John C. LaRue

The decay power law in grid-generated turbulence

The effect of initial conditions on the decay exponent and coefficient and virtual origin in the decay power-law form for the variation of the variance of the turbulent velocity downstream of biplane grids constructed of rods of both round and square cross-section is determined. This effect is determined for data obtained as part of the present study as well as from previous studies. These studies cover a Reynolds number range from 6000 to 68000, mesh sizes of 2.54 and 5.08 cm, and solidities of 0.34 and 0.44. It is shown that the choice of the virtual origin and the use of data in the non-homogeneous portion of the flow can have a significant influence on the value of the parameters in the decay power-law. Criteria are developed to identify the nearly homogeneous and isotropic portion of the flow. These criteria include low values of the velocity skewness, constancy of the skewness of the velocity derivative and balance of the turbulent kinetic energy equation. Results based on data selected by means of these criteria show that the decay exponent and virtual origin are independent of initial conditions such as Reynolds number, mesh size, solidity, and rod shape and surface roughness with values of respectively 1.30 and 0. In contrast and as expected, the decay coefficient is found to be a function of these initial conditions. Thus, the downstream variation of the variance of the turbulent velocity is universally self-similar.

Further results on the thermal mixing layer downstream of a turbulence grid

Simultaneous velocity and temperature measurements in the thermal mixing layer downstream of a partially heated turbulence grid are reported. The temperature data are in good agreement with earlier results. The velocity‐temperature correlations are new and permit a systematic assessment of an earlier similarity theory for the region far downstream of the grid. By selecting the virtual origin appropriately and by reconsidering the role of scalar dissipation in that region, agreement between measurement and prediction with respect to temperature intensity and two mean lateral fluxes, that of temperature and of temperature intensity, is achieved.

Modified gauge for time‐resolved skin‐friction measurements

A new gauge for skin‐friction measurements in turbulent flows involving a wire close to the surface and a second, flush‐mounted film which serves as a constant temperature guard heater is described and evaluated. It is shown that a laminar calibration of the gauge applies to measurements in turbulent flow and that the frequency response of the gauge is appropriate for time‐resolved data in such a flow. Initial results from a turbulent channel flow are presented.

Application of SU-8 as the insulator toward a novel planar microelectrode array for extracellular neural recording

In this research work the design and prototyping of novel planar microelectrode arrays that utilize SU-8 as the insulating material is presented. The microelectrode arrays are fabricated as a key component of an extracellular neural recording system comprising a full customized CMOS analog neural signal readout ASIC for neural signal processing and an 8 by 8 microelectrode array for acquiring the action potentials by means of capacitive coupling. The prototyped multi-channel mixed signal fully-customized CMOS biosensor analog front-end serves as the neural signal readout circuitry, which is comprised of analog signal buffers, high gain amplifier and control/interface. The concept of using epoxy-based photoresist SU-8 as the insulator instead of the conventional silicon nitride or silicon dioxide is due to the existence of nonuniformities and pinholes in the nitride layer. This pinhole free biocompatible SU-8 greatly improves the performance of the insulation layer and provides an approach for the construction of low cost disposable microelectrode arrays.

Tunable infrared filter based on elastic polymer springs

This paper describes the design, fabrication and testing of tunable Fabry-Perot filters. The goal of this research is to develop novel tunable filter with an area of 5x5 mm2 that will be used in infrared gas sensors. This exploits the fact that most gases have unique infrared absorption signatures in the 2-14 µm wavelength region. The filter consists of two thin silicon wafers coated with quarter wave dielectric layers to form wavelength dependent high reflection mirrors and separated by air gaps with an average height of 8, 5.1 and 3.5 μm. The mirrors are supported by four elastic polymer posts (springs) each with an area of 100×100 μm2 made by using photo definable polydimethylsilxane (PDMS). An electrostatic voltage is used to compress the springs, change the airgap height and hence shift the transmission peaks to a shorter wavelength. A finesse of 12 with full width at half maximum (FWHM) of 70 nm, and a peak transmission of 63% were achieved by applying 100 volts on a device with 8 µm post height and wafer thickness of 125 µm. In addition, the measured tunability before and after hard baking of the device was 210 nm and 130 nm respectively. The tunability stayed constant after hard baking the devices and did not show any changes with time. The tunability was also measured on a thinner silicon mirror with 3.5 µm post height. In this case, the filter was tuned 180 nm by applying 10 volts. However, the filter finesse was 3, transmission peak was 40% and FWHM over 200 nm. An antireflection coating was deposited on one side of silicon wafers and a Fabry-Perot filter to study transmission enhancement and satisfactory results were achieved.

Similarity solutions and experiment for turbulent wakes

Available data for the turbulent wake of unheated and heated cylinders are compared to far‐wake solutions obtained from the relevant equations prior to application of any closure assumption. It is found that such comparisons require a new determination of the virtual origin significantly different from traditional values and at the same time provide a sensitive criterion to decide if the stations at which data are collected are sufficiently far downstream for far‐wake solutions to apply.

Design of a CMOS-based multichannel integrated biosensor chip for bioelectronic interface with neurons

In this paper we present the design and prototyping of a 24-channel mixed signal full-customized CMOS integrated biosensor chip for in vitro extracellular recording of neural signals. Design and implementation of hierarchical modules including microelectrode electrophysiological sensors, analog signal buffers, high gain amplifier and control/interface units are presented in detail. The prototype chip was fabricated by MOSIS with AMI C5 0.5 µm, double poly, triple metal layer CMOS technology. The electroless gold plating process is used to replace the aluminum material obtained from the standard CMOS process with biocompatible metal gold in the planner microelectrode array sensors to prevent cell poisoning and undesirable electrochemical corrosion. The biosensor chip provides a satisfactory signal-to-noise ratio for neural signals with amplitudes and frequencies within the range of 600µV – 2mV and 100 Hz to 10KHz, respectively.

The distortion of a passive scalar by two‐dimensional objects

The response of a heated, nearly homogeneous and isotropic turbulent field to a nonuniform rapid distortion upstream of two‐dimensional objects placed in a homogeneous grid generated flow is investigated experimentally. In particular, the effect of the nonuniform distortion on the single‐point statistical properties of the velocity and a passive scalar (temperature), the axial heat flux and higher‐order cross moments are presented. The nearly homogeneous, turbulent flow is produced by a biplane grid of rods and a square mesh grid of electrically heated wire that is placed downstream of the turbulence producing grid. Spatially and temporally resolved, simultaneous measurements of the streamwise turbulent velocity and temperature are obtained upstream of several two‐dimensional objects along the mean stagnation streamline. The effects of the blocking and vortex stretching mechanisms on the root‐mean‐square (RMS) velocity for various ratios of the integral length scale to the characteristic length of the obj...

Development of planar microelectrode array for extracellular characterization of action potentials in sinoatrial node cells

In this research work we prototyped a planar microelectrode arrays that utilize SU-8 as the insulating material for non-invasive chronically characterization of action potentials from SA node cells. The microelectrode arrays are fabricated to form a microchannel based extracellular action potential characterization system which comprises a 2 by 50 linear microelectrode array for acquiring the action potentials with the principle of capacitive coupling and a full customized CMOS analog bio-signal readout ASIC for bio-signal conditioning and processing. The prototyped multi-channel mixed signal fully-customized CMOS biosensor analog front-end serves as the low magnitude action potential signal readout circuitry, which is comprised of analog signal buffers, high gain amplifier and control/interface. The concept of using epoxy-based photoresist SU-8 as the insulator instead of the conventional silicon nitride or silicon dioxide is due to the high insulation property and bio-compatibility of the SU-8 material. This biocompatible planar microelectrode arrays with microchannels provides the biological scientists powerful tools for the long term non-invasive study of action potentials from SA node cells to screen libraries of chemical compounds for new drug candidates.

Temporal and spatial unmixedness downstream of a plate array

The effect of a plate array on a turbulent velocity and turbulent concentration field is determined. Profiles of mean and root-mean-square velocity and concentration, profiles of temporal and spatial unmixedness, and profiles of the variance of the gradient of velocity and the variance of the gradient of concentration are presented. Velocity and concentration integral length scales are compared. A biplane injection grid is used to produce the turbulent concentration and turbulent velocity field. Helium is injected through the jets at the grid nodes as air passes through the grid. The time-resolved velocity and concentration data are obtained using a two-sensor probe that consists of a hot wire and a TSI 1440-20 aspirating concentration probe. The addition of a plate array is shown to decrease the spatial unmixedness to a nearly zero value in about half the downstream distance observed without plates. Further, an increase in dissipation is shown with the array in place that reduces the temporal unmixedness...