David R. Emerson

High Speed Aerodynamic Characteristics of Rarefied Flow past Stationary and Rotating Cylinders

High-speed rarefied flow past both stationary and rotating cylinders is investigated using the direct simulation Monte Carlo method. Test validations are initially compared against experimental drag coefficients for flow past a stationary cylinder. We then compare various aerodynamic characteristics such as the coefficients of lift and drag, as well as the coefficients of pressure, skin friction, and heat transfer for stationary and rotating cylinders. The impact of flow speed and rarefaction on the drag and lift coefficients has also been assessed. In particular, an inverse Magnus effect, which involves a change in direction of the lift force, has been observed in the high subsonic flow regime. The flow characteristics are entirely different in the supersonic regime since the flow is predominantly affected by the Mach number. The effects of rotation rate and incomplete surface accommodation have also been studied for a rotating cylinder.

Recent Advances in Electrowetting Microdroplet Technologies

Electrowetting is potentially one of the most versatile techniques for manipulating submillimetre-sized droplets in microfluidic systems. By applying a voltage between a droplet and an electrode buried beneath the substrate of a microfluidic chip, it is possible to alter the wetting behaviour of the liquid so as to generate surface tension gradients around the droplet. This concept has subsequently become known as electrowetting-on-dielectric (EWOD) and has developed into one of the most popular technologies for droplet manipulation. Combining a large number of individually addressable electrodes onto a single microfluidic chip makes it possible to dispense, transport, split and merge individual droplets without the need for any moving components, and opens up the possibility for fully-controllable ‘digital’ microfluidic systems where the samples and reagents are manipulated using a standard set of droplet handling operations.

mdFoam+: Advanced molecular dynamics in OpenFOAM

This paper introduces mdFoam+, which is an MPI parallelised molecular dynamics (MD) solver implemented entirely within the OpenFOAM software framework. It is open-source and released under the same GNU General Public License (GPL) as OpenFOAM. The source code is released as a publicly open software repository that includes detailed documentation and tutorial cases. Since mdFoam+ is designed entirely within the OpenFOAM C++ object-oriented framework, it inherits a number of key features. The code is designed for extensibility and flexibility, so it is aimed first and foremost as an MD research tool, in which new models and test cases can be developed and tested rapidly. Implementing mdFoam+ in OpenFOAM also enables easier development of hybrid methods that couple MD with continuum-based solvers. Setting up MD cases follows the standard OpenFOAM format, as mdFoam+ also relies upon the OpenFOAM dictionary-based directory structure. This ensures that useful pre- and post-processing capabilities provided by OpenFOAM remain available even though the fully Lagrangian nature of an MD simulation is not typical of most OpenFOAM applications. Results show that mdFoam+ compares well to another well-known MD code (e.g. LAMMPS) in terms of benchmark problems, although it also has additional functionality that does not exist in other open-source MD codes.

Simulation of Low Knudsen Number Isothermal Flow Past a Confined Spherical Particle in a Micro-Pipe

Low Knudsen number isothermal slip flow past a confined spherical particle has been investigated using a specially adapted Navier-Stokes solver. Knudsen numbers covering the continuum and slip-flow regimes (Kn ≤ 10−1 ) are considered while the Reynolds number is varied between 10−3 ≤ Re ≤ 0.5. In addition, blockage effects are studied by varying the ratio between the diameter of the pipe (H) and the diameter of the particle (D). A particularly important aspect of the present study is the proper formulation of the slip-velocity boundary condition over the curved surface of the particle. This is achieved by recasting Maxwell’s conventional velocity-slip equation as a function of the wall shear stress in order to account correctly for the curvature. The results show that blockage effects are extremely important in the continuum regime and cause amplification in the hydrodynamic drag on the particle. However, blockage phenomena are shown to be less important as the Knudsen number is increased. At the upper limit of the slip-flow regime, Kn ≈ 10−1 , blockage amplification effects are reduced by almost 50% for a pipesphere geometry of H/D = 2.Copyright

On the inverse Magnus effect for flow past a rotating cylinder

Flow past a rotating cylinder has been investigated using the direct simulation Monte Carlo method. The study focuses on the occurrence of the inverse Magnus effect under subsonic flow conditions. In particular, the variations in the coefficients of lift and drag have been investigated as a function of the Knudsen and Reynolds numbers. Additionally, a temperature sensitivity study has been carried out to assess the influence of the wall temperature on the computed aerodynamic coefficients. It has been found that both the Reynolds number and the cylinder wall temperature significantly affect the drag as well as the onset of lift inversion in the transition flow regime.

Numerical Simulation of Ion Transport in a Nano-Electrospray Ion Source at Atmospheric Pressure

AbstractUnderstanding ion transport properties from the ion source to the mass spectrometer (MS) is essential for optimizing device performance. Numerical simulation helps in understanding of ion transport properties and, furthermore, facilitates instrument design. In contrast to previously reported numerical studies, ion transport simulations in a continuous injection mode whilst considering realistic space-charge effects have been carried out. The flow field was solved using Reynolds-averaged Navier-Stokes (RANS) equations, and a particle-in-cell (PIC) method was applied to solve a time-dependent electric field with local charge density. A series of ion transport simulations were carried out at different cone gas flow rates, ion source currents, and capillary voltages. A force evaluation analysis reveals that the electric force, the drag force, and the Brownian force are the three dominant forces acting on the ions. Both the experimental and simulation results indicate that cone gas flow rates of ≤250 slph (standard liter per hour) are important for high ion transmission efficiency, as higher cone gas flow rates reduce the ion signal significantly. The simulation results also show that the ion transmission efficiency reduces exponentially with an increased ion source current. Additionally, the ion loss due to space-charge effects has been found to be predominant at a higher ion source current, a lower capillary voltage, and a stronger cone gas counterflow. The interaction of the ion driving force, ion opposing force, and ion dispersion is discussed to illustrate ion transport mechanism in the ion source at atmospheric pressure.n Graphical Abstract