Lumped-element Simulation Research Articles

Simulation of gas mixture dynamics in a pipeline network using explicit staggered-grid discretization

We develop an explicit staggered finite difference discretization scheme for simulating the transport of highly heterogeneous gas mixtures through pipeline networks. This study is motivated by the proposed blending of hydrogen into natural gas pipelines to reduce end use carbon emissions while using existing pipeline systems throughout their planned lifetimes. Our computational method accommodates an arbitrary number of constituent gases with very different physical properties that may be injected into a network with significant spatiotemporal variation. In this setting, the gas flow physics are highly location- and time- dependent, so that local composition and nodal mixing must be accounted for. The resulting conservation laws are formulated in terms of pressure, partial densities and flows, and volumetric and mass fractions of the constituents. We include non-ideal equations of state that employ linear approximations of gas compressibility factors, so that the pressure dynamics propagate locally according to a variable wave speed that depends on mixture composition and density. We derive compatibility relationships for network edge boundary values that are more complex than for a homogeneous gas. The simulation method is evaluated on initial boundary value problems for a single pipe and a small network, is cross-validated with a lumped element simulation, and used to demonstrate a local monitoring and control policy for maintaining allowable concentration levels.

A microfluidic reciprocating intracochlear drug delivery system with reservoir and active dose control

Reciprocating microfluidic drug delivery, as compared to steady or pulsed infusion, has unique features which may be advantageous in many therapeutic applications. We have previously described a device, designed for wearable use in small animal models, that periodically infuses and then withdraws a sub-microliter volume of drug solution to and from the endogenous fluid of the inner ear. This delivery approach results in zero net volume of liquid transfer while enabling mass transport of compounds to the cochlea by means of diffusion and mixing. We report here on an advanced wearable delivery system aimed at further miniaturization and complex dosing protocols. Enhancements to the system include the incorporation of a planar micropump to generate reciprocating flow and a novel drug reservoir that maintains zero net volume delivery and permits programmable modulation of the drug concentration in the infused bolus. The reciprocating pump is fabricated from laminated polymer films and employs a miniature electromagnetic actuator to meet the size and weight requirements of a head-mounted in vivo guinea pig testing system. The reservoir comprises a long microchannel in series with a micropump, connected in parallel with the reciprocating flow network. We characterized in vitro the response and repeatability of the planar pump and compared the results with a lumped element simulation. We also characterized the performance of the reservoir, including repeatability of dosing and range of dose modulation. Acute in vivo experiments were performed in which the reciprocating pump was used to deliver a test compound to the cochlea of anesthetized guinea pigs to evaluate short-term safety and efficacy of the system. These advances are key steps toward realization of an implantable device for long-term therapeutic applications in humans.

Determining The Acoustic Resistance of Small Sound Holes for MEMS Microphones

Capacitive MEMS microphones are now widely used for mobile phones and headsets. The frequency response of MEMS microphones is influenced by the package geometry. This can be applied to tune the performance of the microphone in the system. Packages with small sound inlet holes are acoustic low-pass filters. The cut-off frequency is determined by the diameter of the holes. In previous work we did not find sufficient agreement between a lumped element simulation and the measured frequency response of the microphones. Now we measured the acoustic resistance of small sound inlet holes directly. The measured resistances agree well with calculated values. The frequency response of MEMS microphones was measured for different sound hole diameters and lengths. The results are in good agreement with calculated transfer functions of the package.

Switching noise picked up by a planar dipole antenna mounted near integrated circuits

For applications in which antennas are located near or in integrated circuits (ICs), the switching noise from digital circuits can interfere with the operation of antennas. This paper presents a study of the switching noise picked up by a planar dipole antenna from a divide-by-128 IC located near the antenna. To develop understanding of the measurement results, a lumped-element simulation model has been developed. Quantitative agreements between the measurements and simulations for numerous experiments have been obtained. The measurements indicate that by selecting the signal frequency on the antennas much greater than the circuit frequency, the immunity from switching noise can be improved. The measurements also showed that circuits such as buffers are relatively noisier, and emanate more noise at higher operating frequencies. Finally, the measurements showed that using antennas with a differential or balanced feed structure can substantially reduce the coupling of switching noise (/spl sim/20 dB) which is mostly common-mode in nature.

Applied optimization and circuit design

A qualitative review of applied optimization forms is given, illustrated by reference to the linear circuit design problem. When used to solve very difficult problems, optimization theory becomes extremely vague with many conflicting objectives. These opposing standpoints are discussed and an approach is described which attempts to handle the complications. Working online, many of these nebulous aspects can be considered at run-time. To illustrate this approach a lumped-element simulation of a coaxial cable over a frequency band of 1 2 MHz-1 GHz is described.