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换热器传热管在静水中的挤压膜阻尼研究 总被引:2,自引:0,他引:2
本文通过求解三维Navier-Stokes方程,导出计算挤压膜阻尼的公式,对Mulcahy所提出的模型作了改进,并提出了stokes数的间隙修正因子的概念. 改进后的挤压膜阻尼模型计及管子振幅的影响,在管子振幅很小的情况下,它与Mulcahy提出的模型相似,实际上可把Mulcahy提出的模型看成改进后模型的一个特例. 为了验证改进后的模型,对传热管在空气和静水介质中的阻尼进行了实验研究.结果表明理论计算值与实验测量值符合良好. 相似文献
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本文建立了换热器管两端弹支、中间支承板孔中悬浮的力学模型,把板孔间隙的作用归结为附加质量和附加阻尼、分析了管在多个中间支承板中悬浮时的固有频率和阻尼比,并在静水中作了对比测试。结果表明,计算值与试验值吻合良好. 相似文献
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完成了简谐激励下水中平行圆板的挤压膜振动实验.用最小二乘法识别出非线性粘性挤压膜力模型中的4个系数.不同挤压膜厚、不同频率和不同振幅情况的数值模拟结果与实验值吻合较好.研究结果表明,所用模型可较好地描述粘性挤压膜运动,识别出的系数在一定范围内给出良好精度的数值模拟. 相似文献
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Green函数法解点支承任意板自由振动频率 总被引:2,自引:0,他引:2
1 引言板是工程中经常采用的结构形式之一.沿边界呈对称点支承的悬臂板在土木工程中不乏常见.这种板的自由振动构成了典型板自振问题的一种特殊形式,由于解析法对自由边界条件及支承点的约束条件难以满足,所以常令科技人员感到棘手.H.T.Saliba 曾用4种级数迭加对这种板进行过分析、计算,得出几组计算结果.计算中每个级数取30 相似文献
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本文认为泥沙在波动水流中的沉降与在静水中的沉降不同,即泥沙还受波浪紊动的举力作用而使沉降速度减慢.波浪运动由水面向下逐渐衰减,所以,波浪对泥沙沉降速度的减缓作用也由水面向下逐渐衰减.由此出发推得了相应的理论公式,计算结果与实验数据的比较在规律性上是一致的,但计算值的精度尚嫌不足. 相似文献
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某新型直升机“地面共振”初步分析 总被引:3,自引:0,他引:3
对某新型直升机垂直起落状态“地面共振”初步分析,采用经典的空间动力模型进行计算.文中给出了该机“地面共振”初步计算的计算方法和计算结果,并得出初步计算结论.根据现有条件,该机暂时无法做起落架刚度、阻尼试验,在初步计算中,起落架的全部动力学参数都是根据理论计算或经验公式得出的,包括起落架传递系数,缓冲器和机轮的静压曲线、刚度阻尼特性等.本文推导了这些参数的计算方法以及全机平衡状态计算的计算方法. 相似文献
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阻尼器是机械结构中起到减振作用的重要部件.在各种阻尼器中,油阻尼器的应用非常广泛,这种阻尼器工作时,可能同时存在着粘性和挤压油膜两种阻尼作用.本文以离心机结构中的一种油阻尼器为研究对象,围绕油膜压强和油膜厚度之间的关系展开研究工作.实验采用激振器对阻尼芯进行单向简谐激振,并调节油膜厚度、激振频率、激振功率等参数,进行多组实验,得到油膜压强随油膜厚度的变化曲线,显示出粘性阻尼器随着油膜厚度的减小,逐渐向挤压油膜阻尼器过渡的过程.同时根据挤压油膜理论,采用雷诺方程和有限长轴承近似方法推导出实验用阻尼器模型的油膜压强表达式,将实验结果与理论公式加以对比,并进行一定的分析和讨论. 相似文献
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为了提高小长径比弹丸射击质量,设计了一种大展弦比张开式尾翼,采用AUSM+格式、SST(shearstress transport)湍流模型和隐式算法(lower-upper symmetric Gauss-Seidel implicit method, LU-SGS),求解三维RANS 方程,对前体形状完全相同,不同展弦比的3 种尾翼弹进行了数值模拟,得到了三者在马赫数1.5~3.5 下的气动力特性的差异,分析其原因,并给出了不同展弦比张开式尾翼的适用范围. 计算结果表明:C型弹的升阻比较B 型弹在1.5 马赫数区域附近增加了7% 以上,当马赫数达到2.5 以上时,A 型弹的升阻比大于B 型弹和C 型弹,在3.5 马赫数区域附近A 型弹的升阻比较B 型弹增加了5.4% 以上. 3 种弹丸的俯仰力矩系数随着马赫数的增大而负向减少,且减少的趋势随着展弦比的增加而增大. A 型弹、B 型弹、C 型弹的静稳定裕度的变化范围分别为4%~20.3%,8.5%~23.2%,11.4%~25.6%. 相似文献
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The turbulent flow structure and vortex dynamics of a jet-in-a-crossflow (JICF) problem, which is related to gas turbine blade film cooling, is investigated using the particle-image velocimetry (PIV) technique. A cooling jet emanating from a pipe interacts with a turbulent flat plate boundary layer at a Reynolds number Re∞ = 400,000. The streamwise inclination of the coolant jet is 30° and two velocity ratios (VR = 0.28, VR = 0.48) and two mass flux ratios (MR = 0.28, MR = 0.48) are considered. Jets of air and CO2 are injected separately into a boundary layer to examine the effects of the density ratio between coolant and mainstream on the mixing behavior and consequently, the cooling efficiency. The results show a higher mass flux ratio to enlarge the size of the recirculation region leading to a more pronounced entrainment of hot outer fluid into the wake of the jet. Furthermore, the lateral spreading of the coolant is strongly increased at a higher density ratio. The results of the experimental measurements are used to validate numerical findings. This comparison shows an excellent agreement for mean velocity and higher moment velocity distributions. 相似文献
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The desirable weight-to-axle power ratio for agricultural tractors is determined by the necessity for the optimum utilisation of the available axle power to produce the required drawbar pull at a preselected slip. For a vehicle designed to operate in a given speed range, the weight-to-axle power ratio should be within a particular limit, so that a specific level of conversion efficiency can be maintained. In this paper attempts have been made to select suitable tyres for Indian two-wheel drive tractors operating in sandy clay loam soils on the basis of weight-to-power utiisation and maximum pull-to-optimum weight ratio at a preselected slip using the developed traction prediction equations. A comparison has also been made between the desired and actual weight on a single traction wheel and suitable tyre and tyre normally fitted in Indian two wheel drive tractors up to 35 kW. 相似文献
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Research was conducted to quantify the effect of tire variables (section width, diameter, inflation pressure); soil variables (soil moisture content, initial cone index, initial bulk density); and external variables (travel speed, axle load, number of tire passes) on soil compaction and to develop models to assess compaction in agricultural soils. Experiments were conducted in a laboratory soil bin at the Asian Institute of Technology in three soils, namely: clay soil (CS), silty clay loam soil (SCLS), and silty loam soil (SLS). A dimensional analysis technique was used to develop the compaction models. The axle load and the number of tire passes proved to be the most dominant factors which influenced compaction. Up to 13% increase in bulk density and cone index were observed when working at 3 kN axle load in a single pass using a 8.0–16 tire. Most of the compaction occurred during the first three passes of the tire. It was also found that the aspect ratio, tire inflation pressure and soil moisture content have significant effect on soil compaction. The initial cone index did not show significant effect. The compaction models provided good predictions even when tested with actual field data from previous studies. Thus, using the models, a decision support system could be developed which may be able to provide useful recommendations for appropriate soil management practices and solutions to site-specific compaction problems. 相似文献
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Hydrodynamic characteristics of fluidization in a conical or tapered bed differ from those in a columnar bed because the superficial velocity in the bed varies in the axial direction. Fixed and fluidized regions could coexist and sharp variations in pressure drop could occur, thereby giving rise to a noticeable pressure drop-flow rate hysteresis loop under incipient fluidization conditions. To explore these unique properties, several experiments were carried out using homogeneous, well-mixed, ternary mixtures with three dif- ferent particle sizes at varying composition in gas-solid conical fluidized beds with varying cone angles. The hydrodynamic characteristics determined include the minimum fluidization velocity, bed fluctuation, and bed expansion ratios. The dependence of these quantities on average particle diameter, mass fraction of the fines in the mixture, initial static bed height, and cone angle is discussed. Based on dimensional analysis and factorial design, correlations are developed using the system parameters, i.e. geometry of the bed (cone angle), particle diameter, initial static bed height, density of the solid, and superficial velocity of the fluidizing medium. Experimental values of minimum fluidization velocity, bed fluctuation, and bed expansion ratios were found to agree well with the developed correlations. 相似文献
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开挖应力释放率计算模型 总被引:10,自引:0,他引:10
本文在明确应力释放率和位移释放率的基础上,重点分析开挖面空间效应区的位移释放,并建立了应力释放与位移释放间的关系。文中分析了开挖问题采用平面模型计算时,应对开挖初应力进行修正,并给出了算法。最后与北京地铁的实测资料进行对比。 相似文献
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The present paper investigates the impact of the velocity and density ratio on the turbulent mixing process in gas turbine
blade film cooling. A cooling fluid is injected from an inclined pipe at α=30° into a turbulent boundary layer profile at a freestream Reynolds number of Re ∞ = 400,000. This jet-in-a-crossflow (JICF) problem is investigated using large-eddy simulations (LES). The governing equations
comprise the Navier–Stokes equations plus additional transport equations for several species to simulate a non-reacting gas
mixture. A variation of the density ratio is simulated by the heat-mass transfer analogy, i.e., gases of different density
are effused into an air crossflow at a constant temperature. An efficient large-eddy simulation method for low subsonic flows
based on an implicit dual time-stepping scheme combined with low Mach number preconditioning is applied. The numerical results
and experimental velocity data measured using two-component particle-image velocimetry (PIV) are in excellent agreement. The
results show the dynamics of the flow field in the vicinity of the jet hole, i.e., the recirculation region and the inclination
of the shear layers, to be mainly determined by the velocity ratio. However, evaluating the cooling efficiency downstream
of the jet hole the mass flux ratio proves to be the dominant similarity parameter, i.e., the density ratio between the fluids
and the velocity ratio have to be considered. 相似文献
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Reflection and transmission of an incident plane wave at five types of possible interfaces between two dipo-lar gradient elastic solids are studied in this paper. First, the explicit expressions of monopolar tractions and dipolar trac-tions are derived from the postulated function of strain energy density. Then, the displacements, the normal derivative of displacements, monopolar tractions, and dipolar tractions are used to create the nontraditional interface conditions. There are five types of possible interfaces based on all possible combinations of the displacements and the normal derivative of displacements. These interfacial conditions with consid-eration of microstructure effects are used to determine the amplitude ratio of the reflection and transmission waves with respect to the incident wave. Further, the energy ratios of the reflection and transmission waves with respect to the incident wave are calculated. Some numerical results of the reflection and transmission coefficients are given in terms of energy flux ratio for five types of possible interfaces. The influences of the five types of possible interfaces on the energy parti-tion between the refection waves and the transmission waves are discussed, and the concept of double channels of energy transfer is first proposed to explain the different influences of five types of interfaces. 相似文献
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下承式梁拱组合桥梁的梁拱协作机理研究 总被引:1,自引:0,他引:1
文章将外部静定、内部高次超静定的梁拱组合桥梁分解成梁拱闭合体和吊杆两种构件,并且将吊杆力假定为膜张力,求解出了梁拱组合桥梁在均布荷载作用下拱肋分担的荷载大小及其修正系数,明确了其传力途径,定量地求出了梁拱分担的荷载比例,拱梁的拉压力偶矩大小以及梁拱截面的弯矩,并讨论了拱梁荷载比、弯矩比的影响因素及其关系,完整地揭示了下承式梁拱组合桥梁的梁拱协作机理. 相似文献