8,883 research outputs found
Testing microelectronic biofluidic systems
According to the 2005 International Technology Roadmap for Semiconductors, the integration of emerging nondigital CMOS technologies will require radically different test methods, posing a major challenge for designers and test engineers. One such technology is microelectronic fluidic (MEF) arrays, which have rapidly gained importance in many biological, pharmaceutical, and industrial applications. The advantages of these systems, such as operation speed, use of very small amounts of liquid, on-board droplet detection, signal conditioning, and vast digital signal processing, make them very promising. However, testable design of these devices in a mass-production environment is still in its infancy, hampering their low-cost introduction to the market. This article describes analog and digital MEF design and testing method
Optimal mixing enhancement
We introduce a general-purpose method for optimising the mixing rate of
advective fluid flows. An existing velocity field is perturbed in a
neighborhood to maximize the mixing rate for flows generated by velocity fields
in this neighborhood. Our numerical approach is based on the infinitesimal
generator of the flow and is solved by standard linear programming methods. The
perturbed flow may be easily constrained to preserve the same steady state
distribution as the original flow, and various natural geometric constraints
can also be simply applied. The same technique can also be used to optimize the
mixing rate of advection-diffusion flow models by manipulating the drift term
in a small neighborhood
Closed-loop separation control over a sharp edge ramp using Genetic Programming
We experimentally perform open and closed-loop control of a separating
turbulent boundary layer downstream from a sharp edge ramp. The turbulent
boundary layer just above the separation point has a Reynolds number
based on momentum thickness. The goal of the
control is to mitigate separation and early re-attachment. The forcing employs
a spanwise array of active vortex generators. The flow state is monitored with
skin-friction sensors downstream of the actuators. The feedback control law is
obtained using model-free genetic programming control (GPC) (Gautier et al.
2015). The resulting flow is assessed using the momentum coefficient, pressure
distribution and skin friction over the ramp and stereo PIV. The PIV yields
vector field statistics, e.g. shear layer growth, the backflow area and vortex
region. GPC is benchmarked against the best periodic forcing. While open-loop
control achieves separation reduction by locking-on the shedding mode, GPC
gives rise to similar benefits by accelerating the shear layer growth.
Moreover, GPC uses less actuation energy.Comment: 24 pages, 24 figures, submitted to Experiments in Fluid
Tracer Applications of Noble Gas Radionuclides in the Geosciences
The noble gas radionuclides, including 81Kr (half-life = 229,000 yr), 85Kr
(11 yr), and 39Ar (269 yr), possess nearly ideal chemical and physical
properties for studies of earth and environmental processes. Recent advances in
Atom Trap Trace Analysis (ATTA), a laser-based atom counting method, have
enabled routine measurements of the radiokrypton isotopes, as well as the
demonstration of the ability to measure 39Ar in environmental samples. Here we
provide an overview of the ATTA technique, and a survey of recent progress made
in several laboratories worldwide. We review the application of noble gas
radionuclides in the geosciences and discuss how ATTA can help advance these
fields, specifically determination of groundwater residence times using 81Kr,
85Kr, and 39Ar; dating old glacial ice using 81Kr; and an 39Ar survey of the
main water masses of the oceans, to study circulation pathways and estimate
mean residence times. Other scientific questions involving deeper circulation
of fluids in the Earth's crust and mantle also are within the scope of future
applications. We conclude that the geoscience community would greatly benefit
from an ATTA facility dedicated to this field, with instrumentation for routine
measurements, as well as for research on further development of ATTA methods
Slow decay of concentration variance due to no-slip walls in chaotic mixing
Chaotic mixing in a closed vessel is studied experimentally and numerically
in different 2-D flow configurations. For a purely hyperbolic phase space, it
is well-known that concentration fluctuations converge to an eigenmode of the
advection-diffusion operator and decay exponentially with time. We illustrate
how the unstable manifold of hyperbolic periodic points dominates the resulting
persistent pattern. We show for different physical viscous flows that, in the
case of a fully chaotic Poincare section, parabolic periodic points at the
walls lead to slower (algebraic) decay. A persistent pattern, the backbone of
which is the unstable manifold of parabolic points, can be observed. However,
slow stretching at the wall forbids the rapid propagation of stretched
filaments throughout the whole domain, and hence delays the formation of an
eigenmode until it is no longer experimentally observable. Inspired by the
baker's map, we introduce a 1-D model with a parabolic point that gives a good
account of the slow decay observed in experiments. We derive a universal decay
law for such systems parametrized by the rate at which a particle approaches
the no-slip wall.Comment: 17 pages, 12 figure
On the control and suppression of the Rayleigh-Taylor instability using electric fields
It is shown theoretically that an electric field can be used to control and suppress the classical Rayleigh-Taylor instability found in stratified flows when a heavy fluid lies above lighter fluid. Dielectric fluids of arbitrary viscosities and densities are considered and a theory is presented to show that a horizontal electric field (acting in the plane of the undisturbed liquid-liquid surface), causes growth rates and critical stability wavenumbers to be reduced thus shifting the instability to longer wavelengths. This facilitates complete stabilization in a given finite domain above a critical value of the electric field strength. Direct numerical simulations based on the Navier-Stokes equations coupled to the electrostatic fields are carried out and the linear theory is used to critically evaluate the codes before computing into the fully nonlinear stage. Excellent agreement is found between theory and simulations, both in unstable cases that compare growth rates and in stable cases that compare frequencies of oscillation and damping rates. Computations in the fully nonlinear regime supporting finger formation and roll-up show that a weak electric field slows down finger growth and that there exists a critical value of the field strength, for a given system, above which complete stabilization can take place. The effectiveness of the stabilization is lost if the initial amplitude is large enough or if the field is switched on too late. We also present a numerical experiment that utilizes a simple on-off protocol for the electric field to produce sustained time periodic interfacial oscillations. It is suggested that such phenomena can be useful in inducing mixing. A physical centimeter-sized model consisting of stratified water and olive oil layers is shown to be within the realm of the stabilization mechanism for field strengths that are approximately 2 × 104  V/m
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