Visualization of the homogeneous charge compression ignition/controlled autoignition combustion process using two-dimensional planar laser-induced fluorescence imaging of formaldehyde

The paper reports an investigation into the HCCI/CAI combustion process using the two-dimensional PLIF technique. The PLIF of formaldehyde formed during the low-temperature reactions of HCCI/CAI combustion was exciting by a tunable dye laser at 355nm wavelength and detected by a gated ICCD camera. Times and locations of the two-stage autoignition of HCCI/CAI combustion were observed in a single cylinder optical engine for several fuel blends mixed with n-heptane and iso-octane. The results show, when pure n-heptane was used, the initial formation of formaldehyde and its subsequent burning were closely related to the start of the low temperature heat release stage and the start of the main heat release stage of HCCI combustion respectively. Meanwhile, it was found that the formation of formaldehyde was more affected by the charge temperature than by the fuel concentration. But its subsequent burning or the start of main heat release combustion toke place at those areas where both the fuel concentration and the charge temperature were sufficient high. As a result, it was found that the presence of stratified residual gases affected both the spatial location and the temporal site of autoignition in a HCCI/CAI combustion engine. All studied fuels were found having similar formaldehyde formation timings with n-heptane. This means that the presence of iso-octane did not affect the start of low temperature reactions apparently. However, the heat release during low temperature reaction was significantly reduced with the presence of iso-octane in the studied fuels. In addition, the presence of iso-octane retarded the start of the main combustion stage

Self-similarity in a system with short-time delayed feedback

Using the Poincar\'{e} section technique, we study in detail the dynamical behaviors of delay differential system and find a new type of solutions $S_i$ in short-time delay feedback. Our numerical results remind us to deny the opinion that there are no complex phenomena in short-time delay case. Many similarities between foundamental solution and the new type of solutions are found. We demonstrate that the scales of $S_i$ increase with exponential growth via $i$ in the direction of $\mu$, while decrease with exponential decays in the direction of $x$ or delay time $t_R$.Comment: 4 pages, REVTEX, 4 ps figures, to be published in Phys. Lett.

RR Lyrae in XSTPS: The halo density profile in the North Galactic Cap

We present a catalog of RR Lyrae stars (RRLs) observed by the Xuyi Schmidt Telescope Photometric Survey (XDSS). The area we consider is located in the North Galactic Cap, covering 376.75 sq deg at RA $\approx$ 150 deg and Dec $\approx$ 27 deg down to a magnitude limit of i $\approx$ 19. Using the variability information afforded by the multi-epoch nature of our XDSS data, combined with colors from the Sloan Digital Sky Survey, we are able to identify candidate RRLs. We find 318 candidates, derive distances to them and estimate the detection efficiency. The majority of our candidates have more than 12 observations and for these we are able to calculate periods. These also allows us to estimate our contamination level, which we predict is between 30% to 40%. Finally we use the sample to probe the halo density profile in the 9-49 kpc range and find that it can be well fitted by a double power law. We find good agreement between this model and the models derived for the South Galactic Cap using the Watkins et al. (2009) and Sesar et al. (2010) RRL data-sets, after accounting for possible contamination in our data-set from Sagittarius stream members. We consider non-spherical double power law models of the halo density profile and again find agreement with literature data-sets, although we have limited power to constrain the flattening due to our small survey area. Much tighter constraints will be placed by current and future wide-area surveys, most notably ESA's astrometric Gaia mission. Our analysis demonstrates that surveys with a limited number of epochs can effectively be mined for RRLs. Our complete sample is provided as accompanying online material.Comment: 14 pages, ApJ (in press

Using transfer ratio to evaluate EMC design of adjustable speed drive systems

This paper proposes a way to evaluate the conducted electromagnetic compatibility performance of variable speed drive systems. It is considered that the measured noise level is determined by two factors, the level of the noise source and the conversion efficiency of the propagation path from the source to the measurement equipments. They are corresponding to the two roles played by the converter. On the one hand, a converter provides the noise source and generates the noise current and voltage on the motor side with the cable and the motor. On the other hand, it acts as the propagation path with the DC bus and the rectifier to spread the noise generated on the motor side to the line side. The transfer ratio is defined as the ratio between the CM current on the motor side and the CM current on the line side. It can be used to evaluate the EMC design of a converter because it is independent of the cable and the motor. A simplified model is used to explain this characteristic. It can be measured when the converter is powered off. Verification is carried out by experimental results obtained from a 12-kVA laboratory system.\u

A three dimensional extinction map of the Galactic Anticentre from multi-band photometry

We present a three dimensional extinction map in $r$ band. The map has a spatial angular resolution, depending on latitude, between 3 -- 9\,arcmin and covers the entire XSTPS-GAC survey area of over 6,000\,$\rm deg^2$ for Galactic longitude $\rm 140 \leq$ $l$ $\leq 220\deg$ and latitude $\rm -40\leq$ $b$ $\leq 40\deg$. By cross-matching the photometric catalog of the Xuyi Schmidt Telescope Photometric Survey of the Galactic Anticentre (XSTPS-GAC) with those of 2MASS and WISE, we have built a multi-band photometric stellar sample of about 30 million stars and applied spectral energy distribution (SED) fitting to the sample. By combining photometric data from the optical to the near-infrared, we are able to break the degeneracy between the intrinsic stellar colours and the amounts of extinction by dust grains for stars with high photometric accuracy, and trace the extinction as a function of distance for low Galactic latitude and thus highly extincted regions. This has allowed us to derive the best-fit extinction and distance information of more than 13 million stars, which are used to construct the three dimensional extinction map. We have also applied a Rayleigh-Jeans colour excess (RJCE) method to the data using the 2MASS and WISE colour $(H-W2)$. The resulting RJCE extinction map is consistent with the integrated two dimensional map deduced using the best-fit SED algorithm. However for individual stars, the amounts of extinction yielded by the RJCE method suffer from larger errors than those given by the best-fit SED algorithm.Comment: 20 pages, 18 figures, accepted in MNRA

Loss of mass and stability of galaxies in MOND

The self-binding energy and stability of a galaxy in MOND-based gravity are curiously decreasing functions of its center of mass acceleration towards neighbouring mass concentrations. A tentative indication of this breaking of the Strong Equivalence Principle in field galaxies is the RAVE-observed escape speed in the Milky Way. Another consequence is that satellites of field galaxies will move on nearly Keplerian orbits at large radii (100 - 500 kpc), with a declining speed below the asymptotically constant naive MOND prediction. But consequences of an environment-sensitive gravity are even more severe in clusters, where member galaxies accelerate fast: no more Dark-Halo-like potential is present to support galaxies, meaning that extended axisymmetric disks of gas and stars are likely unstable. These predicted reappearance of asymptotic Keplerian velocity curves and disappearance of "stereotypic galaxies" in clusters are falsifiable with targeted surveys.Comment: 4 pages, 2 figures, ApJ Letter

Refining MOND interpolating function and TeVeS Lagrangian

The phenomena customly called Dark Matter or Modified Newtonian Dynamics (MOND) have been argued by Bekenstein (2004) to be the consequences of a covariant scalar field, controlled by a free function (related to the MOND interpolating function) in its Lagrangian density. In the context of this relativistic MOND theory (TeVeS), we examine critically the interpolating function in the transition zone between weak and strong gravity. Bekenstein's toy model produces too gradually varying functions and fits rotation curves less well than the standard MOND interpolating function. However, the latter varies too sharply and implies an implausible external field effect (EFE). These constraints on opposite sides have not yet excluded TeVeS, but made the zone of acceptable interpolating functions narrower. An acceptable "toy" Lagrangian density function with simple analytical properties is singled out for future studies of TeVeS in galaxies. We also suggest how to extend the model to solar system dynamics and cosmology, and compare with strong lensing data (see also astro-ph/0509590).Comment: accepted for publication in ApJ Letter