Non-acceleration theorem in a primitive equation system is developed to investigate the influences of waves on the mean flow variation against external forcing. Numerical results show that mechanical forcing overwhelm...Non-acceleration theorem in a primitive equation system is developed to investigate the influences of waves on the mean flow variation against external forcing. Numerical results show that mechanical forcing overwhelms thermal forcing in maintaining the mean flow in which the internal mechanical forcing associated with horizontal eddy flux of momentum plays the most important roles. Both internal forcing and external forcing are shown to be active and at the first place for the mean flow variations, whereas the forcing-induced mean meridional circulation is passive and secondary. It is also shown that the effects on mean flow of external mechanical forcing are concentrated in the lower troposphere, whereas those due to wave-mean flow interaction are more important in the upper troposphere. These act together and result in the vertically easterly shear in low latitudes and westerly shear in mid-latitudes. This vertical shear of mean flow is to some extent weakened by thermal forcing.展开更多
Acoustic propagation problems in the sheared mean flow are numerically investigated using different acoustic propagation equations , including linearized Euler equations ( LEE ) and acoustic perturbation equations ( A...Acoustic propagation problems in the sheared mean flow are numerically investigated using different acoustic propagation equations , including linearized Euler equations ( LEE ) and acoustic perturbation equations ( APE ) .The resulted acoustic pressure is compared for the cases of uniform mean flow and sheared mean flow using both APE and LEE.Numerical results show that interactions between acoustics and mean flow should be properly considered to better understand noise propagation problems , and the suitable option of the different acoustic equations is indicated by the present comparisons.Moreover , the ability of APE to predict acoustic propagation is validated.APE can replace LEE when the 3-D flow-induced noise problem is solved , thus computational cost can decrease.展开更多
The gravity wave breaking is crucial to the large-scale circulation of middle atmosphere. In this paper, we follow Lindzen (1981) to draw out the parameterization of two-dimensional gravity wave breaking including ine...The gravity wave breaking is crucial to the large-scale circulation of middle atmosphere. In this paper, we follow Lindzen (1981) to draw out the parameterization of two-dimensional gravity wave breaking including inertial effect. Also we present some properties of critical levels and inertial critical levels.展开更多
To reduce computational costs, an improved form of the frequency domain boundary element method(BEM) is proposed for two-dimensional radiation and propagation acoustic problems in a subsonic uniform flow with arbitr...To reduce computational costs, an improved form of the frequency domain boundary element method(BEM) is proposed for two-dimensional radiation and propagation acoustic problems in a subsonic uniform flow with arbitrary orientation. The boundary integral equation(BIE) representation solves the two-dimensional convected Helmholtz equation(CHE) and its fundamental solution, which must satisfy a new Sommerfeld radiation condition(SRC) in the physical space. In order to facilitate conventional formulations, the variables of the advanced form are expressed only in terms of the acoustic pressure as well as its normal and tangential derivatives, and their multiplication operators are based on the convected Green's kernel and its modified derivative. The proposed approach significantly reduces the CPU times of classical computational codes for modeling acoustic domains with arbitrary mean flow. It is validated by a comparison with the analytical solutions for the sound radiation problems of monopole,dipole and quadrupole sources in the presence of a subsonic uniform flow with arbitrary orientation.展开更多
A study is made of the distribution of the diagnostic quantity vector E and the teleconnection structure of 30-50 (quasi-40) day oscillation, together with the dependence on the conversion of barotropic unstable energ...A study is made of the distribution of the diagnostic quantity vector E and the teleconnection structure of 30-50 (quasi-40) day oscillation, together with the dependence on the conversion of barotropic unstable energy of mean flow in terms of ECWMF daily 500 hPa grid data in winter, indicating that the energy transportation is closely associated with the westerly jet position, with zonal (meridional) propagation in the strong (weak) wind region, that considerable conversion of barotropic energy occurs at the jet exit region where low-frequency oscillation gains energy from the mean flow, leading to maximum kinetic energy for the oscillation observed there, which is marked by evident barotropy in striking contrast to the baroclinicity at low latitudes and that the teleconnection core is related to the center of action in the atmosphere and bound up with the pattern of the west wind.展开更多
Using 19-year satellite altimetric data, variations in the eddy kinetic energy, energy exchanges and interaction between the eddy fi eld and mean fl ow are discussed for the Kuroshio south of Japan. In the seasonal cy...Using 19-year satellite altimetric data, variations in the eddy kinetic energy, energy exchanges and interaction between the eddy fi eld and mean fl ow are discussed for the Kuroshio south of Japan. In the seasonal cycle, the eddy kinetic energy level is a minimum in December/January and a maximum in April/May. In addition to seasonal variations, the eddy kinetic energy undergoes interannual changes. The energy transfers mainly from the mean fl ow to the eddy fi eld in the Kuroshio south of Japan, and dominant energy exchanges mainly occur along the Kuroshio path south of Japan in each year from 1993 to 2011. In addition, there is often barotropic instability south of Honshu. Regarding interactions between the eddy fi eld and mean fl ow, cyclonic and anticyclonic accelerations are also found along the Kuroshio path and they fl ank each other. There is cyclonic acceleration always imposed on southeast of Kyushu, and anticyclonic acceleration dominates south of Honshu from 2001 to mid-2005. Reynolds stress is used to explain the dynamic process of energy exchange. Furthermore, lag-correlation and linear regression analysis show that variability of the energy conversion rate and Reynolds stress involve responses to eddy acceleration at two time scales. The enhanced eddy acceleration induces large Reynolds stress, and enhanced Reynolds stress or barotropic instability further enforces energy transfer from the mean fl ow to the eddy fi eld.展开更多
Several bottom-mounted Acoustic Doppler Current Profiler (ADCP) moorings were deployed in the northern Yellow Sea (NYS) during the four seasons of 2006–2007 and also the summertime of 2009. A synthesis analysis on th...Several bottom-mounted Acoustic Doppler Current Profiler (ADCP) moorings were deployed in the northern Yellow Sea (NYS) during the four seasons of 2006–2007 and also the summertime of 2009. A synthesis analysis on the time-continuous records was performed to examine the characteristics and variations of tidal currents and mean flow over the observation period at these stations. Tidal currents accounted for ~75% of the total kinetic energy, with the absolute dominance of M2 constituent. Visible vertical variations of tidal flow were found on all sites, featured by the decrease of amplitude, increase of rotation rate as well as a decreasing trend of the phase for M2 component with depth. A notable exception was in the central NYS, where the maximum tidal currents occurred in the upper or middle layers (~20–40 m) instead of near the surface (<10 m). The observed mean flow was relatively weak, smaller than 15 cm/s. Velocity on the northern end of Yellow Sea Trough (YST) was characterized by low magnitude and an obvious layered structure vertically. In the Bohai Strait (BS) and the northern slope area, the currents weakened and the flow direction presented a major trend to deflect counterclockwise with depth in most observations. Summertime cyclonic circulation around the Yellow Sea Cold Water Mass (YSCWM), its intensification on the frontal zone and the Yellow Sea Warm Current (YSWC) for the winter season were all evident by our direct current measurements. However, the details of water exchange through the BS appeared partly diff erent from the traditionally-accepted pattern. The vertical diff erences of tidal and mean flow were larger in summer than that in winter, implying the influence of thermal structure to the local currents. Aff ected by the water stratification, mean flow usually reached its maximum near the thermocline in spring and summer, while showing a nearly uniform vertical distribution during winter.展开更多
Ten studied steels including different carbon content, silicon content, and manganese content were deformed in compression over a temperature range of 600 ?C to 1 000 ?C at the strain rate of 1 s-1. The curves of the ...Ten studied steels including different carbon content, silicon content, and manganese content were deformed in compression over a temperature range of 600 ?C to 1 000 ?C at the strain rate of 1 s-1. The curves of the mean flow stressdeformation temperature were drawn up. The mean flow stresses of higher carbon content steels decreased continuously as the applied deformation temperature increased in the whole temperature range, while the mean flow stress of lowest carbon steel displayed an abrupt drop near the two phases region. The reason for the abrupt drop phenomena was explained as the result of phase transformation. The mean flow stresses of steels with high silicon content and low manganese content also have this phenomena.展开更多
Using the active feedback control system on the elastic wave metamaterial,this research concentrates on the sound transmission with the dynamic effective model.The metamaterial is subjected to an incident pressure and...Using the active feedback control system on the elastic wave metamaterial,this research concentrates on the sound transmission with the dynamic effective model.The metamaterial is subjected to an incident pressure and immersed in the external mean flow.The elastic wave metamaterial consists of double plates and the upper and lower four-link mechanisms are attached inside.The vertical resonator is attached by the active feedback control system and connected with two four-link mechanisms.Based on the dynamic equivalent method,the metamaterial is equivalent as a single-layer plate by the dynamic effective parameter.With the coupling between the fluid and structure,the expression of the sound transmission loss(STL)is derived.This research shows the influence of effective mass density on sound transmission properties,and the STL in both modes can be tuned by the acceleration and displacement feedback constants.In addition,the dynamic response and the STL are also changed obviously by different values of structural damping,incident angle(i.e.,the elevation and azimuth angles)and Mach number of the external fluid with the mean flow property.The results for sound transmission by two methods are compared,i.e.,the virtual work principle for double plates and the dynamic equivalent method corresponding to a single one.This paper is expected to be helpful for understanding the sound transmission properties of both pure single-and double-plate models.展开更多
The NCEP/NCAR reanalysis data have been employed to diagnose variations of the zonal mean flow in boreal summer. Two leading EOF modes are found to dominate the spatial and temporal changes of the summertime zonal mea...The NCEP/NCAR reanalysis data have been employed to diagnose variations of the zonal mean flow in boreal summer. Two leading EOF modes are found to dominate the spatial and temporal changes of the summertime zonal mean winds in the troposphere. EOF1 shows the distribution of zonal-mean flow anomalies with higher variance in the North Polar Region, whereas the EOF2 shows the distribution of zonal-mean flow anomalies with higher variance in tropical and extra-tropical regions. The EOF1 and EOF2 have respectively the periodicities similar to those of AO and ENSO. Significant lag correlations have been found between EOF1 and ENSO, and between EOF2 and AO, in the seasons including spring, autumn, and winter. However, no significant correlations have been found between EOF1 in summer and ENSO in any other seasons, and between EOF2 in summer and AO in other seasons, no matter how big the lag that represents number of seasons has been set. These results suggest that the principal modes of summertime zonal mean flow could be statistically separated from each other. Hence, EOF1 and EOF2 are physically related to the AO and ENSO respectively. A theory called quasi-geostrophic non-acceleration theorem has been used to partly explain the possible mechanisms of the maintenance of the two principal modes. The composite differences of the divergence of Eliassen-Palm flux (E-P flux) between positive and negative years as obtained from the time series of EOF1 and EOF2 display the distributions that contribute to the zonal mean wind anomalies represented by EOF1 and EOF2, respectively. The planetary other than the synoptic waves dominate the behaviors of the E-P fluxes, suggesting the crucial role of the planetary waves in the maintenance of the zonal mean flow anomalies. The residual circulation as well as the friction, which cancel the divergence of the E-P flux, also play an important role in some places. These results are very helpful for our better understanding how the anomalous zonal mean flows maintain and how the ENSO and AO influence the global climate variations.展开更多
In this paper we investigate the one-dimensional hyperbolic mean curvatureflow for closed plane curves. More precisely, we consider a family of closed curves F : S1 × [0, T ) → R^2 which satisfies the followin...In this paper we investigate the one-dimensional hyperbolic mean curvatureflow for closed plane curves. More precisely, we consider a family of closed curves F : S1 × [0, T ) → R^2 which satisfies the following evolution equation δ^2F /δt^2 (u, t) = k(u, t)N(u, t)-▽ρ(u, t), ∨(u, t) ∈ S^1 × [0, T ) with the initial data F (u, 0) = F0(u) and δF/δt (u, 0) = f(u)N0, where k is the mean curvature and N is the unit inner normal vector of the plane curve F (u, t), f(u) and N0 are the initial velocity and the unit inner normal vector of the initial convex closed curve F0, respectively, and ▽ρ is given by ▽ρ Δ=(δ^2F /δsδt ,δF/δt) T , in which T stands for the unit tangent vector. The above problem is an initial value problem for a system of partial differential equations for F , it can be completely reduced to an initial value problem for a single partial differential equation for its support function. The latter equation is a hyperbolic Monge-Ampere equation. Based on this, we show that there exists a class of initial velocities such that the solution of the above initial value problem exists only at a finite time interval [0, Tmax) and when t goes to Tmax, either the solution convergesto a point or shocks and other propagating discontinuities are generated. Furthermore, we also consider the hyperbolic mean curvature flow with the dissipative terms and obtain the similar equations about the support functions and the curvature of the curve. In the end, we discuss the close relationship between the hyperbolic mean curvature flow and the equations for the evolving relativistic string in the Minkowski space-time R^1,1.展开更多
Based on the ERA-40 reanalysis data from the European Centre for Medium-Range Weather Forecasts and the output of ECHAM5/MPI-OM, this study investigated the interactions between the quasi-stationary planetary wave (SP...Based on the ERA-40 reanalysis data from the European Centre for Medium-Range Weather Forecasts and the output of ECHAM5/MPI-OM, this study investigated the interactions between the quasi-stationary planetary wave (SPW) and mean flow, and their responses to El Ni o-Southern Oscillation (ENSO) events in the northern hemispheric stratosphere. Results show that the activity of SPW is the strongest in winter, when the SPW propagates along the polar waveguide into the stratosphere and along the low-latitude waveguide to the subtropical tropopause. The analysis of three dimensional SPW structure indicates that the main sources of SPW activity are located over the Eurasian continent and the North Pacific north of 45°N. On the one hand, the two waveguides of the SPW reflect the influence of mean flow on the propagation of the SPW. On the other hand, the upward propagating SPW can interact with the stratospheric mean flow, leading to deceleration of the zonal mean westerly. Furthermore, the SPW exhibits clear responses to ENSO events. During El Ni o winters, the SPW in the stratosphere tends to propagate more upward and poleward. Its interactions with mean flow can induce a dipole pattern in zonal mean zonal winds, with accelerated westerly winds at low-middle latitudes and decelerated westerly winds at high latitudes. The ECHAM5/MPI-OM model reproduces the climatology of the SPW well. Although the simulated SPW is slightly weaker than the observations in the stratosphere, the model's performance has significant improvements compared with other GCMs used in previous studies. However, there are still some problems in the responses of the SPW to ENSO in the model. Although the model reproduces the responses of both the amplitude and the SPW-mean flow interactions to ENSO well in the troposphere, the stratospheric responses are quite weak. Therefore, further studies are needed to improve the simulation of the stratospheric atmospheric circulation and related dynamical processes.展开更多
A multi-dimensional computational fluid dynamics(CFD) approach was proposed in this study aiming to calculate the transfer matrix of an engine exhaust muffler in the conditions with and without mean flow.The CFD model...A multi-dimensional computational fluid dynamics(CFD) approach was proposed in this study aiming to calculate the transfer matrix of an engine exhaust muffler in the conditions with and without mean flow.The CFD model of the muffler with absorptive material defined as porous zone was calibrated with the measured noise reduction without mean flow,and was further employed to study the effect of the mean flow on the acoustic performance of the muffler.Furthermore,the exhaust acoustical source was derived from the calculated transfer matrices of six different additional acoustic loads obtained by the proposed CFD approach as well as the measured tail noise based on a multiload least squares method.Finally,the exhaust noise was predicted based on Thevenin's theorem.The proposed CFD approach was suggested to be able to predict the acoustic performance of a complex muffler considering mean flow(without and with mean flow) and heat transfer,and provide reasonable results of the exhaust noise.展开更多
This article concerns the self-similar solutions to the hyperbolic mean curvature flow (HMCF) for plane curves, which is proposed by Kong, Liu, and Wang and relates to an earlier proposal for general flows by LeFloc...This article concerns the self-similar solutions to the hyperbolic mean curvature flow (HMCF) for plane curves, which is proposed by Kong, Liu, and Wang and relates to an earlier proposal for general flows by LeFloch and Smoczyk. We prove that all curves immersed in the plane which move in a self-similar manner under the HMCF are straight lines and circles. Moreover, it is found that a circle can either expand to a larger one and then converge to a point, or shrink directly and converge to a point, where the curvature approaches to infinity.展开更多
Under the hypothesis of mean curvature flows of hypersurfaces, we prove that the limit of the smooth rescaling of the singularity is weakly convex. It is a generalization of the result due to G.Huisken and C. Sinestra...Under the hypothesis of mean curvature flows of hypersurfaces, we prove that the limit of the smooth rescaling of the singularity is weakly convex. It is a generalization of the result due to G.Huisken and C. Sinestrari in. These apriori bounds are satisfied for mean convex hypersurfaces in locally symmetric Riemannian manifolds with nonnegative sectional curvature.展开更多
Given a family of smooth immersions of closed hypersurfaces in a locally symmetric Riemannian manifold with bounded geometry, moving by mean curvature flow, we show that at the first finite singular time of mean curva...Given a family of smooth immersions of closed hypersurfaces in a locally symmetric Riemannian manifold with bounded geometry, moving by mean curvature flow, we show that at the first finite singular time of mean curvature flow, certain subcritical quantities concerning the second fundamental form blow up. This result not only generalizes a result of Le-Sesum and Xu-Ye-Zhao, but also extends the latest work of Le in the Euclidean case展开更多
Based on the traditional theory of wave mean flow interaction, an improved quasi-geostrophic Eliassen-Palm flux with diabatic heating included is deduced. It is shown that there exists an intrinsic relation between th...Based on the traditional theory of wave mean flow interaction, an improved quasi-geostrophic Eliassen-Palm flux with diabatic heating included is deduced. It is shown that there exists an intrinsic relation between the atmospheric energy cycle derived by Lorenz and the wave energy transfer derived by Eliassen and Palm. From this relation it becomes clear that the energy propagation process of large-scale stationary wave is indeed a part of Lorenz energy cycle, and the energy transform from mean flow to wave equals the global mass integral of the divergence of local wave energy flux or the global integral of local wave energy. The diagnostic results by using NCEP/NCAR reanalysis data suggest that the classical adiabatic Eliassen-Palm flux relation can present only the wintertime wave energy transformation. For other seasons, however, the diabatic effect must be taken into account.展开更多
Inertia-gravity waves play an increasingly important role in the middle atmosphere dynamics. As a result, more attention has been paid to the study of inertia-gravity waves, especially to the middle atmosphere gravity...Inertia-gravity waves play an increasingly important role in the middle atmosphere dynamics. As a result, more attention has been paid to the study of inertia-gravity waves, especially to the middle atmosphere gravity waves. This paper presents some aspects of inertia-gravity waves with emphasis on the propagation. Two methods are used here, namely, geometric optical method and physical optical method. We can see from the study that inertia-gravity waves are similar to planetary waves in some respects and they are different from planetary waves in others.展开更多
The interaction between waves, currents and bottoms in estuarine and coastal regions is ubiquitious, in particular the dynamic mechanism of waves on large-scale slowly varying currents. The wave action concept may be ...The interaction between waves, currents and bottoms in estuarine and coastal regions is ubiquitious, in particular the dynamic mechanism of waves on large-scale slowly varying currents. The wave action concept may be extended and applicated to the study of the mechanism. Considering the effects of moving bottoms and starting from the Navier-Stokes equation of motion of a vinous fluid including the Coriolis force, a generalized mean-flow medel theory for the nearshore region, that is, a set of mean-flow equations and their generalized wave action equation involving the three new kinds of actions termed respectively as the current wave action, the bottom wave action and the dissipative wave action which can be applied to arbitrary depth over moving bottoms and ambient currents with a typical vertical structure, is developed by vertical integration and time-averaglng over a wave peried, thus extending the classical concept, wave action, from the ideal averaged flow conservative system to the real averaged flow dissipative dynamical system, and having a large range of application.展开更多
基金Research Project No.[75-09-01] on medium-range numerical weather forecasts.
文摘Non-acceleration theorem in a primitive equation system is developed to investigate the influences of waves on the mean flow variation against external forcing. Numerical results show that mechanical forcing overwhelms thermal forcing in maintaining the mean flow in which the internal mechanical forcing associated with horizontal eddy flux of momentum plays the most important roles. Both internal forcing and external forcing are shown to be active and at the first place for the mean flow variations, whereas the forcing-induced mean meridional circulation is passive and secondary. It is also shown that the effects on mean flow of external mechanical forcing are concentrated in the lower troposphere, whereas those due to wave-mean flow interaction are more important in the upper troposphere. These act together and result in the vertically easterly shear in low latitudes and westerly shear in mid-latitudes. This vertical shear of mean flow is to some extent weakened by thermal forcing.
基金Supported by the National Natural Science Foundation of China(10902050)the China Postdoctoral Science Foundation Funded Project(20100481138)the Aeronautical Science Foundation of China(20101452017)
文摘Acoustic propagation problems in the sheared mean flow are numerically investigated using different acoustic propagation equations , including linearized Euler equations ( LEE ) and acoustic perturbation equations ( APE ) .The resulted acoustic pressure is compared for the cases of uniform mean flow and sheared mean flow using both APE and LEE.Numerical results show that interactions between acoustics and mean flow should be properly considered to better understand noise propagation problems , and the suitable option of the different acoustic equations is indicated by the present comparisons.Moreover , the ability of APE to predict acoustic propagation is validated.APE can replace LEE when the 3-D flow-induced noise problem is solved , thus computational cost can decrease.
文摘The gravity wave breaking is crucial to the large-scale circulation of middle atmosphere. In this paper, we follow Lindzen (1981) to draw out the parameterization of two-dimensional gravity wave breaking including inertial effect. Also we present some properties of critical levels and inertial critical levels.
基金supported by National Engineering School of Tunis (No.13039.1)
文摘To reduce computational costs, an improved form of the frequency domain boundary element method(BEM) is proposed for two-dimensional radiation and propagation acoustic problems in a subsonic uniform flow with arbitrary orientation. The boundary integral equation(BIE) representation solves the two-dimensional convected Helmholtz equation(CHE) and its fundamental solution, which must satisfy a new Sommerfeld radiation condition(SRC) in the physical space. In order to facilitate conventional formulations, the variables of the advanced form are expressed only in terms of the acoustic pressure as well as its normal and tangential derivatives, and their multiplication operators are based on the convected Green's kernel and its modified derivative. The proposed approach significantly reduces the CPU times of classical computational codes for modeling acoustic domains with arbitrary mean flow. It is validated by a comparison with the analytical solutions for the sound radiation problems of monopole,dipole and quadrupole sources in the presence of a subsonic uniform flow with arbitrary orientation.
基金This work is supported by National Natural Science Foundation of China.
文摘A study is made of the distribution of the diagnostic quantity vector E and the teleconnection structure of 30-50 (quasi-40) day oscillation, together with the dependence on the conversion of barotropic unstable energy of mean flow in terms of ECWMF daily 500 hPa grid data in winter, indicating that the energy transportation is closely associated with the westerly jet position, with zonal (meridional) propagation in the strong (weak) wind region, that considerable conversion of barotropic energy occurs at the jet exit region where low-frequency oscillation gains energy from the mean flow, leading to maximum kinetic energy for the oscillation observed there, which is marked by evident barotropy in striking contrast to the baroclinicity at low latitudes and that the teleconnection core is related to the center of action in the atmosphere and bound up with the pattern of the west wind.
基金Supported by the Knowledge Innovation Program of Chinese Academy of Sciences(No.KZCX2-EW-201)the Basic Research Program of Science and Technology Projects of Qingdao(No.11-1-4-95-jch)
文摘Using 19-year satellite altimetric data, variations in the eddy kinetic energy, energy exchanges and interaction between the eddy fi eld and mean fl ow are discussed for the Kuroshio south of Japan. In the seasonal cycle, the eddy kinetic energy level is a minimum in December/January and a maximum in April/May. In addition to seasonal variations, the eddy kinetic energy undergoes interannual changes. The energy transfers mainly from the mean fl ow to the eddy fi eld in the Kuroshio south of Japan, and dominant energy exchanges mainly occur along the Kuroshio path south of Japan in each year from 1993 to 2011. In addition, there is often barotropic instability south of Honshu. Regarding interactions between the eddy fi eld and mean fl ow, cyclonic and anticyclonic accelerations are also found along the Kuroshio path and they fl ank each other. There is cyclonic acceleration always imposed on southeast of Kyushu, and anticyclonic acceleration dominates south of Honshu from 2001 to mid-2005. Reynolds stress is used to explain the dynamic process of energy exchange. Furthermore, lag-correlation and linear regression analysis show that variability of the energy conversion rate and Reynolds stress involve responses to eddy acceleration at two time scales. The enhanced eddy acceleration induces large Reynolds stress, and enhanced Reynolds stress or barotropic instability further enforces energy transfer from the mean fl ow to the eddy fi eld.
基金Supported by the State Key Program of National Natural Science of China(Nos.41430963,U1706215)the National Science Foundation for Young Scientists of China(No.41506012)
文摘Several bottom-mounted Acoustic Doppler Current Profiler (ADCP) moorings were deployed in the northern Yellow Sea (NYS) during the four seasons of 2006–2007 and also the summertime of 2009. A synthesis analysis on the time-continuous records was performed to examine the characteristics and variations of tidal currents and mean flow over the observation period at these stations. Tidal currents accounted for ~75% of the total kinetic energy, with the absolute dominance of M2 constituent. Visible vertical variations of tidal flow were found on all sites, featured by the decrease of amplitude, increase of rotation rate as well as a decreasing trend of the phase for M2 component with depth. A notable exception was in the central NYS, where the maximum tidal currents occurred in the upper or middle layers (~20–40 m) instead of near the surface (<10 m). The observed mean flow was relatively weak, smaller than 15 cm/s. Velocity on the northern end of Yellow Sea Trough (YST) was characterized by low magnitude and an obvious layered structure vertically. In the Bohai Strait (BS) and the northern slope area, the currents weakened and the flow direction presented a major trend to deflect counterclockwise with depth in most observations. Summertime cyclonic circulation around the Yellow Sea Cold Water Mass (YSCWM), its intensification on the frontal zone and the Yellow Sea Warm Current (YSWC) for the winter season were all evident by our direct current measurements. However, the details of water exchange through the BS appeared partly diff erent from the traditionally-accepted pattern. The vertical diff erences of tidal and mean flow were larger in summer than that in winter, implying the influence of thermal structure to the local currents. Aff ected by the water stratification, mean flow usually reached its maximum near the thermocline in spring and summer, while showing a nearly uniform vertical distribution during winter.
基金Funded by Shanghai Pujiang Program(No.16PJ1430200)
文摘Ten studied steels including different carbon content, silicon content, and manganese content were deformed in compression over a temperature range of 600 ?C to 1 000 ?C at the strain rate of 1 s-1. The curves of the mean flow stressdeformation temperature were drawn up. The mean flow stresses of higher carbon content steels decreased continuously as the applied deformation temperature increased in the whole temperature range, while the mean flow stress of lowest carbon steel displayed an abrupt drop near the two phases region. The reason for the abrupt drop phenomena was explained as the result of phase transformation. The mean flow stresses of steels with high silicon content and low manganese content also have this phenomena.
基金The authors wish to express gratitude for the support provided by the National Natural Science Foundation of China(Grant Nos.11922209,11991031 and 12021002).
文摘Using the active feedback control system on the elastic wave metamaterial,this research concentrates on the sound transmission with the dynamic effective model.The metamaterial is subjected to an incident pressure and immersed in the external mean flow.The elastic wave metamaterial consists of double plates and the upper and lower four-link mechanisms are attached inside.The vertical resonator is attached by the active feedback control system and connected with two four-link mechanisms.Based on the dynamic equivalent method,the metamaterial is equivalent as a single-layer plate by the dynamic effective parameter.With the coupling between the fluid and structure,the expression of the sound transmission loss(STL)is derived.This research shows the influence of effective mass density on sound transmission properties,and the STL in both modes can be tuned by the acceleration and displacement feedback constants.In addition,the dynamic response and the STL are also changed obviously by different values of structural damping,incident angle(i.e.,the elevation and azimuth angles)and Mach number of the external fluid with the mean flow property.The results for sound transmission by two methods are compared,i.e.,the virtual work principle for double plates and the dynamic equivalent method corresponding to a single one.This paper is expected to be helpful for understanding the sound transmission properties of both pure single-and double-plate models.
基金Supported jointly by the National Natural Science Foundation of China under Grant No.40675025the Jiangsu Key Laboratory of Meteorological Disaster under Grant No.KLME060101the National Key Technology R&D Program under Grant No.2007BAC29B02.
文摘The NCEP/NCAR reanalysis data have been employed to diagnose variations of the zonal mean flow in boreal summer. Two leading EOF modes are found to dominate the spatial and temporal changes of the summertime zonal mean winds in the troposphere. EOF1 shows the distribution of zonal-mean flow anomalies with higher variance in the North Polar Region, whereas the EOF2 shows the distribution of zonal-mean flow anomalies with higher variance in tropical and extra-tropical regions. The EOF1 and EOF2 have respectively the periodicities similar to those of AO and ENSO. Significant lag correlations have been found between EOF1 and ENSO, and between EOF2 and AO, in the seasons including spring, autumn, and winter. However, no significant correlations have been found between EOF1 in summer and ENSO in any other seasons, and between EOF2 in summer and AO in other seasons, no matter how big the lag that represents number of seasons has been set. These results suggest that the principal modes of summertime zonal mean flow could be statistically separated from each other. Hence, EOF1 and EOF2 are physically related to the AO and ENSO respectively. A theory called quasi-geostrophic non-acceleration theorem has been used to partly explain the possible mechanisms of the maintenance of the two principal modes. The composite differences of the divergence of Eliassen-Palm flux (E-P flux) between positive and negative years as obtained from the time series of EOF1 and EOF2 display the distributions that contribute to the zonal mean wind anomalies represented by EOF1 and EOF2, respectively. The planetary other than the synoptic waves dominate the behaviors of the E-P fluxes, suggesting the crucial role of the planetary waves in the maintenance of the zonal mean flow anomalies. The residual circulation as well as the friction, which cancel the divergence of the E-P flux, also play an important role in some places. These results are very helpful for our better understanding how the anomalous zonal mean flows maintain and how the ENSO and AO influence the global climate variations.
基金Kong and Wang was supported in part by the NSF of China (10671124)the NCET of China (NCET-05-0390)the work of Liu was supported in part by the NSF of China
文摘In this paper we investigate the one-dimensional hyperbolic mean curvatureflow for closed plane curves. More precisely, we consider a family of closed curves F : S1 × [0, T ) → R^2 which satisfies the following evolution equation δ^2F /δt^2 (u, t) = k(u, t)N(u, t)-▽ρ(u, t), ∨(u, t) ∈ S^1 × [0, T ) with the initial data F (u, 0) = F0(u) and δF/δt (u, 0) = f(u)N0, where k is the mean curvature and N is the unit inner normal vector of the plane curve F (u, t), f(u) and N0 are the initial velocity and the unit inner normal vector of the initial convex closed curve F0, respectively, and ▽ρ is given by ▽ρ Δ=(δ^2F /δsδt ,δF/δt) T , in which T stands for the unit tangent vector. The above problem is an initial value problem for a system of partial differential equations for F , it can be completely reduced to an initial value problem for a single partial differential equation for its support function. The latter equation is a hyperbolic Monge-Ampere equation. Based on this, we show that there exists a class of initial velocities such that the solution of the above initial value problem exists only at a finite time interval [0, Tmax) and when t goes to Tmax, either the solution convergesto a point or shocks and other propagating discontinuities are generated. Furthermore, we also consider the hyperbolic mean curvature flow with the dissipative terms and obtain the similar equations about the support functions and the curvature of the curve. In the end, we discuss the close relationship between the hyperbolic mean curvature flow and the equations for the evolving relativistic string in the Minkowski space-time R^1,1.
基金supported by National Basic Research Program of China(Grant No. 2010CB428603)National Natural Science Foundation of China (Grant Nos. 41025017 and 40775035)
文摘Based on the ERA-40 reanalysis data from the European Centre for Medium-Range Weather Forecasts and the output of ECHAM5/MPI-OM, this study investigated the interactions between the quasi-stationary planetary wave (SPW) and mean flow, and their responses to El Ni o-Southern Oscillation (ENSO) events in the northern hemispheric stratosphere. Results show that the activity of SPW is the strongest in winter, when the SPW propagates along the polar waveguide into the stratosphere and along the low-latitude waveguide to the subtropical tropopause. The analysis of three dimensional SPW structure indicates that the main sources of SPW activity are located over the Eurasian continent and the North Pacific north of 45°N. On the one hand, the two waveguides of the SPW reflect the influence of mean flow on the propagation of the SPW. On the other hand, the upward propagating SPW can interact with the stratospheric mean flow, leading to deceleration of the zonal mean westerly. Furthermore, the SPW exhibits clear responses to ENSO events. During El Ni o winters, the SPW in the stratosphere tends to propagate more upward and poleward. Its interactions with mean flow can induce a dipole pattern in zonal mean zonal winds, with accelerated westerly winds at low-middle latitudes and decelerated westerly winds at high latitudes. The ECHAM5/MPI-OM model reproduces the climatology of the SPW well. Although the simulated SPW is slightly weaker than the observations in the stratosphere, the model's performance has significant improvements compared with other GCMs used in previous studies. However, there are still some problems in the responses of the SPW to ENSO in the model. Although the model reproduces the responses of both the amplitude and the SPW-mean flow interactions to ENSO well in the troposphere, the stratospheric responses are quite weak. Therefore, further studies are needed to improve the simulation of the stratospheric atmospheric circulation and related dynamical processes.
文摘A multi-dimensional computational fluid dynamics(CFD) approach was proposed in this study aiming to calculate the transfer matrix of an engine exhaust muffler in the conditions with and without mean flow.The CFD model of the muffler with absorptive material defined as porous zone was calibrated with the measured noise reduction without mean flow,and was further employed to study the effect of the mean flow on the acoustic performance of the muffler.Furthermore,the exhaust acoustical source was derived from the calculated transfer matrices of six different additional acoustic loads obtained by the proposed CFD approach as well as the measured tail noise based on a multiload least squares method.Finally,the exhaust noise was predicted based on Thevenin's theorem.The proposed CFD approach was suggested to be able to predict the acoustic performance of a complex muffler considering mean flow(without and with mean flow) and heat transfer,and provide reasonable results of the exhaust noise.
基金supported in part by a grant from China Scholarship Councilthe National Natural Science Foundation of China(11301006)the Anhui Provincial Natural Science Foundation(1408085MA01)
文摘This article concerns the self-similar solutions to the hyperbolic mean curvature flow (HMCF) for plane curves, which is proposed by Kong, Liu, and Wang and relates to an earlier proposal for general flows by LeFloch and Smoczyk. We prove that all curves immersed in the plane which move in a self-similar manner under the HMCF are straight lines and circles. Moreover, it is found that a circle can either expand to a larger one and then converge to a point, or shrink directly and converge to a point, where the curvature approaches to infinity.
基金supported partially by the National Natural Science Foundation of China (10871171)the Chinese-Hungarian Sci. and Tech. cooperation (for 2007-2009)
文摘Under the hypothesis of mean curvature flows of hypersurfaces, we prove that the limit of the smooth rescaling of the singularity is weakly convex. It is a generalization of the result due to G.Huisken and C. Sinestrari in. These apriori bounds are satisfied for mean convex hypersurfaces in locally symmetric Riemannian manifolds with nonnegative sectional curvature.
基金supported by the NSFC(11101267,11271132)the Innovation Program of Shanghai Municipal Education Commission(13YZ087)the Science and Technology Program of Shanghai Maritime University(20120061)
文摘Given a family of smooth immersions of closed hypersurfaces in a locally symmetric Riemannian manifold with bounded geometry, moving by mean curvature flow, we show that at the first finite singular time of mean curvature flow, certain subcritical quantities concerning the second fundamental form blow up. This result not only generalizes a result of Le-Sesum and Xu-Ye-Zhao, but also extends the latest work of Le in the Euclidean case
基金the Chinese Academy of Sciences(Grant No.ZKCX2-SW-210) the National Natural Science Foundation of China(Grant Nos.40405016,40475027,40135020,40221503 , 40023001).
文摘Based on the traditional theory of wave mean flow interaction, an improved quasi-geostrophic Eliassen-Palm flux with diabatic heating included is deduced. It is shown that there exists an intrinsic relation between the atmospheric energy cycle derived by Lorenz and the wave energy transfer derived by Eliassen and Palm. From this relation it becomes clear that the energy propagation process of large-scale stationary wave is indeed a part of Lorenz energy cycle, and the energy transform from mean flow to wave equals the global mass integral of the divergence of local wave energy flux or the global integral of local wave energy. The diagnostic results by using NCEP/NCAR reanalysis data suggest that the classical adiabatic Eliassen-Palm flux relation can present only the wintertime wave energy transformation. For other seasons, however, the diabatic effect must be taken into account.
文摘Inertia-gravity waves play an increasingly important role in the middle atmosphere dynamics. As a result, more attention has been paid to the study of inertia-gravity waves, especially to the middle atmosphere gravity waves. This paper presents some aspects of inertia-gravity waves with emphasis on the propagation. Two methods are used here, namely, geometric optical method and physical optical method. We can see from the study that inertia-gravity waves are similar to planetary waves in some respects and they are different from planetary waves in others.
基金This paper was supported bythe Foundationforthe Author of National Excellent Doctoral Dissertation of P.R.China(Grant No.200428) the National Natural Science Foundation of China (Grant Nos .10272072 and 50424913) +1 种基金theShanghai Natural Science Foundation (Grant No.05ZR14048) the Shanghai Leading Academic Discipline Pro-ject (Grant No. Y0103)
文摘The interaction between waves, currents and bottoms in estuarine and coastal regions is ubiquitious, in particular the dynamic mechanism of waves on large-scale slowly varying currents. The wave action concept may be extended and applicated to the study of the mechanism. Considering the effects of moving bottoms and starting from the Navier-Stokes equation of motion of a vinous fluid including the Coriolis force, a generalized mean-flow medel theory for the nearshore region, that is, a set of mean-flow equations and their generalized wave action equation involving the three new kinds of actions termed respectively as the current wave action, the bottom wave action and the dissipative wave action which can be applied to arbitrary depth over moving bottoms and ambient currents with a typical vertical structure, is developed by vertical integration and time-averaglng over a wave peried, thus extending the classical concept, wave action, from the ideal averaged flow conservative system to the real averaged flow dissipative dynamical system, and having a large range of application.