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101.
双局域共振机制声子晶体带隙特性研究 总被引:1,自引:1,他引:0
提出了一种双局域共振机制,结合有限元法对该机制声子晶体结构的带隙生成机理和带隙的影响因素进行了深入分析研究。2个刚性质量体互为刚性边界和振子,由此产生了2个可以互相转化的局域共振单元。双局域共振结构的带隙位置由2个等效单自由度系统的固有频率共同决定,带隙的宽度与2个质量体密度、内振子半径、弹性介质的弹性模量密切相关。研究发现,2个质量体密度差别越大、弹性介质弹性模量越高,带隙的宽度就越大。该机制在一定程度上削弱了基体与振子的紧密耦合关系,该结构具有较大的带隙变化范围,极大地增大了带隙的宽度,为宽频声子晶体结构的研究提供了新的思路。 相似文献
102.
103.
Various diseases are diagnosed using medical imaging used for analysing internal anatomical structures. However, medical images are susceptible to noise introduced in both acquisition and transmission processes. We propose an adaptive data-driven image denoising algorithm based on an improvement of the intersection of confidence intervals (ICI), called relative ICI (RICI) algorithm. The 2D mask of the adaptive size and shape is calculated for each image pixel independently, and utilized in the design of the 2D local polynomial approximation (LPA) filters. Denoising performances, in terms of the PSNR, are compared to the original ICI-based method, as well as to the fixed sized filtering. The proposed adaptive RICI-based denoising outperformed the original ICI-based method by up to 1.32 dB, and the fixed size filtering by up to 6.48 dB. Furthermore, since the denoising of each image pixel is done locally and independently, the method is easy to parallelize. 相似文献
104.
利用参数互异的Fitzhugh-Nagumo神经元构建了含耦合时滞的无标度神经元网络模型,通过数值模拟的方法,提出研究参数异质性和耦合时滞影响下神经元网络的共振动力学.结果发现,当耦合项中不含时滞时,适中的参数异质性能够使得神经元网络对外界弱周期信号的响应达到最优,即适中的参数异质性能够诱导神经元网络的共振响应,而且异质性诱导共振对耦合强度具有鲁棒性.更重要的是,耦合时滞对参数异质性作用下神经元网络的共振特性也有着显著性影响.当时滞约为信号周期的整数倍时,神经元网络能够周期性地出现共振现象,即适当的耦合时滞能够诱导神经元网络的多重共振,而且这种现象在异质性参数的适当范围内都能明显出现. 相似文献
105.
以单晶悬臂梁压电发电装置为研究对象,在考虑压电材料非线性的情况下,利用广义Hamilton原理、Rayleigh-Ritz法、Euler-Bernoulli梁理论及压电元件恒定电场假设建立了悬臂梁压电发电装置的分布式机电耦合模型,通过数值计算分析谐振频率附近解的特性与系统参数及初始条件的关系,揭示了压电材料非线性、外激励参数对系统响应的影响规律,并通过实验验证了解析解的正确性.结果表明,压电材料的非线性特性会导致近似解的共振峰向左偏移,呈现软特性的非线性特征;当激励频率变化时,系统响应存在多解、跳跃等现象,主共振解的真正实现与初始条件的选取有关. 相似文献
106.
论述了双声学共振腔体蜗舌降低离心式风机噪声的新方法,进一步提出了双声学共振腔体蜗舌的解析模型。经实验验证,当采用双声学共振腔体蜗舌时,可降低基频噪声252dB(A)。降低一次谐波噪声163dB(A),总声压级下降97dB(A),比单声学共振腔体蜗舌总声压级多下降34dB(A),风机气动力性能基本不受影响。 相似文献
107.
Cavallaro A Fellner F Matzel KE Stadelmaier U Rupprecht T Böwing B Hohenberger W Bautz W 《Magma (New York, N.Y.)》1998,7(3):179-183
The aim of this study was to determine whether low-field magnetic resonance (MR) imaging can safely and accurately depict
inflammatory changes in patients with anal dynamic graciloplasty, in whom high-field MR imaging is contraindicated and ultrasonography
and computed tomography are inadequate. A 0.2-T field-strength MR examination was performed in six patients with anal dynamic
graciloplasty malfunction in whom reoperation was contemplated. The following sequences were applied:T
2-weighted turbo spinecho with fat saturation,T
1-weighted conventional spin-echo, and contrastenhancedT
1-weighted conventional spin-echo with fat saturation. Results indicated that none of the patients experienced relevant discomfort,
pacemaker malfunction, or electrode dislocation with low-field MR imaging. Inflammatory pelvic changes were visualized in
four patients and atrophy of the transposed gracilis muscle in another. Surgery was thus avoided in the four, who underwent
conservative treatment for their pelvic inflammation. It was concluded that these prelininary results demonstrate the feasibility
of MR imaging with a low field strength in patients with anal dynamic graciloplasty. In such patients, in whom diagnostic
imaging had been problematic, the potential for safe and accurate visualization will be a boon to treatment planning. 相似文献
108.
Viehweg P Heinig A Lampe D Buchmann J Heywang-Köbrunner SH 《Magma (New York, N.Y.)》1998,7(3):141-152
Purpose The aim of this study was to assess the value of contrast-enhanced (c.-e.) MRI in the follow-up of patients with conservatively
treated breast cancer since detection and exclusion of malignancy may interfere significantly with posttherapeutic changes
within the treated breast.
Material and methods A total of 207 patients with a history of limited surgery and radiation therapy underwent MR imaging, 40 patients were examined
0–12 months and 167 patients were examined later than 12 months after radiotherapy. Suspicious or indeterminate findings were
suggested by clinical examination or conventional imaging in 80 studies. In 127 women, MRI was performed within breast tissue
that was difficult to assess due to scarring or dense breast tissue.
Results Recurrent carcinoma was confirmed in 27 patients by surgical biopsy. All 27 carcinomas, except for one with a slow signal
increase, demonstrated early rise of signal intensity on dynamic T1-weighted contrast enhanced images. During the first year
after therapy, the diagnostic accuracy could not be improved by additional use of c.-e. MRI. Differentiation between posttherapeutic
changes and recurrent carcinoma was frequently not possible because of strong and sometimes early and ill-circumscribed enhancement.
Later than 12 months after therapy enhancement decreased significantly, thus the false positive calls could be reduced from
49 (conventional imaging) to 12 (conventional imaging plus MRI). A total of 12 of 26 recurrences and multifocality in 4/5
cases were diagnosed by MR imaging alone at this time interval.
Conclusion In the first year after therapy, c.-e. MRI is only indicated in selected cases. The results later than 12 months emphasize
that c.-e. MRI may contribute significant additional information. It allows better distinction of posttherapeutic fibrosis
from recurrent carcinoma and proved to be able to detect recurrent disease more sensitive and at an earlier stage. 相似文献
109.
J. Gauger N. G. Holzknecht C. -A. Lackerbauer H. Sittek K. E. Fiedler R. Petsch M. Reiser 《Magma (New York, N.Y.)》1996,4(2):93-104
Current studies emphasize the use of array coils to decrease noise and increase the signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR). We applied Tl-weighted and T2-weighted standard nonbreathhold spin echo (SE) sequences and Tl-weighted FLASH, TurboFLASH, T2-weighted spin-echo time (TSE), and heavily T2-weighted half Fourier acquisition single-shot TSE (HASTE) sequences during breathhold for abdominal imaging in 15 normal volunteers. The breathhold scans were performed using both a standard coil and a circular polarized array coil. We analyzed the signal intensity (SI), SNR, and CNR of abdominal organs in all sequences. SNRs increased in all cases by an overall factor of 3 due to an 8% increase in overall Sis and a 50% decrease in noise when applying the array coil. Although the array-coil FLASH sequence performed at least as well as the respective SE sequence, the SNRs of the array-coil TurboFLASH, TSE breathhold, and HASTE sequences were generally lower. We conclude that array-coil imaging significantly improves fast imaging of the abdomen. 相似文献
110.
The principal behaviorally active volatile component (ca. 90% +) of the sex pheromone glands ofLutzomyia longipalpis from Jacobina. Brazil, has been isolated and characterized as a novel homosesquiterpene with the structure 3-methyl--himachalene (C16H26). A minor component (ca. 10%) of the gland extract has also been identified as the sesquiterpene-himachalene (C15H24). This work confirms that there are at least 3 members of theL. longipalpis species complex. 相似文献