共查询到17条相似文献,搜索用时 156 毫秒
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采用计算流体动力学的方法, 研究了微通道内气体在速度滑移和随机表面粗糙度耦合作用下的流动特性. 其中, 利用二阶速度滑移边界条件描述气体的边界滑移, 利用分形几何学建立随机粗糙表面. 研究发现, 综合考虑二阶速度滑移边界条件和随机表面粗糙度在较大的平均Knudsen数范围内 (0.025-0.59) 得到的计算结果与实验数据符合得很好, 而一阶速度滑移边界条件只在平均Knudsen数较小时(<0.1)符合实验结果. 随机表面粗糙度对气体在边界处的滑移有显著影响, 相对粗糙度越大, 速度滑移系数越小. 并针对计算结果, 给出了滑移系数与相对粗糙度近似满足的关系. 随机粗糙表面对气体流动过程中的压强、速度、Poiseuille数也有显著影响.
关键词:
随机表面粗糙度
二阶速度滑移边界条件
分形
微通道 相似文献
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《工程热物理学报》2010,(6)
本文重点研究了微通道内入口效应和粗糙度等因素对流动阻力特性的影响。实验结果表明;对于粗糙度3.5%,水力直径为345μm的微圆管,入口段长度比常规管道短,转捩雷诺数Re_c位于2000~2300,其内入口效应并不影响微管内流体从层流到湍流转捩的拐点位置;内径为112.6μm和169.8μm,高宽比为0.59和0.63以及粗糙分别为1.29%和2.03%的矩形微通道内流动由层流向湍流的转捩发生在Re≈1600~2800之间。实验结果表明,微尺度是微管内流体转捩提前的必要条件,但不是充分条件,拐点提前是由多因素综合作用所造成的结果,其机理有待于进一步研究。 相似文献
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为满足固体激光器用微通道冷却器的换热要求, 根据冷却器结构分别建立了二维和三维物理模型, 利用计算流体力学方法首先对比研究两者的流动特性, 然后考察雷诺数和玻片生热量对微通道流动和传热特性的影响。结果表明:对于类似大平板间的矩形微通道层流流动区域, 其流动及传热特性可直接采用二维简化模型进行模拟分析;对于重点关注的转捩区, 采用三维模型模拟分析更好;当雷诺数增大到转捩点, 流体的传热效果得到明显增强;随着雷诺数的增大, 玻片生热量对通道内最低压力需求的影响逐渐减小;不同玻片生热量对微通道流动影响不可忽略, 对努赛尔数和通道总压降基本无影响。 相似文献
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At Mach number 3.4, visualisation experiments of flow over backward-facing step (BFS) with or without roughness band attached on upstream wall are carried out via traditional schlieren and newly developed nano-tracer-based planar laser scattering (NPLS). The time-averaged flow characteristic of the reattachment region and the instantaneous rich structures of the redeveloping boundary layer in the steamwise-normal plane are both revealed. Additionally, top views in the different planes (y/h = 0.67, 1.00, 1.33, 1.67, 2.00) are imaged with a resolution of 0.064 mm/pixel. By contrasting the NPLS images at different times, the unsteady evolution characteristic of the coherent vortices in the redeveloping boundary layer was discussed. Static wall pressure is measured by a micro-pressure scanning system. The incipient formation positions are pointed out statistically. Without roughness, the longitudinal structures with scales of 1.0h and 1.2h form later and distribute in a longer region compared to that with roughness. Fractal analysis is applied and the averaged fractal dimensions of the overall and sectional flow structures are calculated. If roughness is adopted, the fractal dimension will be larger and the turning point in the sectional fractal dimensions is earlier. However, the dimension tends to be one coincided value in the farther downstream. 相似文献
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Bing-Yang Cao 《Molecular physics》2013,111(10):1403-1410
Rarefied gas flows in rough microchannels are investigated by non-equilibrium molecular dynamics simulations. The surface roughness is modelled by an array of triangular modules. The Maxwell slip model is found to break down due to the surface roughness for gas flows in microchannels with large surface roughness. Non-Maxwell slippage shows that the slip length is smaller than that predicted by the Maxwell model and is nonlinearly related to the mean free path. For larger surface roughness and smaller Knudsen number, the non-Maxwell effect becomes more pronounced. The boundary conditions, generally including velocity slip, no-slip and negative slip, depend not only on the Knudsen number but also on the surface roughness. Simulation results show that A/λ?≈?1 is a good criterion to validate the no-slip boundary condition and A/λ?>?0.3 can be a criterion to judge the occurrence of non-Maxwell slippage, where A is the surface roughness size and?λ?is the mean free path of gas molecules. The permeability enhanced by the surface roughness may be responsible for the roughness-induced non-Maxwell slippage. 相似文献
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Albert?Mihranyan Mortadha?Muhel Maria?Str?mme 《Applied Physics A: Materials Science & Processing》2009,94(2):299-305
The dissolution process of sparingly soluble CaCO3 microparticles and how the fractal surface dimension of the particles changes during dissolution is analyzed. The particles
and the dissolution process are studied using scanning electron microscopy, X-ray diffraction, nitrogen adsorption, laser
diffraction and conductance measurements. Ball milling of the particles is shown to maintain the particle crystallinity, and
to introduce an increased fractal surface dimension in the 1–10 μm size range. Dissolution is found to increase the surface
dimension of initially smooth particles and to maintain the fractal surface roughness of milled particles. The dissolution
process increases the relative number of small particles (50 nm–1 μm) whereas the larger ones decrease in size. The solubility
of the milled fractal particles was ∼1.8 times higher than that for the initially smooth ones. The presented findings show
that developing methods for increasing the fractal surface roughness of particles should be of interest for improving the
solubility of poorly soluble drug candidates. 相似文献
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研究了分形粗糙面的成像问题. 分形粗糙面能够较好的逼近真实环境, 采用带限形式的Weierstrass-Mandelbrot函数建立了分形粗糙面几何模型, 对分形粗糙面参数的选取进行了讨论. 对大尺度粗糙面散射问题提出了一种基于大面元的Kirchhoff近似方法, 采用面元模型来计算粗糙面总的后向散射场以及每一个面元的后向散射场, 并对面元的尺寸选取进行了研究, 通过与解析解进行对比证明了该方法的正确性. 在分形理论建立的确定性粗糙面几何模型与面元的Kirchhoff方法获取的散射场的基础上, 采用正侧视条带式成像模式, 并选用距离多普勒算法对不同分形参数的粗糙面进行合成孔径雷达(SAR) 成像模拟, 结果显示从SAR像中可以清晰地观察到不同分形参数对粗糙面几何轮廓的影响. 该研究包括了从环境模型、电磁模型到SAR成像技术在内的完整的分形环境SAR像模拟过程, 仿真结果显示出分形环境的SAR像特点, 这对基于分形理论的自然环境的遥感探测以及参数反演具有一定的理论支撑作用. 相似文献
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The transition to turbulence in plane Poiseuille flow (PPF) is connected with the presence of exact coherent structures. We here discuss a variety of different structures that are relevant for the transition, compare the critical Reynolds numbers and optimal wavelengths for their appearance, and explore the differences between flows operating at constant mass flux or at constant pressure drop. The Reynolds numbers quoted here are based on the mean flow velocity and refer to constant mass flux. Reynolds numbers based on constant pressure drop are always higher. The Tollmien–Schlichting (TS) waves bifurcate subcritically from the laminar profile at Re = 5772 at wavelength 6.16 and reach down to Re = 2610 at a different optimal wave length of 4.65. Their streamwise localised counter part bifurcates at the even lower value Re = 2334. Three-dimensional exact solutions appear at much lower Reynolds numbers. We describe one exact solutions that has a critical Reynolds number of 316. Streamwise localised versions of this state require higher Reynolds numbers, with the lowest bifurcation occurring near Re = 1018. The analysis shows that the various branches of TS-waves cannot be connected with transition observed near Re ≈ 1000 and that the exact coherent structures related to downstream vortices come in at lower Reynolds numbers and prepare for the transition. 相似文献
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《Journal of Electrostatics》2006,64(7-9):604-610
This work analyzes the effects produced by an EHD actuator on the flow around an airfoil at low Re numbers (Re≈104). The analysis is undertaken from flow visualizations and measurements of the surface pressure distributions. The experiments indicate that, for low Re number, the effects of the actuation depend on the power added to the flow and on the relative distance between the actuator and the separation line. 相似文献
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Motivated by interest in the geometry of high intensity events of turbulent flows, we examine the spatial correlation functions of sets where turbulent events are particularly intense. These sets are defined using indicator functions on excursion and iso-value sets. Their geometric scaling properties are analysed by examining possible power-law decay of their radial correlation function. We apply the analysis to enstrophy, dissipation and velocity gradient invariants Q and R and their joint spatial distributions, using data from a direct numerical simulation of isotropic turbulence at Reλ ≈ 430. While no fractal scaling is found in the inertial range using box-counting in the finite Reynolds number flow considered here, power-law scaling in the inertial range is found in the radial correlation functions. Thus, a geometric characterisation in terms of these sets’ correlation dimension is possible. Strong dependence on the enstrophy and dissipation threshold is found, consistent with multifractal behaviour. Nevertheless, the lack of scaling of the box-counting analysis precludes direct quantitative comparisons with earlier work based on multifractal formalism. Surprising trends, such as a lower correlation dimension for strong dissipation events compared to strong enstrophy events, are observed and interpreted in terms of spatial coherence of vortices in the flow. 相似文献