René Spencer Chatwell

Bio: René Spencer Chatwell is an academic researcher from Technical University of Berlin. The author has contributed to research in topics: Volume viscosity & Supercritical fluid. The author has an hindex of 3, co-authored 10 publications receiving 24 citations.

Diffusion of the carbon dioxide-ethanol mixture in the extended critical region.

Abstract: The effect of traces of ethanol in supercritical carbon dioxide on the mixture's thermodynamic properties is studied by molecular simulations and Taylor dispersion measurements. This mixture is investigated along the isobar p = 10 MPa in the temperature range between T = 304 and 343 K. Along this path, the mixture undergoes two transitions: First, the Widom line is crossed, marking the transition from liquid-like to gas-like conditions. A second transition occurs from the supercritical gas-like domain to a subcritical gas. The Widom line crossover entails inflection points for most of the studied properties, i.e. density, enthalpy, shear viscosity, Maxwell–Stefan and intradiffusion coefficients. On the other hand, the transition between the super- and subcritical regions is found to be generally smooth, an observation that is qualitatively confirmed by experimental Taylor dispersion measurements. Dedicated atomistic simulations show the presence of microheterogeneities due to ethanol self-association along the investigated path, which lead to the mixture's anomalous behavior in its extended critical region.

Bulk viscosity of liquid noble gases.

Abstract: An equation of state for the bulk viscosity of liquid noble gases is proposed. On the basis of dedicated equilibrium molecular dynamics simulations, a multi-mode relaxation ansatz is used to obtain precise bulk viscosity data over a wide range of liquid states. From this dataset, the equation of state emerges as a two-parametric power function with both parameters showing a conspicuous saturation behavior over temperature. After passing a temperature threshold, the bulk viscosity is found to vary significantly over density, a behavior that resembles the frequency response of a one pole low-pass filter. The proposed equation of state is in good agreement with available experimental sound attenuation data.

Acoustic absorption measurement for the determination of the volume viscosity of pure fluids / Messverfahren für die akustischen Absorption zur Bestimmung der Volumenviskosität reiner Fluide

Abstract: Abstract A realistic description of fluid mechanical and acoustic processes requires the volume viscosity of the medium to be known. This work describes how the volume viscosity of pure fluids can be determined by measuring acoustic absorption with the pulse-echo method. The challenge in realizing such a measurement method lies in the separation of the different dissipative effects that superimpose on absorption. Diffraction effects ultimately cause a dissipation of acoustic energy and acoustic reflector surfaces have a small, but finite transmission coefficient. Further, influences of the transducer (in particular its frequency response), as well as the system’s electrical components have to be taken into account. In contrast to the classical approach relying on the amplitude ratio, the absorption is determined by the moments of the amplitude spectrum. The measurement system applied is originally designed for precision measurements of the sound velocity by means of the propagation time difference of two acoustic signals.

Mass transfer through vapour–liquid interfaces: a molecular dynamics simulation study

Abstract: A quasi-stationary molecular dynamics simulation method for studying mass transfer through vapour–liquid interfaces of mixtures driven by gradients of the chemical potential based on the dual contr...

Evaporation driven by conductive heat transport

Abstract: Molecular dynamics simulations are conducted to investigate the evaporation of the truncated ( 2.5 σ ) and shifted Lennard–Jones fluid into vacuum. Evaporation is maintained under stationary condit...

Convergence of Kirkwood–Buff Integrals of Ideal and Nonideal Aqueous Solutions Using Molecular Dynamics Simulations B

Abstract: The computation of Kirkwood–Buff integrals (KBIs) using molecular simulations of closed systems is challenging due to finite system-size effects. One of the problems involves the incorrect asymptotic behavior of the radial distribution function. Corrections to rectify such effects have been proposed in the literature. This study reports a systematic comparison of the proposed corrections (as given by Ganguly et al. J. Chem. Theory Comput. 2013, 9, 1347–1355 and Kruger et al. J. Phys. Chem. Lett. 2013, 4, 4–7) to assess the asymptotic behavior of the RDFs, the KBIs, as well as the estimation of thermodynamic quantities for ideal urea–water and nonideal modified-urea–water mixtures using molecular dynamics simulations. The results show that applying the KBI correction suggested by Kruger et al. on the RDF corrected with the Ganguly et al. correction (denoted as B-KBI) yields improved KBI convergence for the ideal and nonideal aqueous mixtures. Different averaging regions in the running KBIs (correlated or long-range) are assessed, and averaging over the correlated region for large system sizes is found to be robust toward the change in the degree of solvent nonideality and concentration, providing good estimates of thermodynamic quantities. The study provides new insights into improving the KBI convergence, the suitability of different averaging regions in KBIs to estimate thermodynamic properties, as well as the applicability of correction methods to achieve KBI convergence for nonideal aqueous binary mixtures.

Bulk viscosity of liquid noble gases.

Abstract: An equation of state for the bulk viscosity of liquid noble gases is proposed. On the basis of dedicated equilibrium molecular dynamics simulations, a multi-mode relaxation ansatz is used to obtain precise bulk viscosity data over a wide range of liquid states. From this dataset, the equation of state emerges as a two-parametric power function with both parameters showing a conspicuous saturation behavior over temperature. After passing a temperature threshold, the bulk viscosity is found to vary significantly over density, a behavior that resembles the frequency response of a one pole low-pass filter. The proposed equation of state is in good agreement with available experimental sound attenuation data.

Estimation of Tissue Attenuation from Ultrasonic B-Mode Images: Spectral-Log-Difference and Method-of-Moments Algorithms Compared

Abstract: We report on results from the comparison of two algorithms designed to estimate the attenuation coefficient from ultrasonic B-mode scans obtained from a numerical phantom simulating an ultrasound breast scan It is well documented that this parameter significantly diverges between normal tissue and malignant lesions To improve the diagnostic accuracy it is of great importance to devise and test algorithms that facilitate the accurate, low variance and spatially resolved estimation of the tissue's attenuation properties A numerical phantom is realized using k-Wave, which is an open source Matlab toolbox for the time-domain simulation of acoustic wave fields that facilitates both linear and nonlinear wave propagation in homogeneous and heterogeneous tissue, as compared to strictly linear ultrasound simulation tools like Field II k-Wave allows to simulate arbitrary distributions, resolved down to single voxel sizes, of parameters including the speed of sound, mass density, scattering strength and to include power law acoustic absorption necessary for simulation tasks in medical diagnostic ultrasound We analyze the properties and the attainable accuracy of both the spectral-log-difference technique, and a statistical moments based approach and compare the results to known reference values from the sound field simulation