Wyniki wyszukiwania - Biblioteka Nauki

1

Effects of neutral gas friction on the instability of superposed streaming plasmas

100%

Mathur R. P. , Kumar N.

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2011

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tom Vol. 33, nr 4

39-48

EN

The instability of plane interface separating two superposed partially ionized streaming plasmas in a uniform vertical magnetic field in the presence of effect of surface tension is discussed. The dispersion relation has been derived by using the normal mode technique and solved numerically. It has been found that the collision frequency, streaming velocity and ion viscosity have destabilizing influence while surface tension has stabilizing influence on the growth rate of unstable mode of disturbance.

2

Stability analysis from fourth order nonlinear evolution equation for two stokes wave trains in deep water in the presence of air flowing over water

60%

Majumder D. P. , Dhar A. K.

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2009

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tom Vol. 14, no 4

989-1008

EN

Fourth order nonlinear evolution equations are derived for two Stokes wave trains in deep water in the presence of air flowing over water. The importance of the fourth order term in the evolution equation was pointed out by Dysthe (1979). Stability analysis is then made for uniform two Stokes wave trains in the presence of air flowing over water. From these evolution equations the expressions for the maximum growth rate of instability, the wave number at marginal stability and the wave number separation of fastest growing side band are derived and graphs are plotted for the above three expressions against the wave steepness. Significant improvements can be achieved from the results obtained from the two coupled third order nonlinear Schrödinger equations.

3

Positive unstable electrical circuits

60%

Kaczorek T.

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2012

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tom R. 88, nr 5a

187-192

EN

The instability for the positive linear electrical circuits composed of resistors, coils, condensators and voltage (current) sources are addressed. Three different classes of the positive unstable linear electrical circuits are proposed and analyzed. It is shown that positive electrical circuits are unstable for all values of their parameters if the electrical circuit has at least one mesh containing only inductances and source voltages.

PL

W artykule rozpatrywane są niestabilne dodatnie liniowe obwody elektryczne złożone z rezystorów, cewek, kondensatorów i źródeł napięcia lub prądu. Analizowane są trzy różne klasy dodatnich obwodów elektrycznych, które są niestabilne dla wszystkich wartości swoich parametrów tzn. rezystancji, indukcyjności i pojemności. Wykazano, że dodatnie obwody elektryczne są niestabilne dla wszystkich wartości swoich parametrów jeżeli zawierają one przynajmniej jedno oczko złożone tylko z cewek i źródeł napięcia.

4

Niestateczność plastycznego płynięcia metali w procesach wyciskania promieniowego.

60%

Mroczkowski M. , Richert J.

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2003

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tom R. 48, nr 10-11

486-491

PL

Przedstawiono teoretyczne przesłanki prawidłowego doboru kształtu narzędzi do wyciskania promieniowego wyrobów. Za pomocą programów komputerowych DEFORM-2 i FORGE3 analizowano różne warunki płynięcia metalu. Udowodniono, że w przypadku nieprawidłowego doboru kształtu narzędzi dochodzi do niestateczności plastycznego płynięcia, która ujawnia się promieniowym podziałem wyciskanego materiału. Wykazano, że zbyt małe kąty nachylenia powierzchni wykrojów matrycowych przyczyniają się do obwodowego rozciągania warstw zewnętrznych kształtowanego metalu, co prowadzi do niekontrolowanego płynięcia metalu i utraty stateczności plastycznego płynięcia. Wyniki symulacji zostały potwierdzone badaniami eksperymentalnymi procesów wyciskania przeprowadzonymi na zimno i gorąco, a także w stanie półciekłym. Stwierdzono, że symulacje przeprowadzone metodą elementów skończonych pozwalają sprawdzić zachowanie stateczności plastycznego płynięcia, a tym samym ocenić przydatność zaprojektowanych narzędzi do zastosowań praktycznych.

EN

Theoretical premises for correct choice of tool shape for radial extrusion of products have been presented. The different conditions of metal flow were analyzed using DEFORM-2 and FORGE3 programme. It was proved that, in the case of incorrect choice of tool shape, instability of plastic flow occurs, which leads to radial partition of extruded material. It has been show that too small angles of inclination of die surfaces cause a circumferential tension of the external layer of deformed metal, which guides to the uncontrolled metal flow, and the loss of flow stability. The results of simulations have been confirmed by experimental investigations for radial extrusion processes carried out under cold and hot forming conditions, and also in semi-liquid state. It was found that the flow analysis using FEM solutions permits to check the stability of plastic flow and evaluates the usefulness of tools designed.

5

Stability of small but finite amplitude interfacial capillary gravity waves for perturbations in two horizontal directions

60%

Majumder D. P. , Dhar A. K.

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2011

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tom Vol. 16, no 2

425-434

EN

By exact numerical computation Yuen (1984) obtained regions of type-I instability for waves propagating at the interface of two superposed fluids of infinite thickness in which the upper fluid has a constant streaming velocity. In the present paper it is shown that the long wavelength part of these instability regions can be obtained analytically from a fourth order nonlinear evolution equation for small but finite amplitude interfacial capillary gravity waves in the presence of air flowing over water.

6

Thermosolutal convection in Rivlin-Ericksen fluid in the presence of Hall currents in a porous medium in hydromagnetics

60%

Kango S. K. , Singh V.

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tom Vol. 17, no. 1

71-82

EN

The thermosolutal instability of a layer of a Rivlin-Ericksen elastico-viscous fluid is considered in the presence of a uniform horizontal magnetic field to include the Hall currents in a porous medium. For the case of stationary convection, the Hall currents hasten the onset of convection, the magnetic field postpones the onset of convection, medium permeability also postpones the onset of convection in the presence of the Hall currents, whereas the kinematic viscoelasticity has no effect on the onset of convection. The Hall currents, kinematic viscoelasticity, magnetic field, medium permeability and the solute parameter introduce oscillatory modes in the system, which were non-existent in their absence. The case of overstability is also considered wherein the sufficient conditions for the non-existence of overstability are obtained.

7

Evolution of a random field of surface gravity waves in a two fluid domain

51%

Senapati S. , Debsarma S. , Das K. P.

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tom Vol. 17, no. 2

481-493

EN

A spectral transport equation is derived here that governs the evolution of a random field of surface gravity waves in a two layer fluid model. This equation is used to study the stability of an initially homogeneous Lorentz spectrum under long wave length perturbations. It is observed that the effect of randomness is to reduce the growth rate of instability. An increase in the thickness of the upper fluid results in an increase in the extent of instability. It is also found that the extent of instability becomes less for a smaller density difference of the two fluids.

8

Effect of the magnetic field and suspended particles on thermosolutal instability of a rotating Rivlin-Ericksen fluid with varying gravity field in a porous medium

51%

Singh V. , Dixit S.

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tom Vol. 17, no. 2

559-570

EN

In the paper we consider thermal instability of a rotating Rivlin-Ericksen viscoelastic fluid in the presence of suspended particles in a porous medium, the effect of magnetic field with varying gravity field are also studied. It is found that for stationary convection, a Rivlin-Ericksen fluid behaves like an ordinary Newtonian fluid while the magnetic field has both stabilizing and destabilizing effect on the system. Other different aspects affecting stability are also considered.

9

Ocena niestabilności termosprężystej hamulców ciernych w oparciu o kryterium dopuszczalnego odkształcenia osiowego tarczy hamulca

51%

Ścieszka S. , Żołnierz M.

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2006

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tom z. 274

299-310

PL

Ciepło generowane w hamulcach mechanicznych wywołuje odkształcenia termosprężyste tarcz hamulcowych, co prowadzi do zmiany rozkładu nacisków na powierzchniach ciernych tarcz. To wzajemnie powiązane ze sobą oddziaływanie prowadzi do wytworzenia tzw. gorących plam, jeżeli początkowa prędkość hamowania jest wystarczająco wysoka. Tak opisany proces nazywa się niestabilnością termosprężystą. W artykule przedstawiono zastosowanie metody elementów skończonych (MES) do modelowania tego procesu w hamulcach tarczowych maszyn wyciągowych. Przedstawiono wpływ wybranych parametrów konstrukcyjnych (grubość tarczy) i materiałowych tarczy (współczynnika rozszerzalności cieplnej, przewodności cieplnej i modułu sprężystości) na wartość prędkości krytycznej. Jako kryterium oceny przejścia ze sfery stabilności do niestabilności przyjęto kryterium dopuszczlnego odkształcenia osiowego tarczy hamulca.

EN

The heat generated in mechanical brakes causes thermoelastic distortion of disc and affects the distribution of contact pressure. This interaction is unstable if the sliding speed is sufficiently high leading eventually to the development of hot spots. The phenomenon is reffered to as frictionally-excited thermoelstic instability or TEI. This paper describes application of finite element modelling of the phenomenon in mining winding machine friction disc Breakes. The effects of friction material properties, disc material properties and disc thickness on the criterial speed were investigated. The most influential property was found to be the thermal expansion coefficient of disc material. As the criterion for stability/instability evaluation the axial disc diistortion-rise limit was applied in the finite element analysis.

10

Application of element deletion method for numerical analyses of cracking

51%

Lee H. C. , Choi J. S. , Jung K. H. , Im Y. T.

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2009

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tom Vol. 35, nr 2

154-161

EN

Purpose: To develop a numerical algorithm to simulate cracking and its evolution for machining, shearing and multi-pass hot bar rolling processes. Design/methodology/approach: Element deletion method was adopted for developing a numerical algorithm and implemented to a rigid-viscoplastic finite element program. Cockcroft-Latham and specific plastic work criteria were incorporated in the present investigation for simulating cracking and shearing processes. An instability condition for the tension was assumed to be valid to determine a critical damage factor for initiation of possible cracking. Findings: The developed element deletion algorithm was simple to be applied for simulating cracking and shearing patterns for the processes applied. Cockcroft-Latham and specific plastic work fracture criteria were reasonable in predicting the internal and external crack, respectively. Research limitations/implications: The research finding can be utilized for investigating occurrence of external and internal cracking involved with metal forming processes such as Chevron cracking in extrusion. Practical implications: By expanding the current approach to determine a processing map for extrusion the processing condition to prevent Chevron cracking can be determined easily and utilized in industry. Also, the current investigation can be easily expanded to other process design and control. Originality/value: Numerical algorithm based on the element deletion method was developed and implemented to the existing finite element program to examine the processes including cracking phenomenon. The applicability to utilize a critical damage factor for the fracture criteria based on the instability was evaluated.

11

Relaxation and breakup of a cylindrical liquid column

51%

Ali M. , Umemura A.

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2011

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tom Vol. 16, no 1

257-270

EN

Instability of a capillary wave and breakup of a square cylindrical liquid column during its relaxation have been investigated numerically by simulating three-dimensional Navier-Stokes equations. For this investigation a computer code based on the volume-of-fluid (VOF) method has been developed and validated with published experimental results. The result shows that the agreement of numerical simulation is quite good with the experimental data. The code is then used to study the capillary wave and breakup phenomena of the liquid column. The investigation shows the underlying physics during relaxation of the square cylindrical liquid column, illustrates the formation and propagation of the capillary wave and breakup processes. The breakup behavior for the present configuration of the liquid column shows some significant differences from those predicted by conventional jet atomization theories. The formation of the capillary wave is initiated by the surface tension on the sharp edge of the square end of the cylinder and the propagation of the wave occurs due to the effect of surface tension force on the motion of the fluid. The propagation of the capillary wave to the end of the liquid column causes a disturbance in the system and makes the waves unstable which initiates the breakup of the liquid column. The characteristics of the capillary wave show that the amplitude of the swell grows faster than the neck of the wave and that of the tip wave grows much faster than other waves. The velocity of the liquid particle is dominated by the pressure in the liquid column.

12

The local and global instability and vibration of a nonlinear column subjected to Euler's load

51%

Sokół K.

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2010

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tom Vol. 9, nr 1

187-194

EN

In this paper, the results of numerical studies on the local and global instability and vibration of a geometrically nonlinear column subjected to Euler's load are presented. The Hamilton principle was used to formulate the boundary problem. Due to the geometrical nonlinearity, the solution of the problem was performed by means of the perturbation method. The magnitude of the bifurcation load of the nonlinear column, the local and global instability regions and characteristic curves have also been presented.

13

Oscylator RLC z losowymi parametrami rezystancji i indukcyjności

51%

Fajkowski P.

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2009

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tom nr 1

43-47

PL

W artykule prezentowany jest model szeregowego oscylatora RLC, który zaburzany jest losowymi parametrami rezystancji i indukcyjności. Do numerycznej symulacji układu został stworzony program napisany w języku C++. Program bada wpływ losowych zmian parametrów na oscylator RLC. Na podstawie tworzonych wykresów określamy stabilność pracy oscylatora. Głównym zadaniem jest znalezienie przybliżonej wartości losowego parametru, przy którym układ staje się nieprzewidywalny. Świadczyć o tym będą przecięcia trajektorii pojawiające się na portrecie fazowym układu RLC.

EN

The article presents the model of serial RLC oscillator that is disturbed by random parameters of resistance and inductance. Numerical simulation of the system has been realised by the program developed in C++ language. The program examines impact of random parameter changes on the RLC oscillator. Authors determine the stability of the oscillator operation on the grounds of created graphs. The main aim was to find approximate value of a random parameter for which the system becomes unpredictable. This will be indicated by trajectory intersections that occur in the phase portrait of the RLC system.

14

The difficulties in relations with Russia and Arab Spring as a most important factors affecting the security in Europe

41%

MAŠĽAN M.

Acta Scientifica Academiae Ostroviensis. Sectio A, Nauki Humanistyczne, Społeczne i Techniczne

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2016

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nr 7(1)/2016

291-305

EN

The security of Europe is a very important and frequent topic. The required security level needs a lot of financial costs and the strongest European economics spend on this purpose amounts which rank them among the ten world countries with the largest military expenditures. If we take into consideration that countries such as France, Great Britain and Germany spend for this area approximate USD 165 bilion per year, then the effectiveness of these expenses is very questionable because our abitlity to face a danger or only react on existing migration wave is substantially close to zero. Although we are threatened, we are able only to discuss, act collectively, slowly and inefficiently. European community is absolutely not prepared for the management of crisis situations. I do not mean the lack of immediate solutions to stop the migrants flow but the inability to mutually coordinate the activities, serving to protect the borders.

PL

Bezpieczeństwo w Europie jest bardzo ważnym i aktualnym tematem. Odpowiedni poziom bezpieczeństwa wymaga dużych nakładów finansowych. Największe gospodarki europejskie wydają na ten cel sumy, które plasują je wśród dziesięciu krajów na świecie, które mają największe wydatki na cele militarne. Jeśli weźmiemy pod uwagę to, że takie kraje jak Francja, Wielka Brytania i Niemcy wydają na te cele blisko 165 miliardów dolarów rocznie, to zasadność tych wydatków może zostać zakwestionowana, ponieważ nasza umiejętność reagowania na zagrożenie w obliczu fali migracyjnej w zasadzie bliska jest zeru. Pomimo że zagrożenie jest realne, jesteśmy w stanie tylko rozmawiać, działać kolektywnie powoli i nieefektywnie. Europejska społeczność nie jest absolutnie przygotowana na radzenie sobie w sytuacji kryzysowej. Mowa tu nie tyle o braku rozwiązań dla sytuacji związanej z falą migrantów, co o nieumiejętności dwustronnej współpracy w zapewnieniu bezpieczeństwa granic.

15

Analytic and experimental study for light alloy aluminium panels under compression

41%

Fayza A. , Zghal A. , Bayraktar E.

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2007

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tom Vol. 25, nr 1

19-22

EN

Purpose: Aluminium alloys have been indispensable for the progress of many technologies during the last decades. In particular, most stiffeners in aerospace structures are composed of aluminium panels often solicited with elastic and plastic bucking under particular boundary and loading conditions. The purpose of this paper is to enhance the difficulties encountered to predict the incipient elastic-plastic buckling for thin aluminium plates under combined compressive loads. Design/methodology/approach: The used methodology was an analytic non linear approach, validated further with an experimental investigation. In fact, the instability of thin elastic-plastic rectangular panels made of 2024 T45 alloys is analyzed. General concept of the Von Kaman's equation with a set of trigonometric and harmonic functions was used in the analytic model. The computation of buckling loads concerns both elastic and plastic instability solutions. Developments in the plastic range were concerned with the (j2d) deformation and the (J2f) flow constitutive laws. Findings: A methodology to develop this kind of analytic resolution is pointed out and has been illustrated for a set of variables. Several 2d and 3d plots, which can be used to predict incipient buckling strengths for plates with flat initial configurations, have been presented for the various load conditions. Research limitations/implications: In the future it will be possible to apply the investigated analytic procedure to other particular cases. Practical implications: Plots obtained with analytic solution can be used to predict incipient buckling strengths for plates with flat initial configurations are presented for the various tests. The interest of three dimensional representations is to indicate when plastic buckling occurs for a square plate under biaxial loading. Originality/value: This paper presents a stable and low cost analytic solution to deal with instability phenomenon in elastic and plastic range for the design of light alloy aluminium plates. This approach is applied to assess the conditions for which plastic buckling can happen when particularly thin aluminium panels are used. This latter, can be implemented in finite element standard codes.