Parameters in wind-wave frequency spectra and their bearings on spectrum forms and growth

The spectrum variance m0, peak frequency ω0 and peakness factor p are expressed in terms of nondimensional fetch and duration by making use of relations which are derived through comparing and analyzing existing empirical formulas for the growth of significant wave height and period. The main features of spectrum growth as specified by these parameters agree with those of the JONS-WAP experiments. For given wind speed and fetch, the high frequency parts beyond the peaks of shallow water spectra almost coincide with that of the corresponding deep water spectrum, whereas the low frequency parts differ appreciably. The method developed in this paper predicts smaller significant wave height as well as smaller wave period for shallow water spectra in contrast to the theoretical result of Kitaigorodskii ef al, in which the peak frequency, and consequently the significant wave period, remains basically unchanged for different water depths. Spectra are further reduced to a form in which only significant wave height and period are left as parameters, the peakness factor being replaced by the wave steepness through an empirical relation between them. Spectra in this form have been verified by observations.

Study on the growth of wind wave frequency spectra generated by cold waves in the northern East China Sea

The growth of frequency spectra and spectral parameters of wind waves generated by cold waves, a kind of severe weather system, in the northern East China Sea is studied in this paper. Based on a third-generation wave action model(the Simulating WAves Nearshore model), simulations were developed to analyze the spatiotemporal characteristics of wind waves and to output spectral data. It is shown that the cold wave-induced spectra can be well described by the modified Joint North Sea Wave Project spectral form. The growth of wave spectra is comprehensively reflected by the evolution of the three characteristic parameters: peak frequency, spectral peak and wave energy. Besides, the approximations of dependences between spectral parameters and the three types of universal induced factors are obtained with the least squares method and compared systematically. Fetch and peak frequency turn out to be suitable parameters to describe the spectral parameters, while the dependences on the inverse wave age vary in different sea areas. In general, the derived relationships improve on results from previous studies for better practical application of the wind wave frequency spectrum in the northern East China Sea.

A systematic analysis of the radial resonance frequency spectra of the PZT-based (Zr/Ti=52/48) piezoceramic thin disks

In this paper,both the ID radial mode and the equivalent circuit of a piezoceramic disk resonator were theoretically analyzed based on IEEE standards.And then,the radial resonance frequency spectra of the PZT-based(Nb/Ce co-doped Pb(Zro.s2Ti0.48)O3,abbreviated as PZT-NC)piezoceramic circular disks were measured by an impedance analyzer.A set of resonance frequency spectra including six electrical parameters:Z,R,X,Y,G,and B,were used for making a value distinction between three possible resonance frequencies,and between three possible antiresonance frequencies.A new-form Nyquist diagram was depicted to describe the position relations of these characteristic frequencies.Such a complete resonance frequency spectrum was used to perform the accurate calculation of some material constants and electromechanical coupling parameters for the PZT-NC piezoceramics.Further,the frequency dependence of the AC conductive behavior of the specimen was characterized by the complex impedance measurement.The values of AC conductivity at resonance/antiresonance were deduced from the equivalent circuit parameters.Moreover,the Van Dyke circuit model was assigned to each element contribution and the simulated curves showed a nice fitting with the experimental results.Finally,an additional impedance analysis associated with resonance frequency calculation revealed a complicated coupled vibration mode existing in the annular disk specimen.

Presentation of the Berry-Tabor conjecture in Levy plates

The Berry-Tabor(BT)conjecture is a famous statistical inference in quantum chaos,which not only establishes the spectral fluctuations of quantum systems whose classical counterparts are integrable but can also be used to describe other wave phenomena.In this paper,the BT conjecture has been extended to Lévy plates.As predicted by the BT conjecture,level clustering is present in the spectra of Lévy plates.The consequence of level clustering is studied by introducing the distribution of nearest neighbor frequency level spacing ratios P(r),which is calculated through the analytical solution obtained by the Hamiltonian approach.Our work investigates the impact of varying foundation parameters,rotary inertia,and boundary conditions on the frequency spectra,and we find that P(r)conforms to a Poisson distribution in all cases.The reason for the occurrence of the Poisson distribution in the Lévy plates is the independence between modal frequencies,which can be understood through mode functions.

Observing spectral characteristics over a continental shelf and slope in the South China Sea

Using in situ measurement data from May-June, 1998, and data from the Asian seas international acoustics experiment （ASIAEX） from 2001 in the South China Sea （SCS）, the spectral density function and the dissipa- tion spectrum function are estimated. In the inffa-gravity wave （IGW） band, the power spectra of velocity （u, v, w） are universal functions with respect to characteristic frequencies, which correspond to the peak fre- quencies of the dissipation spectrum （PFDS）. This suggests that high-frequency internal waves in the IGW band have similar dynamical characteristics. In addition, the evolution of these characteristic frequencies is explored and its highest value is 8.8 cph （cycles per hour, 1 cph=2.778× 10-3 Hz）.

Impact of Impeller Stagger Angles on Pressure Fluctuation for a Double-Suction Centrifugal Pump

Pressure fluctuation may cause high amplitude of vibration of double-suction centrifugal pumps, but the impact of impeller stagger angles is still not well understood. In this paper, pressure fluctuation experiments are carried out for five impeller configurations with different stagger angles by using the same test rig system. Results show that the stagger angles exert negligible effects on the characteristics of head and efficiency. The distributions of pressure fluctuations are relatively uniform along the suction chamber wall, and the maximum pressure fluctuation amplitude is reached near the suction inlet tongue region. The pressure fluctuation characteristics are affected largely by impeller rotation, whose dominant frequencies include impeller rotation frequency and its harmonic frequencies, and half blade passage frequency. The stagger angle exerts a small effect on the pressure fluctuations in the suction chamber while a great effect on the pressure fluctuation in volute casing, especially on the aspect of decreasing the amplitude on blade passage frequency. Among the tested cases, the distribution of pressure fluctuations in the volute becomes more uniform than the other impeller configurations and the level of pressure fluctuation may be reduced by up to 50% when the impeller stagger angle is close to 24° or 360°.The impeller structure pattern needs to be taken into consideration during the design period, and the halfway staggered impeller is strongly recommended.

Synthesis,and Experimental and Theoretical Characterization of 3-(4-(Dimethylamino) benzylidene)-1,5-dioxaspiro[5.5] Undecane-2,4-dione

The title molecule, 3-（4-（dimethylamino）benzylidene）-l,5-dioxaspiro[5.5] unde- cane-2,4-dione （I）, was synthesized and characterized by elemental analysis, IR, UV-vis spectra and X-ray diffraction analysis. The compound belongs to the triclinic system, space group Pi with a = 6.3640（6）, b = 7.7404（8）, c = 16.2890（18） A, α = 86.860（2）, β = 85.837（2）, γ = 79.6720（10）°, V = 786.60（14） A3, D, = 1.331 g/cm3, and F（000） = 336. Geometrical structure of the title compound was optimized by density functional theory （DFT） using B3LYP method with 6-31G＊＊ as the basis set. The vibrational frequencies were calculated by the DFT method and the results are consistent with the observed frequencies. The electronic absorption spectra were studied with the time- dependent density functional theory （TD-DFT）, showing the calculation results in good agreement with the corresponding experimental data.

Iterative physical optics model for electromagnetic scattering and Doppler analysis

An iterative physical optics（IPO） model is proposed to solve extra large scale electric electromagnetic（EM） scattering from randomly rough surfaces. In order to accelerate the convergence of the IPO model, the forward-backward methodology and its modification with underrelaxation iteration are developed to simulate the rough surface scattering; the local iteration methodology and the fast far field approximation（Fa FFA） in the matrix-vector product are proposed to reduce greatly the computational complexity. These techniques make Monte Carlo simulations possible. Thus, the average Doppler spectra of backscattered signals obtained from the simulations are compared for different incident angles and sea states. In particular, the simulations show a broadening of the Doppler spectra for a more complicated sea state at a low grazing angle（LGA）.

Analysis of shock wave unsteadiness using space and time correlations applied to shadowgraph flow visualization data

Unsteady flow characteristics of a normal shock wave,a lambda foot,and a separated turbulent boundary layer are investigated within a unique test section with supersonic inlet flow.The supersonic wind tunnel facility,containing this test section,provides a Mach number of approximately 1.54 at the test section entrance.Digitized shadowgraph flow visualization data are employed to visualize shock wave structure within the test section.These data are analyzed to determine shock wave unsteadiness characteristics,including grayscale spectral energy variations with frequency,as well as time and space correlations,which give coherence and time lag properties associated with perturbations associated with different flow regions.Results illustrate the complexity and unsteadiness of shock-wave-boundary-layerinteractions,including event frequencies from grayscale spectral energy distributions determined using a Lagrangian approach applied to shock wave location,and by grayscale spectral energy distributions determined using ensemble-averaging applied to multiple closely-located stationary pixel locations.Auto-correlation function results and two-point correlation functions(in the form of magnitude squared coherence)quantify the time-scales of periodic events,as well as the coherence of flow perturbations associated with different locations,over a range of frequencies.Associated time lag data provide information on the originating location of perturbation events,as well as the propagation direction and event sequence associated with different flow locations.Additional insight into spatial variations of time lag and flow coherence is provided by application of magnitude squared coherence analysis to multiple locations,relative to a single location associated with the normal shock wave.