TASK Quarterly   Scientific Bulletin of the Centre of Informatics - Tricity Academic Supercomputer & networK   ISSN 1428-6394

Volume 15, Number 1, 2011


Contents:



Abstracts:

  • A.Ambroziak, P.Klosowski, L.Pyrzowski, Elasto-Viscoplastic Properties of AA2017 Aluminium Alloy

    This paper describes a procedure for the identification of material parameters for the elasto-viscoplastic Bodner-Partom model. A set of viscoplastic parameters is identified for AA2017 aluminium alloy. The evaluation of material parameters for the Bodner-Partom constitutive equations is carried out using tensile tests. Numerical simulations of material behaviour during constant strain-rate tests are compared with direct uniaxial tensile experiments. A review of the models applied to concisely describe aluminium alloy is also given.

  • W.G.Li, Effect of Exit Blade Angle, Viscosity and Roughness in Centrifugal Pumps Investigated by CFD Computation

    It has long been known that the exit blade angle plays a very important role in the performance of a centrifugal pump handling water or viscous oil. The effect of exit blade angle on the performance and flow of centrifugal pumps is usually investigated experimentally. However, due to the high cost and limited data that can be obtained by experiment, currently there is a great need for this effect to be studied numerically by means of computational fluid dynamics (CFD). At present, extensive comparisons between experiment and simulation regarding the performance of viscous oil and flow in centrifugal pumps with different exit blade angles are not available. Hydraulic performance and flow details in the impeller and the volute of a centrifugal pump with an exit blade angle of 44o were investigated numerically with the CFD code Fluent, using water and viscous oil as the working fluid, respectively. The effect of exit blade angle was then elucidated by comparing the performance and flow with that of a pump with an exit blade angle of 20o. It was determined that the results for the performance and flow obtained by means of CFD were in qualitative agreement with the observations. The sudden-rising-head effect is dependent on roughness and viscosity. The unstable zone in the theoretical head curve of the impeller at a low flow rate was attributed to the strong reaction of the volute to the flow in the vicinity of the impeller exit. The flow in the impeller with a large exit blade angle was subject to separation near the blade pressure side, however, a large exit blade angle helped improve the pump performance of viscous oil.

  • A.Goralczyk, P.Chaja, A.Adamkowski, Method for Calculating Performance Characteristics of Hydrokinetic Turbines

    This paper presents a numerical algorithm and its computer implementation (program) developed to determine the operational characteristics of axial flow hydrokinetic turbines. The program is based on the Vortex-Lattice Method (VLM), which proved useful for predicting hydrodynamic characteristics of screw propellers. In order to validate the software, we carried out a series of experiments on a laboratory test stand at the Institute of Fluid-Flow Machinery at the Polish Academy of Sciences (IMP PAN) in Gdansk. From the practical point of view, the agreement between the calculated and experimental results was satisfactory. Therefore, the developed numerical method can be a useful tool for the analysis of operating conditions and in the design of hydrokinetic turbines.

  • J.Swirydczuk, P.Doerffer, M.Szymaniak, Unsteady Flow through the Gap of Savonius Turbine Rotor

    The paper presents a numerical analysis of unsteady flow through the gap of a Savonius turbine rotor, performed for varying integral turbine operating parameters calculated for the case under study as well as obtained from experiments described in the literature. Changes in the unsteady flow pattern, including the development and interaction of numerous vortex structures in the vicinity of the gap, and their relevance to the varying integral parameters are discussed. We conclude that the structure responsible for the intensity of the flow through the gap is a vortex which forms in the vicinity of the gap at certain phases of rotor revolution, and that a possible way to improve the efficiency of operation of a Savonius turbine is to control the intensity of this vortex by modifying the shape of the blade edges in the vicinity of the gap.

  • K.Tesch, M.Banaszek, Variational Method of Finding Streamlines in Ring Cascades for Creeping Flows

    This paper presents a new analytical method of finding streamlines for creeping flows in a ring cascade composed of an infinite number of infinitely thin blades. An analytical solution was obtained by minimizing the dissipation functional by means of the variational calculus. The necessary condition for the optimum of the functional yields the Stokes equation after certain additional assumptions are introduced. We considered different variants of fixing of the inlet and outlet of the cascade.

  • E.A.Anagnostakis, Wavefunction-Engineering of Intersubband THz-Laser Nanoheterointerfaces

    A novel THz-luminescence laser nanoheterointerfacial scheme of the intersubband, longer-wavelength limit, mid-infrared functionality type has been designed on the basis of optically-pumped dual-resonant tunnelling of conductivity electrons within an appropriately energetically-determined configuration of five subbands hosted by two communicating asymmetric, approximately rectangular quantum wells (QWs).

    The employed upper laser-action level is the second excited subband of the nanostructure back, wider QW and is provided with electrons via resonant tunnelling from the first excited subband of the nanostructure front QW populated through remotely ignited optical pumping out of the local fundamental subband.

    On the other hand, the first excited back-QW subband functions as the lower laser action level, directly delivering the received electrons to the local fundamental subband via a fast vertical longitudinal optical phonon scattering. From there, they are recycled back to the nanostructure front QW fundamental subband by virtue of a second, reverse sense resonant-tunnelling-mediated normal charge transport mechanism.

    A nanophotonics application of the scheme predicts laser operability in the 15-THz range.

  • S.Gulkowski, J.M.Olchowik, K.Cieslak, P.P.Moskvin, Modelling of the Interface Evolution During Si Layer Growth on a Partially Masked Substrate

    High-quality thin Si layers obtained from the solution by epitaxial lateral overgrowth (ELO) can play a crucial role in photovoltaic applications. The laterally overgrown parts of the layer are characterized by a lower dislocation density than that of the substrate. The height and width of the layer depend on several factors, such as the technological conditions of liquid phase expitaxy (LPE), growth temperature, cooling rate and the geometry of the system (mask filling factor). Therefore, it is crucial to find the optimal set of technological parameters in order to obtain very thin structures with a maximum width (high aspect ratio).

    This paper presents a computational study of Si epilayer growth on a line-pattern masked silicon substrate from Si-Sn rich solution. To solve this problem, a mixed Eulerian-Lagrangian approach was applied. The concentration profile was calculated by solving the two-dimensional diffusion equation with appropriate boundary conditions. The growth velocity was determined on the basis of gradients of concentration in the border of the interface. Si interface evolution from the opened window was demonstrated.

  • J.Dow, A.A.Zolotovsky, Band Structure of Ruthenate Superconductors

    We investigate the band structure of ruthenates using the first-principles method of density functional theory (DFT). We calculated the band structure and density of states at the Fermi level for superconductors Ba2YRuO6 and Sr2YRuO6. Our calculations show that the maximum critical superconducting transition temperature Tc=93K can be obtained for Ba2YRuO6.

  • G.Nabiyouni, M.J.Fesharaki, A.Zolotovsky, Preparation, Magnetic Studies and Band Structure Calculation of NiFe2O4 Nanoparticles

    We have undertaken a comprehensive theoretical study of the band structure, density of states, dependence of the Curie point and saturation magnetization on the size of NiFe2O4 nanoparticles prepared by the conventional ceramic method. Commercially available NiFe2O4 powder was first annealed in an oxygen environment in a furnace at 1100oC for 3h. The X-ray diffraction pattern indicated that the sample was single-phase at this stage. The average grain size estimated by scanning electron microscopy (SEM) was in the range of 300 to 350nm. The magnetic behavior of the sample at room temperature was studied by means of a superconducting quantum interference device (SQUID). The Curie temperature of the nickel ferrite powder was measured using an LCR meter. The measurement of the Curie temperature and saturation magnetization indicated that a decrease in the grain size leads to a decrease in the Curie temperature and in the saturation magnetization. The small value of saturation magnetization was attributed to a spin-glass-like surface layer on the nanocrystalline nickel ferrite with a ferrimagnetically aligned core (H.Nathani and S.Gubbala 2004 J.Mater. Sci. and Engin. B 111 95). Good agreement was obtained between theory and experimental results.

  • P.Moskvin, M.Antonov, G.Olchowik, J.M.Olchowik, A.Zdyb, S.Gulkowski, W.Sadowski, Polyassociative Model of A2B6 Semiconductor Melt and p-T-x Phase Equilibria in Zn-Cd-Te System

    This work presents a numerical analysis of p-T-x phase equilibrium in the Zn-Cd-Te ternary system in the framework of the polyassociative solution model. On the basis of the experimental data on p-T-x phase equilibrium in the ternary system, thermodynamic functions describing the formation of liquid associates were found. It was shown that the results of the mixing of components in the Zn-Cd-Te ternary melt are related to the occurrence of ZnCdTe and ZnCdTe3 associates. Dissociation parameters of these complexes were calculated and subsequently used in order to efficiently describe p-T-x phase equilibrium of the system in a wide temperature range.