A driblet of magnetic fluids (MFs) falls on an annular magnet, forming a closed liquid ring. The magnetized MFs can produce liquid support due to magnetostatic force. The air cushion enclosed by the ...MFs sealing ring may generate gas support as the magnet bottom combines with a substrate. The supporting capacity supplied by the liquid-gas contributes to friction reduction. Research shows such supporting is affected by the surface magnetic field and field distribution. Tribological results confirm that low friction can be obtained since the tribo-pairs are separated by the supporting force and the friction originates from the fluid viscosity. Such design would be significant for solving the “cold welding” as well as the “stick-slip” phenomenon, especially in precise sliding machine.
•Gas supporting force is obtained based on magnetic fluids seal.•The supporting force is mainly determined by the sealing capacity.•Effects of magnetic field intensity and distribution on the force were studied.•Direct contact between friction pairs can be avoided due to the supporting force.•Ultralow friction and absence of stick-slip are achieved based on such construction.
Temperature distribution in the metallic plate influenced by high frequency electromagnetic wave depends on thermal and electrical properties of the plate material. The goal of the paper is to ...present the significance of the temperature dependence of the electrical resistivity and its influence on the plate heating. A mathematical model of the problem was established and an analytical closed-form solution the problem was presented. The active power of the electromagnetic wave was calculated by Poynting vector. Temperature field was obtained by integral-transform technique. Numerical examples were presented for three different materials (copper, aluminum, and steel). Calculated results show the large influence of temperature dependent electrical resistance on heating of the metallic plate.
Magnetic fields measured by Voyager 1 show that the spacecraft crossed or was crossed by the termination shock on about 16 December 2004 at 94.0 astronomical units. An estimate of the compression ...ratio of the magnetic field strength B (± standard error of the mean) across the shock is B₂/B₁ = 3.05 ± 0.04, but ratios in the range from 2 to 4 are admissible. The average B in the heliosheath from day 1 through day 110 of 2005 was 0.136 ± 0.035 nanoteslas, approximately4.2 times that predicted by Parker's model for B. The magnetic field in the heliosheath from day 361 of 2004 through day 110 of 2005 was pointing away from the Sun along the Parker spiral. The probability distribution of hourly averages of B in the heliosheath is a Gaussian distribution. The cosmic ray intensity increased when B was relatively large in the heliosheath.
Purpose: Hemodynamics is important in the initiation, growth, and rupture of intracranial aneurysms. Since intracranial aneurysms are small, a high-field MR system with high spatial resolution and ...high SNR is desirable for this hemodynamic analysis. The purpose of this study was to investigate whether the accuracy of MR fluid dynamic (MRFD) results based on 3D phase-contrast MR (3D PC MR, non-electrocardiogramECG-gated 4D Flow MRI) data from a human cerebrovascular phantom and human healthy subjects obtained by a 7T MR system was superior to those by a 3T MR system.Methods: 3D PC MR and 3D time of flight MR angiography (3D TOF MRA) imaging were performed on a 3T MR system and a 7T MR system for a human cerebrovascular phantom and 10 healthy human subjects, and MRFD analysis was performed using these data. The MRFD results from each MR system were then compared with the following items based on the computational fluid dynamics (CFD) results: 3D velocity vector field; correlation coefficient (R), angular similarity index (ASI), and magnitude similarity index (MSI) of blood flow velocity vectors.Results: In the MRFD results of 3D velocity vectors of the cerebrovascular phantom, noise-like vectors were observed near the vascular wall on the 3T MR system, but no noise was observed on the 7T MR system, showing results similar to those of CFD. In the MRFD results of the cerebrovascular phantom and healthy subjects, the correlation coefficients R, ASI, and MSI of the 7T MR system were higher than those of the 3T MR system, and ASI and MSI of healthy human subjects were significantly different between the two systems.Conclusions: The accuracy of high spatial resolution MRFD using the 7T MR system exceeded that of the 3T MR system.
A magnetorheological fluid (MRF) performs differently under different magnetic field strength. This study examined the fretting friction and wear characteristics of MRFs under a range of magnetic ...field strengths and oscillation frequencies. The fretting friction and wear behaviors of MRF are investigated using a fretting friction and wear tester. The surfaces of specimen are examined by optical microscopy and 3D surface profilometer before and after the tests and wear surface profiles, the wear volume loss and wear coefficient for each magnetic field strength are evaluated. The results show that the friction and wear properties of MRF change according to the magnetic field strength and oscillation frequency.
•Fretting friction and wear characteristics of MRF is examined.•The friction coefficients increased with increasing magnetic field strength.•The coefficient of friction decreased with increasing oscillation frequency.•Wear volume and coefficient become worse with increasing magnetic field strength.
Previous studies have shown that under some conditions, noise fluctuations in an fMRI time-course are dominated by physiological modulations of the image intensity with secondary contributions from ...thermal image noise and that these two sources scale differently with signal intensity, susceptibility weighting (TE) and field strength. The SNR of the fMRI time-course was found to be near its asymptotic limit for moderate spatial resolution measurements at 3 T with only marginal gains expected from acquisition at higher field strengths. In this study, we investigate the amplitude of image intensity fluctuations in the fMRI time-course at magnetic field strengths of 1.5 T, 3 T, and 7 T as a function of image resolution, flip angle and TE. The time-course SNR was a similar function of the image SNR regardless of whether the image SNR was modulated by flip angle, image resolution, or field strength. For spatial resolutions typical of those currently used in fMRI (e.g., 3 × 3 × 3 mm
3), increases in image SNR obtained from 7 T acquisition produced only modest increases in time-course SNR. At this spatial resolution, the ratio of physiological noise to thermal image noise was 0.61, 0.89, and 2.23 for 1.5 T, 3 T, and 7 T. At a resolution of 1 × 1 × 3 mm
3, however, the physiological to thermal noise ratio was 0.34, 0.57, and 0.91 for 1.5 T, 3 T and 7 T for TE near T2*. Thus, by reducing the signal strength using higher image resolution, the ratio of physiologic to image noise could be reduced to a regime where increased sensitivity afforded by higher field strength still translated to improved SNR in the fMRI time-series.
Penumbral Structure and Outflows in Simulated Sunspots Rempel, M.; Schüssler, M.; Cameron, R. H. ...
Science (American Association for the Advancement of Science),
07/2009, Letnik:
325, Številka:
5937
Journal Article
Recenzirano
Sunspots are concentrations of magnetic field on the visible solar surface that strongly affect the convective energy transport in their interior and surroundings. The filamentary outer regions ...(penumbrae) of sunspots show systematic radial outward flows along channels of nearly horizontal magnetic field. These flows were discovered 100 years ago and are present in all fully developed sunspots. By using a comprehensive numerical simulation of a sunspot pair, we show that penumbral structures with such outflows form when the average magnetic field inclination to the vertical exceeds about 45 degrees. The systematic outflows are a component of the convective flows that provide the upward energy transport and result from anisotropy introduced by the presence of the inclined magnetic field.
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