The numerical approximation of high frequency wave propagation is important in many applications.Examples include the simulation of seismic,acoustic,optical waves and microwaves.When the frequency of the waves is high...The numerical approximation of high frequency wave propagation is important in many applications.Examples include the simulation of seismic,acoustic,optical waves and microwaves.When the frequency of the waves is high,this is a difficult multiscale problem.The wavelength is short compared to the overall size of the computational domain and direct simulation using the standard wave equations is very expensive.Fortunately,there are computationally much less costly models,that are good approximations of many wave equations precisely for very high frequencies.Even for linear wave equations these models are often nonlinear.The goal of this paper is to review such mathematical models for high frequency waves,and to survey numerical methods used in simulations.We focus on the geometrical optics approximation which describes the infinite frequency limit of wave equations.We will also discuss finite frequency corrections and some other models.展开更多
We construct a numerical scheme based on the Liouville equation of geometric optics coupled with the Geometric Theory of Diffraction(GTD)to simulate the high frequency linear waves diffracted by a half plane.We first ...We construct a numerical scheme based on the Liouville equation of geometric optics coupled with the Geometric Theory of Diffraction(GTD)to simulate the high frequency linear waves diffracted by a half plane.We first introduce a condition,based on the GTD theory,at the vertex of the half plane to account for the diffractions,and then build in this condition as well as the reflection boundary condition into the numerical flux of the geometrical optics Liouville equation.Numerical experiments are used to verify the validity and accuracy of this new Eulerian numerical method which is able to capture the moments of high frequency and diffracted waves without fully resolving the high frequency numerically.展开更多
As an important equipment for sea state remote sensing, high frequency surface wave radar （HFSWR） has received more and more attention. The conventional method for wave height inversion is based on the ratio of the ...As an important equipment for sea state remote sensing, high frequency surface wave radar （HFSWR） has received more and more attention. The conventional method for wave height inversion is based on the ratio of the integration of the second-order spectral continuum to that of the first-order region, where the strong external noise and the incorrect delineation of the first- and second-order Doppler spectral regions due to spectral aliasing are two major sources of errors in the wave height. To account for these factors, two more indices are introduced to the wave height estimation, i.e., the ratio of the maximum power of the second-or- der continuum to that of the Bragg spectral region （RSCB） and the ratio of the power of the second harmonic peak to that of the Bragg peak （RSHB）. Both indices also have a strong correlation with the underlying wave height. On the basis of all these indices an empirical model is proposed to estimate the wave height. This method has been used in a three-months long experiment of the ocean state measuring and analyzing ra- dar, type S （OSMAR-S）, which is a portable HFSWR with compact cross-loop/monopole receive antennas developed by Wuhan University since 2006. During the experiment in the Taiwan Strait, the significant wave height varied from 0 to 5 m. The significant wave heights estimated by the OSMAR-S correlate well with the data provided by the Oceanweather Inc. for comparison, with a correlation coefficient of 0.74 and a root mean square error （RMSE） of 0.77 m. The proposed method has made an effective improvement to the wave height estimation and thus a further step toward operational use of the OSMAR-S in the wave height extraction.展开更多
Based on the quasi-harmonic analysis of 11 d vector ocean currents obtained from two high frequency sur- face wave radars located at Zhujiajian Island and Shengshan Island, the spatial distribution characteristics of ...Based on the quasi-harmonic analysis of 11 d vector ocean currents obtained from two high frequency sur- face wave radars located at Zhujiajian Island and Shengshan Island, the spatial distribution characteristics of surface tidal currents in the open sea area to the east of the Zhoushan Islands of Zhejiang Province, China are studied. The following conclusions are drawn from the analysis： the tidal current pattern in the open sea area to the east of Zhoushan Islands is primarily regular semidiurnal, which is significantly affected by the shallow water constituents. The directions of the major axes of tidal current ellipses of M2 lie approx- imately in the NW-SE direction. With the increasing of distance away from the coast, the directions of the tidal current ellipses gradually shift toward the E-W direction. The tidal currents are mainly reversing cur- rents. The spatial distribution of probable maximum current velocities decreases gradually from northeast to southwest which is basically in accordance with the spatial distribution of measured maximum current velocities. The residual currents near the coast are larger than those far away from the coast. The directions of the residual currents are basically north by east, and the angle to the due north increases gradually with the increasing distance away from the coast. The topography shows a certain impact on the spatial distri- bution of shallow water constituents, the rotation of tidal currents, the probable maximum currents and the residual currents.展开更多
The high frequency gravitational waves （around lOS-lO12 Hz） could interact with a specially designed electro- magnetic resonance system. It is found that the power of transverse perturbative photon flux （PPF） of a...The high frequency gravitational waves （around lOS-lO12 Hz） could interact with a specially designed electro- magnetic resonance system. It is found that the power of transverse perturbative photon flux （PPF） of an electromagnetic resonance system can be improved significantly by virtue of an astigmatic Caussian beam. Cor- respondingly the signal-to-noise ratio （SNR） would also be improved. When the eccentric ratio of waist satisfying w0x ： w0y 〉 1, the peak value of signal photon flux could be raised by 2-4 times with typical systematic parameters, while the background photon flux would be depressed. Therefore, the ratio of transverse PPF to background photon flux （i.e., SNR） can be further improved 3-8 times with dimensionless amplitude of relic gravitational wave ht = 10-36.展开更多
In this paper, on the basis of the working principles of high frequency ground wave radar for retrieval of ocean wave and sea wind elements were used to systematically study the data obtained from contrast validation ...In this paper, on the basis of the working principles of high frequency ground wave radar for retrieval of ocean wave and sea wind elements were used to systematically study the data obtained from contrast validation test in Zhoushan sea area of Zhejiang Province on Oct. 2000, to validate the accuracy of OSMAR2000 for wave and wind parameters, and to analyze the possible error caused when using OSMAR2000 to retrieve ocean parameters.展开更多
This paper discusses the basic categories of noise in detecting high frequency gravitational waves in the microwave band （-0.1-10GHz）, which contain shot noise from the laser and the thermal radiation photons, therm...This paper discusses the basic categories of noise in detecting high frequency gravitational waves in the microwave band （-0.1-10GHz）, which contain shot noise from the laser and the thermal radiation photons, thermal noise from statistical fluctuation of the thermal photons and fluctuation of the temperature, radiation press noise on the fractal membrane, the noise caused by the scattering of the Gaussian Beam （GB） in the detecting tube and noise in the microwave radiometers. The analysis shows that a reasonable signal-to-noise ratio may be achieved for a detecting device with the fixed power of GB （105 W）, only when the temperature of the environment is no more than T=I K, and the optimal length of the microwave radiometers is about 0.3 m.展开更多
To describe properties of the high frequency gravitational wave （HFGW） propagating through the vacuum gravitational field in Robertson-Walker background space-time, we calculated its energy momentum pseudo-tensor （...To describe properties of the high frequency gravitational wave （HFGW） propagating through the vacuum gravitational field in Robertson-Walker background space-time, we calculated its energy momentum pseudo-tensor （EMPT） in the limit of short wavelengths by taking the Brill-Hartle average on the second order perturbation of the Einstein tensor over several wavelengths. By rewriting the EMPT as a form of perfect fluid, the dynamical back-reaction of HIFGW on the background spacetime was discussed. The result shows that the energy density of HFGW, which is in the gauge we chose, is positive definite. The HFGW serves as a source for curving the background space-time and affects the dynamical evolution and time evolution of the scale factor of the Robertson-Walker metric.展开更多
Severe vibration of underground structures may be induced under blast loads. According to the characteristics of the explosion-induced ground shock wave, a new-type damper, inverse control magneto-rheological（MR） da...Severe vibration of underground structures may be induced under blast loads. According to the characteristics of the explosion-induced ground shock wave, a new-type damper, inverse control magneto-rheological（MR） damper was designed to control the vibration, The high-frequency performance test of the MR damper was carried out on the small shaking table. It is shown that the performance can be modeled by use of the modified Bouc-Wen model, and the Parameters of the model keep stable in the range of 15--50 Hz.展开更多
Surface acoustic wave（SAW） resonators with Pt/AlN/Si and Pt/AlN/Pt/Si configurations were fabricated by lift-off photolithography techniques. High-temperature performances of both configurations were investigated fo...Surface acoustic wave（SAW） resonators with Pt/AlN/Si and Pt/AlN/Pt/Si configurations were fabricated by lift-off photolithography techniques. High-temperature performances of both configurations were investigated for temperature sensor applications. AlN films grown on Ptcoated Si substrates exhibit a lower（002） preferred orientation than those grown on Si substrates. The center frequencies of Pt/AlN/Si and Pt/AlN/Pt/Si configurations at room temperature are 424.1 and 456.4 MHz, respectively.The SAW was limited by Pt bottom electrodes to propagate in AlN layer. The temperature coefficient of frequency（TCF） values of Pt/AlN/Si and Pt/AlN/Pt/Si configurations are-51.6 × 10^-6 and-69.2 × 10^-6℃^-1, respectively.Compared with that of Pt/AlN/Si configuration, the TCF value of Pt/AlN/Pt/Si configuration decreases by 34.1 %.AlN resonator with the Pt floating bottom electrodes provides a large, quasi-constant temperature sensitivity which is suitable for temperature sensor applications.展开更多
OSMAR071 is the latest product of the OSMAR （ocean state monitor and analysis radar） series of high frequency surface wave radar （HFSWR）, which was developed by the Radiowave Propagation Laboratory of Wuhan Univer...OSMAR071 is the latest product of the OSMAR （ocean state monitor and analysis radar） series of high frequency surface wave radar （HFSWR）, which was developed by the Radiowave Propagation Laboratory of Wuhan University. It adopts a modified Barrick waveheight inversion model. The modifications are introduced to improve the model＇s performances under the effect of noises and interferences and in the case of broad beam radar detection. The two unknown coefficients in the modified model are figured out by fitting the HFSWR significant waveheight results to those output from a wave buoy located in the radiating coverage of the radar site. The model is applied to inverse the waveheights from radar data for the duration from Dec. 1st, 2008 to Feb. 25th, 2009, and then the radar waveheights are compared with the buoy measurements. Results show that the rms difference between radar-derived significant waveheights and those from the buoy is 0.38 m and the correlation coefficient between the two series is 0.66. This study describes OSMAR071 observation of significant waveheight with relatively satisfactory accuracy during about three months.展开更多
We conducted the drifter trajectory tracking experi- ment for two drifters in the East China Sea, in which the currents were detected by two multi-frequency HFSWRs using the Lagrangetracking method. The tracking drift...We conducted the drifter trajectory tracking experi- ment for two drifters in the East China Sea, in which the currents were detected by two multi-frequency HFSWRs using the Lagrangetracking method. The tracking drifter trajectory shows good agree- ment with the GPS records by qualitative and quantitative analysis that indicates that drifter tracking by HFSWR is valid. In the 12- hour tracking for drifters P1 and P2, the average errors are 1.84 kin, 1.73 kin, and the maximum errors are 3.52 km and 3.12 km, respec- tively. The current measurement is evaluated by an acoustic Doppler current profilers （ADCP） in-situ observation, and it is the first multi-frequency HFSWR ocean surface monitoring experiment in China. In addition, the main aspects （HFSWR current measurement error, wind, and wave） influencing the tracking accuracy are dis- cussed, and especially the wind factor＇s impact is analyzed through the wind filed data.展开更多
The author argues in this document that initial vacuum state values possibly responsible for GW generation in relic conditions in the initial onset of inflation may have a temporary unsqueezed, possibly even coherent ...The author argues in this document that initial vacuum state values possibly responsible for GW generation in relic conditions in the initial onset of inflation may have a temporary unsqueezed, possibly even coherent initial value, which would permit in certain models classical coherent initial gravitational wave states. Furthermore, several arguments pro and con as to if or not initial relic GW should be high frequency will be presented, with the reason given why earlier string models did NOT favor low frequency relic GW from the big bang. What is observed is that large higher dimensions above our 4 Dimensional space time, if recipients of matter-energy from collapse and re birth of the universe are enough to insure low relic GW. The existence of higher dimensions, in itself if the additional dimensions are small and compact will have no capacity to lower the frequency limit values of relic GW, as predicted by Giovannini, et al. in 1995.展开更多
We review the level set methods for computing multi-valued solutions to a class of nonlinear first order partial differential equations,including Hamilton-Jacobi equations,quasi-linear hyperbolic equations,and conserv...We review the level set methods for computing multi-valued solutions to a class of nonlinear first order partial differential equations,including Hamilton-Jacobi equations,quasi-linear hyperbolic equations,and conservative transport equations with multi-valued transport speeds.The multivalued solutions are embedded as the zeros of a set of scalar functions that solve the initial value problems of a time dependent partial differential equation in an augmented space.We discuss the essential ideas behind the techniques,the coupling of these techniques to the projection of the interaction of zero level sets and a collection of applications including the computation of the semiclassical limit for Schr¨odinger equations and the high frequency geometrical optics limits of linear wave equations.展开更多
The power spectrum of primordial tensor perturbations Pt increases rapidly in the high frequency region if the spectral index nt 〉 0. It is shown that the amplitude of relic gravitational waves ht （5×109 Hz） v...The power spectrum of primordial tensor perturbations Pt increases rapidly in the high frequency region if the spectral index nt 〉 0. It is shown that the amplitude of relic gravitational waves ht （5×109 Hz） varies from 10-36 to 10-25 while rtt varies from -6.25 × 10-3 to 0.87. A high frequency gravitational wave detector proposed by F,-Y, Li detects gravitational waves through observing the perturbed photon flux that is generated by interaction between relic gravitational waves and electromagnetic field. It is shown that the perturbative photon flux N1x （5 × 109 Hz） varies from 1.40× 10-4 s-i to 2.85× 107 s-i while nt varies from -6.25 ×10-3 to 0.87, Correspondingly, the ratio of the transverse perturbative photon flux N1x to the background photon flux varies from 10-28 to 10-16.展开更多
Extremely powerful astrophysical electromagnetic（EM） systems could be possible sources of highfrequency gravitational waves（HFGWs）. Here, based on properties of magnetars and gamma-ray bursts（GRBs）, we address ...Extremely powerful astrophysical electromagnetic（EM） systems could be possible sources of highfrequency gravitational waves（HFGWs）. Here, based on properties of magnetars and gamma-ray bursts（GRBs）, we address ＂Gamma-HFGWs＂（with very high-frequency around 1020 Hz） caused by ultra-strong EM radiation（in the radiation-dominated phase of GRB fireballs） interacting with super-high magnetar surface magnetic fields（～1011 T）.By certain parameters of distance and power, the Gamma-HFGWs would have far field energy density ？gw around10-6, and they would cause perturbed signal EM waves of～10-20 W/m2 in a proposed HFGW detection system based on the EM response to GWs. Specially, Gamma-HFGWs would possess distinctive envelopes with characteristic shapes depending on the particular structures of surface magnetic fields of magnetars, which could be exclusive features helpful to distinguish them from background noise. Results obtained suggest that magnetars could be involved in possible astrophysical EM sources of GWs in the very high-frequency band, and Gamma-HFGWs could be potential targets for observations in the future.展开更多
In this paper, we propose a tailored-finite-point method for the numerical simulation of the Helmholtz equation with high wave numbers in heterogeneous medium. Our finite point method has been tailored to some particu...In this paper, we propose a tailored-finite-point method for the numerical simulation of the Helmholtz equation with high wave numbers in heterogeneous medium. Our finite point method has been tailored to some particular properties of the problem, which allows us to obtain approximate solutions with the same behaviors as that of the exact solution very naturally. Especially, when the coefficients are piecewise constant, we can get the exact solution with only one point in each subdomain. Our finite-point method has uniformly convergent rate with respect to wave number k in L^2-norm.展开更多
High frequency surface wave radar (HFSWR) is well proved to have over the horizon (OTH) detection capability to weak aerial targets, such as concealed airplanes or cruise missiles. The most important problem of detect...High frequency surface wave radar (HFSWR) is well proved to have over the horizon (OTH) detection capability to weak aerial targets, such as concealed airplanes or cruise missiles. The most important problem of detection of fast and small targets using HFSWR is earlier warning, i.e. enlargement of detection range oftargets. Therefore, the detection threshold should be decreased as low as possible, but numerous false alarms are brought about at the same time. On this condition, conventional track initiation techniques, which normally require the probability of false alarm to be at the level of 10-6, will initiate enormous false tracks and lead to abnormal operation of tracking system. An adaptive modified hough transform (AMHT) track initiator is proposed accordingly and the relation of detection range to the performance of track initiator is analyzed in this paper. Simulations are performed to confirm the capability of track initiation to fast and small targets in dense clutter by AMHT track initiator. The tolerable probability of false alarm of detector can reach the level of 10 -3 . And it performs better than track initiator based on modified hough transform (MHT).展开更多
Abstract. Due to the incapability of high frequency surface wave radar （HFSWR） to detect undercurrent parameters, a new algorithm is proposed to apply a three-dimensional （3D） nonlinear barotropic shallow sea and ...Abstract. Due to the incapability of high frequency surface wave radar （HFSWR） to detect undercurrent parameters, a new algorithm is proposed to apply a three-dimensional （3D） nonlinear barotropic shallow sea and continental shelf model in coordinate system to the inversion of undercurrent. The calculation domain of this model is the area detected by HFSWR. Considering the benthal topography of the detected area and the ocean dynamic parameters, such as surface current, wind and wave detected by HFSWR, the relation between surface current and undercurrent is established in this model, Accordingly, the undercurrent parameters of corresponding area are obtained. The inversion results agree with the law of ocean dynamics and reproduce the movement of undercurrent.展开更多
This paper studies the development on the first order sea clutter cross section for bistatic high frequency surface wave radar (HFSWR). Based on the received first order electric field expression, a closed-form of cro...This paper studies the development on the first order sea clutter cross section for bistatic high frequency surface wave radar (HFSWR). Based on the received first order electric field expression, a closed-form of cross sections is derived to account for the case of receiving antenna array being mounted on the shipborne platform. The uniform linear motion and sway motion components are assumed to be responsible for the observed differences in comparison with the bistatic fixed antenna case. Correspondingly, simulations are conducted to study the sea clutter spectral characteristics for these two cases versus different system parameters and sea state conditions. It is shown numerically that the forward motion component will spread the Bragg lines severely and the influence triggered by the sway motion can be explained as the Bessel function modulation of the ordinary sea clutter spectra. The obtained results have important implications in the application of shipborne HFSWR technology to ocean remote sensing and target detection.展开更多
文摘The numerical approximation of high frequency wave propagation is important in many applications.Examples include the simulation of seismic,acoustic,optical waves and microwaves.When the frequency of the waves is high,this is a difficult multiscale problem.The wavelength is short compared to the overall size of the computational domain and direct simulation using the standard wave equations is very expensive.Fortunately,there are computationally much less costly models,that are good approximations of many wave equations precisely for very high frequencies.Even for linear wave equations these models are often nonlinear.The goal of this paper is to review such mathematical models for high frequency waves,and to survey numerical methods used in simulations.We focus on the geometrical optics approximation which describes the infinite frequency limit of wave equations.We will also discuss finite frequency corrections and some other models.
文摘We construct a numerical scheme based on the Liouville equation of geometric optics coupled with the Geometric Theory of Diffraction(GTD)to simulate the high frequency linear waves diffracted by a half plane.We first introduce a condition,based on the GTD theory,at the vertex of the half plane to account for the diffractions,and then build in this condition as well as the reflection boundary condition into the numerical flux of the geometrical optics Liouville equation.Numerical experiments are used to verify the validity and accuracy of this new Eulerian numerical method which is able to capture the moments of high frequency and diffracted waves without fully resolving the high frequency numerically.
基金The National Natural Science Foundation of China under contract No.61371198the National Special Program for Key Scientific Instrument and Equipment Development of China under contract No.2013YQ160793the Natural Science Foundation of Jiangsu Province of China under contract No.BK2012199
文摘As an important equipment for sea state remote sensing, high frequency surface wave radar （HFSWR） has received more and more attention. The conventional method for wave height inversion is based on the ratio of the integration of the second-order spectral continuum to that of the first-order region, where the strong external noise and the incorrect delineation of the first- and second-order Doppler spectral regions due to spectral aliasing are two major sources of errors in the wave height. To account for these factors, two more indices are introduced to the wave height estimation, i.e., the ratio of the maximum power of the second-or- der continuum to that of the Bragg spectral region （RSCB） and the ratio of the power of the second harmonic peak to that of the Bragg peak （RSHB）. Both indices also have a strong correlation with the underlying wave height. On the basis of all these indices an empirical model is proposed to estimate the wave height. This method has been used in a three-months long experiment of the ocean state measuring and analyzing ra- dar, type S （OSMAR-S）, which is a portable HFSWR with compact cross-loop/monopole receive antennas developed by Wuhan University since 2006. During the experiment in the Taiwan Strait, the significant wave height varied from 0 to 5 m. The significant wave heights estimated by the OSMAR-S correlate well with the data provided by the Oceanweather Inc. for comparison, with a correlation coefficient of 0.74 and a root mean square error （RMSE） of 0.77 m. The proposed method has made an effective improvement to the wave height estimation and thus a further step toward operational use of the OSMAR-S in the wave height extraction.
基金The National High Technology Research and Development Program of China(863 Program)under contract No.2012AA091701the Fundamental Research Fund for the Central Universities of China under contract No.2012212020211
文摘Based on the quasi-harmonic analysis of 11 d vector ocean currents obtained from two high frequency sur- face wave radars located at Zhujiajian Island and Shengshan Island, the spatial distribution characteristics of surface tidal currents in the open sea area to the east of the Zhoushan Islands of Zhejiang Province, China are studied. The following conclusions are drawn from the analysis： the tidal current pattern in the open sea area to the east of Zhoushan Islands is primarily regular semidiurnal, which is significantly affected by the shallow water constituents. The directions of the major axes of tidal current ellipses of M2 lie approx- imately in the NW-SE direction. With the increasing of distance away from the coast, the directions of the tidal current ellipses gradually shift toward the E-W direction. The tidal currents are mainly reversing cur- rents. The spatial distribution of probable maximum current velocities decreases gradually from northeast to southwest which is basically in accordance with the spatial distribution of measured maximum current velocities. The residual currents near the coast are larger than those far away from the coast. The directions of the residual currents are basically north by east, and the angle to the due north increases gradually with the increasing distance away from the coast. The topography shows a certain impact on the spatial distri- bution of shallow water constituents, the rotation of tidal currents, the probable maximum currents and the residual currents.
基金Supported by the National Natural Science Foundation of China under Grant Nos 11205254 and 61501069the Fundamental Research Funds for the Central Universities under Grant No 106112016CDJXY300002
文摘The high frequency gravitational waves （around lOS-lO12 Hz） could interact with a specially designed electro- magnetic resonance system. It is found that the power of transverse perturbative photon flux （PPF） of an electromagnetic resonance system can be improved significantly by virtue of an astigmatic Caussian beam. Cor- respondingly the signal-to-noise ratio （SNR） would also be improved. When the eccentric ratio of waist satisfying w0x ： w0y 〉 1, the peak value of signal photon flux could be raised by 2-4 times with typical systematic parameters, while the background photon flux would be depressed. Therefore, the ratio of transverse PPF to background photon flux （i.e., SNR） can be further improved 3-8 times with dimensionless amplitude of relic gravitational wave ht = 10-36.
基金Supported by Natural Science Fund Project of Guangdong Province, Grant No.04001308, and National 863 Project, Grant No.2002AA639150, and No.2002AA639480
文摘In this paper, on the basis of the working principles of high frequency ground wave radar for retrieval of ocean wave and sea wind elements were used to systematically study the data obtained from contrast validation test in Zhoushan sea area of Zhejiang Province on Oct. 2000, to validate the accuracy of OSMAR2000 for wave and wind parameters, and to analyze the possible error caused when using OSMAR2000 to retrieve ocean parameters.
基金supported by the National Basic Research Program of China (Grant No 2003CB716300)the National Natural Science Foundation of China (Grant No 10575140)CAEP Foundation (Grant Nos 2008 T0401 and 2008 T0402)
文摘This paper discusses the basic categories of noise in detecting high frequency gravitational waves in the microwave band （-0.1-10GHz）, which contain shot noise from the laser and the thermal radiation photons, thermal noise from statistical fluctuation of the thermal photons and fluctuation of the temperature, radiation press noise on the fractal membrane, the noise caused by the scattering of the Gaussian Beam （GB） in the detecting tube and noise in the microwave radiometers. The analysis shows that a reasonable signal-to-noise ratio may be achieved for a detecting device with the fixed power of GB （105 W）, only when the temperature of the environment is no more than T=I K, and the optimal length of the microwave radiometers is about 0.3 m.
基金the National Basic Research Program of China (No.2003 CB 716300)the National Natural Science Foundation of China (No.10575140)+1 种基金the Nature Science Foundation of Chongqing (No. 8562)the Foundation of China Academy of Engineering Physics.
文摘To describe properties of the high frequency gravitational wave （HFGW） propagating through the vacuum gravitational field in Robertson-Walker background space-time, we calculated its energy momentum pseudo-tensor （EMPT） in the limit of short wavelengths by taking the Brill-Hartle average on the second order perturbation of the Einstein tensor over several wavelengths. By rewriting the EMPT as a form of perfect fluid, the dynamical back-reaction of HIFGW on the background spacetime was discussed. The result shows that the energy density of HFGW, which is in the gauge we chose, is positive definite. The HFGW serves as a source for curving the background space-time and affects the dynamical evolution and time evolution of the scale factor of the Robertson-Walker metric.
基金Supported by National Nature Fund and National Civil-Defense Office
文摘Severe vibration of underground structures may be induced under blast loads. According to the characteristics of the explosion-induced ground shock wave, a new-type damper, inverse control magneto-rheological（MR） damper was designed to control the vibration, The high-frequency performance test of the MR damper was carried out on the small shaking table. It is shown that the performance can be modeled by use of the modified Bouc-Wen model, and the Parameters of the model keep stable in the range of 15--50 Hz.
基金financially supported by the National Nature Science Foundation of China (No. 61223002)Sichuan Youth Science and Technology Innovation Research Team Funding (No. 2011JTD0006)
文摘Surface acoustic wave（SAW） resonators with Pt/AlN/Si and Pt/AlN/Pt/Si configurations were fabricated by lift-off photolithography techniques. High-temperature performances of both configurations were investigated for temperature sensor applications. AlN films grown on Ptcoated Si substrates exhibit a lower（002） preferred orientation than those grown on Si substrates. The center frequencies of Pt/AlN/Si and Pt/AlN/Pt/Si configurations at room temperature are 424.1 and 456.4 MHz, respectively.The SAW was limited by Pt bottom electrodes to propagate in AlN layer. The temperature coefficient of frequency（TCF） values of Pt/AlN/Si and Pt/AlN/Pt/Si configurations are-51.6 × 10^-6 and-69.2 × 10^-6℃^-1, respectively.Compared with that of Pt/AlN/Si configuration, the TCF value of Pt/AlN/Pt/Si configuration decreases by 34.1 %.AlN resonator with the Pt floating bottom electrodes provides a large, quasi-constant temperature sensitivity which is suitable for temperature sensor applications.
基金Supported by the National High Technology Research and Development Program of China (863 Program) (2001AA631050)the National Natural Science Foundation of China (60571065)Open fund of State Key Laboratory of Offshore Marine Environment (Xiamen University)
文摘OSMAR071 is the latest product of the OSMAR （ocean state monitor and analysis radar） series of high frequency surface wave radar （HFSWR）, which was developed by the Radiowave Propagation Laboratory of Wuhan University. It adopts a modified Barrick waveheight inversion model. The modifications are introduced to improve the model＇s performances under the effect of noises and interferences and in the case of broad beam radar detection. The two unknown coefficients in the modified model are figured out by fitting the HFSWR significant waveheight results to those output from a wave buoy located in the radiating coverage of the radar site. The model is applied to inverse the waveheights from radar data for the duration from Dec. 1st, 2008 to Feb. 25th, 2009, and then the radar waveheights are compared with the buoy measurements. Results show that the rms difference between radar-derived significant waveheights and those from the buoy is 0.38 m and the correlation coefficient between the two series is 0.66. This study describes OSMAR071 observation of significant waveheight with relatively satisfactory accuracy during about three months.
基金Supported by the National High Technology Research and Development Program of China (863 Program) (2006AA09A303)
文摘We conducted the drifter trajectory tracking experi- ment for two drifters in the East China Sea, in which the currents were detected by two multi-frequency HFSWRs using the Lagrangetracking method. The tracking drifter trajectory shows good agree- ment with the GPS records by qualitative and quantitative analysis that indicates that drifter tracking by HFSWR is valid. In the 12- hour tracking for drifters P1 and P2, the average errors are 1.84 kin, 1.73 kin, and the maximum errors are 3.52 km and 3.12 km, respec- tively. The current measurement is evaluated by an acoustic Doppler current profilers （ADCP） in-situ observation, and it is the first multi-frequency HFSWR ocean surface monitoring experiment in China. In addition, the main aspects （HFSWR current measurement error, wind, and wave） influencing the tracking accuracy are dis- cussed, and especially the wind factor＇s impact is analyzed through the wind filed data.
文摘The author argues in this document that initial vacuum state values possibly responsible for GW generation in relic conditions in the initial onset of inflation may have a temporary unsqueezed, possibly even coherent initial value, which would permit in certain models classical coherent initial gravitational wave states. Furthermore, several arguments pro and con as to if or not initial relic GW should be high frequency will be presented, with the reason given why earlier string models did NOT favor low frequency relic GW from the big bang. What is observed is that large higher dimensions above our 4 Dimensional space time, if recipients of matter-energy from collapse and re birth of the universe are enough to insure low relic GW. The existence of higher dimensions, in itself if the additional dimensions are small and compact will have no capacity to lower the frequency limit values of relic GW, as predicted by Giovannini, et al. in 1995.
基金the National Science Foundation under Grant DMS05-05975Osher’s research was supported by AFOSR Grant FA9550-04-0143NSF DMS-0513394 and the Sloan Foundation。
文摘We review the level set methods for computing multi-valued solutions to a class of nonlinear first order partial differential equations,including Hamilton-Jacobi equations,quasi-linear hyperbolic equations,and conservative transport equations with multi-valued transport speeds.The multivalued solutions are embedded as the zeros of a set of scalar functions that solve the initial value problems of a time dependent partial differential equation in an augmented space.We discuss the essential ideas behind the techniques,the coupling of these techniques to the projection of the interaction of zero level sets and a collection of applications including the computation of the semiclassical limit for Schr¨odinger equations and the high frequency geometrical optics limits of linear wave equations.
基金Supported by National Natural Science Foundation of China(11305181,11322545,11335012)Open Project Program of State Key Laboratory of Theoretical Physics,Institute of Theoretical Physics,Chinese Academy of Sciences,China(Y5KF181CJ1)
文摘The power spectrum of primordial tensor perturbations Pt increases rapidly in the high frequency region if the spectral index nt 〉 0. It is shown that the amplitude of relic gravitational waves ht （5×109 Hz） varies from 10-36 to 10-25 while rtt varies from -6.25 × 10-3 to 0.87. A high frequency gravitational wave detector proposed by F,-Y, Li detects gravitational waves through observing the perturbed photon flux that is generated by interaction between relic gravitational waves and electromagnetic field. It is shown that the perturbative photon flux N1x （5 × 109 Hz） varies from 1.40× 10-4 s-i to 2.85× 107 s-i while nt varies from -6.25 ×10-3 to 0.87, Correspondingly, the ratio of the transverse perturbative photon flux N1x to the background photon flux varies from 10-28 to 10-16.
基金Supported by National Natural Science Foundation of China(11605015,11375279,11205254,11647307)the Fundamental Research Funds for the Central Universities(106112017CDJXY300003,106112017CDJXFLX0014)
文摘Extremely powerful astrophysical electromagnetic（EM） systems could be possible sources of highfrequency gravitational waves（HFGWs）. Here, based on properties of magnetars and gamma-ray bursts（GRBs）, we address ＂Gamma-HFGWs＂（with very high-frequency around 1020 Hz） caused by ultra-strong EM radiation（in the radiation-dominated phase of GRB fireballs） interacting with super-high magnetar surface magnetic fields（～1011 T）.By certain parameters of distance and power, the Gamma-HFGWs would have far field energy density ？gw around10-6, and they would cause perturbed signal EM waves of～10-20 W/m2 in a proposed HFGW detection system based on the EM response to GWs. Specially, Gamma-HFGWs would possess distinctive envelopes with characteristic shapes depending on the particular structures of surface magnetic fields of magnetars, which could be exclusive features helpful to distinguish them from background noise. Results obtained suggest that magnetars could be involved in possible astrophysical EM sources of GWs in the very high-frequency band, and Gamma-HFGWs could be potential targets for observations in the future.
基金the NSFC Projects No.10471073No.10676017the National Basic Research Program of China under the grant 2005CB321701
文摘In this paper, we propose a tailored-finite-point method for the numerical simulation of the Helmholtz equation with high wave numbers in heterogeneous medium. Our finite point method has been tailored to some particular properties of the problem, which allows us to obtain approximate solutions with the same behaviors as that of the exact solution very naturally. Especially, when the coefficients are piecewise constant, we can get the exact solution with only one point in each subdomain. Our finite-point method has uniformly convergent rate with respect to wave number k in L^2-norm.
文摘High frequency surface wave radar (HFSWR) is well proved to have over the horizon (OTH) detection capability to weak aerial targets, such as concealed airplanes or cruise missiles. The most important problem of detection of fast and small targets using HFSWR is earlier warning, i.e. enlargement of detection range oftargets. Therefore, the detection threshold should be decreased as low as possible, but numerous false alarms are brought about at the same time. On this condition, conventional track initiation techniques, which normally require the probability of false alarm to be at the level of 10-6, will initiate enormous false tracks and lead to abnormal operation of tracking system. An adaptive modified hough transform (AMHT) track initiator is proposed accordingly and the relation of detection range to the performance of track initiator is analyzed in this paper. Simulations are performed to confirm the capability of track initiation to fast and small targets in dense clutter by AMHT track initiator. The tolerable probability of false alarm of detector can reach the level of 10 -3 . And it performs better than track initiator based on modified hough transform (MHT).
文摘Abstract. Due to the incapability of high frequency surface wave radar （HFSWR） to detect undercurrent parameters, a new algorithm is proposed to apply a three-dimensional （3D） nonlinear barotropic shallow sea and continental shelf model in coordinate system to the inversion of undercurrent. The calculation domain of this model is the area detected by HFSWR. Considering the benthal topography of the detected area and the ocean dynamic parameters, such as surface current, wind and wave detected by HFSWR, the relation between surface current and undercurrent is established in this model, Accordingly, the undercurrent parameters of corresponding area are obtained. The inversion results agree with the law of ocean dynamics and reproduce the movement of undercurrent.
基金supported by the National Natural Science Foundation of China(61471144)
文摘This paper studies the development on the first order sea clutter cross section for bistatic high frequency surface wave radar (HFSWR). Based on the received first order electric field expression, a closed-form of cross sections is derived to account for the case of receiving antenna array being mounted on the shipborne platform. The uniform linear motion and sway motion components are assumed to be responsible for the observed differences in comparison with the bistatic fixed antenna case. Correspondingly, simulations are conducted to study the sea clutter spectral characteristics for these two cases versus different system parameters and sea state conditions. It is shown numerically that the forward motion component will spread the Bragg lines severely and the influence triggered by the sway motion can be explained as the Bessel function modulation of the ordinary sea clutter spectra. The obtained results have important implications in the application of shipborne HFSWR technology to ocean remote sensing and target detection.
