67,007 research outputs found
Stellar collisions in galactic centers: black hole growth and color gradients
We study the effects of stellar collisions, particularly on feeding massive
black holes (BHs) and color gradients, in realistic galactic centers. We find
that the mass released by stellar collisions is not sufficient to account for
the present BH mass in galactic centers, especially in bright galaxies. This
study, together with the study by Magorrian & Tremaine (1999) on tidal
disruption of stars by massive BHs, implies that the material for BH growth
(especially in galaxies brighter than ~10^9 Lsun) can only come from other
sources, for example, the mass released by stellar evolution in the initial ~1
Gyr of the galaxy's lifetime, or the gas that sinks to the galactic center in a
galaxy merger. We also analyze how the color of a stellar system is affected by
collisions of stars. We find that collisions between main-sequence stars cannot
cause observable color gradients in the visible bands at projected radius
R>0.1" in M31, M32 and other nearby galactic centers. This result is consistent
with the lack of an observable color gradient in M32 at R>0.1". At even smaller
radii, the color differences caused by collisions between main-sequence stars
are at most 0.08 mag at R=0.02". The averaged blueing due to stellar collisions
in the region R<0.1" of M32 should not be larger than 0.06 mag in color index
U-V and 0.02 mag in V-I. The observed blueing in the center of the galaxy M31
(in a 0.14"x0.14" box) must be caused by some mechanism other than collisions
between main-sequence stars.Comment: 27 pages, including 10 figures, MNRAS in pres
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Study on Actuator Line Modeling of Two NREL 5-MW Wind Turbine Wakes
The wind turbine wakes impact the efficiency and lifespan of the wind farm. Therefore, to improve the wind plant performance, research on wind plant control is essential. The actuator line model (ALM) is proposed to simulate the wind turbine efficiently. This research investigates the National Renewable Energy Laboratory 5 Million Watts (NREL 5-MW) wind turbine wakes with Open Field Operation and Manipulation (OpenFOAM) using ALM. Firstly, a single NREL 5-MW turbine is simulated. The comparison of the power and thrust with Fatigue, Aerodynamics, Structures, and Turbulence (FAST) shows a good agreement below the rated wind speed. The information relating to wind turbine wakes is given in detail. The top working status is proved at the wind speed of 8 m/s and the downstream distance of more than 5 rotor diameters (5D). Secondly, another case with two NREL 5-MW wind turbines aligned is also carried out, in which 7D is validated as the optimum distance between the two turbines. The result also shows that the upstream wind turbine has an obvious influence on the downstream one
Transmission of Water Waves under Multiple Vertical Thin Plates
The transmission of water waves under vertical thin plates, e.g., offshore floating breakwaters, oscillating water column wave energy converters, and so on, is a crucial feature that dominates the hydrodynamic performance of marine devices. In this paper, the analytical solution to the transmission of water waves under multiple 2D vertical thin plates is firstly derived based on the linear potential theory. The influences of relevant parameters on the wave transmission are discussed, which include the number of plates, the draft of plates, the distance between plates and the water depth. The analytical results suggest that the transmission of progressive waves gradually weakens with the growth of the number and draft of plates, and under the conditions of given number and draft of plates, the distribution of plates has significant influence on the transmission of progressive waves. The results of this paper contribute to the understanding of the transmission of water waves under multiple vertical thin plates, as well as the suggestion on optimal design of complex marine devices, such as a floating breakwater with multiple plates
Study of and decays
We study the charmless two-body
and three-body decays. We obtain to agree with the
recent LHCb measurement. However, we find that is unable to explain the
LHCb observation of , which implies the possibility for other
contributions, such as that from the resonant decay with as a higher-wave baryon state. For
, we show that ,
which are consistent with the current data of , respectively. Our results also support the relation of ,
given by the previous study.Comment: 8 pages, 1 figure, revised version accepted by EPJ
Design of a 2.4 GHz High-Performance Up-Conversion Mixer with Current Mirror Topology
In this paper, a low voltage low power up-conversion mixer, designed in a Chartered 0.18 μm RFCMOS technology, is proposed to realize the transmitter front-end in the frequency band of 2.4 GHz. The up-conversion mixer uses the current mirror topology and current-bleeding technique in both the driver and switching stages with a simple degeneration resistor. The proposed mixer converts an input of 100 MHz intermediate frequency (IF) signal to an output of 2.4 GHz radio frequency (RF) signal, with a local oscillator (LO) power of 2 dBm at 2.3 GHz. A comparison with conventional CMOS up-conversion mixer shows that this mixer has advantages of low voltage, low power consumption and high-performance. The post-layout simulation results demonstrate that at 2.4 GHz, the circuit has a conversion gain of 7.1 dB, an input-referred third-order intercept point (IIP3) of 7.3 dBm and a noise figure of 11.9 dB, while drawing only 3.8 mA for the mixer core under a supply voltage of 1.2 V. The chip area including testing pads is only 0.62×0.65 mm2
Parameter Sensitivity Analysis of Social Spider Algorithm
Social Spider Algorithm (SSA) is a recently proposed general-purpose
real-parameter metaheuristic designed to solve global numerical optimization
problems. This work systematically benchmarks SSA on a suite of 11 functions
with different control parameters. We conduct parameter sensitivity analysis of
SSA using advanced non-parametric statistical tests to generate statistically
significant conclusion on the best performing parameter settings. The
conclusion can be adopted in future work to reduce the effort in parameter
tuning. In addition, we perform a success rate test to reveal the impact of the
control parameters on the convergence speed of the algorithm
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