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981.
防护林带:湍流的数学模型与计算机模拟 总被引:3,自引:0,他引:3
虽然防护林用于减小风速、控制热量和水汽传递及污染物扩散、
改善气候与环境、增加作物产量等已经有几百年了, 但直到近几十年,
人们才开始系统地研究防护林空气动力学的遮蔽机制.在本综述中,我
们考察了绕防护林带的流动与湍流控制机制,最新的模型与数值模拟
研究情况;通过数值模拟与实验数据的比较,来了解防护林带结构与防
风效果之间的关系;讨论数值分析如何及为什么能够得到所需要的结果.
本文将从多孔隙防护林带流动基本方程组的推导开始,讨论数值模型及
模拟过程,对附体与分离流动进行预测;分析了遮蔽机制与动量交换;对
风向、防护林密度、宽度和三维性对流动与湍流的影响作了系统的论
述.还对热流和土壤水分蒸发的新模型及数值模拟作了简述.最后,我们
对网络工作站、群和高性能分布式并行计算机及其对防护林带模型预
报能力的提高作了讨论. 相似文献
982.
给出两水平固壁间两层不可压缩理想流体中二维非线性界面波的演化方程,首先建立出这个演化方程,并由此方程在一定条件下得到二维非线性界面长波满足的近似方程,然后从理论上证明这个长波近似方程包含了以下两个描述一阶界面升高的著名的浅水孤立波方程;Korteweg-de Vries(KdV)方程和Kadomtsev-Petvishvili(KP)方程,所得特殊结果与前人的一致,表明所建立的二维非线性界面波演化方程正确且具有一般性。 相似文献
983.
A nonlinear Galerkin/ Petrov- least squares mixed element (NGPLSME) method for the stationary Navier-Stokes equations is presented and analyzed. The scheme is that Petrov-least squares forms of residuals are added to the nonlinear Galerkin mixed element method so that it is stable for any combination of discrete velocity and pressure spaces without requiring the Babuska-Brezzi stability condition. The existence, uniqueness and convergence ( at optimal rate ) of the NGPLSME solution is proved in the case of sufficient viscosity ( or small data). 相似文献
984.
A class of recently developed explicit algebraic stress models based on tensorially quadratic stress--strain relations [7] is subjected to a systematical realizability analysis. It is found that these models, which are of particular interest for their rigorous derivation from linear second-moment closure models, tend to produce inappropriate unrealizable results like negative turbulence energy components, even in simple shear flows. The cause of the defect is identified in conjunction with a set of realizability-furnishing constraints on the model coefficients. With the exception of the silent normal stress component in accelerated flow, the nature and rationale of the explicit algebraic stress model suggested by Gatski and Speziale [7] can be extended to maintain the realizability principle. Results obtained from the corresponding quasi-realizable quadratic eddy-viscosity model are reported in comparison with other nonlinear modelling practices. 相似文献
985.
There are different excitation mechanisms that cause fatal damages due to undesirable vibrations in heat exchanger tube bundles
subjected to cross-flow. One of them is the fluid-damping-controlled instability (galloping) that is characterised by a sudden
appearance of large amplitudes of the tubes exclusively in cross-flow direction. This paper reports on investigations using
an experimental set-up in a wind tunnel where the galloping mechanism in a tube bundle can be observed as an isolated phenomenon.
The apparatus allows to realise several tube bundle configurations and geometry's of real heat exchangers. The position of
a flexible test tube with a linear iso-viscoelastic mounting inside the tube array is variable. The test tube is equipped
with dynamical pressure sensors which are placed directly under pressure holes inside the tube. For the investigation of the
acting fluid forces the non-stationary pressure distribution is measured simultaneously at 30 points on the circumference
in mid plane and at 15 points in line along the tube together with the tube motion. The acting fluid forces are determined
by integration of the whole pressure field process. The study gives insights into the effect of the fluid-damping-controlled
instability that is still not fully understood. Moreover, a flow visualization gives an impression of the mechanism at relevant
Reynolds-numbers. The results show that in case of instability due to galloping the correlation length of the forces acting
along the tube axis increases suddenly to large values. The fluid forces are correlated well for the whole tube when galloping
is dominant. The exciting fluid forces show harmonic character and lead to a classical resonance behaviour. Instead of a simple
free vibration test in vacuum or still air, which is done mostly for fluid excited structures, the damping coefficient of
the oscillating system is determined under operating conditions on the basis of the measured fluid forces. A comparison of
the results with those of a free vibration test in still air is shown.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
986.
本文运用流体力学、结构动力学及变分原理推导了结构-流体耦合系统的动力学方程,并对水下结构物在运行时的自振特征及动力响应给出了数值计算分析。 相似文献
987.
An analysis is presented to investigate the effect of radiation on mixed convection from a horizontal flat plate in a saturated porous medium. Both a hot surface facing upward and a cold surface facing downward are considered in the analysis. The conservation equations that govern the problem are reduced to a system of nonlinear ordinary different equations. The important parameters of this problem are the radiation parameter R, the buoyancy parameter B, and the freestream to wall temperature ratio T ∞/T w for the case of a hot surface or the wall to freestream to wall temperature T w /T ∞ for the case of a cold surface. 相似文献
988.
本文介绍了电热炮的基本原理、实验装置结构、等离子体发生器和发射实验结果。实验结果表明,电热炮原理实验的装置是成功的,已将19.3g复合弹丸加速到1.85km/s,穿透30mm厚的钢板。 相似文献
989.
990.