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90°弯管二次流非定常动力学特性研究进展
Advances in Unsteady Dynamics of Secondary Flow in 90° Pipe Bends
90°弯管在工程中应用十分广泛,试验研究表明弯管内二次流(迪恩涡)的非定常运动可致使管道出现热疲劳和机械疲劳,严重危害工业管道系统安全运行。早期研究仅关注弯管内时均水力特性,管内二次流非定常动力学特性研究较匮乏。迪恩涡摆动现象作为90°弯管湍流的基础流体力学问题近年来逐渐受到关注。笔者从上游进流条件、弯管处流动分离、流动结构、摆动特征频率、二次流影响范围和流动控制等方面出发,对以往迪恩涡摆动现象研究成果展开讨论。经对比分析现有研究成果指出,现阶段对迪恩涡摆动现象的认识仍不全面,其动力学特性和时空演化机理尚不明晰,且缺乏高精度三维流场数据,提出在迪恩涡摆动现象的触发机制、本征模态、摆动特征频率、空间结构和流动控制方法等方面仍值得进一步探索和研究。
[Objective] The unsteady motion of secondary flow, namely Dean vortices, in bent pipes can induce thermal fatigue and mechanical fatigue, posing a serious threat to the safe operation of industrial piping systems. In recent years, the swirl switching phenomenon, as a fundamental fluid mechanics issue in turbulent flow through 90° bends, has gradually attracted widespread academic attention. However, most existing studies focus mainly on time-averaged hydraulic characteristics such as pressure distribution and mean velocity distribution in bends, while research on the unsteady dynamic characteristics of secondary flow in pipes remains relatively limited. [Methods] Through the extensive literature review, this study collected and organized existing research results on the swirl switching phenomenon in turbulent flow through 90° bends. From the perspectives of upstream inflow conditions, flow separation in bends, flow structures, oscillation characteristic frequencies, the influence range of secondary flow, and flow control, the research progress on the swirl switching phenomenon is systematically summarized. [Results] The correlation between the swirl switching phenomenon and upstream inflow conditions in spatially developing turbulent flow through 90° bends still requires further verification. Flow separation on the inner side of the bend is not a necessary condition for triggering Dean vortex oscillation. Significant discrepancies remain regarding the dominant proper orthogonal decomposition modes representing Dean vortex oscillation. The characteristic frequencies of the swirl switching phenomenon extracted using POD analysis are lower than those obtained by other analysis methods, and extracting characteristic frequencies based on variations of single local physical quantities exhibits considerable randomness. The spatial scale of Dean vortex oscillation, its interaction with the streamwise main flow, and its spatiotemporal evolution mechanisms still require further investigation. The triggering mechanism of the swirl switching phenomenon remains unclear, and how to effectively control it also requires more in-depth research on the underlying flow mechanisms. [Conclusion] Current domestic and international understanding of the swirl switching phenomenon remains incomplete. Its dynamic characteristics and spatio-temporal evolution mechanisms are still unclear, and high-precision three-dimensional flow field data are lacking. It is proposed that further exploration and investigation of the triggering mechanisms, intrinsic modes, characteristic oscillation frequencies, spatial structures, and flow control methods of the swirl switching phenomenon should be conducted using high-fidelity direct numerical simulations and advanced three-dimensional time-resolved flow field measurement techniques.
圆管湍流 / 90°弯管 / 二次流 / 迪恩涡摆动 / 动力学特性
turbulent pipe flow / 90° bends / secondary flow / Dean swirl switching / dynamic characteristics
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Flow visualization was performed on a single short elbow piping by means of two-dimensional particle image velocimetry. The piping was designed as a 1/7-scale model of a section of the cold-leg piping of a Japan sodium-cooled fast reactor. This study characterized the periodic motions and flow structures that appeared in and downstream of the elbow and potentially affected flow-induced vibrations. The flow field that related flow separation and frequency characteristics of the flow velocity fluctuation were explored for Reynolds number from 0.3 × 106 to 1.0 × 106, which belonged to the post-critical regime. Experimental results show that flow structures are not strongly dependent on Reynolds number in this range. Frequency analysis for the velocity fluctuation in terms of Strouhal number (St) reveals that there exist not only two kinds of vortices with different shedding periods, but also one periodic flow in the circumferential direction. In the flow separation region, vortices are periodically emitted with St ≈ 0.5, while those with about 1.0 are shed in a shear flow region located between the separation region and the pipe center. Moreover, a periodic motion with St ≈ 0.5 appeared in the circumferential direction in the vicinity near the separation region. These values of St were not strongly dependent on Reynolds number in this study.
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It has been found experimentally that the turbulent pipe flow through a mitred, right-angle bend produces a downstream secondary circulation which does not conform to the twin-circulatory flow usually to be found in pipe bends. The secondary flow is dominated by a single circulation about the axis in either a clockwise or an anticlockwise sense, between which it switches abruptly at a low, random frequency. The phenomenon is explained in terms of the asymmetry of the inner wall separation and the turbulent axial circulation generated in the upstream flow.
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利用RNG k-ε模型对旋流雾化喷嘴中的直径为2 mm的细小直角弯管流道的流场进行了三维数值模拟，获得了流道内的压力分布和不同截面的迪恩涡结构。结果表明：沿着流动方向，弯曲段流道中内外壁面的压差先增大后减小，内壁面45°截面附近，压力下降到最低，最大下降幅度为74.1%。弯管流道中迪恩涡形成于弯曲段10°截面附近，迪恩涡的旋转中心，从两侧中心位置附近向两侧上端偏移，在65°截面偏移方向发生转折，最后又回到两侧中心位置。迪恩涡的边界和强度先增大后减小，在下游直管段中消失。
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The 3D numerical simulation was carried out on the field in a small right-angle bend flow channel of 2 mm diameter in the swirl atomizing nozzle using RNG k-<i>ε</i> model,so as to obtian the pressure distribution and dean vortices structure in different cross sections in the flow channel.The results show that along the flow direction,the pressure difference between the inside and the outside increases first and then decreases in bending section.The pressure drops to the lowest near the 45° section of the inside internal wall where the biggest drop is 74.1%.Dean vortices form near 10° section of the bending section.And the rotational centers of the dean vortices move from the centers of both side to the above,the migration directions turn at 65° section,finally return to the side near the center positions.At the same time the boundary and intensity of the dean vortices first increase then decrease,until disappear in the downstream straight pipe.
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Large-eddy simulations are performed to investigate turbulent flows through 90° pipe bends that feature unsteady flow separation, unstable shear layers, and an oscillation of the Dean vortices. Single bends with curvature radii of one- and three-pipe diameters are considered at the Reynolds number range 5000–27 000. The numerically computed distributions of the time-averaged velocities, Reynolds stress components, and power spectra of the velocities are validated by comparison with particle image velocimetry measurements. The power spectra of the overall forces onto the pipe walls are determined. The spectra exhibit a distinct peak in the high frequency range that is ascribed to vortex shedding at the inner side of the bends and shear layer instability. At the largest Reynolds number the spectra also exhibit an oscillation at a frequency much lower than that commonly observed at vortex shedding from separation. It turns out that the associated flow pattern is similar to the swirl switching phenomenon earlier found in experimental studies with which the present results are compared. It is shown that the low frequency oscillation perceptible on the entire wall is caused by the two Dean vortices whose strength vary in time and which as such alternately dominate the flow field.
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Turbulent flow through 90° pipe bends, for four different curvatures, has been investigated using large eddy simulations. In particular, the origin of the so-called swirl switching phenomenon, which is a large scale oscillation of the flow after the bend, has been studied for different bend curvature ratios. A classification of the phenomenon into a high and a low frequency switching, with two distinct physical origins, is proposed. While the high frequency switching stems from modes formed at the bend, and becomes increasingly important for sharp curvatures, the low frequency switching originates from very-large-scale motions created in the upstream pipe flow.
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Swirl-switching is a low-frequency oscillatory phenomenon which affects the Dean vortices in bent pipes and may cause fatigue in piping systems. Despite thirty years worth of research, the mechanism that causes these oscillations and the frequencies that characterise them remain unclear. Here we show that a three-dimensional wave-like structure is responsible for the low-frequency switching of the dominant Dean vortex. The present study, performed via direct numerical simulation, focuses on the turbulent flow through a $90^{\\circ }$ pipe bend preceded and followed by straight pipe segments. A pipe with curvature 0.3 (defined as ratio between pipe radius and bend radius) is studied for a bulk Reynolds number $Re=11\\,700$, corresponding to a friction Reynolds number $Re_{\\unicode[STIX]{x1D70F}}\\approx 360$. Synthetic turbulence is generated at the inflow section and used instead of the classical recycling method in order to avoid the interference between recycling and swirl-switching frequencies. The flow field is analysed by three-dimensional proper orthogonal decomposition (POD) which for the first time allows the identification of the source of swirl-switching: a wave-like structure that originates in the pipe bend. Contrary to some previous studies, the flow in the upstream pipe does not show any direct influence on the swirl-switching modes. Our analysis further shows that a three-dimensional characterisation of the modes is crucial to understand the mechanism, and that reconstructions based on two-dimensional POD modes are incomplete.
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Very large-scale motions in the form of long regions of streamwise velocity fluctuation are observed in the outer layer of fully developed turbulent pipe flow over a range of Reynolds numbers. The premultiplied, one-dimensional spectrum of the streamwise velocity measured by hot-film anemometry has a bimodal distribution whose components are associated with large-scale motion and a range of smaller scales corresponding to the main turbulent motion. The characteristic wavelength of the large-scale mode increases through the logarithmic layer, and reaches a maximum value that is approximately 12–14 times the pipe radius, one order of magnitude longer than the largest reported integral length scale, and more than four to five times longer than the length of a turbulent bulge. The wavelength decreases to approximately two pipe radii at the pipe centerline. It is conjectured that the very large-scale motions result from the coherent alignment of large-scale motions in the form of turbulent bulges or packets of hairpin vortices.
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Pipe bends disrupt the flow, resulting in an asymmetric velocity field across the pipe diameter (D). We examined the recovery length required for the flow to return to a symmetric velocity profile downstream of a sharp elbow. The wall-resolved Large Eddy Simulation (LES) approach was applied to reproduce turbulent fluid flow at Reynolds numbers (Re) of 5600 and 10,000. An additional case in the transitional laminar-turbulent-laminar regime was analyzed at Re=1400. This analysis explored the behavior of the Dean vortices downstream of the elbow and revealed that, in turbulent cases, these vortices reverse their vorticity direction in the region between 8 D and 10 D. However, they eventually decay in structure as far as 25 D from the elbow. Flow asymmetry was analyzed in a 100 D long pipe section downstream of the elbow using four different criteria: wall shear stress (WSS), streamwise velocity, its fluctuations, and vorticity fields. This study found that in turbulent flows, the distance required for flow recovery is a few tens of D and decreases with increasing Re. However, in the transitional case, the flow separation within the elbow induces instabilities that gradually diminish downstream, and flow asymmetry persists even longer than the 100 D length of our outlet pipe section. WSS proved sensitive for detecting asymmetry near walls, whereas flow profiles better revealed bulk asymmetry. It was also shown that asymmetry indicators derived from velocity fluctuations and vorticity were less sensitive than those obtained from streamwise velocity.
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