204 research outputs found
Heating and cooling of coronal loops observed by SDO
Context: One of the most prominent processes suggested to heat the corona to
well above 10^6 K builds on nanoflares, short bursts of energy dissipation.
Aims: We compare observations to model predictions to test the validity of
the nanoflare process.
Methods: Using extreme UV data from AIA/SDO and HMI/SDO line-of-sight
magnetograms we study the spatial and temporal evolution of a set of loops in
active region AR 11850.
Results: We find a transient brightening of loops in emission from Fe xviii
forming at about 7.2 MK while at the same time these loops dim in emission from
lower temperatures. This points to a fast heating of the loop that goes along
with evaporation of material that we observe as apparent upward motions in the
image sequence. After this initial phases lasting for some 10 min, the loops
brighten in a sequence of AIA channels showing cooler and cooler plasma,
indicating the cooling of the loops over a time scale of about one hour. A
comparison to the predictions from a 1D loop model shows that this observation
supports the nanoflare process in (almost) all aspects. In addition, our
observations show that the loops get broader while getting brighter, which
cannot be understood in a 1D model.Comment: 9 pages, 7 figures, accepted by A&
Statistics of Flares Sweeping across Sunspots
Flare ribbons are always dynamic, and sometimes sweep across sunspots.
Examining 588 (513 M-class and 75 X-class) flare events observed by Transition
Region and Coronal Explorer (TRACE) satellite and Hinode Solar Optical
Telescope (SOT) from 1998 May to 2009 May, we choose the event displaying that
one of the flare ribbons completely sweeps across the umbra of a main sunspot
of the corresponding active region, and finally obtain 20 (7 X-class and 13
M-class) events as our sample. In each event, we define the main sunspot
completely swept across by the flare ribbon as A-sunspot, and its nearby
opposite polarity sunspots, B-sunspot. Observations show that the A-sunspot is
a following polarity sunspot in 18 events, and displays flux emergence in 13
cases. All the B-sunspots are relatively simple, exhibiting either one main
sunspot or one main sunspot and several small neighboring sunspots (pores). In
two days prior to the flare occurrence, the A-sunspot rotates in all the cases,
while the B-sunspot, in 19 events. The total rotating angle of the A-sunspot
and B-sunspot is 193 degrees on average, and the rotating directions, are the
same in 12 events. In all cases, the A-sunspot and B-sunspot manifest shear
motions with an average shearing angle of 28.5 degrees, and in 14 cases, the
shearing direction is opposite to the rotating direction of the A-sunspot. We
suggest that the emergence, the rotation and the shear motions of the A-sunspot
and B-sunspot result in the phenomenon that flare ribbons sweep across sunspots
completely.Comment: 12 pages, 3 figures, 2 tables, accepted by ApJ Letter
Conversion from mutual helicity to self-helicity observed with IRIS
Context. In the upper atmosphere of the Sun observations show convincing
evidence for crossing and twisted structures, which are interpreted as mutual
helicity and self-helicity. Aims. We use observations with the new Interface
Region Imaging Spectrograph (IRIS) to show the conversion of mutual helicity
into self-helicity in coronal structures on the Sun. Methods. Using far UV
spectra and slit-jaw images from IRIS and coronal images and magnetograms from
SDO, we investigated the evolution of two crossing loops in an active region,
in particular, the properties of the Si IV line profile in cool loops. Results.
In the early stage two cool loops cross each other and accordingly have mutual
helicity. The Doppler shifts in the loops indicate that they wind around each
other. As a consequence, near the crossing point of the loops (interchange)
reconnection sets in, which heats the plasma. This is consistent with the
observed increase of the line width and of the appearance of the loops at
higher temperatures. After this interaction, the two new loops run in parallel,
and in one of them shows a clear spectral tilt of the Si IV line profile. This
is indicative of a helical (twisting) motion, which is the same as to say that
the loop has self-helicity. Conclusions. The high spatial and spectral
resolution of IRIS allowed us to see the conversion of mutual helicity to
self-helicity in the (interchange) reconnection of two loops. This is
observational evidence for earlier theoretical speculations.Comment: A&A, in pres
Interaction between a fast rotating sunspot and ephemeral regions as the origin of the major solar event on 2006 December 13
The major solar event on 2006 December 13 is characterized by the
approximately simultaneous occurrence of a heap of hot ejecta, a great
two-ribbon flare and an extended Earth-directed coronal mass ejection. We
examine the magnetic field and sunspot evolution in active region NOAA AR
10930, the source region of the event, while it transited the solar disk centre
from Dec. 10 to Dec. 13. We find that the obvious changes in the active region
associated with the event are the development of magnetic shear, the appearance
of ephemeral regions and fast rotation of a smaller sunspot. Around the area of
the magnetic neutral line of the active region, interaction between the fast
rotating sunspot and the ephemeral regions triggers continual brightening and
finally the major flare. It is indicative that only after the sunspot rotates
up to 200 does the major event take place. The sunspot rotates at
least 240 about its centre, the largest sunspot rotation angle which
has been reported.Comment: 4 pages, 6 figures, ApJ Letters inpres
Numerical Investigation of Two-stage Vapor Compression System with Simultaneous Vapor and Liquid Injection
Two-stage vapor compression systems can be advantageous over single-stage systems by providing improved system performance, lower discharge temperature and reduced throttling losses. These systems employ various intermediate configurations, such as liquid injection and vapor injection. This paper presents a configuration for a R1234ze(E) two-stage air-conditioning system using a turbo compressor with two injection ports, one for vapor injection and the second for liquid injection. The liquid injection is used to cool the motor and electronics. A component-based representation and solution approach was used to simulate the two-stage compression system with simultaneous vapor and liquid injection at steady state. The turbo compressor was represented using a customized performance map. The condenser and the evaporator were modeled using finite-volume approach. A parametric study was conducted to assess the impact of the following three variables on the system performance: vapor injection ratio, condenser air flow rate, and discharge pipe pressure drop. The simulation results show that as the vapor injection ratio increased, the system performance undergoes a tradeoff between an enhanced subcooling effect (and thus enhanced unit refrigeration capacity) and a decreased suction mass flow rate. Maximum COP occurs when the vapor injection ratio was 0.1. The results also show that as the condenser air flow rate increased, both the capacity and power consumption (including fan power) increased monotonically, and COP increased first and then decreased. At 75% load, the COP improvement at the optimum flow rate was marginally less than 0.5%. Lastly, higher discharge pipe pressure drop increased the discharge pressure. It showed very small effect on the overall system performance at the condition selected for the current study
Computational fluid dynamics-based hull form optimization using approximation method
With the rapid development of the computational technology, computational fluid dynamics (CFD) tools have been widely used to evaluate the ship hydrodynamic performances in the hull forms optimization. However, it is very time consuming since a great number of the CFD simulations need to be performed for one single optimization. It is of great importance to find a high-effective method to replace the calculation of the CFD tools. In this study, a CFD-based hull form optimization loop has been developed by integrating an approximate method to optimize hull form for reducing the total resistance in calm water. In order to improve the optimization accuracy of particle swarm optimization (PSO) algorithm, an improved PSO (IPSO) algorithm is presented where the inertia weight coefficient and search method are designed based on random inertia weight and convergence evaluation, respectively. To improve the prediction accuracy of total resistance, a data prediction method based on IPSO-Elman neural network (NN) is proposed. Herein, IPSO algorithm is used to train the weight coefficients and self-feedback gain coefficient of ElmanNN. In order to build IPSO-ElmanNN model, optimal Latin hypercube design (Opt LHD) is used to design the sampling hull forms, and the total resistance (objective function) of these hull forms are calculated by Reynolds averaged Navier–Stokes (RANS) method. For the purpose of this paper, this optimization framework has been employed to optimize two ships, namely, the DTMB5512 and WIGLEY III ships, and these hull forms are changed by arbitrary shape deformation (ASD) technique. The results show that the optimization framework developed in this study can be used to optimize hull forms with significantly reduced computational effort
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