A Molecular Dynamics Study on Obstructed Flow Around Single and Two Paratactic Circular Cylinders

Obstructed flow around single and two paratactic circular cylinders were investigated with two-dimensional molecular dynamics simulation (MDS) methods in the view of discrete particles. The transient and time-averaged profiles of streamline and density were obtained in order to analyze the characteristics of the wake flow. For single cylinder case, different flow patterns, i.e. Stokes flow, steady vortices flow, periodic vortices-shedding flow with the Ka´rma´n vortex street and supersonic flow, were divided based on Reynolds number (Re), 4<Re<12, 12<Re<20, 20<Re<62 and Re>62 respectively. For two paratactic cylinders case, different flow patterns, namely periodic vortices-shedding flow, periodic vortices-shedding flow with gap-flow, bistable flow and synchronized vortex-shedding flow, were observed with different center-to-center pitch ratios (D*/d*), D*/d* = 1.0, 1.0<D*/d*<1.1, 1.1<D*/d*<1.8 and D*/d*>1.8 respectively. The results show qualitative or quantitative agreement with those obtained from experiments and other MDS and indicate that most macroscopic obstructed flow patterns still exist even in nanoscale.

Download Full-text

Single crystal structure and molecular dynamics analysis of a myo-inositol derivative

Acta Crystallographica Section B Structural Science ◽

10.1107/s0108768100005309 ◽

2000 ◽

Vol 56 (4) ◽

pp. 738-743

Author(s):

Jan Dillen ◽

Martin W. Bredenkamp ◽

Mare-Loe Prinsloo

Keyword(s):

Crystal Structure ◽

Molecular Dynamics ◽

Single Crystal ◽

Room Temperature ◽

Quantitative Agreement ◽

Dynamics Simulation ◽

X Ray Diffraction ◽

Field Methods ◽

X Ray ◽

Empirical Force Field

The crystal structure of 5-O-tert-butyldimethylsilyl-3,4-O-carbonyl-1,2-O-cyclohexylidene-2-oxo-3-oxa-4-bornanylcarbonyl-D-myo-inositol has been studied by single-crystal X-ray diffraction at both room temperature and 173 K. At room temperature, the tert-butyldimethylsilyl group exhibits dynamical disorder. A molecular dynamics simulation was used to model the disorder and this indicates that the group librates between two stable conformations in the crystal. Approximate relative energies of the different forms and energy barriers for the transition were obtained by empirical force field methods. Calculations of the thermal motion of the atoms are in good qualitative, but fair to poor quantitative agreement with the X-ray data.

Download Full-text

Quantification of wake unsteadiness for low-Re flow across two staggered cylinders

Proceedings of the Institution of Mechanical Engineers Part C Journal of Mechanical Engineering Science ◽

10.1177/0954406219866478 ◽

2019 ◽

Vol 233 (19-20) ◽

pp. 6892-6909 ◽

Cited By ~ 13

Author(s):

Wei Zhang ◽

Xiaojun Li ◽

Zuchao Zhu

Keyword(s):

Sensitivity Analysis ◽

Shear Layer ◽

Unstable Mode ◽

Flow Patterns ◽

Temporal Variations ◽

Wake Flow ◽

Circular Cylinders ◽

Flow Unsteadiness ◽

Far Wake ◽

Perturbation Growth

This work performs a numerical investigation on the two-dimensional flow across two circular cylinders in staggered arrangement at Re = 100. The seaparting distances between the centers of the cylinders are D/ d = 4–10 with Δ D/ d = 2 and T/ d = 0.0–2.0 with Δ T/ d = 0.5 in the streamwise and transverse directions, respectively, in which d is the cylinder diameter. Although the low- Re flow across staggered cylinders has been studied in a number of works, the authors mainly concerned about the identification and transition of various flow patterns. In this work, our objective is to quantitatively reveal the characteristics of flow unsteadiness as affected by the two separating distances. The flow unsteadiness is assessed from several aspects, including the spatial distributions and temporal variations of instantaneous flow patterns, fluctuating characteristic quantities, and fluctuating flow in the gap and in the near- and far-wake regions. To investigate the inherent instability of the flow, the global linear stability and sensitivity analysis is further carried out to demonstrate the unstable mode of perturbation growth and the critical flow patterns that destabilize the flow. The numerical results reveal that the wake flow between the two centerlines and beside the upstream cylinder is the most intensely perturbed. The flow around the downstream cylinder exhibits great fluctuation as perturbed by the destabilized shear layer of the upstream cylinder. The flow downstream of both cylinders shows multiple peak fluctuation of velocity because of the complex interactions between the destabilized shear layer and the wake vortices, resulting in the bidirectional transverse propagation of fluctuation. The stability analysis demonstrates that the unstable mode of perturbation growth is more significant in the far-wake region as the two cylinders are placed in proximity; the sensitivity analysis shows that the gap flow is crucial for the flow destabilization at small D, while the wake flow of cylinder- B is more significant for large D.

Download Full-text