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81.
82.
利用实验室风浪槽内测得的波面序列资料估计风浪外频谱。通过与实测风浪内频谱的比较,研究实测风浪外频谱的谱形特征,探讨海浪外频谱与内频谱的相似性问题。此外,还检验一种理论海浪外频谱。 相似文献
83.
报道一种实时大视角、全景深获取水下图象的新方法。该方法试图解决在无人摇控潜器(ROV)和自治式潜器 (UUV)走航式获取水下清晰图象时 ,遇到的激光在水下传输过程中后向散射噪声问题 ,建立了一种具有宽视角、全景深性能的水下激光扫描系统。用这种方法和系统在空气中和水中做了大量的实验 ,并讨论了这种方法和系统的应用前景。 相似文献
84.
85.
The effect of the cone spacing of a conieal structure on the ice force is studied by model experiments. The ice force reduction coefficient presented in this paper expresses the relationship between the ice force and the arrangement of cones. The experiments prove that the mode of the ice failure before the boundary of upward-downward bending cone (UDBC) is crushing. A conclusion can also be drawn from the experiments that the ice force on the boundary of UDBC is by far less than that on a vertical pile with the same diameter. Moreover, the ice force frequencies on upright and inve-rted cones are obtained under the field condition of the platform JZ20-2, respectively. The results show that the alterna-tion of the ice force on UDBC can hardly induce resonance of platform JZ20-2. 相似文献
86.
Size‐frequency distributions can support reliable inferences concerning population dynamics of brachiopods, but only a few data are available so far. In this study, length and width frequency distributions of dead specimens of the Recent brachiopods Joania cordata and Argyrotheca cuneata from the Marine Protected Area ‘Secche di Tor Paterno’, Central Tyrrhenian Sea, Italy (41°35′ N, 12°20′ E), are reported in order to add new data about size‐frequency distributions of brachiopods. The studied specimens came from death assemblages in the coralligenous substrate, in the Posidonia oceanica meadows, and in the sand channels. The observed patterns vary from left‐skewed (J. cordata) to right‐skewed (A. cuneata), indicating respectively a low and high mortality of smaller individuals. Significant differences between the coralligenous substrate and the P. oceanica meadow were observed for both species, revealing a variation among different habitats. All length and width distributions are clearly polymodal, but the biological meaning of the peaks is difficult to interpret, as the two species seem to have a 2‐year life span. A biometric analysis of shell sizes revealed that length and width are the most variable parameters during the growth of the animal. 相似文献
87.
We report a theoretical investigation of an elastic and slender fluid-conveying pipe with a top-end excitation subjected to uniform cross flows. Considering the mean drag force and the time varying vortex-induced lift force which is modeled using a nonlinear van der Pol oscillator, the nonlinear partial differential equations of the motion of coupled fluid-structure system are constructed and simplified to a reduced-order model through the Galerkin-type discretization. By virtue of quasi-static displacement conditions, the characteristics of vortex-induced vibration of the pipe are evaluated for the first two lock-in modes. The results show that the top-end excitation can increase the vibration amplitude of the pipe when the cross-flow speed is out of the lock-in regions. When the cross-flow speed is within the lock-in region, however, the top-end oscillation causes a transition between quasi-periodic and periodic in the responses of the pipe, significantly reducing or increasing the vibration amplitudes depending on the excitation acceleration and frequency. This finding has an important guidance in suppressing vortex-induced vibrations by balancing the internal fluid velocity and the top-end excitation. 相似文献
88.
Abstract Applicability of log-Gumbel (LG) and log-logistic (LL) probability distributions in hydrological studies is critically examined under real conditions, where the assumed distribution differs from the true one. The set of alternative distributions consists of five two-parameter distributions with zero lower bound, including LG and LL as well as lognormal (LN), linear diffusion analogy (LD) and gamma (Ga) distributions. The log-Gumbel distribution is considered as both a false and a true distribution. The model error of upper quantiles and of the first two moments is analytically derived for three estimation methods: the method of moments (MOM), the linear moments method (LMM) and the maximum likelihood method (MLM). These estimation methods are used as methods of approximation of one distribution by another distribution. As recommended in the first of this two-part series of papers, MLM turns out to be the worst method, if the assumed LG or LL distribution is not the true one. It produces a huge bias of upper quantiles, which is at least one order higher than that of the other two methods. However, the reverse case, i.e. acceptance of LN, LD or Ga as a hypothetical distribution, while the LG or LL distribution is the true one, gives the MLM bias of reasonable magnitude in upper quantiles. Therefore, one should avoid choosing the LG and LL distributions in flood frequency analysis, especially if MLM is to be applied. 相似文献
89.
AbstractFloods, as extreme hydrological phenomena, can be described by more than one correlated characteristic, such as peak, volume and duration. These characteristics should be jointly considered since they are generally not independent. For an ungauged site, univariate regional flood frequency analysis (FA) provides a limited assessment of flood events. A recent study proposed a procedure for regional FA in a multivariate framework. This procedure represents a multivariate version of the index-flood model and is based on copulas and multivariate quantiles. The performance of the proposed procedure was evaluated by simulation. However, the model was not tested on a real-world case study data. In the present paper, practical aspects are investigated jointly for flood peak (Q) and volume (V) of a dataset from the Côte-Nord region in the province of Quebec, Canada. The application of the proposed procedure requires the identification of the appropriate marginal distribution, the estimation of the index flood and the selection of an appropriate copula. The results of the case study show that the regional bivariate FA procedure performed well. This performance depends strongly on the performance of the two univariate models and, more specifically, the univariate model of Q. The results show also the impact of the homogeneity of the region on the performance of the univariate and bivariate models.
Editor D. Koutsoyiannis 相似文献
90.
A frequency response function change (FRFC) method to detect damage location and extent based on the change in the frequency response functions of a shear building under the effects of ground excitation was proposed in this paper. The damage identification equation was derived from the motion equations of the system before and after the occurrence of the damage. Efforts to make the FRFC method less model‐dependent were made. Intact system matrices, which could be estimated using the measured data without the need for an analytical model, and the frequency response functions were required for the FRFC method. The effects of measurement noise and model parameter error in the FRFC method were studied numerically. The proposed FRFC method was validated by experimental studies of a six‐story steel building structure with single and multiple damage cases. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献