in electromagnetic molds. The important shield effect as a function of the screen location is studied by means of a mercury pool simulating electromagnetic casting.

Thermally Assisted Fluorescence: A New Technique for Local Flame Temperature Measurement

The theoretical basis of a new technique called thermally assisted atomic fluorescence for measurement of local spatial-temporal flame temperatures is given. In this method, the ratio of fluorescence signals from a radiatively excited level and a higher collisionally excited level or from two higher collisionally excited levels of an inorganic probe, such as In or TI, is related to the flame temperature. The conditions necessary for the measured flame temperature to be identical to the translational (Boltzmann) flame temperature are given. The experimental system and conditions for making flame temperature measurements, which consists basically of a single pulsed dye laser and a gated detector, are described. The assumption of steady-state conditions is discussed as well as several anomalies in terms of fluorescence from certain energy levels of Tl. The accuracy and precision of the measured flame temperatures for several methane-air flames are discussed.

The Fe−3Si steel etching technique for local strain measurement

The Fe−3Si steel etching technique for measuring localized plastic deformation is described. The technique is well suited for revealing plastic strain in the vicinity of notches, cracks and other stress concentrations, both on the micro- and macroscale. The method is capable of yielding quantitative information about the strain distribution on the surface and in the interior of test specimens whose plastic-flow properties model the behavior of engineering alloys, in general, and low- and medium-strength steels, in particular. Methods of melting and fabricating the steel for this purpose, specimen preparation, and metallographic procedures are explained. Mechanical properties of the steel and its special etching response are described. In addition, results of some plastic-zone determinations are presented to illustrate the technique's capabilities, and other applications are mentioned.

Mobile dislocation density during inhomogenous deformation in Au(14at%Cu)

The processes of nucleation and propagation of Portevin–LeChatelier deformation bands during tensile deformation of the alloy AA5754 have been investigated by a special optical technique for local strain measurements, combining both a line-scan and a CMOS camera to record the changes of surface strain, applying digital image correlation. Examples for bands of types A and B are observed and discussed with respect to the micro-mechanisms of their nucleation and propagation, emphasizing local stress concentrations in addition to the well-known effects of dynamic strain aging and dislocation interactions for explaining the PLC behavior. Finally the nucleation and propagation of PLC bands are simulated by computations based on a simple constitutive model.

CHEMICAL SCAVENGER PROBE STUDIES OF ATOM AND EXCITED MOLECULE REACTIVITY IN ACTIVE NITROGEN FROM A SUPERSONIC STREAM

A quartz chemical scavenger probe has been developed to study the local composition of supersonic electrically discharged gas streams. The probe samples the central portion of a nonequilibrium jet and allows direct comparison with other local measurement techniques (e.g. differential catalytic detectors) for determining active species concentrations. Active nitrogen from a Mach 3 stream was sampled and reacted inside the probe with one of the scavenger gases NO, NH3, or C2H4at 18.8 mm Hg and at an average temperature of 500 °K. Limiting values of the NO destruction rate and the HCN production rate were observed; however, NH3destruction exhibited no plateau. The observed maximum rate of NO destruction was 2.1 times as large as the NO flow rate at the light titration end-point. This difference is attributed to a reaction of NO, added in excess of the titration end-point flow, with metastable electronically excited molecules formed within the discharge zone. The converging-diverging supersonic nozzle-glow discharge source used in these experiments apparently delivers metastable excited molecules to the reaction zone in a higher relative concentration than do the more conventional subsonic electrical discharge flow systems used for most previous active nitrogen studies.

A steady-state technique for local heat-transfer measurement and its application to the flat plate

A steady-state technique of heat-transfer measurement has been developed based on the method of Seban, Emery & Levy (1959) whereby energy is dissipated by the Joule effect in a thin metal sheet on the surface of a model. For the present application, use was made of very thin but mechanically resistant films of metal of very nearly constant thickness, obtained by a simple mirror-silvering technique. The present investigation was prompted by the desire to make very local measurements of heat transfer for application in regions where large variations in convective heat flux and therefore in temperature could be expected.Comparison between theory and experiment has been made in the simple case of a flat plate with constant heat flux for which a rigorous computation could be made based on the theory of Chapman & Rubesin (1949). The model was so conceived that the heat losses were small enough to be neglected. Therefore no corrections, which are often inaccurate, were needed for the experimental results, contrary to what is generally done when using other techniques for heat-transfer measurements. The excellent agreement between theory and experiment gives complete confidence in the method. The theoretical analysis showed that the measurements are simply related to the results that could be obtained in the case of an isothermal surface, because of the constant ratio that exists between the corresponding heat-transfer coefficients.