共查询到19条相似文献,搜索用时 187 毫秒
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本文设计了具有相同平均沟槽密度的三种排列类型的一维沟槽结构: 密排列、周期间隔排列和两种准周期间隔排列, 并采用数值模拟和实验验证相结合的方法研究了一维沟槽结构在不同排列下的流体减阻特性. 模拟计算分析流场特征和总阻力, 发现相对于密排列和周期间隔排列的沟槽结构, 准周期间隔排列具有更好的减阻特性, 并且这一结果得到减阻实验的验证. 通过流场分布特性进一步分析沟槽结构的减阻机理. 机理分析发现高速流在经一维准周期结构的扰动波调制后形成了准周期间隔排列的速度条纹相, 这有效地抑制了大涡在流向和展向上的形成, 从而实现较大幅度的减阻. 同时对比分析沟槽排列结构调制展向涡和流向涡各自对流动减阻的贡献, 结果表明, 调制流向涡对减阻的作用更大.
关键词:
流体减阻
沟槽结构
准周期 相似文献
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采用二维粒子图像测速仪(2DPIV)对槽道内涡波流场进行实验研究,用POD技术对2DPIV瞬态速度矢量场进行主导模态重构,得到槽道内的平均流速和湍流动能分布;采用大涡PIV方法对湍流动能耗散率分布进行计算.结果表明:重构流场表征了原始流场的主导结构,剔除了噪声等干扰信息;大涡PIV方法能有效地估算动能耗散率的分布;湍流动能在壁面附近较小,在接近槽道中心区域湍流动能越来越大,呈现出射流的特征;动能耗散率的峰值出现在壁面附近和槽道中心区域,动能耗散率随着远离壁面程度的增加先降低后逐渐增加直至达到峰值. 相似文献
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在低Reynolds数条件下,翼型绕流的上表面边界层由于抗逆压梯度能力变差容易发生流动分离,从而形成长层流分离泡.分离泡通常是非定常的,会诱发边界层的转捩、再附并形成湍流边界层.这个过程会使翼型的气动性能急剧下降,并伴随着强非线性效应.转捩后形成的湍流边界层也会产生高摩擦阻力.针对这种现象,文章以NACA0012翼型为例,通过隐式大涡模拟研究了有效的主动控制方案.为了统一分离控制技术和湍流边界层减阻技术,研究了在平板或槽道湍流中取得较好控制效果的壁面垂向反向控制方案.首先利用隐式大涡模拟研究了低Reynolds数条件下NACA0012翼型绕流的流场特征.其次分析并验证了反向控制方案在分离区控制流场的可行性,发现反向控制在分离区的作用相当于基于流场信息的壁面抽吸控制,且控制具有实时性和高效性,控制抽吸了前缘的低能流体,使得翼型前缘附面层变薄,并增强了其抗逆压梯度的能力,较大程度提高了翼型的气动性能.最后在湍流边界层验证了其减阻控制效果,发现反向控制阻断了流向涡的法向输运,抑制了涡结构的发展,并减弱了猝发过程,使得湍流的高摩阻力得到了有效降低. 相似文献
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在零压梯度平板湍流边界层流动中安装垂直流向高度为h的扰流板,诱导流场产生横向涡,研究横向涡影响下湍流/非湍流界面特性沿流向的发展。结果表明,在本实验条件下,整个流动经历了从湍流边界层到流动分离和再附,再向湍流边界层恢复的过程。在扰流板下游约18h距离后,扰流板尾迹的影响逐渐衰退,壁面剪切对湍流强度的贡献开始逐渐恢复,在扰流板下游约55h距离后,湍流边界层再次充分发展。与此同时,由于扰流板后流场流动结构拟序性的增强,湍流/非湍流界面的分形维度受扰流板影响而减小,表明脱落涡有使界面多尺度特性、三维性减弱的趋势。此外,界面高度的概率密度分布受扰流板影响呈现显著的右偏,主要与扰流板增强喷射运动强度,使得界面更容易抬升相关。流动结构及界面特性受扰流板影响后的流向演化有同步变化的模式,扰流板对界面特性影响主要集中于(-5~18)h的流向范围。 相似文献
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可压缩边界层转捩问题与湍流问题一直是制约高超声速飞行器发展的关键基础问题,也是近年来流体力学领域研究的热点问题.采用直接数值模拟方法,获得了空间发展的Ma=2.25超声速湍流边界层流场,通过对湍流边界层的发展状态进行评估,得出有效的Reynolds数Reθ范围约为2 600~4 600.对壁面摩阻系数开展了分解,获得了各分量的占比,对充分发展的湍流边界层进行1阶和高阶统计分析,包括形状因子、壁面律、平坦因子与偏斜因子、Reynolds应力、脉动涡量等,得到了剪切Reynolds数与动量Reynolds数之间的关系式,分析了湍流边界层壁面律的分层特性,发现湍流的间歇特性主要分布在y+ < 30的区域,并且法向速度脉动的间歇性远高于另外两者,3个方向上的Reynolds应力分布和涡量分布都存在较大差异.通过两点相关性分析和Lagrange涡结构,对近壁区湍流结构进行了分析,包括流向平面和展向平面,发现流向脉动速度的相关区域流向尺度较长,呈现狭长的特性,并且流向平面的相关系数与壁面存在一定的夹角;而在边界层外层,流向速度脉动相关区域的流向尺度变短而展向尺度增加,呈现宽胖型.研究结果进一步加深了对超声速湍流边界层的认识,为下一步湍流边界层的流动控制奠定了基础. 相似文献
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Mechanism of controlling turbulent channel flow with the effect of spanwise Lorentz force distribution 下载免费PDF全文
A direct numerical simulation(DNS) is performed to investigate the control effect and mechanism of turbulent channel flow with the distribution of spanwise Lorentz force. A sinusoidal distribution of constant spanwise Lorentz force is selected, of which the control effects, such as flow characters, mean Reynolds stress, and drag reductions, at different parameters of amplitude A and wave number k_x are discussed. The results indicate that the control effects vary with the parameter A and k_x. With the increase of A, the drag reduction rate D_r first increases and then decreases rapidly at low k_x,and slowly at high k_x. The low drag reduction(or even drag increase) is due to a weak suppression or even the enhancements of the random velocity fluctuation and mean Reynolds stress. The efficient drag reduction is due to the quasi-streamwise vortex structure induced by Lorentz force, which contributes to suppressing the random velocity fluctuation and mean Reynolds stress, and the negative vorticity improves the distribution of streamwise velocity. Therefore, the optimal control effect with a drag reduction of up to 58% can be obtained. 相似文献
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Turbulent control and drag reduction in a channel flow via a bidirectional traveling wave induced by spanwise oscillating Lorentz force have been investigated in the paper. The results based on the direct numerical simulation (DNS) indicate that the bidirectional wavy Lorentz force with appropriate control parameters can result in a regular decline of near-wall streaks and vortex structures with respect to the flow direction, leading to the effective suppression of turbulence generation and significant reduction in skin-friction drag. In addition, experiments are carried out in a water tunnel via electro-magnetic (EM) actuators designed to produce the bidirectional traveling wave excitation as described in calculations. As a result, the actual substantial drag reduction is realized successfully in these experiments. 相似文献
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《中国科学:物理学 力学 天文学(英文版)》2010,(10)
Direct numerical simulation (DNS) of incompressible magnetohydrodynamic (MHD) turbulent channel flow has been performed under the low magnetic Reynolds number assumption.The velocity-electric field and electric-electric field correlations were studied in the present work for different magnetic field orientations.The Kenjeres-Hanjalic (K-H) model was validated with the DNS data in a term by term manner.The numerical results showed that the K-H model makes good predictions for most components of the velocity-electric field correlations.The mechanisms of turbulence suppression were also analyzed for different magnetic field orientations utilizing the DNS data and the K-H model.The results revealed that the dissipative MHD source term is responsible for the turbulence suppression for the case of streamwise and spanwise magnetic orientation,while the Lorentz force which speeds up the near-wall fluid and decreases the production term is responsible for the turbulence suppression for the case of the wall normal magnetic orientation. 相似文献
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Turbulent Boundary Layer Control via a Streamwise Travelling Wave Induced by an External Force 下载免费PDF全文
Turbulent boundary layer control via a streamwise travelling wave is investigated based on direct numerical simulation of an incompressible turbulent channel flow. The streamwise travelling wave is induced on one side wall of the channel by a spanwise external force, e.g., Lorenz force, which is con~ned in the viscous sublayer. As the control strategy used in this study has never been examined, we pay our attention to its efficiency of drag control. It is revealed that the propagating direction of the travelling wave, i.e., the downstream or upstream propagating direction with respect to the streamwise flow, has an important role on the drag control, leading to a significant drag reduction or enhancement for the parameters considered. The coherent structures of turbulent boundary layer are altered and the underlying mechanisms are analysed. The results obtained provide physical insight into the understanding of turbulent boundary layer control. 相似文献
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由条带和流向涡的循环再生构成的近壁自维持过程(self-sustaining process, SSP)是壁湍流产生和维持的重要机制. 文章通过对最小槽道的直接数值模拟(direct numerical simulation, DNS)获得近壁自维持过程的流场数据, 采用正规正交分解法(proper orthogonal decomposition, POD)对该数据进行分析, 获得了不同流向和展向尺度的特征模态, 通过将Navier-Stokes方程在这些模态上进行投影, 得到近壁自维持过程的降阶模型, 并采用DNS数据对降阶模型的预测能力进行了评价. 该模型被初步应用于大涡模拟近壁模型的构造. 相似文献
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A spanwise heterogeneity of streamwise drag is known to lead to the formation of large secondary motions of Prandtl's second kind. Based on the data sets extracted from direct numerical simulations (DNS) of fully developed turbulent channel flow where streamwise stripes of free-slip surface with varying spanwise extension are introduced, we investigate the topological structure of the secondary motions. We find a complex restructuring of the secondary motion with increasing extent of free-slip/no-slip region where the width of the free-slip region in viscous units appears to be one important governing parameter for the vortex formation. The most striking feature of this restructuring is a change in the rotational direction of the major vortex pair such that the related high- and low-momentum pathways are found at different locations. The present results reveal that the spanwise inhomogeneity of the Reynolds stress distribution is strongly related to the observed change of rotational direction. In addition, it is shown that the vorticity source remains largely unchanged and mainly restricted to a rather small region close to the discontinuity in the boundary condition, despite the fact that the topology of secondary motions substantially changes with variation of the spanwise length scale. This suggests a complex interplay between the vortices that are generated at the surface discontinuities and the surrounding flow. 相似文献
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LIU Nansheng LU Xiyun & ZHUANG Lixian Department of Mechanics Mechanical Engineering University of Science Technology of China Hefei China 《中国科学G辑(英文版)》2004,47(4):463-476
Rotating turbulence occurs extensively in nature and engineering circumstances. Meanwhile, understanding physical mechanisms of the rotating turbulence is important to the fundamental research of turbulence. The turbulent flow in rotating frames undergoes two kinds of Coriolis force effects. First, a secondary flow is induced in the case that there is a mean vorticity component perpendicular to the rotating axis. Second, there are augmenting or suppressing effects on the turbulence if there i… 相似文献
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In the transitional channel flow, the large-scale intermittent structure of localised turbulence, which is called the turbulent stripe pattern, can be found in the form of stripe arrangement. The structure of the turbulent stripe pattern is an oblique laminar–turbulent banded pattern and is inclined with respect to the streamwise direction. We performed direct numerical simulation at a transitional Reynolds number and very low-rotation numbers, and focused on the turbulent stripe pattern in the plane Poiseuille flow subjected to spanwise system rotation. We captured the turbulent stripe pattern in a rotating channel flow and found the augmentation and diminution of the turbulent stripe pattern were affected by the spanwise rotation. The contents of the discussion are the spatial size of the turbulent stripe pattern on the basis of the instantaneous flow fields, the energy spectra, and various statistics relating to the spanwise velocity component that characterise the turbulent stripe pattern. The turbulent stripe pattern was found to contain kinetic energy that was larger in very weakly rotating flows than in the static system. It was also found that the magnitude of the spanwise secondary flow increases, while the quasi-laminar region is wider at a very lowrotation number. 相似文献