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26 results on '"Walther, J.H."'

8. On the water-carbon interaction for use in molecular dynamics simulations of graphite and carbon nanotubes

Author

Werder, T., Walther, J.H., Jaffe, R.L., Halicioglu, T., and Koumoutsakos, P.

Subjects
Graphite -- Chemical properties, Carbon -- Chemical properties, Molecular dynamics -- Research, Chemicals, plastics and rubber industries
Abstract

A systematic molecular dynamics study shows that the contact angle of a water droplet on graphite changes significantly as a function of the water-carbon interaction energy. As variation of the droplet size is observed in the range from 1000 to 17500 water molecules and the line tension is determined to be positive.

Published
2003

10. Carbon nanotubes in water: structural characteristics and energetics

Author

Walther, J.H., Jaffe, R., Halicioglu, T., and Koumoutsakos, P.

Subjects
Chemistry, Physical and theoretical -- Research, Carbon -- Spectra, Molecular structure -- Analysis, Water -- Electric properties, Chemicals, plastics and rubber industries
Published
2001

18. Phase-locked stereoscopic PIV measurements of the turbulent swirling flow in a dynamic model of a uniflow-scavenged two-stroke engine cylinder

Author

Ingvorsen, K.M. (author), Meyer, K.E. (author), Walther, J.H. (author), Mayer, S. (author), Ingvorsen, K.M. (author), Meyer, K.E. (author), Walther, J.H. (author), and Mayer, S. (author)

Abstract

It is desirable to use computational fluid dynamics for the optimization of in-cylinder processes in large two-stroke low-speed uniflowscavenged marine diesel engines. However, the complex nature of the turbulent swirling in-cylinder flow necessitates experimental data for validation of the used turbulence models. In the present work, the flow in a dynamic scale model of a uniflow-scavenged cylinder is investigated experimentally. The model has a transparent cylinder and a movable piston driven by a linear motor. The flow is investigated using phase-locked stereoscopic particle image velocimetry (PIV) and time resolved laser Doppler anemometry (LDA). Radial profiles of the phase-averaged mean velocities are computed from the velocity fields recorded with PIV and the validity of the obtained profiles is demonstrated by comparison with reference LDA measurements. Radial profiles are measured at five axial positions for 15 different times during the engine cycle and shows the temporal and spatial development of the swirling in-cylinder flow. The tangential velocity profiles in the bottom of the cylinder near the end of the scavenging process are characterized by a concentrated swirl resulting in wake-like axial velocity profiles and the occurrence of a vortex breakdown. After scavenge port closing the axial velocity profiles indicate that large transient swirl-induced structures exists in the cylinder. Comparison with profiles obtained under steady-flow conditions shows that the steady profiles in general will not be representative for the dynamic conditions. The temporal development of the swirl strength is investigated by computing the angular momentum. The swirl strength shows an exponential decay from scavenge port closing to scavenge port opening corresponding to a reduction of 34%.

Published
2013

19. Continuum simulations of water flow past fullerene molecules

Author

Popadić, A., Praprotnik, M., Koumoutsakos, P., Walther, J.H., Popadić, A., Praprotnik, M., Koumoutsakos, P., and Walther, J.H.

Abstract

We present continuum simulations of water flow past fullerene molecules. The governing Navier-Stokes equations are complemented with the Navier slip boundary condition with a slip length that is extracted from related molecular dynamics simulations. We find that several quantities of interest as computed by the present model are in good agreement with results from atomistic and atomistic-continuum simulations at a fraction of the cost. We simulate the flow past a single fullerene and an array of fullerenes and demonstrate that such nanoscale flows can be computed efficiently by continuum flow solvers, allowing for investigations into spatiotemporal scales inaccessible to atomistic simulations.

Published
2015
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23. Hydrophobic hydration of C60 and carbon nanotubes in water

Author

Walther, J.H., Jaffe, R.L., Kotsalis, E.M., Werder, T., Halicioglu, T., and Koumoutsakos, P.

Subjects
*MOLECULAR dynamics, *SIMULATION methods & models, *HYDROPHOBIC surfaces, *FULLERENES, *MOLECULES
Abstract

We perform molecular dynamics (MD) simulations to study the hydrophobic–hydrophilic behavior of pairs of C60 fullerene molecules and single wall carbon nanotubes in water. The interaction potentials involve a fully atomistic description of the fullerenes or carbon nanotubes and the water is modeled using the flexible SPC model. Both unconstrained and constrained MD simulations are carried out. We find that these systems display drying, as evidenced by expulsion of the interstitial water, when the C60 and carbon nanotubes are separated by less than 12, and 9–10 A˚, respectively. From the constrained simulations, the computed mean force between two carbon nanotubes in water exhibits a maximum at a tube spacing of 5.0 A˚ which corresponds to approximately one unstable layer of interstitial water molecules. The main contribution to the force stems from the van der Waals attraction between the carbon surfaces. The minimum in the potential of mean force has a value of −17 kJ mol−1 A˚−1 at a tube spacing of 3.5 A˚. [Copyright &y& Elsevier]

Published
2004
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