Model identification and FE simulations: Effect of different yield loci and hardening laws in sheet forming

The bi-axial experimental equipment [Flores, P., Rondia, E., Habraken, A.M., 2005a. Development of an experimental equipment for the identification of constitutive laws (Special Issue). International Journal of Forming Processes] developed by Flores enables to perform Bauschinger shear tests and successive or simultaneous simple shear tests and plane strain tests. Flores investigates the material behavior with the help of classical tensile tests and the ones performed in his bi-axial machine in order to identify the yield locus and the hardening model. With tests performed on one steel grade, the methods applied to identify classical yield surfaces such as [Hill, R., 1948. A theory of the yielding and plastic flow of anisotropic materials. Proceedings of the Royal Society of London A 193, 281–297; Hosford, W.F., 1979. On yield loci of anisotropic cubic metals. In: Proceedings of the 7th North American Metalworking Conf. (NMRC), SME, Dearborn, MI, pp. 191–197] ones as well as isotropic Swift type hardening, kinematic Armstrong–Frederick or Teodosiu and Hu hardening models are explained. Comparison with the Taylor–Bishop–Hill yield locus is also provided. The effect of both yield locus and hardening model choices is presented for two applications: plane strain tensile test and Single Point Incremental Forming (SPIF).     

Investigation of operational parameters for an industrial CFB combustor of coal, biomass and sludge

The combustion of coal and/or biomass （sludge, wood waste, RDF, etc.） in a circulating fluidized bed has been a commercial topper for over 20 years, and references to principles and applications are numerous and widespread although few data are presented concerning the operation of large scale CFB-units. The authors studied the CFB-combustion at UPM-Kymmene （Ayr）, a major paper mill relying for its steam production upon the combustion of coal （80-85 %）, wood bark （5-10%） and wastewater treatment sludge （5-10%）. The maximum capacity of the CFB is 58 MWth. A complete diagnostic of the operation was made, and additional tests were performed to assess the operating mode. The plant schematics, relevant dimensions and process data are given. To assess the operation of the UPM-CFB, it is important to review essential design parameters and principles of CFB combustors, which will be discussed in detail to include required data, heat balance and flowrates, operating versus transport velocity, kinetics and conversion （including the possible effect of the Bouduard reaction if carbon is present）. Since the residence time in the riser and the cyclone efficiency determine the burnout of circulating fuel-particles, the UPM-CFB was subjected to a stimulus response technique using nickel oxide as tracer. Results illustrate the efficiency of the cyclone separation and the number of recycle loops for particles of a given size. Results will also be used to assess the cyclone operation and efficiency and to comment upon expected and measured carbon conversion.     

Experimental analysis of the steady-state behaviour of beam systems with discontinuous support

This paper deals with the experimental analysis of the long-term behaviour of periodically excited linear beams supported by a one-sided spring or an elastic stop. Numerical analysis of the beams showed subharmonic, quasi-periodic and chaotic behaviour. Furthermore, in the beam system with the one-sided spring three different routes leading to chaos were found. Because of the relative simplicity of the beam systems and the variety of calculated nonlinear phenomena, experimental setups are made of the beam systems to verify the numerical results. The experimental results correspond very well with the numerical results as far as the subharmonic behaviour is concerned. Measured chaotic behaviour is proved to be chaotic by calculating Lyapunov exponents of experimental data.

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Sommario Il presente lavoro concerne l'analisi sperimentale del comportamento a regime di travi lineari, su supporti elastici nonlineari discontinui, eccitate periodicamente. L'analisi numerica dei sistemi in esame ha evidenziato risposte subarmoniche, quasi-periodiche e caotiche, nonchè l'esistenza, nel caso di trave con una molla laterale, di tre differenti percorsi verso il caos. La relativa semplicità dei sistemi di travi ha consentito di procedere ad una verifica sperimentale dei risultati numerici e della varietà dei fenomeni nonlineari da essi evidenziati. La corrispondenza fra risultati sperimentali e numerici è molto buona nel caso di risposta subarmonica. Il comportamento caotico sperimentale è stato convalidato attraverso il calcolo degli esponenti di Lyapunov a partire dai relativi dati.

     

Experimental investigation of solute transport in large,homogeneous and heterogeneous,saturated soil columns

Laboratory tracer experiments were conducted to investigate solute transport in 12.5-m long, horizontally placed soil columns during steady saturated water flow. Two columns having cross-sectional areas of 10×10cm² were used: a uniformly packed homogeneous sandy column and a heterogeneous column containing layered, mixed, and lenticular formations of various shapes and sizes. The heterogeneous soil column gradually changed, on average, from coarse-textured at one end to fine-textured at the other end. NaCl breakthrough curves (BTC's) in the columns were measured with electrical conductivity probes inserted at 50- or 100-cm intervals. Observed BTC's in the homogeneous sandy column were relatively smooth and sigmoidal (S-shaped), while those in the heterogeneous column were very irregular, nonsigmoidal, and exhibited extensive tailing. Effective average pore-water velocities (v _eff) and dispersion coefficients (D _eff) were estimated simultaneously by fitting an analytical solution of the convection-dispersion equation to the observed BTC's. Velocity variations in the heterogeneous medium were found to be much larger than those in the homogeneous sand. Values of the dispersivity,=D _eff/v _eff, for the homogeneous sandy column ranged from 0.1 to 5.0 cm, while those for the heterogeneous column were as high as 200cm. The dispersivity for transport in both columns increased with travel distance or travel time, thus exhibiting scale-dependency. The heterogeneous soil column also showed the effects of preferential flow, i.e., some locations in the column showed earlier solute breakthrough than several locations closer to the inlet boundary. Spatial fluctuations in the dispersivity could be explained qualitatively by the particular makeup of the heterogeneities in the column.     

Surface versus bulk nucleation of dislocations during contact

The indentation of single crystals by a periodic array of flat rigid contacts is analyzed using discrete dislocation plasticity. Plane strain analyses are carried out with the dislocations all of edge character and modeled as line singularities in a linear elastic solid. The limiting cases of frictionless and perfectly sticking contacts are considered. The effects of contact size, dislocation source density, and dislocation obstacle density and strength on the evolution of the mean indentation pressure are explored, but the main focus is on contrasting the response of crystals having dislocation sources on the surface with that of crystals having dislocation sources in the bulk. When there are only bulk sources, the mean contact pressure for sufficiently large contacts is independent of the friction condition, whereas for sufficiently small contact sizes, there is a significant dependence on the friction condition. When there are only surface dislocation sources the mean contact pressure increases much more rapidly with indentation depth than when bulk sources are present and the mean contact pressure is very sensitive to the strength of the obstacles to dislocation glide. Also, on unloading a layer of tensile residual stress develops when surface dislocation sources dominate.     

Theoretical analysis of the dynamic behavior of hysteresis elements in mechanical systems

Many machine elements in common engineering use exhibit the characteristic of “hysteresis springs”. Plain and rolling element bearings that are widely used in motion guidance of machine tools are typical examples. The study of the non-linear dynamics caused by such elements becomes imperative if we wish to achieve accurate control of such machines.

This paper outlines the properties of rate-independent hysteresis and shows that the calculation of the free response of a single-degree-of-freedom (SDOF) mass-hysteresis-spring system is amenable to an exact solution. The more important issue of forced response is not so, requiring other methods of treatment. We consider the approximate describing function method and compare its results with exact numerical simulations. Agreement is good for small excitation amplitudes, where the system approximates to a linear mass-spring-damper system, and for very large amplitudes, where some sort of mass-line is approached. Intermediate values however, show high sensitivity to amplitude variations, and no regular solution is obtained by either approach. This appears thus to be an inherent property of the system pointing to the need for developing further analysis methods.     

Flow measurement around a model ship with propeller and rudder

For the design of hull forms with better resistance and propulsive performance, it is essential to understand flow characteristics, such as wave and wake development, around a ship. Experimental data detailing the local flow characteristics are invaluable for the validation of the physical and numerical modeling of computational fluid dynamics (CFD) codes, which are recently gaining attention as efficient tools for hull form evaluation. This paper describes velocity and wave profiles measured in the towing tank for the KRISO 138,000 m³ LNG carrier model with propeller and rudder. The effects of propeller and rudder on the wake and wave profiles in the stern region are clearly identified. The results contained in this paper can provide an opportunity to explore integrated flow phenomena around a model ship in the self-propelled condition, and can be added to the International Towing Tank Conference benchmark data for CFD validation as the previous KCS and KVLCC cases.     

Experimental study of the Kolmogorov refined similarity variable

Recently Thoroddsen and Van Atta (1992, Phys. Fluids A4, 2592) showed that Kolmogorov's refined similarity hypothesis (1962, J. Fluid Mech. 13, 77; 82) is supported by experimental data from a wind-tunnel study of a cylinder wake, at Re_λ of 550. We show here that the probability density of the Kolmogorov similarity variable approaches a Gaussian distribution. The data also suggest that it may obey an even more demanding conditional similarity, which leads to important conclusions regarding the scaling exponents.     

Measurements in the vaneless diffuser of a radial flow compressor

Results from an experimental study of flow behaviour at the inlet of a vaneless diffuser of a centrifugal compressor are presented. Measurements from a crossed hot-wire probe are given for operating points having inlet flow coefficients ranging from 0.006 to 0.019 at different Reynolds numbers. Instantaneous, time-averaged, and phase-averaged absolute velocity and flow angle at the diffuser inlet are deduced from the hot-wire signals after correction for mean density variations. These results show how flow behaviour varies in stable, rotating stall and surge regimes of compressor operation