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Abstract: The active electrocaloric (EC) regenerator exploiting electric conversion into thermal energy has recently become important for developing a new generation of heat-management devices. We analyze an active EC regenerator numerically. We establish a temperature span across the regenerator ΔT by commuting a liquid crystalline (LC) unit between regions with and without an external electric field E. In modelling, we use Landau-de Gennes mesoscopic approach, focusing on the temperature regime where isotropic (paranematic) and nematic phase order compete. We determined conditions enabling a large enough value of ΔT suitable for potential applications. In particular, (i) the vicinity of the paranematic-nematic (P–N) phase transition, (ii) large enough latent heat of the transition, (iii) strong enough applied external field (exceeding the critical field Ec at which the P–N transition becomes gradual), and (iv) relatively short contact times between LC unit and heat sink and heat source reservoirs are advantageous. Our analysis reveals that Δ T >> 1K could be achieved using appropriate LC material.
Keywords: liquid crystals, electrocaloric effect, phase behavior
Published in DKUM: 18.09.2024; Views: 0; Downloads: 2
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Abstract: The main goal of the master's thesis was the analysis of air resistance on the body in motion in a model that does not require solving the Navier-Stokes equations, but works on the basis of mechanics and statistical physics. The model was a Monte Carlo (MC) simulation of the motion of ideal gas molecules in a closed container in which a body was placed, moving along one of the axes. For the most part of calculations, the approach was used when the body was fixed in the middle of the simulation cell, and one of the components of the molecular velocity had an additional term that simulated the flow, as if the body was moving at this speed in the opposite direction. First of all, a linear dependence of the drag force on speed was found for low flow speed for a flat plate, which was predicted by linear drag law. For high molecular flow rates, the quadratic dependence predicted by the Bernoulli equation was clearly observed. The results of calculating the corresponding resistivity coefficients for the flat plate were in agreement with the analytical values for both regimes of speeds. By analogy, a simulation was made for a spherical body, which also demonstrated a strong quadratic dependence at high speeds and the drag coefficient value is approximately equal to the analytical one. In the following, we studied systematically ellipsoids with circular cross-section, where we varied the ratio between semiaxes in the direction of motion and perpendicular direction, respectively. The results for the ellipsoid showed that the drag coefficient value is maximum for a flat plate (a limiting case of an ellipsoid, when the semiaxis in the direction of motion tends to 0) and decreases with stretching of the body along the flow axis. When the Maxwell distribution of molecular speeds that was mainly used was replaced with uniform Root-Mean-Square (RMS) speed the results for drag coefficient were slightly different.
Keywords: Air resistance, drag force, quadratic drag law, drag coefficient, Monte Carlo (MC) simulation, Maxwell distribution.
Published in DKUM: 13.10.2021; Views: 1038; Downloads: 64
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Abstract: Splošna teorija relativnosti je že skoraj stoletje nazaj napovedala obstoj gravitacijskih valov, vendar nam je šele razvoj tehnologije omogočil neposredno opazovanje njegovega fizikalnega vpliva. Ta dosežek ni le ponovno potrdil splošne teorije relativnosti, temveč nam je podal revolucionaren način pridobivanja informacij na področju astrofizike, neodvisno od spektra elektromagnetnega valovanja, s čimer smo nazadnje pridobili tudi nov vidik razumevanja številnih astrofizikalnih objektov in procesov ter razvoja zgodnjega vesolja. Za odkritje obstoja gravitacijskih valov je bilo izkazano precejšnje zanimanje, tudi s strani širše javnosti, saj je bilo njihovo odkritje izpostavljeno v različnih medijih. Mladoletniki, na katere imajo mediji prav največji vpliv, pogosto iščejo dodatne informacije o aktualni temi, bodisi s samostojnim raziskovanjem ali pri svojem učitelju (v tem primeru učitelju fizike). Učni načrt fizike za gimnazijo ne vsebuje vsebin v zvezi z odkritjem in opisom gravitacijskih valov, zato bi bilo treba za vsaj osnovno razumevanje tega kompleksnega pojava dijakom zagotoviti ustrezno fizikalno predznanje v obliki poglobitve ali dopolnitve nekaterih učnih vsebin. S kvalitativnim raziskovalnim pristopom je bilo ugotovljeno, da je učni načrt za fiziko fleksibilen v tolikšni meri, da ob uvodni poglobitvi in dopolnitvi nekaterih vsebin omogoča zadostno fizikalno predznanje, potrebno za razumevanje vsebin o gravitacijskih valovih, v okviru vsebinsko nerazporejenih ur pa celo v formalnem smislu omogoča njihovo obravnavo. Veljavnost in učinkovitost tega spoznanja sta bili preverjeni z didaktičnim eksperimentom.
Keywords: gravitacijski valovi (GV), splošna teorija relativnosti, učni načrt za fiziko, didaktični eksperiment.
Published in DKUM: 02.10.2017; Views: 1623; Downloads: 117
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Abstract: Nematic liquid crystalline structures within square wells are studied numerically using both Lebwohl-Lasher lattice semimicroscopic and the Landau-de Gennes mesoscopic approach. At lateral boundary wall strong planar anchoring is enforced. The cell thickness h along the z Cartesian coordinate is assumed to be smaller than the characteristic square well size R. Using semimicroscopic modelling we restrict to effectively two-dimensional systems which we study in terms of the tensor nematic order parameter. We consider impact of appropriate nanoparticles (NPs) on nematic configuration for cases where R becomes comparable to the biaxial order parameter correlation length. In this case a star-like order reconstruction biaxial profile could be formed in absence of NPs. We demonstrate existence of a rich variety of different nematic structures, including topological defects, which are enabled by presence of appropriate NPs.
Keywords: liquid crystals, nanoparticles, nematic ordering, square well, software simulation, visualization
Published in DKUM: 14.06.2017; Views: 1261; Downloads: 372
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Abstract: We studied numerically external field induced memory effects in randomly perturbed nematic liquid crystals. Random anisotropy nematic-type lattice model was used. The impurities imposing orientational disorder were randomly spatially distributed with the concentration p below the percolation threshold. Simulations were carried for finite temperatures, where we varied p, interaction strength between LC molecules, and impurities and external field B. In the {B, T} plane we determined lines separating short range—quasi long range and quasi long range—long range order. Furthermore, crossover regime separating external field and random field dominated regime was estimated. We calculated remanent nematic ordering in samples at B = 0 as a function of the previously experienced external field strength B.
Keywords: liquid crystal, topological defects, random disorder
Published in DKUM: 14.06.2017; Views: 1283; Downloads: 343
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