排序方式: 共有24条查询结果,搜索用时 31 毫秒
1.
R.Babaei N.Hatami K.Asgari N.Varahram P.Davami 《铸造》2006,55(4):373-377
铸造过程中熔融金属内的固体渣粒的运动轨迹是非常重要的。文中给出了粒子传输轨迹的数学模型,该数学模型采用显示法来确定粒子的位置。模拟研究结果与用其它数学模型的模拟结果比较,吻合良好。该模型可用于固体粒子位置的确定,并应用到SUTCAST软件中。 相似文献
2.
Pooria Varahram Somayeh Mohammady Borhanuddin Mohd Ali 《Wireless Personal Communications》2013,72(2):1125-1137
Peak-to-average power ratio (PAPR) is one of the main drawbacks in orthogonal frequency division multiplexing (OFDM) systems. High PAPR forces the power amplifier to back off in order to operate in its linear region, which degrades the power efficiency of the system. Several PAPR reduction techniques have been developed, but most of them have not considered both complexity and PAPR reduction. In this paper, a novel PAPR reduction scheme based on the insertion of dummy sequences to an enhanced partial transmit sequence is proposed. By applying this scheme the PAPR performance is enhanced compared to the conventional methods while the complexity is significantly reduced. Numerical analysis is carried out with OFDM signal and QPSK modulation. 相似文献
3.
A. Varahram B. Breidenstein T. Hassel F.-W. Bach H.J. Maier 《Forschung im Ingenieurwesen》2014,78(3-4):145-149
Exploration for deep-geothermal resources is still a costly and high risk operation. The research association “Geothermal Energy and High Performance Drilling Research Program, gebo” was founded to improve the cost effectiveness of geothermal energy production in particular by developing new drilling concepts for deep geological layers. A mono-diameter borehole casing concept is developed to achieve this ambitious aim. To accomplish this, a special type of casing is essential, structured from folded tubulars for expandable casing applications featuring clover-like cross sections and expandable to a cylindrical form when the casing is at its downhole position. A smaller borehole volume and a reduction of drilling time and casing related costs are the main advantages of this method compared to conventional drilling. Design, material selection and realization of the folded tubulars take place at the Institut für Werkstoffkunde (Materials Science). The evaluations of the results are based on residual stress analysis and the measurement of the geometry after each deformation step at the Institute of Production Engineering and Machine Tools (IFW). 相似文献
4.
Measurements of austempering kinetics and mechanical properties are presented as a function of austempering time over the
range 1–4320 min for different combinations of austempering temperature (275, 315, 370 and 400 °C) and austenitizing temperature
(870, 900 and 950 °C) for a ductile iron of composition 3.5% C, 2.6% Si, 0.48% Cu, 0.96% Ni, 0.27% Mo and 0.25% Mn. The austempering
kinetics are used to calculate processing windows for the three austenitizing temperatures. The mechanical properties are
analysed to show that the processing windows accurately predict the austempering times over which the mechanical properties
satisfy the ASTM standard. The analysis shows the role of austenitizing temperature, austempering temperature and time in
optimizing the mechanical properties.
This revised version was published online in November 2006 with corrections to the Cover Date. 相似文献
5.
R. Hadian M. Emamy N. Varahram N. Nemati 《Materials Science and Engineering: A》2008,490(1-2):250-257
The effects of both Li modification and cooling rate on the microstructure and tensile properties of an in situ prepared Al–15%Mg2Si composite were investigated. It was found that the addition of 0.3%Li reduces the average size of Mg2Si primary particles from 30 to 6 μm. The effect of cooling rate was investigated by the use of a mold with different section test bars. The results showed an increase in both UTS and elongation values with reduction in section thicknesses corresponding to increasing cooling rates. Adding Li also raised the tensile strength and elongation values and reduced the number of decohered particles observed in fracture surfaces thereby increasing the alloy's ductility. Data scatter and unexpected low tensile values of 3 mm sections were attributed to casting defects observed in fracture surfaces. Large clusters of Mg2Si particles and eutectic cell boundaries were found to be potential crack propagation paths in this alloy. 相似文献
6.
In this paper the peak to average power ratio (PAPR) reduction and digital predistortion effects in orthogonal frequency division multiplexing (OFDM) systems are investigated. By applying a predistortion technique called complex gain memory predistortion (CGMP), power amplifier works at higher power efficiency. The proposed enhanced partial transmit sequence scheme is applied for PAPR reduction and integration with CGMP technique results in increasing in OFDM system efficiency and prolonged battery life. Simulation and results are examined with actual power amplifier and OFDM signal with quadrature phase shift keying (QPSK) modulation. 相似文献
7.
Ahmad Bahmani Nader Hatami Naser Varahram Parviz Davami Mohsen Ostad Shabani 《The International Journal of Advanced Manufacturing Technology》2013,64(9-12):1313-1321
In the present work, a mathematical model was developed based on finite difference method to predict the microporosity distribution in A356 aluminum alloy casting. Heat, mass, and gas conservation equations were solved in this model. Moreover, Darcy’s equation was considered in the mushy zone. Results show that the distribution and concentration of microporosities in cast parts vary with both cooling rate and initial gas content. Simulation results were compared with experimental data where proportionally good agreement with experimental results was found. Finally, a complex cast part was simulated presenting the ability of the model to predict the porosities in industrial cast parts. 相似文献
8.
Pooria Varahram Borhanuddin Mohd Ali 《International Journal of Communication Systems》2012,25(4):543-552
In this paper, the effects of peak‐to‐average power ratio reduction in nonlinear power amplifiers (PAs) by considering memory effects and of digital predistortion are investigated. A new predistortion technique is proposed, which is called the complex gain memory predistortion (CGMP) method. The CGMP is applied to compensate the dynamic memory effects of PAs. The conventional partial transmit sequence method is applied for peak‐to‐average power ratio reduction, and combining it with the CGMP results in efficiency enhancement and spectrum efficiency improvement. Simulation and results are examined with the two types of PAs and with an OFDM signal with quadrature phase‐shift keying modulation. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
9.
Khalid Al-Hussaini Borhanuddin M. Ali Pooria Varahram Shaiful Jahari Hashim Ronan Farrell 《Telecommunication Systems》2017,65(2):297-307
In this paper, we propose a novel technique to reduce the crest factor (CF) in orthogonal frequency division multiplexing systems. It consists of two inverse fast Fourier transform (IFFT) blocks, the input symbols of the first IFFT are the mapped symbols, whereas the input symbols of the second IFFT are the summations of the absolute value of the real part of the outer signal constellation points and zero symbols. First, the output of the two IFFT blocks is partitioned into four subblocks, which are subsequently used to rearrange the subblocks with padding zeros in a specific manner. Then, a new optimization scheme is introduced, in which only a single two-phase sequence and four iterations need to be applied. Numerical analysis shows that the proposed hybrid technique achieves better CF reduction performance with significantly lower complexity and better bit error rate performance than the existing scrambling (multiplicative) and additive CF techniques. 相似文献
10.
Emad Galin Moghaddam Neda Karimzadeh Naser Varahram Parviz Davami 《Metallurgical and Materials Transactions A》2013,44(8):3826-3835
Particulate VC-reinforced high-manganese austenitic steel matrix composites with different vanadium and tungsten contents were synthesized by conventional alloying and casting route. Microstructural characterizations showed that the composites processed by in situ precipitation of the reinforcements were composed of V8C7 particulates distributed in an austenitic matrix. It was observed that addition of tungsten to austenite increases work-hardening rate of subsurface layer during pin-on disk wear test. The maximum abrasive wear resistance was achieved at tungsten content equal to 2 wt pct. However, excessive addition of tungsten promoted the formation of W3C phase and reduced the abrasive wear resistance because of decrease in distribution homogeneity and volume fraction of the reinforcing VC particles. 相似文献