Effect of Transitional Nonequilibrium on the Molecular–Dissociation Rate in a Hypersonic Shock Wave

The problem of the influence of a nonequilibrium (non–Maxwellian( distribution of translational energy over the degrees of freedom of molecules on the rate of their dissociation in a hypersonic shock wave is considered. An approximate beam—continuous medium model, which was previously applied to describe a hypersonic flow of a perfect gas, was used to study translational nonequilibrium. The degree of dissociation of diatomic molecules inside the shock–wave front, which is caused by the nonequilibrium distribution over the translational degrees of freedom, is evaluated. It is shown that the efficiency of the first inelastic collisions is determined by the dissociation rate exponentially depending on the difference in the kinetic energy of beam molecules and dissociation barrier.     

The rheological behaviour of HDPE/LDPE blends

The rheological behaviour of three types of HDPE/LDPE blends at several compositions (various weight percentages of LDPE) has been studied with the aid of a capillary rheometer and three different capillaries. In particular, entrance effects and shear viscosities have been determined over a wide shear rate region and at different temperatures. Thus activation energies could also be evaluated. Synergistic effects are evidenced when the relevant properties of the homopolymers parents are not too different from one another.     

A photoviscoelastic analysis of time-dependent stresses in a polyphase system

The method of photoviscoelastic stress analysis is used to predict time-dependent stress redistributions in a polyphase-material system having a viscoelastic binder and subjected to applied exteernal-loading conditions. The polyphase-material model studied is composed of a photoviscoelastic matrix material and contains rigid inclusions and voids, thus simulating a threephase composite system. In order to perform the study, a photoviscoelastic model material is developed. An epoxy-resin system consisting primarily of Shell Epon 828 and Epon 871, optimized to display the properties desirable for such application, is utilized. The time-dependent stess distributions obtained by the photoviscoelastic analysis are compared with results obtained by applying the “correspondence rule” to a finite-element solution for the elastic stress field of a mathematical model of the three-phase material system. The comparison of results indicates that the technique of photoviscoelastic stress analysis is extremely applicable to complex models such as the one studied. The feasibility of this application to more complex polyphase models with varying loading conditions is indicated.     

Multi-scale Measurement of (Amorphous) Polymer Deformation: Simultaneous X-ray Scattering,Digital Image Correlation and In-situ Loading

This paper presents a method to investigate the behaviour of polymers on different scales during deformation using simultaneously collected synchrotron X-ray scattering, digital image correlation (DIC) and tensile loading. The method is demonstrated through experiments made on specimens of amorphous polycarbonate. Deformation is measured in-situ, simultaneously across different scales from the macroscopic deformation, measured using sensors on the tensile machine, to the full-field mesoscopic deformation, measured using DIC, down to the deformation of the nano-scale structure, studied using small and wide angle X-ray scattering (SAXS/WAXS). The DIC reveals highly inhomogeneous deformations that render conventional techniques for measuring deformation, such as extensiometers, virtually useless. The X-ray scattering is measured in several spatial points during continuous loading giving the evolution of the microstructure with respect to both spatial location and load level. The spatial mapping of the scattering reveals characters that would not be observed when only measuring at the centre point or measuring on a large area of the specimen, e.g. wide beam SAXS/WAXS or small angle neutron scattering (SANS). With these data, the macroscopic and the mesoscopic deformation can be correlated to the behaviour of the microstructure providing relevant information when developing micro-mechanical based constitutive models. The experimental results shown here indicate a direct correlation between the major principal strain direction and the maximum anisotropy direction of the SAXS patterns. The current approach can be extended to any kind of polymeric materials or polymer-based nano-composites.     

The effect of molecular weight on the rheological properties of poly(dimethylsiloxane) filled with fumed silica

The viscometric, stress relaxation, and stress growth rheological properties were measured for various molecular weight PDMS fluids filled with fumed silica. The stress growth function exhibited significant overshoot, when the continuous phase molecular weight was slightly greater than the entanglement molecular weight; however, significant overshoot peaks were not observed, when the continuous phase molecular weight was less than or much greater than the molecular weight between entanglements. The experimentally observed transient rheological properties are rationalized in terms of a molecular model, where interparticle interactions occur via entanglements of the polymer adsorbed on the silica surface. When the molecular weight of the adsorbed polymer is greater than the entanglement molecular weight, the strength of the interparticle interaction will increase substantially and the particle diffusivity will substantially decrease.     

Influence of concentrated lateral loads on the elastic stability of cylinders in bending

The character of the instability and the degradation of the moment-carrying capacity are found by Mylar model experiments for cylinders in bending when subjected to concentrated lateral loads. Lateral loads can seriously degrade the moment capability of cylinders. Critical combinations of moment and lateral load cause two distinct modes of failure—collapse and snapping. Collapse modes exhibit buckles which cover the compression half of the cylinder and are critical for large values of moment and small values of lateral load. Snapping modes of failure involve a single dimple and exist for smaller values of moment and larger values of lateral load.     

Application of faraday's effect in static and dynamic holographic photoelasticity

The basic principle of applying Faraday's effect to achieve the separation of fringes in static and dynamic holographic photoelasticity, and a study and application of Faraday's light rotator are described in this paper. It is proposed that Faraday's light rotator be used for automating photoelastic instrumentation for measuring isoclinics and the decimal orders of isochromatic fringes.     

Calculation of the rotational transition probabilities of diatomic molecules when they collide with heavy particles

The collisional rotational transition probabilities for molecule-molecule and molecule-atom interactions in three-dimensional space are calculated. The quasiclassical approach developed in [1] is used. Expressions are obtained that are suitable for practical calculations of single-quantum and double-quantum rotational transitions in diatomic molecules. The collisional rotational transition probabilities are averaged over the Maxwell velocity distribution and their dependence on the gas temperature is obtained. To illustrate the method the results of a calculation of the probabilities for HCl-HCl, HCl-He, CO-CO interaction are presented.     

A composite three-dimensional photoelastic method

Care must be taken in preparation and testing of three-dimensional composite photoelastic models. Some problems encountered in modeling the prototype and during model testing are: model-material failure, loss of fringe pattern in slicing, inherent shrinkage response in freezing, inadequate bonding between materials, and modular ratio difficulties. The selection of the correct plastic can eliminate the first four problems, but the correct modular ratio between the matrix and the insert has to be obtained. This investigation illustrated the behavior of commercially manufactured plastics as inserts, with a matrix material of Epon 828 epoxy. The effective moduli of elasticity of these plastics are reported for pure tension and for flexure. Since the manufactured plastics produced varying results, the use of Epon 828 epoxy as an insert was investigated. The inserts were cast in tygon tubing and their curing cycle was altered from that used for the matrix material to produce a different effective modulus of elasticity. The Epon 828 inserts gave excellent results in the beams. The use of the same material for matrix and insert eliminates many of the problems associated with composite three-dimensional photoelasticity.     

Flow of a dipolar fluid between parallel plates

The flow of a dipolar fluid between two parallel plates with and without heat transfer is studied. The following cases are discussed:

1. Isothermal flow due to the relative motion of the plates,
2. Isothermal flow due to a constant pressure gradient with the plates at rest,
3. Nonisothermal flow with linearly varying plate temperatures.

Case (ii) is of particular interest to the experimentalists as it shows the effect of the material constants even when there are no externally applied dipolar tractions on the plates.     

Epon 828 epoxy: A new photoelastic-model material

The photoelastic properties of Epon 828 are discussed. The epoxy is evaluated experimentally for its time-edge effect, optical creep, stress-optic relations, and mechanical properties at room and elevated temperatures. Epon 828 is extremely clear, has good transparency, and can be use for photoelastic stress analysis at room and elevated temperatures. It has a low-fringe constant at room temperature as well as at elevated temperatures coupled with a high critical modulus of elasticity. Epon 828 can easily be cast stress-free and has relatively small amounts of optical and mechanical creep. The machining characteristics of Epon 828 are excellent and it cements easily with itself and other epoxy materials.     

Influence of vibration-dissociation coupling on heat transfer and hypersonic drag

A numerical investigation is carried out within the framework of the multicomponent total viscous shock layer model [6, 7], according to which when Re 100 the flow near a blunt body can be divided into a shock wave zone and a viscous shock layer. At the inner edge of the shock wave the generalized Rankine-Hugoniot relations are imposed, and in the shock layer the complete system of viscous shock layer equations is solved with allowance for vibrational relaxation and nonequilibrium dissociation and ionization reactions.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 3, pp. 141–151, May–June, 1990.     

Adaptive control of MEMS gyroscope using global fast terminal sliding mode control and fuzzy-neural-network

An adaptive control of MEMS gyroscope using global fast terminal sliding mode control (GTSMC) and fuzzy-neural-network (FNN) is presented for micro-electro-mechanical systems (MEMS) vibratory gyroscopes in this paper. This approach gives a new global fast terminal sliding surface, which will guarantee that the designed control system can reach the sliding surface and converge to equilibrium point in a shorter finite time from any initial state. In addition, the proposed adaptive global fast terminal sliding mode controller can real-time estimate the angular velocity and the damping and stiffness coefficients. Moreover, the main feature of this scheme is that an adaptive fuzzy-neural-network is employed to learn the upper bound of model uncertainties and external disturbances, so the prior knowledge of the upper bound of the system uncertainties is not required. All adaptive laws in the control system are derived in the same Lyapunov framework, which can guarantee the globally asymptotical stability of the closed-loop system. Numerical simulations for a MEMS gyroscope are investigated to demonstrate the validity of the proposed control approaches.     

Photoviscoplastic investigations of slip-line fields and principal-stress trajectories in viscoelastoplastic strip for drawing with back tension

Photoviscoplasticity is applied to the drawing with back tension through rollers and wedge-shaped dies in model tests of plane-strain state. The effects of the back tension on the slip-line fields and on the principal-stress trajectories for the strips are discussed in the light of experimental results obtained from the model tests. Actual strips are considered as viscoelastoplastic media in the actual drawing processes. The model specimens used are softened celluloid as the viscoelastoplastic medium for the strip and Araldite as the glassy elastic medium for the dies. At a suitable experimental temperature, the viscoelastoplastic strip is drawn viscoplastically through the gap between the dies mentioned above. The slip-line fields, the principal-stress trajectories in the strip and the normal pressure on the contact surface are greatly influenced by the effect of the back tension.     

An exploratory investigation of cumulative shock fatigue

A simple device for producing cumulative shock loading in solids is described. The device uses a ballistic-impact-driven projectile to introduce high-stress waves into a solid. The impact time and load amplitude can be varied to produce fracture in one or several impacts in PMMA rods. The wavefront approached a square wave shape. Materials other than PMMA were loaded to failure to demonstrate the versatility of the device. Fracture morphologies observed with optical and scanning-electron microscopy are described.     

The gamma-ray-irradiation method applied to three-dimensional thermal photoelasticity

It has recently been shown that a gamma-ray irradiation fixes the birefringence in photoelastic models of Araldite B. The method has hitherto only been used to fix the birefringence in specimens subjected to constant forces. This work applies the irradiation method to determine the thermal stresses in a thick-walled cylinder with a temperature gradient along the radius. The model behaved in the following way during the experiment. The material relaxed when the temperature gradient was applied, and the model was irradiated when it was stress free. Real stresses, which were equal to the thermal stresses but with opposite sign, appeared when the temperature gradient was removed. As the problem is mathematically two-dimensional, it is possible to determine the birefringence by means of the slicing technique. Good agreement was obtained between the stress determined experimentally and those calculated analytically. The irradiation sensitivity of a number of materials has been determined, and some new materials have been used that are superior in two important ways to that used earlier in that they require a much lower irradiation dose and their mechanical properties are considerably better.     

Sharpening and multiplication of moiré fringes

Moiré fringe sharpening is produced when the widths of clear and opaque bands in a pair of super-imposed screens are complementary. Fringe multiplication is produced when the frequency of lines in the reference screen exceeds the frequency in the specimen screen by an integral factor. The number of moiré fringes then formed corresponds to the greater frequency. The conditions of formation of these moiré patterns are presented and demonstrated. Intensity distribution across projected images of moiré fringes is discussed.     

Allowance for the effect of nonisothermicity on heat transfer in channels during the laminar movement of liquids having a linear law of fluidity

Nonisothermal heat exchange during the laminar flow of liquids having a linear law of fluidity in the thermal initial section of channels is analyzed. The calculation for the condition that t_ω =const takes into account the effect of the temperature on the zero-point fluidity and the coefficient of instability of the structure when the thermal conductivity (diffusion) depends on the shear stress.     

On the flow between two counter-rotating infinite plane disks

This paper studies the boundary-value problem arising from the behaviour of a fluid occupying the region -1≦x≦1 between two rotating disks, rotating about a common axis perpendicular to their planes when the disks are rotating with the same speed Ω₀ but in the opposite sense. The equations which describe the axially symmetric similarity solutions of this problem are $$\varepsilon H^{iv} + HH''' + GG' = 0$$ $$\varepsilon G'' + HG' - H'G = 0$$ with the boundary conditions $$H( \pm 1) = H'( \pm 1) = 0$$ $$G( - 1) = - 1,{\text{ }}G(1) = 1$$ where ?=v/2Ω₀ and v is the kinematic viscosity. The existence of an odd solution is established. This particular solution satisfies many special conditions, for example, G′ (x, ?)>0. Moreover, precise estimates are obtained on the size and behaviour of the solution as ? ↓ 0.     

Investigation of the Dynamic Response of Swirling Flows Based on LES and Experiments

The dynamic response of a swirling flow undergoing vortex breakdown is investigated via Large Eddy Simulation (LES) and experiments in a water flow facility. The investigation is carried out following previous work on the link between thermoacoustic combustion instabilities and coherent structures in lean premixed gas turbine combustors. The velocity field transfer function is obtained in LES from the Unit Impulse Response determined via application of a low intensity broadband noise perturbation of the inflow mass flow rate and the Wiener-Hopf filtering method. In the experiments, harmonic fluctuations in the water flow rate through the swirler are generated via a piston mounted on the side wall of the test facility and activated with a low frequency linear motor. The velocity field transfer function is then obtained via phase averaging applied to Particle Image Velocimetry snapshots which are collected at prescribed values of the harmonic phase. The analysis, which is carried out in terms of coherent structures identified via Proper Orthogonal Decomposition, gives numerical transfer functions with amplitude and phase consistent with the experimental ones.