High-order Bragg forward scattering and frequency shift of low-frequency underwater acoustic field by moving rough sea surface

Acoustic scattering modulation caused by an undulating sea surface on the space-time dimension seriously affects underwater detection and target recognition.Herein,underwater acoustic scattering modulation from a moving rough sea surface is studied based on integral equation and parabolic equation.And with the principles of grating and constructive interference,the mechanism of this acoustic scattering modulation is explained.The periodicity of the interference of moving rough sea surface will lead to the interference of the scattering field at a series of discrete angles,which will form comb-like and frequency-shift characteristics on the intensity and the frequency spectrum of the acoustic scattering field,respectively,which is a high-order Bragg scattering phenomenon.Unlike the conventional Doppler effect,the frequency shifts of the Bragg scattering phenomenon are multiples of the undulating sea surface frequency and are independent of the incident sound wave frequency.Therefore,even if a low-frequency underwater acoustic field is incident,it will produce obvious frequency shifts.Moreover,under the action of ideal sinusoidal waves,swells,fully grown wind waves,unsteady wind waves,or mixed waves,different moving rough sea surfaces create different acoustic scattering processes and possess different frequency shift characteristics.For the swell wave,which tends to be a single harmonic wave,the moving rough sea surface produces more obvious high-order scattering and frequency shifts.The same phenomena are observed on the sea surface under fully grown wind waves,however,the frequency shift slightly offsets the multiple peak frequencies of the wind wave spectrum.Comparing with the swell and fully-grown wind waves,the acoustic scattering and frequency shift are not obvious for the sea surface under unsteady wind waves.

High-Order Method with Moving Frames to Compute the Covariant Derivatives of Vectors on General 2D Curved Surfaces

The covariant derivative is a generalization of differentiating vectors.The Euclidean derivative is a special case of the covariant derivative in Euclidean space.The covariant derivative gathers broad attention,particularly when computing vector derivatives on curved surfaces and volumes in various applications.Covariant derivatives have been computed using the metric tensor from the analytically known curved axes.However,deriving the global axis for the domain has been mathematically and computationally challenging for an arbitrary two-dimensional(2D)surface.Consequently,computing the covariant derivative has been difficult or even impossible.A novel high-order numerical scheme is proposed for computing the covariant derivative on any 2D curved surface.A set of orthonormal vectors,known as moving frames,expand vectors to compute accurately covariant derivatives on 2D curved surfaces.The proposed scheme does not require the construction of curved axes for the metric tensor or the Christoffel symbols.The connectivity given by the Christoffel symbols is equivalently provided by the attitude matrix of orthonormal moving frames.Consequently,the proposed scheme can be extended to the general 2D curved surface.As an application,the Helmholtz‐Hodge decomposition is considered for a realistic atrium and a bunny.

Dual solutions in boundary layer flow of a moving fluid over a moving permeable surface in presence of prescribed surface temperature and thermal radiation

An analysis of the heat transfer for a boundary layer forced convective flow past a moving permeable flat surface parallel to a moving fluid is presented. Prescribed surface temperature at the boundary is considered, A thermal radiation term in the energy equation is considered. The similarity solutions for the problem are obtained and the reduced ordinary differential equations are solved numerically. To support the validity of the numerical results, a comparison is made with the available results for some particular cases of this study. Dual solutions exist when the surface and the fluid move in the opposite directions.

Calculation of stratum surface principal curvature based on a moving least square method

With the east section of the Changji sag Zhunger Basin as a case study, both a principal curvature method and a moving least square method are elaborated. The moving least square method is introduced, for the first time, to fit a stratum surface. The results show that, using the same-degree base function, compared with a traditional least square method, the moving least square method can produce lower fitting errors, the fitting surface can describe the morphological characteristics of stratum surfaces more accurately and the principal curvature values vary within a wide range and may be more suitable for the prediction of the distribution of structural fractures. The moving least square method could be useful in curved surface fitting and stratum curvature analysis.

Finite Element Simulation of Processes Involving Moving Heat Sources. Application to Welding and Surface Treatment

A wide range of welding and surface treatment processes involve the use of a heat source which is moving at a constant speed over the component. The numerical simulation of such processes implies a transient analysis using a very refined mesh in order to follow properly the path of the heat source. The 3D-mesh size can be very large if one consider the welds length or the heat-treated surface size in industrial components. To reduce the computational time to acceptable values, several techniques have been investigated. The first type is to use analytical methods such as Rosenthal equations. The second type of solutions consists in performing a transient analysis using adaptive meshing. But, for a large proportion of the involved processes, practical experience demonstrates the existence of quasi steady state conditions over the major part of the heat source path. Numerical algorithms have therefore been developed to directly compute the steady temperature, metallurgical phase proportion and stress distributions. This paper gives a general overview of the different numerical methods used to simulate welding and surface treatment processes with a special emphasis on the steady state calculation. The benefits and limitations of each of them are discussed and applications are presented.

Boundary layer flow on a moving surface in otherwise quiescent pseudo-plastic non-Newtonian fluids

A theoretical analysis for the boundary layer flow over a continuous moving surface in an otherwise quiescent pseudo-plastic non-Newtonian fluid medium was presented. The types of potential flows necessary for similar solutions to the boundary layer equations were determined and the solutions were numerically presented for different values of power law exponent.

On the Boundary Layer Flow over a Moving Surface in a Fluid with Temperature-Dependent Viscosity

This paper examines a boundary layer flow over a continuously moving heated flat surface with velocity??in a streaming flow with velocity??and with temperature dependent viscosity. The momentum and the energy equations are coupled through the viscous dissipation term. The coupled boundary layer equations are transformed into a self-similar form using an appropriate similarity variable. An efficient numerical technique is used to solve the self-similar boundary layer equations. It is shown that at low enough values for the velocity ratio , an increase in viscous dissipation enhances greatly the local heat transfer leading to temperature overshoots adjacent to the wall. The viscosity variation parameter is shown to have significant effects on the temperature dependent viscosity and the velocity and temperature distribution within the boundary layer.

Viscous Slip MHD Flow over a Moving Sheet with an Arbitrary Surface Velocity

The magnetohydrodynamic（MHD） flow induced by a stretching or shrinking sheet under slip conditions is studied.Analytical solutions based on the boundary layer assumption are obtained in a closed form and can be applied to a flow configuration with any arbitrary velocity distributions. Seven typical sheet velocity profiles are employed as illustrating examples. The solutions to the slip MHD flow are derived from the general solution and discussed in detail. Different from self-similar boundary layer flows, the flows studied in this work have solutions in explicit analytical forms. However, the current flows require special mass transfer at the wall, which is determined by the moving velocity of the sheet. The effects of the slip parameter, the mass transfer at the wall, and the magnetic field on the flow are also demonstrated.

Group Method Analysis of MHD Mixed Convective Flow Past on a Moving Curved Surface with Suction

The group-theorytic approach is applied for solving the problem of the unsteady MHD mixed convective flow past on a moving curved surface. The application of two-parameter groups reduces the number of independent variables by two, and consequently the system of governing partial differential equations with boundary conditions reduces to a system of ordinary differential equations with appropriate boundary conditions. The obtained ordinary differential equations are solved numerically using the shooting method. The effects of varying parameters governing the problem are studied. A comparison with previous work is presented.

Numerical Simulation of Sloshing with Large Deforming Free Surface by MPS-LES Method

Moving particle semi-implicit （MPS） method is a fully Lagrangian particle method which can easily solve problems with violent free surface. Although it has demonstrated its advantage in ocean engineering applications, it still has some defects to be improved. In this paper, MPS method is extended to the large eddy simulation （LES） by coupling with a sub-particle-scale （SPS） turbulence model. The SPS turbulence model turns into the Reynolds stress terms in the filtered momentum equation, and the Smagorinsky model is introduced to describe the Reynolds stress terms. Although MPS method has the advantage in the simulation of the free surface flow, a lot of non-free surface particles are treated as free surface particles in the original MPS model. In this paper, we use a new free surface tracing method and the key point is ＂neighbor particle＂. In this new method, the zone around each particle is divided into eight parts, and the particle will be treated as a free surface particle as long as there are no ＂neighbor particles＂ in any two parts of the zone. As the number density parameter judging method has a high efficiency for the free surface particles tracing, we combine it with the neighbor detected method. First, we select out the particles which may be mistreated with high probabilities by using the number density parameter judging method. And then we deal with these particles with the neighbor detected method. By doing this, the new mixed free surface tracing method can reduce the mistreatment problem efficiently. The serious pressure fluctuation is an obvious defect in MPS method, and therefore an area-time average technique is used in this paper to remove the pressure fluctuation with a quite good result. With these improvements, the modified MPS-LES method is applied to simulate liquid sloshing problems with large deforming free surface. Results show that the modified MPS-LES method can simulate the large deforming free surface easily. It can not only capture the large impact pressure accurately on rolling tank wall but also can generate all physical phenomena successfully. The good agreement between numerical and experimental results proves that the modified MPS-LES method is a good CFD methodology in free surface flow simulations.

The Fabrication of Microstructure Surface of Superhydrophobic Coating by Surface Gelation Technology

The microstructured surface of materials were fabricated by a two-step acid-base catalyzed sol-gel process. In fluorinated polymer with PTFE doping, the well-proportioned composite sols were prepared using sol-gel processing under the hydrochloric acid and deficiency of water conditions. After the substrate was coated by composite sols, and the gelation treatment on the surface of composite coating, the micrometer-scale and nanometer-scale hierarchical structures were formed in surface layer of material. XPS and TEM technologies were employed to identify that the gelation occurs just on the surface of composite coating. The morphology of coating surface was observed by SEM and AFM technologies. The microstructured surface of material can be fabricated using this inexpensive and easily controlled method on low surface energy resin materials, the super-hydrophobic coatings materials can be prepared.

MODEL B-I FOR MACRO RECTANGULAR ELEMENT AND PROBLEMS OF INFLUENCE SURFACE FOR PLATE STRUCTURES

Model B-I for marco rectangular element is presented for the first time in this paper. To establish the influence surf ace for resultant R of bending plates, a number of generalized distributive loads q are defined. It is shown by numerical examples that Model B-I and the formula for the generalized distributive loads advanced in this paper are featured by high accuracy, low memory space and flexibility in practical application, and that they are especially effective for plate structures subject to moving loads, such as the two-dimensional continuous plates of highway bridges and the flat stabs in piled jetty engineering.

On study of horizontal thin film flow of Sisko fluid due to surface tension gradient

The present paper is concerned with the steady thin film flow of the Sisko fluid on a horizontal moving plate, where the surface tension gradient is a driving mechanism. The analytic solution for the resulting nonlinear ordinary differential equation is obtained by the Adomian decomposition method （ADM）. The physical quantities are derived including the pressure profile, the velocity profile, the maximum residue time, the stationary points, the volume flow rate, the average film velocity, the uniform film thickness, the shear stress, the surface tension profile~ and the vorticity vector. It is found that the velocity of the Sisko fluid film decreases when the fluid behavior index and the Sisko fluid parameter increase, whereas it increases with an increase in the inverse capillary number. An increase in the inverse capillary number results in an increase in the surface tension which in turn results in an increase in the surface tension gradient on the Sisko fluid film. The locations of the stationary points are shifted towards the moving plate with the increase in the inverse capillary number, and vice versa locations for the stationary points are found with the increasing Sisko fluid parameter. Furthermore, shear thinning and shear thickening characteristics of the Sisko fluid are discussed. A comparison is made between the Sisko fluid film and the Newtonian fluid film.

THE SURFACE PATH OF THE KUROSHIO EXTENSION'S WESTERN SECTOR AND THE EDDIES ON BOTH SIDES

The surface path of the Kuroshio Extension’s western sector and the eddies on both sides are systematically analysed based on the GEK-measured surface current and temperature-salinity data from 1955-1985.The main results are shown as follows : 1) According to the position and the features of distribution pattern, the surface path of the Kuroshio Extension’s western sector is classified into two kinds (straight and meander ) and seven types (f. ne, Sc, Ui or Vi,Vdi or Udi ,f+v). The straight kind accounts for 1/3 and the meander kind accounts for 2/3. 2) The warm eddies on the northern side originate mostly from the area off Sanriku and Joban of Japan. Their moving paths lie in two patterns: Pattern I, eddy moves northeastward or northward ; Pattern II. eddy rotates about the original area.The cold eddies on the southern side originate mainly from the area off Boso Peninsula. Their moving paths also lie in two patterns: Pattern III,eddy moves from west to east; Pattern IV.eddy moves from north to south.

土壤表面与运动目标复合电磁散射的矩量法研究

为了满足土壤表面上方雷达目标检测、识别和分类等的需要,采用空间域合成法生成高度起伏遵守Alpha-stable分布的土壤表面,选用Topp模型模拟土壤的介电常数,运用矩量法研究了锥形波入射条件下土壤与其上方运动的三角形截面柱的复合电磁散射问题,通过数值计算得到了复合散射系数随散射角的变化曲线,分析了等边三角形和不规则三角形截面柱在土壤表面上方横向平移、纵向平移、转动以及电磁波入射角对复合散射系数的影响。数值计算结果表明:复合散射系数随散射角振荡地变化,等边三角形截面柱的纵向平移和转动对复合散射系数有较大的影响,而横向平移对复合散射系数的影响很小;不规则三角形截面柱的横向平移和纵向平移对复合散射系数产生的影响很小,转动对复合散射系数的影响较大。有关结果可为民用、军用领域工程问题提供理论依据和实践方法。

往复式涡环生成装置性能的多目标优化

为了将涡环更好地应用到送风等流体输送领域,需要提高涡环生成频率以获得更高的流体输送效率。本文设计一种能够提高涡环生成频率的往复式涡环生成装置,该装置在一个活塞运动周期中可以生成两个涡环,有效地利用了活塞往复运动过程中的推力。针对涡环远距离输送的特点,设计并进行实验;基于响应曲面法与NSGA-II算法对往复式涡环生成装置性能进行多目标优化,使其能够将涡环输送更远距离。结果表明:涡环1与涡环2的输送距离难以同时达到最大值,涡环平均输送距离最大为295 cm,相比优化前增加了2.25%;单个涡环输送距离最大为377 cm,相比优化前增加了6.2%。

基于响应面法的隧道复合式路面力学响应分析

为揭示移动荷载作用下公路隧道复合式路面结构力学响应,通过现场试验与有限元法相结合及模型精度验证的手段,基于响应面试验设计方法分析有仰拱和无仰拱的隧道复合式路面力学响应。结果表明,无论有仰拱,还是无仰拱,轴载对各项力学指标的影响最大,混凝土基层厚度对各项力学指标的影响较小,水平荷载对混凝土面板板底拉应力和沥青面层层间剪应力的影响较大、对沥青面层层底拉应变影响较小;无仰拱时围岩模量对各项力学指标的影响大于有仰拱;有仰拱时混凝土面层厚度对沥青面层层底拉应变的影响大于沥青面层厚度,混凝土面层厚度对沥青面层层间剪应力的影响大于基层厚度,无仰拱时影响相反;有仰拱隧道复合式路面结构轴载换算系数得到混凝土面层设计轴次是现行规范的1.8倍,无仰拱是1.3倍;有仰拱、无仰拱时混凝土面层疲劳寿命分别比设计轴载大17和8个数量级,沥青面层疲劳寿命均比设计轴载大1个数量级。研究结果可为隧道复合式路面结构设计研究提供参考。

矿区地表形变时空演化分析方法研究

用传统监测方法获取的离散点数据只能描述特定时间的空间形变状态,无法反映矿区地表形变整体时空演化。本文介绍了一种矿区地表形变时空模型的构建方法,通过对空间多项式引入时间项变为时空多项式,并基于移动最小二乘法顾及局部特征,提出时空多项式和移动最小二乘法结合完成模型拟合。实验分析表明,选取合适时空项阶次的模型具有良好的拟合精度,实现了对矿区地表形变的时空趋势拟合,能够直观地反映矿区地表形变的整体时空演化趋势和规律。

GPS高程拟合模型方法的研究与分析

以某地区68个点作为实验数据,选择其中的49个点作为已知点来确定该地区的GPS高程拟合模型,其余19个点作为检校点来检验GPS高程拟合模型的精度,采用二次多项式曲面拟合、双三次多项式曲面拟合和移动三角面多项式曲面拟合来构建高程拟合模型。通过实验分析出,这3种方法的精度均能达到四等水准测量精度的要求,并且移动三角面多项式曲面拟合模型的计算精度最高。

基于光谱共焦的缸套内表面微织构形貌在线测量方法研究

为实现缸套内表面微织构形貌的在线检测,设计一种非接触式测量系统。采用基于EtherCAT通信协议开发的伺服系统,通过高精度伺服电机带动光谱共焦位移传感器,一次采集多个微织构的不同截面轮廓,经特征提取与坐标变换,形成单个织构的三维点云数据。通过光强值出现局部极小值确定边缘位置,采用稳健高斯回归滤波方法滤除误差,使用移动最小二乘法拟合曲面,最终获得微织构的精准三维形貌。试验结果表明:该系统与超景深显微镜测得的直径最大差值为5.7μm,深度最大差值为0.45μm,具有较高的重复性和精度,可作为缸套内表面微织构形貌在线检测的一种手段。