Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

Recent progress in flapping wing aerodynamics and aeroelasticity

Journal of Physics D: Applied Physics published the first Plasma Roadmap in 2012 consisting of the individual perspectives of 16 leading experts in the various sub-fields of low temperature plasma science and technology. The 2017 Plasma Roadmap is the first update of a planned series of periodic updates of the Plasma Roadmap. The continuously growing interdisciplinary nature of the low temperature plasma field and its equally broad range of applications are making it increasingly difficult to identify major challenges that encompass all of the many sub-fields and applications. This intellectual diversity is ultimately a strength of the field. The current state of the art for the 19 sub-fields addressed in this roadmap demonstrates the enviable track record of the low temperature plasma field in the development of plasmas as an enabling technology for a vast range of technologies that underpin our modern society. At the same time, the many important scientific and technological challenges shared in this roadmap show that the path forward is not only scientifically rich but has the potential to make wide and far reaching contributions to many societal challenges.

/pdf/the-2017-plasma-roadmap-low-temperature-plasma-science-and-1x1mdft0yo.pdf

The 2017 Plasma Roadmap: Low temperature plasma science and technology

1. Introduction 2. Fixed, rigid wing aerodynamics 3. Flexible wing aerodynamics 4. Flapping wing aerodynamics.

/pdf/aerodynamics-of-low-reynolds-number-flyers-4v0j5o71pp.pdf

Aerodynamics of Low Reynolds Number Flyers

Water tunnel experiments on a flexible airfoil plunging with constant amplitude have been carried out for Reynolds numbers of 0 to 27000. Peaks in thrust coefficient at intermediate values of airfoil stiffness were observed at both zero and non-zero Reynolds numbers, indicating that a degree of flexibility is beneficial at low Reynolds numbers. Time-averaged velocity fields and momentum flux data revealed a broader, higher-velocity jet in cases of optimum airfoil stiffness. Stronger vortices, separated by a larger lateral distance, characterised the corresponding instantaneous velocity fields. The flexibility causes the airfoil to pitch passively; the phase angle of the pitch was found to lead the plunge. Pitch amplitude and trailing-edge amplitude were found to be single-valued functions of pitch phase angle. The shape characteristics of the airfoil could therefore be described by the pitch phase angle only. Thrust coefficient was found to be a function of only two parameters: Strouhal number and pitch phase angle. For each Strouhal number, a peak in thrust coefficient was observed at a particular value of the pitch phase angle. The optimum pitch phase angle was found to tend to a limit of 105±5 degrees at very large Strouhal numbers. A significant thrust benefit was observed over very stiff airfoils when the optimum flexibility is utilized.

Flexible flapping airfoil propulsion at low Reynolds numbers

We present an experimental investigation of the flow structure and vorticity field in the wake of a NACA-0012 airfoil pitching sinusoidally at small amplitude and high reduced frequencies. Molecular tagging velocimetry is used to quantify the characteristics of the vortex array (circulation, peak vorticity, core size, spatial arrangement) and its downstream evolution over the first chord length as a function of reduced frequency. The measured mean and fluctuating velocity fields are used to estimate the mean force on the airfoil and explore the connection between flow structure and thrust generation.Results show that strong concentrated vortices form very rapidly within the first wavelength of oscillation and exhibit interesting dynamics that depend on oscillation frequency. With increasing reduced frequency the transverse alignment of the vortex array changes from an orientation corresponding to velocity deficit (wake profile) to one with velocity excess (reverse Karman street with jet profile). It is found, however, that the switch in the vortex array orientation does not coincide with the condition for crossover from drag to thrust. The mean force is estimated from a more complete control volume analysis, which takes into account the streamwise velocity fluctuations and the pressure term. Results clearly show that neglecting these terms can lead to a large overestimation of the mean force in strongly fluctuating velocity fields that are characteristic of airfoils executing highly unsteady motions. Our measurements show a decrease in the peak vorticity, as the vortices convect downstream, by an amount that is more than can be attributed to viscous diffusion. It is found that the presence of small levels of axial velocity gradients within the vortex cores, levels that can be difficult to measure experimentally, can lead to a measurable decrease in the peak vorticity even at the centre of the flow facility in a flow that is expected to be primarily two-dimensional.

MTV measurements of the vortical field in the wake of an airfoil oscillating at high reduced frequency

To determine the types of applications for which plasma-based water treatment (PWT) is best suited, the treatability of 23 environmental contaminants was assessed through treatment in a gas discharge reactor with argon bubbling, termed the enhanced-contact reactor. The contaminants were treated in a mixture to normalize reaction conditions and convective transport limitations. Treatability was compared in terms of the observed removal rate constant (k obs). To characterize the influence of interfacial processes on k obs, a model was developed that accurately predicts k obs for each compound, as well as the contributions to k obs from each of the three general degradation mechanisms thought to occur at or near the gas–liquid interface: 'sub-surface', 'surface' and 'above-surface'. Sub-surface reactions occur just underneath the gas–liquid interface between the contaminants and dissolved plasma-generated radicals, contributing significantly to the removal of compounds that lack surfactant-like properties and so are not highly concentrated at the interface. Surface reactions occur at the interface between the contaminants and dissolved radicals, contributing significantly to the removal of surfactant-like compounds that have high interfacial concentrations. The contaminants' interfacial concentrations were calculated using surface-activity parameters determined through surface tension measurements. Above-surface reactions are proposed to take place in the plasma interior between highly energetic plasma species and exposed portions of compounds that extend out of the interface. This mechanism largely accounts for the degradation of surfactant-like contaminants that contain highly hydrophobic perfluorocarbon groups, which are most likely to protrude from the interface. For a few compounds, the degree of exposure to the plasma interior was supported by new and previously reported molecular dynamics simulations results. By reviewing the predicted contributions from the three general mechanisms, it was determined that surface concentration is the dominant factor determining a compound's treatability. These insights indicate that PWT would be most viable for the treatment of surfactant-like contaminants.

https://iopscience.iop.org/article/10.1088/1361-6463/50/1/014003/pdf

Plasma-based water treatment: development of a general mechanistic model to estimate the treatability of different types of contaminants*

Three experimental techniques have been used to investigate the impact ignition of reactive materials. The three techniques are direct impact, indirect impact, and two‐step impact. For the first two techniques, time‐resolved light spectroscopy was used to identify reaction species from solid PTFE/Al reactive material. A common observation for these two techniques is that heating and some reaction was observed during initial impact of the PTFE/Al reactive material but the majority of the reaction appeared to occur following material breakup and subsequent impact with a secondary surface. There was no spectral evidence for aluminum‐fluorine combustion. For the two‐step impact technique, the reactive material was initially pulverized as it passed through a thin plate and then subsequently ignited when the debris cloud impacted a rigid anvil. All three experiments were observed with high‐speed photography.

Reactive Materials Studies

Qualitative and quantitative measurements have been made in the near field of a tabbed jet. Past results have shown that the addition of a passive mixing tab causes rapid large-scale mixing of the core fluid with the surrounding fluid. The tabbed jet concept has been further modified by placing secondary tabs on both sides of the primary tab. The secondary tabs, which are angled away from the core flow, provide an attachment surface for the jet core fluid as it is directed into the ambient region. This modification significantly increases the flux of vorticity into the flow, and the rate at which the jet core and ambient fluid interact. (Author)

Enhancement of passive mixing tabs by the addition of secondary tabs

This paper describes the development of a stereoscopic molecular tagging velocimetry (sMTV) technique for measuring the instantaneous three-component velocity field over a plane in a flow. The calibration technique employed allows the stereo imaging parameters to be determined directly from a target image without the direct physical measurement of those parameters. Effects of index of refraction variations are accounted for approximately in the calibration method. Sensitivity analysis and target test experiments for a typical optical setup indicate maximum error levels for the in-plane components to be nominally the same as two-component MTV measurements. The error in the out-of-plane velocity component can be one to three times larger, depending on the stereo viewing angle. Experimental results are presented to validate the sMTV technique against other known measured results, along with an application to the highly three-dimensional flow field near the tip of a model propeller.

Douglas Bohl

Papers

MTV measurements of the vortical field in the wake of an airfoil oscillating at high reduced frequency

Plasma-based water treatment: development of a general mechanistic model to estimate the treatability of different types of contaminants*

Reactive Materials Studies

Enhancement of passive mixing tabs by the addition of secondary tabs

Development of stereoscopic molecular tagging velocimetry