Hieu H. Nguyen

Biocompatibility Implications of Polypyrrole Synthesis Techniques

Polypyrrole (PPy) is an inherently conducting polymer that has shown great promise for biomedical applications within the nervous system. However, to effectively use PPy as a biomaterial implant, it is important to understand and reproducibly control the electrical properties, physical topography and surface chemistry of the polymer. Although there is much research published on the use of PPy in various applications, there is no systematic study linking the methodologies used for PPy synthesis to PPys basic polymeric properties (e.g., hydrophilicity, surface roughness), and to the biological effects these properties have on cells. Electrochemically synthesized PPy films differ greatly in their characteristics depending on synthesis parameters such as dopant, substrate and thickness, among other parameters. In these studies, we have used three dopants (chloride (Cl), tosylate (ToS), polystyrene sulfonate (PSS)), two substrates (gold and indium tin oxide-coated glass), and a range of thicknesses, to measure and compare the biomedically important characteristics of surface roughness, contact angle, conductivity, dopant stability and cell adhesion (using PC-12 cells and Schwann cells). As predicted, we discovered large differences in roughness depending on the dopant used and the thickness of the film, while substrate choice had little effect. From contact angle measurements, PSS was found to yield the most hydrophilic material, most likely because of free charges from the long PSS chains exposed on the surface of the PPy. ToS-doped PPy films were tenfold more conductive than Cl- or PSS-doped films. X-ray photoelectron spectroscopy studies were used to evaluate dopant concentrations of PPy films stored in water and phosphate buffered saline over 14 days, and conductance studies over the same timeframe measured electrical stability. PSS proved to be the most stable dopant, though all films experienced significant decay in conductivity and dopant concentration. Cell adhesion studies demonstrated the dependence of cell outcome on film thickness and dopant choice. The strengths and weaknesses of different synthesis parameters, as demonstrated by these experiments, are critical design factors that must be leveraged when designing biomedical implants. The results of these studies should provide practical insight to researchers working with conducting polymers, and particularly PPy, on the relationships between synthesis parameters, polymeric properties and biological compatibility.

Wind turbine loads during simulated thunderstorm microbursts

The International Electrotechnical Commission (IEC) standard 61400-1 for the design of wind turbines does not explicitly address site-specific conditions associated with anomalous atmospheric events or conditions. Examples of off-standard atmospheric conditions include thunderstorm downbursts, hurricanes, tornadoes, low-level jets, etc. This study is focused on the simulation of thunderstorm downbursts using a deterministic-stochastic hybrid model and the prediction of wind turbine loads resulting from the simulated thunderstorm event’s wind field. The wind velocity field model for thunderstorm downburst simulation is first discussed; in this model, downburst winds are generated separately from non-turbulent and turbulent parts. The non-turbulent part is based on an available analytical model, while the turbulent part is simulated as a stochastic process using standard turbulence power spectral density functions and coherence functions adjusted by information on parameters such as the thunderstorm’s translation velocity. In an incremental manner, we address the chief influences of the wind velocity fields associated with downbursts—namely, large wind speeds and rapid direction changes during the storm—by simulating various velocity fields and studying associated turbine loads. The turbine loads are generated using stochastic simulation of the aeroelastic response for a model of the selected utility-scale 5 MW turbine. While we believe this study is likely the first one to directly address the influence of thunderstorm downbursts on turbine loads, we make some controls-related assumptions in this work—for one, we allow for significant yaw errors, during periods of rapid wind direction change, in computing loads; additionally, for brief periods when high winds are in excess of cut-out, the turbine is assumed to continue to operate with similar blade pitch control rates as for winds close to and below the cut-out speed. While these assumptions do influence the loads experienced, the various cases included in this study serve to illustrate how they do so. Moreover, the study highlights the need for enhancements to models for aerodynamic loads computation that can more accurately address large yaw error, yaw control, blade pitch control, and transitions from turbine operating to possibly parked states that are especially important in dealing with transient events such as thunderstorm downbursts. Finally, comparisons of the turbine response to downbursts with discrete events such as in the “extreme direction change” and “extreme coherent gust with direction change” load cases specified in the IEC standard are presented, and brief remarks are made about these comparisons. Such comparisons serve to indicate how turbine loads during thunderstorm downbursts can be quite different from those specified in the IEC standard’s design load cases. Simulation procedures, as outlined here for simulation of downburst-related inflow wind fields, are not difficult to include in site assessment for regions where thunderstorms occur frequently. They might also be considered in future standards-related design load case definitions.

Directional Effects of Shear Combined with Compression on Bridge Elastomeric Bearings

Elastomeric bearings are widely used in bridge supports to accommodate thermal and other movements. The study presented in this paper extends an earlier investigation of two-dimensional bearing performance to three dimensions. Large-deformation rubber hyperelasticity is reviewed and a theoretical model is described with the steel-reinforced bearing subjected to compression in the direction through the thickness followed by shear in various lateral directions, including bridge longitudinal and transverse directions. Computations are carried out using the general-purpose, finite-element analysis computer program, ABAQUS. Conclusions are drawn regarding the effects of shear direction on bearing behavior.

In vitro activity of sparfloxacin (CI-978; AT-4140) against clinical isolates from cancer patients

The in vitro activity of sparfloxacin, a new quinolone, was compared with those of ciprofloxacin and fleroxacin against gram-positive and gram-negative bacteria, greater than 90% of which were isolated from blood culture specimens of cancer patients. Sparfloxacin was extremely active against Acinetobacter species, Aeromonas hydrophila, Citrobacter diversus, Enterobacter species, Escherichia coli, Klebsiella species, Proteus vulgaris, and Serratia marcescens (inhibiting greater than 90% of these isolates at a concentration of 0.5 microgram/ml) and moderately active against Pseudomonas species, other Proteus species, and Citrobacter freundii. Sparfloxacin inhibited greater than 90% of staphylococci (including methicillin-resistant and coagulase-negative strains) at a concentration of 0.12 microgram/ml and greater than 90% of streptococci (including Streptococcus pneumoniae) at a concentration of 1.0 microgram/ml. It was also active against Bacillus cereus, Enterococcus species, and Corynebacterium jeikeium, organisms that have become fairly common in cancer patients.

Comparative efficacies of cilofungin (Ly121019) and amphotericin B against disseminated Candida albicans infection in normal and granulocytopenic mice.

The efficacies of cilofungin (Ly121019), a semisynthetic lipopeptide antifungal agent, and amphotericin B in the treatment of disseminated candidiasis in normal and neutropenic mice were compared. In mice infected with 2 x 10(6) CFU of Candida albicans, treatment with cilofungin in twice-daily doses of 25 or 35 mg/kg of body weight by intraperitoneal injection for 10 days gave survival rates of 83 and 90%. In contrast, there was 97% mortality in infected controls receiving 2 x 10(6) CFU intravenously and 93% survival in mice treated with 1 mg of amphotericin B per kg once a day. Mice rendered granulocytopenic by the administration of cyclophosphamide showed survival rates of 83 and 80% when treated with 25 or 35 mg of cilofungin per kg for 10 days compared with 43% survival rate in mice treated with 1 mg of amphotericin B per kg (P = 0.0030 and P = 0.0080, respectively). Similar results were obtained when the two antifungal agents were administered for a period of 30 days. Administration of 25 or 35 mg of cilofungin per kg twice a day to granulocytopenic mice receiving 10(6) CFU of C. albicans gave survival rates of 93% and 93% compared with 53% survival with amphotericin B. With 15 mg of cilofungin per kg twice a day for 10 days, a survival rate of 43 to 50% was observed in both normal and granulocytopenic mice compared with 56 and 60%, respectively, when this dosage was continued for 30 days. Cilofungin eradicated C. albicans from the kidneys, spleens, and livers of surviving animals. No toxic effects were observed with any of the dosage regimens used. The clearance of C. albicans from the kidneys, spleens, livers, and brains in normal mice was studied following infection with 5 x 10(5) and 1 x 10(5) intravenously. The mice in the treatment groups received 25 mg of cilofungin per kg twice a day for 10 days. In 8 to 12 days, this treatment was able to clear the organisms from the kidneys, spleens, and livers of mice infected with 5 x 10(5) C. albicans. Mice infected with 10(5) C. albicans and treated with cilofungin (25 mg/kg) twice a day for 10 days had no organisms in the kidney, spleen, and liver at days 8, 2, and 8, respectively. There was 1-log-unit reduction in C. albicans counts in brain tissue from mice of one of the treated groups between 2 h and 2 days postinfection, after which the numbers of organisms remained the same until day 12. These data demonstrate the efficacy of cilofungin in the treatment of disseminated C. albicans infections in normal and granulocytopenic mice. The treatment regimen used in this study was able to clear C. albicans from the kidneys, spleen, and liver but not from brain tissue.

Thunderstorm downburst risks to wind farms

As has been noted in recent experiences, thunderstorms can cause damage to turbine units that are part of a wind farm because of the correlated wind inflow fields that these units can be subjected to simultaneously. We extend a model for simulating thunderstorm downburst events and their effects on a single turbine unit to situations where multiple units in a wind farm are subjected to correlated input from moving downbursts. The end-to-end simulation we describe involves first the generation of a downburst touchdown with associated downburst wind field and subsequent movement in the vicinity of a wind farm, comprised of a 6 × 4 rectilinear arrangement of 5 MW 90-m hub-height turbines. The downburst model includes a slowly varying (non-turbulent) component and a turbulent component. The wind field model is developed and calibrated based on Doppler radar data on recorded downbursts. Results show that downbursts with different physical characteristics can lead to highly variable loads on units in the wind farm.

Simulation of Thunderstorm Downbursts and Associated Wind Turbine Loads

This study is focused on simulation of thunderstorm downbursts and associated wind turbine loads. We first present a thunderstorm downburst model, in which the wind field is assumed to result from the summation of an analytical mean field and stochastic turbulence. The structure and evolution of the downburst wind field based on the analytical model are discussed. Loads are generated using stochastic simulation of the aeroelastic response for a model of a utility-scale 5-MW turbine. With the help of a few assumptions, particularly regarding control strategies, we address the chief influences of wind velocity fields associated with downbursts—namely, large wind speeds and large, rapid wind direction changes—by considering different storm scenarios and studying associated turbine loads. These scenarios include, first, an illustrative case to understand details related to the turbine response simulation; this is followed by a study involving a different storm touchdown location relative to the turbine as well as a critical case where a shutdown sequence is included. Results show that the availability of and assumptions in wind turbine control systems during a downburst clearly influence overall system response. Control system choices can significantly mitigate turbine loads during downbursts. Results also show that different storm touchdown locations result in distinct characteristics in inflow wind fields and, hence, in contrasting turbine response.

Extreme and fatigue loads on wind turbines during thunderstorm downbursts: The influence of alternative turbulence models

Downburst winds are inherently transient and non-stationary by nature. Field records have confirmed the often very rapid variation in time of both mean wind speed and wind direction during a downburst. Turbulence characteristics in a downburst are also different from those in the neutral boundary layer. Studies have indicated that turbulence power spectra during a downburst can deviate significantly from more traditional spectra used for the neutral boundary layer. Estimated power spectra from downburst records indicate additional turbulence energy in the mid-frequency range and often they do not follow the −5/3 slope in the inertial subrange. In this study, three turbulence models that can be combined with a non-turbulent downburst wind field model are presented. The influence of these three models on extreme and fatigue loads for a 5-MW wind turbine is investigated first. Then, a parametric study on the longitudinal turbulence length scale, turbulence intensity, and turbulence vertical component in down...

Simulation of Nonlinear Waves on Offshore Wind Turbines and Associated Fatigue Load Assessment

By using a global simulation framework that employs a domain-decomposition strategy for computational efficiency, this study investigates the effects of fully nonlinear (FNL) waves on the fatigue loads exerted on the support structure (monopile) of a fixed-bottom offshore wind turbine. A comparison is made with more conventional linear wave hydrodynamics. The FNL numerical wave solver is invoked only on specific sub-domains where nonlinearities are detected; thus, only locally in space and time, a linear wave solution is replaced by the FNL results as input to the Morison equation used for the hydrodynamic loads. The accuracy and efficiency of this strategy allows long timedomain simulations where strongly nonlinear free-surface phenomena, like imminent breaking waves, are accounted for in the prediction of structural loads. The unsteady nonlinear free-surface problem governing the propagation of gravity waves is formulated using potential theory and a higher-order boundary element method (HOBEM) is used to discretize Laplace’s equation. The FNL solver is employed and associated hydrodynamic loads are predicted in conjunction with aerodynamic loads on the rotor of a 5-MW wind turbine using the NREL open-source software, FAST. We assess fatigue loads by means of both time- and frequency-domain methods. This study shows that the use of linear theory-based hydrodynamics can lead to significant underestimation of fatigue loads and damage.© 2014 ASME

Fatigue Reliability Analysis for Brace–Column Connection Details in a Semisubmersible Hull

Semisubmersible floating platforms used in offshore deep or ultradeep water environments have hull structures that are comprised of vertical cylinders (columns) connected by braces, pontoons, etc. Several of the connections between these various members are susceptible to fatigue damage. In fatigue damage assessment or fatigue reliability analysis, a global structural response analysis is typically carried out using a finite element (FE) model where internal forces or stresses in the various members are evaluated for specified sea states measured at the site. Of specific interest in the present study is the fatigue reliability analysis of brace-column connection details in a semisubmersible hull unit for selected Brazilian environmental conditions. Stress concentration factors (SCFs) for the selected critical hot spots are applied to the nominal component stresses due to axial forces and biaxial bending. The hot-spot stress response spectra are used with various spectral methods—referred to as Rayleigh, modified Rayleigh (with bandwidth correction), and Dirlik—to estimate fatigue damage using Miners rule. Uncertainties in some parameters used in the fatigue life assessment are considered and the probability of fatigue failure in the last operational year of the structure is estimated.