67 research outputs found
SzemcsĂ©s anyagok dinamikai folyamatainak kĂsĂ©rleti vizsgálata = Experimental studies of dynamical processes in granular materials
A szemcsĂ©s anyagok alapvetĹ‘ áramlási tulajdonságait vizsgálva javaslatot tettĂĽnk a lejtĹ‘n megfigyelhetĹ‘ homogĂ©n áramlásokra vonatkozĂł Ăşn. folyási törvĂ©ny mĂłdosĂtott alakjára, mely a kĂsĂ©rleti adatok jobb skálázását adja. Megmutattuk, hogy a kĂĽlönbözĹ‘ anyagokra mĂ©rt folyási törvĂ©ny meredeksĂ©ge szisztematikusan nĹ‘ a rĂ©zsűszög növelĂ©sĂ©vel. Meghatároztuk a lejtĹ‘n megfigyelhetĹ‘ áramlás kĂĽlönbözĹ‘ mĂłdusait, Ă©s a homogĂ©n áramlás instabilitását vizsgálva megmutattuk, hogy a szemcsĂ©s anyag effektĂv sĂşrlĂłdása a nyĂrás növelĂ©sĂ©vel elĹ‘ször (sűrűbb áramlásnál) növekszik, majd ez a trend megfordul Ă©s (az anyag nyĂrás hatására törtĂ©nĹ‘ ritkulásával párhuzamosan) csökkenni kezd. Meghatároztuk a szemcsĂ©s rĂ©tegbe beprĂ©selt lĂ©gzárvány alapvetĹ‘ dinamikai tulajdonságait. A lassĂş nyĂrásnak kitett szemcsĂ©s anyagban megfigyelhetĹ‘ nyĂrási lokalizáciĂł geometriai tulajdonságait vizsgálva megmutattuk, hogy kĂĽlönbözĹ‘ sĂşrlĂłdási egyĂĽtthatĂłjĂş rĂ©tegekbĹ‘l állĂł szemcsĂ©s közegben a nyĂrási zĂłna a rĂ©tegek határán irányt változtat, mely jelensĂ©g párhuzamba állĂthatĂł a fĂ©nytörĂ©s jelensĂ©gĂ©vel. | The basic properties of granular flows were investigated experimentally. We have proposed a modified flow rule for granular flows on an inclined plane, which provides better scaling of the experimental data. Investigating various materials we have shown, that the slope of the flow rule increases systematically with increasing angle of repose. We have determined the various flow regimes of inclined plane flows. By carefully analyzing the instabilities of homogeneous flows we have shown that the effective friction of the granular material (for dense flows) increases with increasing shear rate, but above a certain shear rate this trend turns over and (together with decreasing density) the effective friction starts to decrease. We have determined the basic dynamical properties of air inclusions pressed into a granular layer. Geometrical properties of shear localization in slowly sheared granular materials were also studied. We have shown that in layered granular materials where the layers have different effective frictions, the shear zones are refracted at the layer boundaries, analogously to light refraction
High speed imaging of traveling waves in a granular material during silo discharge
We report experimental observations of sound waves in a granular material
during resonant silo discharge called silo music. The grain motion was tracked
by high speed imaging while the resonance of the silo was detected by
accelerometers and acoustic methods. The grains do not oscillate in phase at
neighboring vertical locations, but information propagates upward in this
system in the form of sound waves. We show that the wave velocity is not
constant throughout the silo, but considerably increases towards the lower end
of the system, suggesting increased pressure in this region, where the flow
changes from cylindrical to converging flow. In the upper part of the silo the
wave velocity matches the sound velocity measured in the same material when
standing (in the absence of flow). Grain oscillations show a stick-slip
character only in the upper part of the silo.Comment: 5 pages, 5 figures, accepted to Phys. Rev.
Nem-egyensúlyi morfológiák dinamikája = Dynamics of non-equilibrium morphologies
ElmĂ©leti mĂłdszereket Ă©s számĂtĂłgĂ©pes szimuláciĂłkat használtunk az elsĹ‘rendű fázisátalakulásokhoz kapcsolĂłdĂł morfolĂłgiák kĂ©pzĹ‘dĂ©si dinamikájának vizsgálatára. Ennek keretĂ©ben sűrűsĂ©g funkcionál- Ă©s fázismezĹ‘ modelleket dolgoztunk ki a tĂşlhűtött egy- Ă©s többkomponensű folyadĂ©kokban, ill. oxidĂĽvegekben törtĂ©nĹ‘ homogĂ©n nukleáciĂł leĂrására. A modellparamĂ©terek rögzĂtĂ©se atomisztikus szimuláciĂłkbĂłl ill. kĂsĂ©rletekbĹ‘l származĂł felĂĽleti jellemzĹ‘k segĂtsĂ©gĂ©vel törtĂ©nt. A nukleáciĂłs gát magasságára vonatkozĂł paramĂ©ter-mentes jĂłslataink jĂłl egyeznek az atomisztikus szimuláciĂłkbĂłl ill. kĂsĂ©rletekbĹ‘l származĂł eredmĂ©nyekkel. FázismezĹ‘ elmĂ©letet dolgoztunk ki az egyensĂşlytĂłl távoli, binĂ©r polikristályos megszilárdulás modellezĂ©sĂ©re kĂ©t- Ă©s három dimenziĂłban. Ennek segĂtsĂ©gĂ©vel olyan komplex, a csĂrakĂ©pzĹ‘dĂ©s Ă©s növekedĂ©s kölcsönhatásával kialakulĂł morfolĂłgiák kĂ©pzĹ‘dĂ©sĂ©t Ărtunk le elsĹ‘kĂ©nt, mint a rendezetlen dendrites, szferolitos, Ă©s kristálykĂ©ve alakzatokĂ©. Alacsony dimenziĂłs, kĂĽlönbözĹ‘ kĂ©miai összetevĹ‘ket tartalmazĂł rendszereket vizsgáltunk, ahol a rĂ©szecskĂ©k diffĂşziĂłs mozgást vĂ©geznek Ă©s közöttĂĽk kĂ©miai reakciĂłk jöhetnek lĂ©tre (reakciĂł-diffĂşziĂł modellek). A rendszerben lezajlĂł nem-egyensĂşlyi fázisátalakulást a mintába befagyott rendezetlensĂ©g jelenlĂ©tĂ©ben tanulmányoztuk renormálási csoport mĂłdszerrel Ă©s numerikus szimuláciĂłval. MegállapĂtottuk, hogy kellĹ‘en erĹ‘s rendezetlensĂ©g mellett az átalakulás egy Ăşn. vĂ©gtelenĂĽl rendezetlen fix-ponttal ĂrhatĂł le. | We applied theoretical methods and computer simulations to investigate morphologies forming during first order transformations and their dynamics. Along these lines, density functional and phase field models have been developed for describing homogeneous crystal nucleation in undercooled one- and two component liquids, and oxide glasses. The model parameters have been fixed using interfacial properties from atomistic simulations or experiment. Our parameter free predictions for the height of the nucleation barrier are in a good agreement with results from atomistic simulations or experiment. We have worked out a phase field theory for modeling polycrystalline solidification in binary alloys far-from-equilibrium in two and three dimensions. Using this approach, we were able to describe the formation of complex morphologies appearing via interacting nucleation and growth, such as the disordered dendritic, spherulitic, and crystal sheaf structures. We have studied multi-component low dimensional systems in which the particles show diffusive motion and chemical reactions take place between them (reaction-diffusion problems). In the presence of quenched disorder we have investigated the properties of non-equilibrium phase transitions by renormalization group method and by numerical simulations. We have shown that for strong enough disorder the transition is controlled by a so called infinite disorder fixed point
Komplex rendszerek dinamikája = Dynamics of complex systems
TĂ©relmĂ©leti mĂłdszerekkel vizsgáltuk a tĂşlhűtött folyadĂ©kbĂłl a kristályos fázisba átvezetĹ‘ kritikus fluktuáciĂłk tulajdonságait. Megmutattuk, hogy a Ginzburg-Landau sorfejtĂ©sen alapulĂł modellek kielĂ©gĂtĹ‘ pontossággal adják meg a nukleáciĂłs gát magasságát. A polikristályos megszilárdulás 3D leĂrására a kristálytani orientáciĂł kvaterniĂł-reprezentáciĂłján alapulĂł fázismezĹ‘ elmĂ©letet dolgoztunk ki, mellyel olyan komplex alakzatok kĂ©pzĹ‘dĂ©sĂ©t modelleztĂĽnk, mint az egymással kölcsönhatĂł dendritek, a szferolitok szĂ©les skálája, ill. a shish-kebab morfolĂłgia. Egyszerű dinamikus sűrűsĂ©g funkcionál elmĂ©let keretĂ©ben a kristályos megszilárdulás mikroszkopikus vonatkozásait vizsgáltuk a kristály nukleáciĂłt megelĹ‘zĹ‘ amorf prekurzor megjelenĂ©sĂ©tĹ‘l, a diffĂşziĂłs instabilitásokon át, a versengĹ‘ diffĂşziĂł kontrollált Ă©s diffĂşziĂł mentes modusok mintázatkĂ©pzĹ‘dĂ©sben játszott szerepĂ©ig. Klasszikus Ă©s kvantum rendszerek nem-egyensĂşlyi relaxáciĂłját vizsgáltuk gyorshűtĂ©si folyamatok során ahol a rendszer kezdĹ‘ állapotát kĂĽlönbözĹ‘ feltĂ©telekkel szabályoztuk. Tanulmányoztuk a dinamikai folyamat során kialakulĂł fĂĽrtök Ă©s a fázisokat elválasztĂł határrĂ©tegek tulajdonságait Ă©s vizsgáltuk a mintába befagyott rendezetlensĂ©g szerepĂ©t is. | We have used field theoretic models to characterize the heterophase fluctuations that drive the system from un-dercooled liquid to the crystalline state. We have shown that models relying on Ginzburg-Landau expanded free energy predict the nucleation barrier fairly accurately. We have developed a phase-field theory relying on the qua-ternion representation when describing crystallographic orientation in 3D. Using this approach, formation of complex solidification patterns such as interacting dendrites, a variety of spherulites, and the shish-kebab mor-phology has been modeled. Using a simple dynamical density functional theory, we have explored the micro-scopic aspects of crystallization, including the formation of amorphous nucleation precursors, the diffusional in-stabilities, and the role competing diffusionless and diffusion controlled growth modes play in pattern formation. We have studied non-equilibrium relaxation of classical and quantum systems following a quench in which the initial state of the system is prepared in different forms. We have investigated the properties of the clusters, as well as the behavior of the interface which separates the evolving phases. We have also studied the role of quenched disorder in such processes
Effects of grain shape on packing and dilatancy of sheared granular materials
Granular material exposed to shear shows a variety of unique phenomena:
Reynolds dilatancy, positional order and orientational order effects may
compete in the shear zone. We study granular packings consisting of macroscopic
prolate, oblate and spherical grains and compare their behaviour. X-ray
tomography is used to determine the particle positions and orientations in a
cylindrical split bottom shear cell. Packing densities and the arrangements of
individual particles in the shear zone are evaluated. For anisometric
particles, we observe the competition of two opposite effects. One the one
hand, the sheared granulate is dilated, but on the other hand the particles
reorient and align with respect to the streamlines. Even though aligned
cylinders in principle may achieve higher packing densities, this alignment
compensates for the effect of dilatancy only partially. The complex
rearrangements lead to a depression of the surface above the well oriented
region while neigbouring parts still show the effect of dilation in the form of
heaps. For grains with isotropic shapes, the surface remains rather flat.
Perfect monodisperse spheres crystallize in the shear zone, whereby positional
order partially overcompensates dilatancy effects. However, already slight
deviations from the ideal monodisperse sphere shape inhibit crystallization.Comment: 12 pages, 13 figures, accepted in Soft Matte
Heaping, Secondary Flows and Broken Symmetry in Flows of Elongated Granular Particles
In this paper we report experiments where we shear granular rods in
split-bottom geometries, and find that a significant heap of height of least
40% of the filling height can form at the particle surface. We show that
heaping is caused by a significant secondary flow, absent for spherical
particles. Flow reversal transiently reverses the secondary flow, leading to a
quick collapse and slower regeneration of the heap. We present a symmetry
argument and experimental data that show that the generation of the secondary
flow is driven by a misalignment of the mean particle orientation with the
streamlines of the flow. This general mechanism is expected to be important in
all flows of sufficiently anisometric grains.Comment: Accepted for Soft Matte
Rapid granular flows on a rough incline: phase diagram, gas transition, and effects of air drag
We report experiments on the overall phase diagram of granular flows on an
incline with emphasis on high inclination angles where the mean layer velocity
approaches the terminal velocity of a single particle free falling in air. The
granular flow was characterized by measurements of the surface velocity, the
average layer height, and the mean density of the layer as functions of the
hopper opening, the plane inclination angle and the downstream distance x of
the flow. At high inclination angles the flow does not reach an x-invariant
steady state over the length of the inclined plane. For low volume flow rates,
a transition was detected between dense and very dilute (gas) flow regimes. We
show using a vacuum flow channel that air did not qualitatively change the
phase diagram and did not quantitatively modify mean flow velocities of the
granular layer except for small changes in the very dilute gas-like phase.Comment: 10 pages, 16 figures, accepted to Phys. Rev.
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