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[en] Shock Hugoniot can be used to express the response of a material to shocks, and the compression ratio of the shock can be determined by the Hugoiot locus. When the shock is strong, it will become radiating, and the radiation will affect the Hugoniot. The role of radiation on the Hugoniot condition is studied in the paper. For the radiative flux-dominated shocks, the radiative flux if large enough may render the structure of the shock Hugoniot locus totally different with the case for the pure hydrodynamic shock: the two branches with one in quadrant I and the other in quadrant III are reversed into two in quadrants IV and II, respectively, correspondingly the compression ratio may be larger than the limiting value (γ+1)/(γ-1) for ideal gases with index γ. For the radiative shock in which the radiative heat wave propagates supersonically, a threshold value for the net radiative flux to the preshock is also defined which determines whether the Hugoniot locus is reversed and the compression ratio exceeds the limiting value. Numerical results also verify the reversal of the Hugoniot locus of the shocks if the net radiative flux to the preshock exceeds the threshold value.
[en] Highlights: • The HFPE CR is self-adaptive for the change of combustion parameters. • The self-adaptive adjustment can avoid knocking or post-combustion. • The self-adaptive adjustment can extend the operation range of the HCCI combustion. • The self-adaptive features are analyzed by dynamic calculations and a prototype test. • A glow plug was used to assist the ignition of HCCI combustion in a HFPE. - Abstract: The Hydraulic Free Piston Engine (HFPE) piston motion profile is controlled by the composite force of the in-cylinder gas and hydraulic fluid. Based on the analysis of HFPE piston motion by dynamic calculations, it is found that the piston motion profile and compression ratio (CR) are self-adaptive for the variation of combustion parameters. As the combustion timing is advanced, the piston commutation time is advanced, and the CR is decreased. Further, the existence of this self-adaptive features are demonstrated by a prototype test of HFPE, and it also proves that HFPE has self-adaptive features for the variation of mixture concentration. The self-adaptive features can make HFPE effectively avoid the phenomenon of knocking or post-combustion, make it stable of the maximum in-cylinder gas pressure and maximum value of heat release rate, reduce the loss of indicated work, and keep the output power relatively stable.
[en] We apply the nonequilibrium Green's function method based on density functional theory to investigate the electronic and transport properties of waved zigzag and armchair graphene nanoribbons. Our calculations show that out-of-plane mechanical deformations have a strong influence on the band structures and transport characteristics of graphene nanoribbons. The computed I-V curves demonstrate that the electrical conductance of graphene nanoribbons is significantly affected by deformations. The relationship between the conductance and the compression ratio is found to be sensitive to the type of the nanoribbon. The results of our study indicate the possibility of mechanical control of the electronic and transport properties of graphene nanoribbons.
[en] Text is a media that is often used to convey information in both wired and wireless-based network. One limitation of the wireless system is the network bandwidth. In this study we implemented a text compression application with lossless compression technique using combination of Burrows wheeler transform, move to front, and Huffman coding methods. With the addition of the compression of the text, it is expected to save network resources. This application provides information about compression ratio. From the testing process, it concludes that text compression with only Huffman coding method will be efficient when the number of text characters are above 400 characters, meanwhile text compression with burrows wheeler transform, move to front, and Huffman coding methods will be efficient when the number of text characters are above 531 characters. Combination of these methods are more efficient than just Huffman coding when the number of text characters are above 979 characters. The more characters that are compressed and the more patterns of the same symbol, the better the compression ratio.
[en] Huygens' geometric construction explaining wave motion has a well-known problem with unphysical back-propagation due to the spherical nature of the secondary wavelets. We solve this by analytically continuing the surface of integration. If the surface is a sphere SR of radius R, this is done by complexifying R to α = R + ia. The resulting complex sphere Sα is shown to be equivalent to the real tangent disk bundle with base SR consisting of all disks with radius a tangent to SR. Huygens' secondary source points are thus replaced by disks, and his secondary wavelets by well-focused pulsed beams propagating outward. This solves the back-propagation problem. The generalized Huygens principle is a completeness relation for these pulsed-beam wavelets enabling a pulsed-beam representation of all radiation fields. Furthermore, this yields a natural and extremely efficient way to compute radiation fields numerically because all pulsed beams missing a given observer can be ignored with minimal error. Increasing the disk radius a sharpens the focus of the pulsed beams, which in turn raises the compression ratio of the representation.
[en] Highlights: • The effect of starting force and injection position on the ignition was experimented. • The large starting force does not absolutely lead to the high ignition efficiency. • Some ignition features that differ from conventional engines were revealed. • Optimal ignition efficiency was obtained at a certain injection position. - Abstract: The free-piston engine alternator (FPEA) is an unconventional engine. The initialization of the new engine differs from the conventional engine due to the feature that it eliminates the crankshaft and flywheel system. This study performed an experiment to analyze the ignition characteristics of a compression ignited diesel FPEA. Meanwhile, the effects of injection position and starting force on the ignition were also investigated. The experimental result shows that the larger starting force does not absolutely bring about higher ignition efficiency for the initialization of FPEA, although it brings in a greater possibility for the engine to gain a higher compression ratio from its resonant reciprocation. The large, but inappropriate starting force may lead to a serious incomplete combustion for the ignition, and the level of combustion completion of ignition is influenced by the injection triggering advance position (ITAP). The experiment suggests that properly enlarging the ITAP can enhance the combustion efficiency of the ignition and is beneficial to enlarge the compression ratio of opposite cylinder, but an excessive ITAP may advance the injection and ignition cycle and further induce a misfire for the FPEA.
[en] Aim: To estimate the acceptable compression ratio of full-field digital mammography (FFDM) using the Joint Photographic Experts Group (JPEG) 2000 compression algorithm. Materials and methods: Eighty cases that included images of 40 masses (20 benign, 20 malignant) and 40 microcalcifications (20 benign, 20 malignant) were collected. The images were compressed to five different lossy ratios: 20:1, 40:1, 60:1, 80:1, and 100:1, and four radiologists independently determined whether the compressed group was distinguishable from the control group. The ratio of the compressed group that was rated indistinguishable from the control group was compared for each reviewer, and the results were analysed for agreements of three or more reviewers. Results: The ability to distinguish the compressed image from the control group is given as a range across the four reviewers: 0-1.3% (0/80 to 1/80) of the 20:1, 0-2.5% (0/80 to 2/80) of the 40:1, 5-7.5% (4/80 to 6/80) of the 60:1, 10-37.5% (8/80 to 30/80) of the 80:1, and 30-87.5% (24/80 to 70/80) of the 100:1. For three compression groups (20:1, 40:1, and 60:1), three or more reviewers agreed that there was a distinguishable difference for 0/80, 0/80, and 3/80 images, respectively. Thus, the compressed images do not differ significantly from the control group (p > 0.05). However, the 80:1 and 100:1 compressed images were different for 9/80 and 29/80 images, respectively, which is significantly different from the control group (p < 0.05). Conclusion: The lossy 60:1 compression ratio for FFDM is visually identical to the control image and, therefore, potentially acceptable for primary interpretation.
[en] Knowing the engineering properties of geomaterials is imperative to make the right decision while designing and executing any geotechnical project. For the economical and safe geotechnical design, quick characterization of the compressibility properties of the cohesive soil is often desirable; these properties are indeed tedious to determine through actual tests. Therefore, correlating the consolidation parameters of the soils with its index properties has a great significance in the geotechnical engineering field. Several attempts have been made in the past to develop correlations between the consolidation parameters and index properties of the cohesive soils, within certain limitations. However, there is still a need to develop such correlations based on the extensive database, composing of unified plasticity range of soils, i.e., low to high plasticity. In the current study, 148 undisturbed soil specimens were obtained from different areas of Pakistan. Out of which 120 samples were utilized to develop correlations, and 28 samples were used to check the validity of the developed correlations. In order to enhance the index properties database, 30 more bentonite mixed soil samples were prepared and tested accordingly. Correlations to envisage different consolidation parameters such as compression index, compression ratio and coefficient of volume compressibility were developed using 150 cohesive soil samples of low to high plasticity. In addition, the performance of these developed correlations was verified on a set of 40 soil samples and compared with the performance of different correlations available in the literature. The percentage deviation in the prediction of compressibility characteristics through developed correlations in the present study was found to be very less, which endorsed the excellent reliability of the developed correlations. (author)
[en] The application of jet compressors to closed Brayton cycle MPD systems has been suggested previously. In the present paper the feasibility of jet compression for MPD systems is investigated and some preliminary test results from a single-stage jet compressor loop are given. Thepurposeoftheexperimentalprogrammeis, (1) to simulate the operation of a jet compressor under conditions comparable to those of an actual MPD plant, and (2) to study the physics of momentum transfer of two high velocity gas (or vapour) streams of very different molecular weights. A jet compressor in which both gases mix at equal forward velocity has the same theoretical compression ratio as a reversible engine. The upper limit for the compression ratio achievable for a given set of input parameters (stagnation properties of both gases) can be calculated from purely thermodynamic principles. The expressions for the performance of an ideal jet compressor were applied to two typical high temperature MPD cycles. The overall cycle efficiency is somewhat lower than obtainable with rotating machinery, but acceptable for special applications. In the experimental programme a single-stage test facility was designed and operated. The driving gas was caesium vapour, the driven gas helium. In the design of the system the objective was to simulate the temperatures, pressures and working media of an MPD plant on a small scale. The facility is capable of handling up to 12 g/sec of the primary stream (caesium) and up to 2.5 g of the secondary stream (helium). The caesium is fed by a diaphragm pump into a refractory metal boiler at a pressure of up to 8.0 atmospheres and then superheated to approximately 1700°K. The superheated vapour expands in a supersonic nozzle and transfers part of its momentum to the low temperature helium stream. The helium is circulated in a closed loop which is equipped with a throttle valve to simulate the pressure drop which would occur in an MPD generator. During some preliminary experiments the test loop was successfully operated at a caesium vapour temperature of 1590*K, and a helium pressure rise of 36% at a flow rate of 1.12 g/sec was achieved. The experimental results have been reasonably close to the expected values. Additional experimental results with the single-stage compressor will be presented. (author)
[en] Fractal image compression explores the self-similarity property of a natural image and utilizes the partitioned iterated function system (PIFS) to encode it. This technique is of great interest both in theory and application. However, it is time-consuming in the encoding process and such drawback renders it impractical for real time applications. The time is mainly spent on the search for the best-match block in a large domain pool. In this paper, a spatial correlation genetic algorithm (SC-GA) is proposed to speed up the encoder. There are two stages for the SC-GA method. The first stage makes use of spatial correlations in images for both the domain pool and the range pool to exploit local optima. The second stage is operated on the whole image to explore more adequate similarities if the local optima are not satisfied. With the aid of spatial correlation in images, the encoding time is 1.5 times faster than that of traditional genetic algorithm method, while the quality of the retrieved image is almost the same. Moreover, about half of the matched blocks come from the correlated space, so fewer bits are required to represent the fractal transform and therefore the compression ratio is also improved