Relativistic electrons generated at Earth's quasi-parallel bow shock.

Plasma shocks are the primary means of accelerating electrons in planetary and astrophysical settings throughout the universe. Which category of shocks, quasi-perpendicular or quasi-parallel, accelerates electrons more efficiently is debated. Although quasi-perpendicular shocks are thought to be more efficient electron accelerators, relativistic electron energies recently observed at quasi-parallel shocks exceed theoretical expectations. Using in situ observations at Earth's bow shock, we show that such relativistic electrons are generated by the interaction between the quasi-parallel shock and a related nonlinear structure, a foreshock transient, through two betatron accelerations. Our observations show that foreshock transients, overlooked previously, can increase electron acceleration efficiency at a quasi-parallel shock by an order of magnitude. Thus, quasi-parallel shocks could be more important in generating relativistic electrons, such as cosmic ray electrons, than previously thought

On The Leakage and Rotordynamic Force Coefficients of Pump Annular Seals Operating with Air/Oil Mixtures: Measurements and Predictions

LecturesIn the subsea oil and gas industry, multiphase pumps and wet gas compressors add pressure to the process fluid thus enabling long distance tie back system that eliminates topside facilities such as an oil and gas separation station. One challenge to construct a reliable multiphase pump or a wet gas compressor is to engineer their ability to withstand a gas-liquid mixture whose gas volume fraction (GVF) or liquid volume fraction (LVF) changes over time. The mixture GVF or LVF affects the static and dynamic forced performance of secondary flow components, namely seals, and which may lead to an increase in both rotor lateral or axial vibrations. The lecture presents measurements of leakage and dynamic force coefficients for six annular seals (see Fig. 1, L = 46 mm, D = 127 mm) for multiple-stage submersible pumps and operating with an air in oil mixture ranging from pure liquid to just air. Each seal has a distinct clearance configuration: one is a plain seal with a small clearance (c=0.203 mm), and another has a larger (worn) clearance (c=0.274 mm); a third seal introduces a wavy clearance (cm= 0.191 mm) that produces a significant centering stiffness; a fourth seal has a shallow groove pattern (cr=0.211); and the fifth seals have a stepped clearance (narrow to wide and wide to narrow). At a shaft speed of 3.5 krpm, an air in ISO VG 10 oil mixture with an inlet GVF varying discretely from 0 to 0.9 feeds a test seal at a supply pressure Ps of 2.5 bar(a). The test mixture mass flow rate decreases continuously with an increase in the inlet GVF. The seals Copyright© 2018 by Turbomachinery Laboratory, Texas A&M Engineering Experiment Station 2 operating with a pure liquid (GVF=0) show frequency independent force coefficients. On the other hand, operation with a mixture produces direct and cross-coupled stiffnesses that vary greatly with frequency, in particular the direct stiffness hardens with excitation frequency. The direct damping coefficients, on the other hand, are not functions of excitation frequency, albeit dropping rapidly in magnitude as the GVF increases. The lecture details comparisons of leakage and force coefficients among the various test seals. The three-wave seal produces the greatest direct stiffness and damping coefficients, as well as the largest effective damping coefficient. The worn surface (largest clearance) seal produces the smallest force coefficients and leaks the most. Operation with a large GVF produces little damping, albeit more than predicted. For all the test seals, the whirl frequency ratio is around 50%. The step clearance seal (with the narrow clearance facing the incoming flow produces a significant negative direct stiffness that could easily impact the static stability of a pump as it reduces its natural frequency. Predictions of seal force coefficients derived from a homogeneous bulk flow match well with the test data for operation with a pure oil and a small GVF ~ 0.2. The discrepancy between the prediction and test data grows rapidly for operation with a larger gas content, GVF > 0.2. Hence, more accurate predictive models are in urgent need of development. The experimental results reveal the best characteristics of certain annular seal configurations, thus aiding to better design and understand the operation of centrifugal pumps handing multiple-phase flows

Leakage and Force Coefficients for Pump Annular Seals Operating With Air/Oil Mixtures: Measurements Vs Predictions and Air Injection to Increase Seal Dynamic Stiffness

LectureThe lecture presents measurements of leakage and dynamic force coefficients for six annular seals operating with an air in oil mixture ranging from pure liquid to just air. Each seal has a distinct clearance configuration: one is a plain seal with a small clearance, and another has a larger (worn) clearance ; a third seal introduces a wavy clearance that produces a significant centering stiffness; a fourth seal has a shallow groove pattern ; and the fifth and sixth seals have a stepped clearance (narrow to wide and wide to narrow). The tests in a plain seal supplied with gas injection (GVF~0?0.6) in the oil stream demonstrate the seal recovers its dynamic stiffness, hence its usage to recover rotor stability. Air injection into a liquid stream drops the mixture sound speed to make it highly compressible; hence the hardening of the seal direct stiffness

The Study of Fluorescence Thermal Measurement Based on DSP

AbstractA FFT algorithm with the DSP processor based on TMS320C5402 and fluorescence thermal measurement based on DSP are proposed. Basing on the Fast Fourier transform (FFT), the fluorescence lifetime is obtained from the tangent function of the phase angle of the first non-zeroth items of the FFT result. This method has a series of advantages such as quick calculation, high accuracy and immunity to the base signal, so the stability, real time and speediness of fluorescence thermal measurement system can be advantaged

Self-assembled platinum nanoparticles on sulfonic acid-grafted graphene as effective electrocatalysts for methanol oxidation in direct methanol fuel cells

© 2016, Nature Publishing Group. All rights reserved. In this article, sulfonic acid-grafted reduced graphene oxide (S-rGO) were synthesized using a one-pot method under mild conditions, and used as Pt catalyst supports to prepare Pt/S-rGO electrocatalysts through a self-assembly route. The structure, morphologies and physicochemical properties of S-rGO were examined in detail by techniques such as atomic force microscope (AFM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The S-rGO nanosheets show excellent solubility and stability in water and the average particle size of Pt nanoparticles supported on S-rGO is ~3.8 nm with symmetrical and uniform distribution. The electrocatalytic properties of Pt/S-rGO were investigated for methanol oxidation reaction (MOR) in direct methanol fuel cells (DMFCs). In comparison to Pt supported on high surface area Vulcan XC-72 carbon (Pt/VC) and Pt/rGO, the Pt/S-rGO electrocatalyst exhibits a much higher electrocatalytic activity, faster reaction kinetics and a better stability. The results indicate that Pt/S-rGO is a promising and effective electrocatalyst for MOR of DMFCs

Crystal chemistry and electron localization in Sn‐doped Fe3O4

For Fe3−xSnxO4 at 300 K and x≳0.1, a paramagnetic quadrupole doublet pattern, in addition to the usual two magnetic patterns, is observed in the 57Fe NGR spectra. This doublet is due to A site Fe2+ whose concentration for 0.1?x?0.5 is such that approximately equal amounts of Fe2+ and Fe3+ are on the B site. The 200 kG field at the Sn4+ site at 300 K is smaller than that in NiFe2O4. At 80 K a paramagnetic quadrupole doublet is observed in the 119Sn NGR spectrum and is due to Sn2+.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87558/2/390_1.pd

Optimal control of large quantum systems: assessing memory and runtime performance of GRAPE

Gradient Ascent Pulse Engineering (GRAPE) is a popular technique in quantum optimal control, and can be combined with automatic differentiation (AD) to facilitate on-the-fly evaluation of cost-function gradients. We illustrate that the convenience of AD comes at a significant memory cost due to the cumulative storage of a large number of states and propagators. For quantum systems of increasing Hilbert space size, this imposes a significant bottleneck. We revisit the strategy of hard-coding gradients in a scheme that fully avoids propagator storage and significantly reduces memory requirements. Separately, we present improvements to numerical state propagation to enhance runtime performance. We benchmark runtime and memory usage and compare this approach to AD-based implementations, with a focus on pushing towards larger Hilbert space sizes. The results confirm that the AD-free approach facilitates the application of optimal control for large quantum systems which would otherwise be difficult to tackle.Comment: 14 pages, 6 figures, 1 tabl