全文获取类型
收费全文 | 771篇 |
免费 | 15篇 |
专业分类
公路运输 | 96篇 |
综合类 | 161篇 |
水路运输 | 449篇 |
铁路运输 | 60篇 |
综合运输 | 20篇 |
出版年
2024年 | 2篇 |
2023年 | 2篇 |
2022年 | 8篇 |
2021年 | 16篇 |
2020年 | 16篇 |
2019年 | 9篇 |
2018年 | 1篇 |
2017年 | 29篇 |
2016年 | 27篇 |
2015年 | 45篇 |
2014年 | 55篇 |
2013年 | 47篇 |
2012年 | 63篇 |
2011年 | 75篇 |
2010年 | 67篇 |
2009年 | 69篇 |
2008年 | 61篇 |
2007年 | 60篇 |
2006年 | 48篇 |
2005年 | 32篇 |
2004年 | 16篇 |
2003年 | 9篇 |
2002年 | 10篇 |
2001年 | 3篇 |
2000年 | 2篇 |
1999年 | 2篇 |
1998年 | 2篇 |
1997年 | 1篇 |
1996年 | 2篇 |
1994年 | 1篇 |
1992年 | 3篇 |
1990年 | 1篇 |
1988年 | 1篇 |
1987年 | 1篇 |
排序方式: 共有786条查询结果,搜索用时 8 毫秒
781.
随着声呐信号处理技术的发展,其对系统的软、硬件资源要求越来越高.利用数字下变频技术,可极大地降低其信号处理资源要求.结合Matlab,仿真研究整个下变频过程,验证分析水声信号数字下变频的正确性.在此基础上,应用Verilog语言移植算法到FPGA内部,在Signaltap II下观察过程变量及结果,并导入Matlab进行数据分析.该技术对于声呐信号处理具有实用价值,并已成功应用于某声呐设备. 相似文献
782.
[Objective]This paper proposes a method for calculating the acoustic and vibration response of underwater cylindrical shell structures based on land-based vibration test results.[Methods]An axisymmetric boundary element method (BEM) is introduced to describe the radiation acoustic field of the shell. The relationship of acoustic pressure at the nodal point of the generatrix with velocity is obtained by solving the numerical solution of the boundary element integral equation, then the acoustic radiation impedance matrix of the outer surface of the shell and acoustic transfer vector (ATV) are constructed. Based on the assumption that the low-order vibration mode of an underwater cylindrical shell is the same as that of an onshore cylindrical shell, combined with the modes and acoustic radiation impedance matrix of an onshore cylindrical shell, the modal added mass and damping are calculated. [Results]The natural frequency calculation formula of the underwater cylindrical shell is established on the basis of the onshore mode, and the calculation method of underwater vibration response and acoustic radiation characteristics with the vibration response in air as input is obtained based on mode superposition method.[Conclusion ] The numerical results of a typical cylindrical shell with internal structure show that the method meets the engineering accuracy requirements. © 2023 Chinese Journal of Ship Research. All rights reserved. 相似文献
783.
某型救生船的流体动力性能和动力定位能力评估 总被引:1,自引:0,他引:1
简要介绍了某救生船的流体动力性能试验和多推力器推力分配方法,并对某救生船在稳定的风、浪、流作用下保持固定位置和艏向的能力予以评估. 相似文献
784.
[Objectives]This study seeks to expand the bandgap frequency band, reduce the bandgap starting frequency and analyze and optimize the bandgap parameters of acoustic metamaterials. [Methods]The influence of geometrical and material parameters on the bandgap properties of acoustic metamaterials is analyzed, and a method for maximizing the bandgap width is proposed. The multi-objective optimization problem is converted into a single objective optimization problem by normalizing the bandgap frequency coefficients. Structural material conversion is achieved via the material selection optimization method, and the optimization equations of bandgap parameters are established on the basis of weight-lightening. For chiral acoustic metamaterials, the material properties (density and wave velocity) and geometric parameters (scatterer diameter, ligament thickness and coating thickness) are defined as design variables, and the comprehensive optimization of structural parameters and material selection of acoustic metamaterials based on weight-lightening are implemented. [Results]The optimization results show that the bandgap width increases by 27.7% and the lower bound frequency decreases by 1048 Hz, thereby achieving the goal of expanding the bandgap width based on lightweight acoustic metamaterials. The acoustic transmission analysis of the finite chiral acoustic metamaterial structure is then carried out to verify the effectiveness of the proposed method. [Conclusions]The results show that the goal of lightweight acoustic metamaterials can be effectively achieved by integrating the comprehensive optimization of structural parameters and materials. As such, this study provides references for the design of new-type acoustic metamaterials. © 2023 Authors. All rights reserved. 相似文献
785.
786.