The Experts below are selected from a list of 66162 Experts worldwide ranked by ideXlab platform
Lambertus Hesselink - One of the best experts on this subject based on the ideXlab platform.
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high fidelity simulation of an ultrasonic standing wave thermoacoustic engine with bulk viscosity effects
55th AIAA Aerospace Sciences Meeting, 2017Co-Authors: Jeffrey Lin, Carlo Scalo, Lambertus HesselinkAbstract:We have carried out boundary-layer-resolved, unstructured fully-compressible Navier--Stokes simulations of an ultrasonic standing-wave thermoacoustic engine (TAE) model. The model is constructed as a quarter-wavelength engine, approximately 4 mm by 4 mm in size and operating at 25 kHz, and comprises a thermoacoustic stack and a coin-shaped cavity, a design inspired by Flitcroft and Symko (2013). Thermal and viscous boundary layers (order of 10 $\mathrm{\mu}$m) are resolved. Vibrational and rotational Molecular Relaxation are modeled with an effective bulk viscosity coefficient modifying the viscous stress tensor. The effective bulk viscosity coefficient is estimated from the difference between theoretical and semi-empirical attenuation curves. Contributions to the effective bulk viscosity coefficient can be identified as from vibrational and rotational Molecular Relaxation. The inclusion of the coefficient captures acoustic absorption from infrasonic ($\sim$10 Hz) to ultrasonic ($\sim$100 kHz) frequencies. The value of bulk viscosity depends on pressure, temperature, and frequency, as well as the relative humidity of the working fluid. Simulations of the TAE are carried out to the limit cycle, with growth rates and limit-cycle amplitudes varying non-monotonically with the magnitude of bulk viscosity, reaching a maximum for a relative humidity level of 5%. A corresponding linear model with minor losses was developed; the linear model overpredicts transient growth rate but gives an accurate estimate of limit cycle behavior. An improved understanding of thermoacoustic energy conversion in the ultrasonic regime based on a high-fidelity computational framework will help to further improve the power density advantages of small-scale thermoacoustic engines.
Bengterik Mellander - One of the best experts on this subject based on the ideXlab platform.
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resolving distribution of Relaxation times in poly propylene glycol on the crossover region
arXiv: Soft Condensed Matter, 2004Co-Authors: Enis Tuncer, Maurizio Furlani, Bengterik MellanderAbstract:In this paper, a recently developed numerical technique [{\em Tuncer E and Guba{\'n}ski S M, IEEE Trans Diel El Insul {\bf 8}(3)(2001) 310-320}] is applied to poly(propylene glycol) complex dielectric data to extract more information about the Molecular Relaxation processes. The method is based on a constrained-least-squares (\clsq) data fitting procedure together with the Monte Carlo (\mc) method. We preselect the number of Relaxation times with no {\em a-priori} physical assumption, and use the Debye single Relaxation as ``kernel'', then the obtained weighting factors at each \mc step from the \clsq method builds up a Relaxation time spectrum. When the analysis is repeated for data at different temperatures a {\em Relaxation-image} is created. The obtained Relaxation are analyzed using the Lorentz (Cauchy) distribution, which is a special form of the L{\'e}vy statistics. In the present report the $\beta$ and $\alpha$ Relaxations are resolved for the \ppg. A comparison of the Relaxations to those earlier reported in the literature indicate that the presented method provides additional information compared to methods based on empirical formulas. The distribution of Relaxation times analysis is especially useful to probe the crossover region where the $\alpha$- and $\beta$- Relaxations merge and the results show that the Relaxation after the crossover region at higher temperatures is Arrhenius-type as the $\beta$-Relaxation. Moreover, this Relaxation is more likely to be the continuation of the $\beta$-Relaxation, but with a different activation energy.
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resolving distribution of Relaxation times in poly propylene glycol on the crossover region
Journal of Applied Physics, 2004Co-Authors: Enis Tuncer, Maurizio Furlani, Bengterik MellanderAbstract:In this article, a recently developed numerical technique [E. Tuncer and S. M. Gubanski, IEEE Trans. Dielectr. Electr. Insul. 8, 310 (2001)] is applied to poly(propylene glycol) (PPG) complex dielectric data to extract more information about the Molecular Relaxation processes. The method is based on a constrained-least-squares (C–LSQ) data fitting procedure together with the Monte Carlo method. We preselect the number of Relaxation times with no a priori physical assumption, and use the Debye single Relaxation as “kernel,” then the obtained weighting factors at each MC step from the C–LSQ method builds up a Relaxation time spectrum. When the analysis is repeated for data at different temperatures a Relaxation image is created. The obtained Relaxation are analyzed using the Lorentz (Cauchy) distribution, which is a special form of the Levy statistics. In the present report the β and α Relaxations are resolved for the PPG. A comparison of the Relaxations to those earlier reported in the literature indicate ...
G. P. Johari - One of the best experts on this subject based on the ideXlab platform.
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dielectric Relaxation and crystallization of ultraviscous melt and glassy states of aspirin ibuprofen progesterone and quinidine
Journal of Pharmaceutical Sciences, 2007Co-Authors: G. P. Johari, S Kim, Ravi Mysore ShankerAbstract:ABSTRACT Molecular Relaxation in ultraviscous melt and glassy states of aspirin, ibuprofen, progesterone, and quinidine has been studied by dielectric spectroscopy. The asymmetric Relaxation spectra is characterized by the Kohlrausch distribution parameter of 0.46 ± 0.02 for aspirin to 0.67 ± 0.02 for progesterone. The dielectric Relaxation time varies with the temperature, T , according to the Vogel–Fulcher–Tammann Equation, log 10 ( τ 0 ) = A VFT + [ B VFT /( T − T 0 )], where A VFT , B VFT , and T 0 are empirical constants. The extrapolated τ 0 at calorimetric glass-softening temperature is close to the value expected. The equilibrium permittivity, e 0 , is lowest for ibuprofen which indicates an antiparallel orientation of dipoles in its liquid's hydrogen-bonded structure. A decrease in e 0 with time shows that ultraviscous aspirin, progesterone, and quinidine begin to cold-crystallize at a relatively lower temperature than ibuprofen. e 0 of the cold-crystallized phases are, 4.7 for aspirin at 290 K, 2.55 for ibuprofen at 287 K, 2.6 for progesterone at 320 K, and 3.2 for quinidine at 375 K. It is argued that hydrogen-bonding, the Kohlrausch parameter, extent of localized motions and the long-range diffusion times all determine the physical and chemical stability of an amorphous pharmaceutical during storage.
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Relaxations and nano phase separation in ultraviscous heptanol alkyl halide mixture
Journal of Chemical Physics, 2007Co-Authors: G Power, G. P. JohariAbstract:To gain insight into the effects of liquid-liquid phase separation on Molecular Relaxation behavior we have studied an apparently homogeneous mixture of 5-methyl-2-hexanol and isoamylbromide by dielectric spectroscopy over a broad temperature range. It shows two Relaxation regions, widely separated in frequency and temperature, with the low-frequency Relaxation due to the alcohol and the high-frequency Relaxation due to the halide. In the mixture, the equilibrium dielectric permittivity es of the alcohol is 41% of the pure state at 155.7K and es of isoamylbromide is ∼86% of the pure state at 128.7K. The difference decreases for the alcohol component with decreasing temperature and increases for the isoamylbromide component. The Relaxation time τ of 5-methyl-2-hexanol in the mixture at 155.7K is over five orders of magnitude less than in the pure state, and this difference increases with decreasing temperature, but τ of isoamylbromide in the mixture is marginally higher than in the pure liquid. This shows ...
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dynamics of a molecule s growth ultrasonic Relaxation studies
Journal of Chemical Physics, 1995Co-Authors: M G Parthun, G. P. JohariAbstract:Longitudinal velocity and attenuation of ultrasonic waves traveling through a liquid at different temperatures have been measured as its molecules combine irreversibly to form large entities and thereby decrease the diffusivity and increase the configurational restrictions to their dynamics. In addition, the number of covalent bonds formed are measured by calorimetry. From these data, the longitudinal modulus and compliance are calculated, and the Molecular Relaxation time and related properties are deduced and interpreted in terms of the number of covalent bonds formed, by a formalism that connects the size of the molecules in the liquid with its elastic behavior. This Relaxation time increases monotonically with increase in the molecule’s size, tending to infinity as the number of covalent bonds formed approaches the Avogadro number. The complex plane plots of the modulus and compliance have a shape that is described by a skewed arc function, with a temperature dependent exponent γ, that ranges in values from 0.33–0.31 for modulus and 0.39–0.45 for compliance. Departure from this shape suggest significant contributions from nonzero shear viscosity for relatively small size of molecules, and contributions from a faster or β‐Relaxation process when the Molecular size is large. The study reveals the behavior of a liquid as it is altered by a permanent change in the size of the diffusing entities.
Jeffrey Lin - One of the best experts on this subject based on the ideXlab platform.
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high fidelity simulation of an ultrasonic standing wave thermoacoustic engine with bulk viscosity effects
55th AIAA Aerospace Sciences Meeting, 2017Co-Authors: Jeffrey Lin, Carlo Scalo, Lambertus HesselinkAbstract:We have carried out boundary-layer-resolved, unstructured fully-compressible Navier--Stokes simulations of an ultrasonic standing-wave thermoacoustic engine (TAE) model. The model is constructed as a quarter-wavelength engine, approximately 4 mm by 4 mm in size and operating at 25 kHz, and comprises a thermoacoustic stack and a coin-shaped cavity, a design inspired by Flitcroft and Symko (2013). Thermal and viscous boundary layers (order of 10 $\mathrm{\mu}$m) are resolved. Vibrational and rotational Molecular Relaxation are modeled with an effective bulk viscosity coefficient modifying the viscous stress tensor. The effective bulk viscosity coefficient is estimated from the difference between theoretical and semi-empirical attenuation curves. Contributions to the effective bulk viscosity coefficient can be identified as from vibrational and rotational Molecular Relaxation. The inclusion of the coefficient captures acoustic absorption from infrasonic ($\sim$10 Hz) to ultrasonic ($\sim$100 kHz) frequencies. The value of bulk viscosity depends on pressure, temperature, and frequency, as well as the relative humidity of the working fluid. Simulations of the TAE are carried out to the limit cycle, with growth rates and limit-cycle amplitudes varying non-monotonically with the magnitude of bulk viscosity, reaching a maximum for a relative humidity level of 5%. A corresponding linear model with minor losses was developed; the linear model overpredicts transient growth rate but gives an accurate estimate of limit cycle behavior. An improved understanding of thermoacoustic energy conversion in the ultrasonic regime based on a high-fidelity computational framework will help to further improve the power density advantages of small-scale thermoacoustic engines.
Axel J Zeitler - One of the best experts on this subject based on the ideXlab platform.
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thermal decoupling of Molecular Relaxation processes from the vibrational density of states at terahertz frequencies in supercooled hydrogen bonded liquids
Journal of Physical Chemistry Letters, 2014Co-Authors: Juraj Sibik, S R Elliott, Axel J ZeitlerAbstract:At terahertz frequencies, the libration-vibration motions couple to the dielectric Relaxations in disordered hydrogen-bonded solids. The interplay between these processes is still poorly understood, in particular at temperatures below the glass transition temperature, Tg, yet this behavior is of vital importance for the Molecular mobility of such materials to remain in the amorphous phase. A series of polyhydric alcohols were studied at temperatures between 80 and 310 K in the frequency range of 0.2–3 THz using terahertz time-domain spectroscopy. Three universal features were observed in the dielectric losses, ϵ″(ν): (a) At temperatures well below the glass transition, ϵ″(ν) comprises a temperature-independent microscopic peak, which persists into the liquid phase and which is identified as being due to librational/torsional modes. For 0.65 Tg < T < Tg, additional thermally dependent contributions are observed, and we found strong evidence for its relation to the Johari–Goldstein secondary β-Relaxation pr...
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thermal decoupling of Molecular Relaxation processes from the vibrational density of states at terahertz frequencies in supercooled hydrogen bonded liquids
Journal of Physical Chemistry Letters, 2014Co-Authors: Juraj Sibik, S R Elliott, Axel J ZeitlerAbstract:At terahertz frequencies, the libration-vibration motions couple to the dielectric Relaxations in disordered hydrogen-bonded solids. The interplay between these processes is still poorly understood, in particular at temperatures below the glass transition temperature, Tg, yet this behavior is of vital importance for the Molecular mobility of such materials to remain in the amorphous phase. A series of polyhydric alcohols were studied at temperatures between 80 and 310 K in the frequency range of 0.2-3 THz using terahertz time-domain spectroscopy. Three universal features were observed in the dielectric losses, ϵ″(ν): (a) At temperatures well below the glass transition, ϵ″(ν) comprises a temperature-independent microscopic peak, which persists into the liquid phase and which is identified as being due to librational/torsional modes. For 0.65 Tg < T < Tg, additional thermally dependent contributions are observed, and we found strong evidence for its relation to the Johari-Goldstein secondary β-Relaxation process. (b) Clear spectroscopic evidence is found for a secondary β glass transition at 0.65 Tg, which is not related to the fragility of the glasses. (c) At temperatures above Tg, the losses become dominated by primary α-Relaxation processes. Our results show that the thermal changes in the losses seem to be underpinned by a universal change in the hydrogen bonding structure of the samples.