Michael Sprague

A simple three-dimensional vortex micromixer

We demonstrate rapid homogenous micromixing at low Reynolds numbers in an easily fabricated and geometrically simple three-dimensional polystyrene vortex micromixer. Micromixing is critically important for miniaturized analysis systems. However, rapid and effective mixing at these small scales remains a persistent challenge. We compare our micromixer’s performance against a two-dimensional square-wave design by examining its effectiveness in mixing solutions of dissimilar concentration as well as suspension solutions comprised of microparticles. Numerical simulations confirm our experimental observations and provide insights on the self-rotational mixing dynamics achieved with our simple geometry at low Reynolds numbers. This rapid, robust, and easily fabricated micromixer is amenable readily to large scale integration.

Stability of Taylor–Couette flow in a finite-length cavity with radial throughflow

Linear stability analysis predicts that a radial throughflow in a Taylor–Couette system will alter the stability of the flow, but the underlying physics for the stabilization of the flow is unclear. We investigate the impact of radial inflow and outflow on Taylor vortex flow and wavy vortex flow in a finite-length cavity via direct numerical simulation using a three-dimensional spectral method. The numerical simulations are consistent with linear stability predictions in that radial inflow and strong radial outflow have a stabilizing effect, while weak radial outflow destabilizes the system slightly. A small radial outflow velocity enhances the strength of the Taylor vortices resulting in destabilization of the base flow, whereas strong radial outflow and radial inflow reduce vortex strength, thus stabilizing the system. The transition to wavy vortex flow is unaffected by small radial outflow, but is stabilized for radial inflow. For strong radial outflow the wavy vortex flow includes localized dislocatio...

Tailored Taylor vortices

The stability of circular Couette flow in discontinuous axisymmetric geometries is investigated using numerical simulations and physical experiments. By contouring the geometry of the inner cylinder, Taylor vortices can be made to appear in discrete sections along the length of the cylinder while adjoining sections remain stable. The disparate flows are connected by transition regions that arise from the stability of the axially nonuniform base flow state. The geometry of the inner cylinder can be tailored to produce the simultaneous onset of Taylor vortices of different wavelength in neighboring sections. In another variant, a stack of inner cylinders of common radius are made to rotate independently to produce adjacent regions of stable and unstable flow.

Continuously tailored Taylor vortices

Modified axisymmetric, finite-length Taylor–Couette (TC) cells with stationary outer cylinder and rotating inner cylinder are designed in an effort to produce simultaneous onset of toroidal vortices of continuously varying wavelength along the gap. For a given axial variation in the inner radius, the axial variation in the outer radius can be chosen such that at every axial position, the criterion for the onset of Taylor vortices in a corresponding classical TC cell is met. In one scenario, a conical inner cylinder is chosen and the shape of the outer cylinder is then determined by locally satisfying the onset criterion. In another scenario, the inner and outer radii are chosen such that the onset criterion is locally satisfied and the axial rate of change in the classical onset wave number is held constant. In both cases, the modified cells possess a large-scale meridional circulation wrought by the finite Ekman (Bodewadt) pumping on the inner (outer) cylinder walls. Using direct numerical simulation, it...

Three-dimensional flow induced by the torsional motion of a cylinder

The flow induced outside a highly flexible cylindrical sheet executing pure torsional motion is studied. The problem is governed by the torsional Reynolds number R= γa2/ν, where γ is the axial rate of rotation, a is the cylinder radius and ν is the fluid kinematic viscosity. An interesting feature of this problem is that the axial pressure gradient of the primary flow induces a weak transverse flow in the meridional plane. The axial component of this motion takes the form of a wall jet. The high Reynolds number asymptotics for the shear stress parameters and the radially entrained flow are presented and compared with full numerical results computed over the large range of Reynolds numbers 10− 2 ≤ R ≤ 106.

Turbulent Flow Simulation at the Exascale: Opportunities and Challenges Workshop: August 4-5, 2015, Washington, D.C.

This report details the findings and recommendations from the Turbulent Flow Simulation at the Exascale: Opportunities and Challenges Workshop, which was held August, 4–5, 2015, and was sponsored by the U.S. Department of Energy (DOE) Office of Advanced Scientific Computing Research (ASCR). The workshop objectives were to define and describe the challenges and opportunities that computing at the exascale will bring to turbulent-flow simulations in applied science and technology. The need for accurate simulation of turbulent flows is evident across the DOE appliedscience and engineering portfolios, including combustion, plasma physics, nuclear-reactor physics, wind energy, and atmospheric science. The workshop brought together experts in turbulent-flow simulation, computational mathematics, and high-performance computing. Building upon previous ASCR workshops on exascale computing, participants defined a research agenda and path forward that will enable scientists and engineers to continually leverage, engage, and direct advances in computational systems on the path to exascale computing.