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101.
When materials processing is conducted in air surroundings by use of an impinging plasma jet, the ambient air will be entrained
into the materials processing region, resulting in unfavorable oxidation of the feedstock metal particles injected into the
plasma jet and of metallic substrate material. Using a cylindrical solid shield may avoid the air entrainment if the shield
length is suitably selected and this approach has the merit that expensive vacuum chamber and its pumping system are not needed.
Modeling study is thus conducted to reveal how the length of the cylindrical solid shield affects the ambient air entrainment
when materials processing (spraying, remelting, hardening, etc.) is conducted by use of a turbulent or laminar argon plasma
jet impinging normally upon a flat substrate in atmospheric air. It is shown that the mass flow rate of the ambient air entrained
into the impinging plasma jet cannot be appreciably reduced unless the cylindrical shield is long enough. In order to completely
avoid the air entrainment, the gap between the downstream-end section of the cylindrical solid shield and the substrate surface
must be carefully selected, and the suitable size of the gap for the turbulent plasma jet is appreciably larger than that
for the laminar one. The overheating of the solid shield or the substrate could become a problem for the turbulent case, and
thus additional cooling measure may be needed when the entrainment of ambient air into the turbulent impinging plasma jet
is to be completely avoided. 相似文献
102.
This paper investigates the effects of the horizontal projection on the spill flame ejected from an enclosure. Experiments were carried out in a reduced-scale enclosure and a horizontal projection was installed above the opening. The spill flame beneath a horizontal projection is composed of a horizontal momentum-dominated rectangle region and a buoyancy-dominated semicircle region. The overall averaged excess air ratio and the averaged entrainment coefficient of the spill flame beneath a horizontal projection were investigated. A theoretical model for the total flame length was developed. The model predictions agree well with the measured values. 相似文献
103.
104.
滑动表面卷吸速度为零时的热弹流润滑分析 总被引:7,自引:4,他引:3
提出了接触处两表面等速反向运动的圆柱间在一定条件下赤存在足够厚的稳态热弹流膜,此时固体表面呈现中凹状、且凹陷深度随勒荷的增加而增大,随表面速度的增加而减小。 相似文献
105.
T.S. Lee H.T. Low W.D. Huang 《International Journal of Computational Fluid Dynamics》2013,27(5):381-391
The four-equation two-fluid model of two-component flow has been developed to study the effects of air entrainment and air release on the pressure transient due to the rapid valve closure. A nearly-implicit hydrodynamic numerical scheme is applied. Numerical experiments show that entrained, entrapped and released gases decrease the pressure wave frequency, produce asymmetric pressure surges with respect to the static pressure and result in the difference between up-surge duration and down-surge duration. These results are consistent with the experimental and field data observed by other investigators. A decrease in pressure peaks is also discovered in the given conditions. 相似文献
106.
C. Turan P. M. Carrica L. Weber 《International Journal of Computational Fluid Dynamics》2013,27(3-4):137-153
Strong flow entrainment has been observed downstream of spillways constructed with flow deflectors. This water entrainment has important environmental and ecological impacts because it improves the mixing of powerhouse and spillway flows, but may negatively impact fish migration or create adverse flow conditions. Most studies found in the literature attempt to explain this entrainment with turbulent mixing. Both reduced-scale hydraulic models and single-phase, isotropic RANS models grossly under-predict the degree of entrainment observed in prototypes. In this paper, an anisotropic model that accounts for the bubble volume fraction and attenuation of the normal velocity fluctuations at the free surface is presented. The model adequately predicts the main mechanisms causing water entrainment and compares well against experimental data for a round surface jet and for Brownlee Dam at model scale. It is shown that appropriate entrainment can only be captured if the turbulence anisotropy and the two-phase nature of the flow are modelled. 相似文献
107.
108.
A detailed investigation into the phenomena of induction of air using a novel arrangement of the ultrasonic horn (tip is located just above the liquid surface) has been made with the quantification of the extent of induction in terms of the air entrainment rate and the gas–liquid mass transfer coefficient for the transfer of air into the system. The measurement of air entrainment rate was found to be quite difficult and hence focus was kept on the quantification in terms of the gas–liquid mass transfer coefficient. The effect of ultrasonic power dissipation and type of the liquid medium (water, sodium chloride and sodium laruyl sulphate [surfactant] solution) on the mass transfer coefficient has been studied and correlations have been developed for the prediction of the same. Comparison with the mechanically agitated surface aerators has enabled us to understand the controlling mechanism in the induction and subsequent distribution of the air i.e. turbulence or convective motion. The present work should open an entirely new field of research in the area of design of sonochemical gas–liquid reactors operating possibly as a combination of gas-inducing reactors and cavitational reactors. 相似文献
109.
T. S. Lee 《国际流体数值方法杂志》1994,19(2):89-103
In pumping installations such as sewage pumping stations, where gas content and air entrainment exist, the computation of fluid pressure transients in pipelines becomes grossly inaccurate when a constant wave speed is assumed. An accurate numerical model with gas release and absorption has been developed in this paper and used to compute the fluid pressure transients in the pumping mains of selected pumping installations. Free and dissolved gases in the transported fluid and cavitation at vapour pressure are also modelled. When compared with the gas-free case, computations show that entrained, entrapped or released gases amplify the positive pressure peak, increase surge damping and produce asymmetric pressure surges. While the upsurge with air entrainment in the pipelines was considerably amplified, the downsurge was only marginally reduced. The computed results show good agreement with the data available. 相似文献
110.
Although the ultrasonic treatment of molten aluminum has been studied for long period, there is still much to be revealed for this process. Many studies have focused on the investigation of acoustic cavitation and streaming under the horn tip and their effects on the treatment efficiency. However, to the best of our knowledge, no attempt has been done to explain phenomena occurring near or on the melt free surface. Thus, the goal of this study is to investigate phenomena occurring at the free surface during ultrasound irradiation and clarify their possible influence on the ultrasound treatment performance. The results of high temperature and water model experiments reveal that ultrasound irradiation significantly promotes the formation of alumina particles on the melt free surface around sonotrode, and part of these particles can be entrained into aluminum melts. Furthermore, TEM observation results suggested that the entrained alumina inclusions can serve as nucleation sites for the primary Al3Zr compounds. Most importantly, the oxidation and entrainment of particles from free surface are likely to be controllable by the immersion depth of sonotrode into molten aluminum. 相似文献