首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   22313篇
  免费   2415篇
  国内免费   10篇
电工技术   1433篇
综合类   413篇
化学工业   9771篇
金属工艺   382篇
机械仪表   488篇
建筑科学   1014篇
矿业工程   191篇
能源动力   236篇
轻工业   2134篇
水利工程   159篇
石油天然气   76篇
无线电   773篇
一般工业技术   4025篇
冶金工业   390篇
原子能技术   61篇
自动化技术   3192篇
  2023年   700篇
  2022年   439篇
  2021年   868篇
  2020年   791篇
  2019年   685篇
  2018年   723篇
  2017年   536篇
  2016年   833篇
  2015年   994篇
  2014年   1065篇
  2013年   1733篇
  2012年   884篇
  2011年   835篇
  2010年   1085篇
  2009年   1217篇
  2008年   729篇
  2007年   680篇
  2006年   493篇
  2005年   460篇
  2004年   366篇
  2003年   341篇
  2002年   228篇
  2001年   187篇
  1998年   253篇
  1997年   176篇
  1996年   245篇
  1995年   238篇
  1994年   190篇
  1993年   251篇
  1992年   181篇
  1990年   169篇
  1989年   191篇
  1988年   151篇
  1987年   174篇
  1986年   190篇
  1985年   168篇
  1984年   190篇
  1983年   184篇
  1982年   164篇
  1981年   208篇
  1980年   172篇
  1979年   174篇
  1977年   151篇
  1976年   158篇
  1975年   207篇
  1974年   193篇
  1973年   365篇
  1972年   213篇
  1971年   153篇
  1968年   153篇
排序方式: 共有10000条查询结果,搜索用时 13 毫秒
101.
Summary This paper presents a unique approach, named mathematical regularization, to remove the nearly-singular and singular integrals occurring in the boundary integral formulations for the solution of the boundary value problems with a pathologica integration boundary due to the very near or coinciding parts of the total boundary. Nonsingular boundary integral equations are derived for thin-walled structure problems. In the case of crack-like problems, we present two kinds of the nonsingular integral representations of the secondary fields and the derivative boundary integral equations.  相似文献   
102.
Mechanische Sensoren zur Drehzahl- und Lageerfassung bei elektrischen Antrieben verursachen Kosten, vergrößern das Bauvolumen des Antriebs, bedingen die Datenübertragung zwischen Regler und Sensor und schränken das Einsatzgebiet des Antriebs ein. Daher wurden verschiedene Konzepte zum Ersatz dieser mechanischen Sensoren entwickelt. Sie unterscheiden sich durch den Aufwand, die benötigte Genauigkeit (Pumpenantrieb Servoantrieb) und dem gewünschten Betriebsbereich.In diesem Beitrag wird die Realisierung des sensorlosen Betriebs einer permanenterregten Synchronmaschine durch ein Kalman-Filter beschrieben. Durch Simulation und durch praktische Untersuchungen konnte gezeigt werden, daß die Schätzung von Lage und Drehzahl die Genauigkeitsanforderungen für eine indirekte Lageregelung erfüllt. Eine solche Lageregelung besitzt keinen mechanischen Sensor mehr.  相似文献   
103.
Summary Theoretical and experimental analyses have been carried out for determining the injection condition below which the formation of air core does not take place in the course of flow of a time-independent power-law fluid through a swirl nozzle. Analytical solution lends one distinct value of generalized Reynolds number at the inlet to a nozzle below which the air core is not formed. Experiments reveal that there exist two limiting values of such generalized Reynolds number regarding the formation of air core in a nozzle. One value being the upper limit below which steady flow occurs without air core, the other one is the lower limit above which steady flow with fully developed air core persists. In between these two limiting values, there prevails a transition zone through which fully developed air core is set up within the nozzle. For all the nozzles, theoretical results are in fair agreement with the experimental values of upper limit of generalized Reynolds numbers with respect to steady flow without air core. Amongst all the pertinent independent geometrical parameters of a nozzle, the orifice-to-swirl chamber-diameter ratio has the remarkable influence on generalized Reynolds number describing the initiation of air core.Nomenclature D 1 Swirl chamber diameter - D 2 Orifice diameter - D s Diameter of tangential entry ports - E A non-dimensional parameter defined by Eq. (9) - E R A non-dimensional parameter defined by Eq. (25) - K Flow consistency index - L 1 Length of the swirl chamber - n Flow behaviour index - P Static pressure inside the nozzle - P b Back-pressure of the nozzle - Q Volume flow rate - R Radius vector or longitudinal coordinate with respect to spherical coordinate system (Fig. 3) - R 1 Radius of the swirl chamber - R 2 Radius of the orifice - Generalized Reynolds number at the inlet to the nozzle - Limiting value of generalized Reynolds number describing initiation of air core - R z Radius at any section - r Radial distance from the nozzle axis - r a Air core radius - u Longitudinal component of velocity with respect to spherical coordinate system (Fig. 3) - V r Radial velocity component - V z Axial velocity component - V Tangential velocity component - Tangential velocity at inlet to the nozzle - v Component of velocity in the axial plane perpendicular toR (Fig. 3) - w Component of velocity perpendicular to axial plane with respect to the spherical coordinate system (Fig. 3) - z Distance along the nozzle axis from its inlet plane - Half of the spin chamber angle - Boundary layer thickness measured perpendicularly from the nozzle wall - 2 Boundary layer thickness at the orifice - Angle, which a radius vector makes with the nozzle axis, in spherical coordinate system (Fig. 3) - Density of the fluid - Running coordinate in the azimuthal direction with respect to the cylindrical polar coordinate system as shown in Fig. 3 - Circulation constant With 8 Figures  相似文献   
104.
Dr. P. Thieler 《Computing》1978,19(4):303-312
LetA be an×n-matrix with the property I–A<1. LetY be an approximation of the inverse ofA. This paper shows how to get a componentwise error estimate forY, that does not require too much numerical effort but generally presents better results than global error estimates do. Although proved by means of interval mathematics, the given error estimate can also be calculated in absence of any implementation of interval arithmetic.
Über komponentenweise Fehlerabschätzungen für inverse Matrizen
Zusammenfassung SeiA einen×n-Matrix mit der Eigenschaft I–A<1. SeiY eine Approximation der Inversen vonA. In dieser Arbeit wird gezeigt, wie man eine komponentenweise Fehlerabschätzung fürY erhalten kann, deren Berechnung nicht sehr aufwendig ist, die aber im allgemeinen schärfer ist als globale Fehlerabschätzungen. Obwohl mit intervallmathematischen Mitteln bewiesen, kann die angegebene Fehlerabschätzung auch berechnet werden, wenn keine Intervallarithmetik implementiert ist.


This research was supported in part by Sonderforschungsbereich 72-Approximation und Optimierung, University of Bonn.  相似文献   
105.
The present paper contains a stability concept for discretization methods of a certain, very general classM, which is optimal (in the sense of yielding the best general, two-sided error bounds) without being more restrictive than any of the classical stability definitions. The optimal stability functional Ψh related to it depends on the linear part of the discretization operator, and has the important property that Ψh [δ] may be of orderq+1, i.e. Ψh [δ] = O(h q+1), even if the local error δ only has orderq, δ = O(h q). This result may be used for the construction of methods with maximum order. Its application to linear cyclic methods, for example, furnishes a new approach to the theory of linearM-cyclick-step methods of maximum order.  相似文献   
106.
The purpose of this paper is to show that for continuous functions the related quadratic splines converge without any assumption on the spline grid. The points of the interpolatory grid can be chosen between the corresponding points of the spline grid with a division ratio from \(\frac{{\sqrt 2 }}{2}\) to \(1 - \frac{{\sqrt 2 }}{2}\) . In the case of continuously differentiable functions the division ratio can even be taken between 0 and 1; in addition, the order of convergence is increased. For twice differentiable functions the full order of convergence is obtained. Analogous results about the convergence of histo splines are proved.  相似文献   
107.
An accurate acceptance-rejection algorithm is devised and tested. The procedure requires an average of less than 3 uniform deviates whenever the standard deviation of the distribution is at least 4, and this number decreases monotonically to 2.63 as . Variable parameters are permitted, and no subroutines for sampling from other statistical distributions are needed.This research was supported by the Austrian Research Council (Fonds zur Förderung der wissenschaftlichen Forschung).  相似文献   
108.
In this paper, two kinds of partial ordering for symmetric matrices are related to each other, namely, the natural partial ordering ≤ generated by the coneK of elementwise nonnegative matrices, and the definite partial ordering \( \leqslant \cdot\) generated by the coneK D of nonnegative definite matrices. The main result of this paper shows how a matrix interval in the sense of the definite partial ordering can be enclosed between optimal bounds with respect to the natural partial ordering. By means of this result, it is possible to compute a numerically practicable inclusion based on the natural partial ordering from a given inclusion of some matrix with the definite partial ordering. In this way, an always and moreover quadratically convergent method of elementwise enclosing the square root of a positive definite, symmetric matrix can be constructed.  相似文献   
109.
An iterative method of solving quasilinear hyperbolic equations of the type $$ - [p_1 (u_x ,x,t)]_x + [p_2 (u_t ,x,t)]_t = f(x,t)$$ in the domain (0, 1)×(0,T) is proposed. For each given initial-boundary-value problem of this type with boundary conditions of the first kind (second kind), a conjugate problem of the same type that has boundary conditions of the second kind (first kind) is defined. From the relations connecting the solutions of a pair of conjugate problems, a series of wave equations is created. The method proposed consists in calculating the solutions of the wave equations of this series. Theoretical proof of the convergence of the solutions of the wave equations to the solutions of the conjugate quasilinear problems is left as an open question. However, numerical results are presented to demonstrate that, under favorable circumstances, the solutions of the wave equations do converge to the solutions of the conjugate quasilinear problems.  相似文献   
110.
Summary Combined unsteady convection from an isothermal horizontal cylinder in a stream flowing vertically upwards has been investigated. Numerical solutions of the unsteady boundary-layer equations have been obtained at any station along the cylinder using the series truncation method. Solutions which are valid near the front and near stagnation points have been obtained using standard finite-difference methods. A series solution in powers of time has been obtained with which the numerical solutions has been checked.
Unstetige, gemischte Konvektion um einen isothermen Kreiszylinder
Zusammenfassung Es wird eine kombinierte, unstetige Konvektion eines isothermen, horizontalen Zylinders in einer vertikal nach oben gerichteten Strömung untersucht. Numerische Lösungen der unstetigen Grenzschichtgleichungen werden an jeder Stelle längs des Zylinders durch die Verwendung der Reihenabbruchsmethode erhalten.Nahe der Vorderseite und nahe bei den Staupunkten gültige Lösungen werden durch Verwendung üblicher Methoden der finiten Differenzen erhalten. Mit Hilfe einer Reihenlösung in Potenzen der Zeit wird das numerische Ergebnis überprüft.

Notation a radius of the cylinder - g acceleration of gravity - G r Grashof number =g|T|a 3/v 2 - Q heat transfer - R e Reynolds number =U 0 a/v - T temperature of fluid in the boundary layer - T 0 temperature of the ambient fluid - T 1 temperature of the cylinder - T temperature difference=T 1T 0 - t time - U 0 free stream - x co-ordinate measuring distance round the cylinder - y co-ordinate measuring distance normal to the cylinder - G r /R e 2 - coefficient of expansion - coefficient of kinematic viscosity - w skin friction With 7 Figures  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号