Table of Content

01 February 2019, Volume 40 Issue 2

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Articles

Preface: symposium on turbulence structures and aerodynamic heat/force (STSAHF2018)-scientific significance of turbulence research

Cunbiao LEE, Zuoli XIAO, Shiyi CHEN

2019, 40(2):  181-184.  doi:10.1007/s10483-019-2440-6

Abstract ( 554 )   HTML   PDF (68KB) ( 205 )  

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Dissipation function in turbulent plane Poiseuille and Couette flows subject to spanwise rotations

Zhenhua XIA, Yipeng SHI, Qingdong CAI, Jie GAI

2019, 40(2):  185-192.  doi:10.1007/s10483-019-2422-6

Abstract ( 513 )   HTML   PDF (286KB) ( 143 )  

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The dissipation function in turbulent plane Poiseuille flows (PPFs) and plane Couette flows (PCFs) subject to spanwise rotations is analyzed. It is found that, in the PCFs without system rotations, the mean part is constant while the fluctuation part follows a logarithmic law, resulting in a similar logarithmic skin friction law as PPFs. However, if the flow system rotates in the spanwise direction, no obvious dependence on the rotation number can be evaluated. In the PPFs with rotations, the dissipation function shows an increase with the rotation number, while in the PCFs with rotations, when the rotation number increases, the dissipation function first decreases and then increases.

Thirty years of turbulence study in China

Bohua SUN

2019, 40(2):  193-214.  doi:10.1007/s10483-019-2427-9

Abstract ( 439 )   HTML   PDF (505KB) ( 273 )  

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This paper provides a comprehensive introduction to the major progress in transition and turbulence studies in China over the past thirty years. Despite the author's best efforts, there are some unavoidable omissions. We expect that with the continued deepening of turbulence research, Chinese scholars will make increasing contributions in this field, which will certainly have strong influence on the establishment of a China School of Turbulence.

Non-equilibrium turbulent phenomena in the flow over a backward-facing ramp

Le FANG, Hongkai ZHAO, Weidan NI, Jian FANG, Lipeng LU

2019, 40(2):  215-236.  doi:10.1007/s10483-019-2428-6

Abstract ( 510 )   HTML   PDF (1795KB) ( 195 )  

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Non-equilibrium turbulence phenomena have raised great interests in recent years. Significant efforts have been devoted to non-equilibrium turbulence properties in canonical flows, e.g., grid turbulence, turbulent wakes, and homogeneous isotropic turbulence (HIT). The non-equilibrium turbulence in non-canonical flows, however, has rarely been studied due to the complexity of the flows. In the present contribution, a directnumerical simulation (DNS) database of a turbulent flow is analyzed over a backwardfacing ramp, the flow near the boundary is demonstrated, and the non-equilibrium turbulent properties of the flow in the wake of the ramp are presented by using the characteristic parameters such as the dissipation coefficient C_∊ and the skewness of longitudinal velocity gradient S_k, but with opposite underlying turbulent energy transfer properties. The equation of Lagrangian velocity gradient correlation is examined, and the results show that non-equilibrium turbulence is the result of phase de-coherence phenomena, which is not taken into account in the modeling of non-equilibrium turbulence. These findings are expected to inspire deeper investigation of different non-equilibrium turbulence phenomena in different flow conditions and the improvement of turbulence modeling.

Neural network as a function approximator and its application in solving differential equations

Zeyu LIU, Yantao YANG, Qingdong CAI

2019, 40(2):  237-248.  doi:10.1007/s10483-019-2429-8

Abstract ( 628 )   HTML   PDF (1564KB) ( 497 )  

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A neural network (NN) is a powerful tool for approximating bounded continuous functions in machine learning. The NN provides a framework for numerically solving ordinary differential equations (ODEs) and partial differential equations (PDEs) combined with the automatic differentiation (AD) technique. In this work, we explore the use of NN for the function approximation and propose a universal solver for ODEs and PDEs. The solver is tested for initial value problems and boundary value problems of ODEs, and the results exhibit high accuracy for not only the unknown functions but also their derivatives. The same strategy can be used to construct a PDE solver based on collocation points instead of a mesh, which is tested with the Burgers equation and the heat equation (i.e., the Laplace equation).

Effect of glow discharge on hypersonic flat plate boundary layer

Chi LI, Yunchi ZHANG

2019, 40(2):  249-260.  doi:10.1007/s10483-019-2424-9

Abstract ( 532 )   HTML   PDF (830KB) ( 90 )  

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Glow discharge is introduced as an artificial disturbance to investigate the evolution of first-and second-mode instabilities in a hypersonic flat plate boundary layer. Experiments are conducted in a Mach 6.5 quiet wind tunnel using Rayleigh scattering visualization and particle image velocimetry (PIV). Detailed analysis of the experimental observations is provided. It is found that the artificially introduced 17 kHz disturbance, which belongs to the first-mode frequency band, can effectively enhance first-mode waves. Moreover, it can enhance second-mode waves even more intensely. Possible mechanisms to explain this phenomenon are discussed.

Revisiting coherent structures in low-speed turbulent boundary layers

Xianyang JIANG

2019, 40(2):  261-272.  doi:10.1007/s10483-019-2445-8

Abstract ( 464 )   HTML   PDF (925KB) ( 158 )  

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Coherent structures are essential for the momentum exchange and turbulence production in wall-bounded turbulent flows. Diversified coherent structures have been observed in turbulent boundary layers, and hairpin-based vortices dominate most of the relevant literature. However, there is no consensus yet on the origin and forming mechanism of hairpin vortices. Herein, five cornerstones pertaining to the framework of hairpin-based coherent structures are reviewed, and three different hairpin generation mechanisms are clarified. Next, the time-resolved tomographic particle image velocimetry (Tomo-PIV) is used in an early turbulent boundary layer (Re_θ=420) to investigate the origin of hairpin vortices. The timelines reveal a triangular bulge in the low-speed streak (LSS), and the initial roll-up occurs at two sides of it. Meanwhile, the material surfaces manifest as a three-dimensional (3D) wave structure in the LSS, which may support the model of a soliton-like coherent structure (SCS). Subsequently, the method of Lagrangian-averaged vorticity deviation is used to detect early vortices. We find that the 3D wave structure is flanked by two vortices, thus confirming the roll-up of timelines and demonstrating the advantage of the Lagrangian criteria in capturing structures in complex flows. These results suggest that various coherent structures may evolve from the metamorphosis of 3D wave structures and their later interaction. Finally, the limitations of traditional experimental and post-processing tools are discussed.

Experimental and numerical study of flow structures of the second-mode instability

Yiding ZHU

2019, 40(2):  273-282.  doi:10.1007/s10483-019-2430-9

Abstract ( 466 )   HTML   PDF (520KB) ( 228 )  

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Flow structures of a Mach 6 transitional boundary layer over a 260 mm long flared cone are investigated by the particle image velocimetry (PIV). Particle images near the curved wall are initially transformed into surface-fitted orthogonal coordinates and spliced with their 180°-symmetric images to satisfy a no-slip condition at the wall. The results are then reversely transformed to the physical domain. Direct numerical simulation (DNS) is also performed to validate the experimental results. The experimental and numerical results are in agreement, indicating a strong dilatation process within the second-mode instability.

Transition control of Mach 6.5 hypersonic flat plate boundary layer

Yunchi ZHANG, Chi LI

2019, 40(2):  283-292.  doi:10.1007/s10483-019-2423-8

Abstract ( 511 )   HTML   PDF (732KB) ( 103 )  

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An artificial disturbance is introduced into the boundary layer over a flat plate to investigate the effect on the transition process in the Mach 6.5 wind tunnel at Peking University. A linear stability theory (LST) is utilized to predict the evolution of the eigenmodes, and the frequency of the artificial disturbance is chosen according to the LST results. The artificial disturbance is generated by glowing discharge on the surface of the plate close to the leading edge. The Rayleigh-scattering visualization and particle image velocimetry (PIV) measurements are performed. By comparing the experimental results with artificial disturbances with those under the natural condition (without artificial disturbances), the present paper shows that the second-mode instability waves are significantly stimulated by the artificial disturbances, and the boundary layer transition is effectively triggered.

Effects of finite-size neutrally buoyant particles on the turbulent channel flow at a Reynolds number of 395

Zhaosheng YU, Chenlin ZHU, Yu WANG, Xueming SHAO

2019, 40(2):  293-304.  doi:10.1007/s10483-019-2426-8

Abstract ( 507 )   HTML   PDF (845KB) ( 155 )  

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A direct-forcing fictitious domain (DFFD) method is used to perform fully resolved numerical simulations of turbulent channel flows laden with large neutrally buoyant particles. The effects of the particles on the turbulence (including the mean velocity, the root mean square (RMS) of the velocity fluctuation, the probability density function (PDF) of the velocity, and the vortex structures) at a friction Reynolds number of 395 are investigated. The results show that the drag-reduction effect caused by finite-size spherical particles at low particle volumes is negligibly small. The particle effects on the RMS velocities at Re_τ=395 are significantly smaller than those at Re_τ=180, despite qualitatively the same effects, i.e., the presence of particles decreases the maximum streamwise RMS velocity near the wall via weakening the large-scale streamwise vortices, and increases the transverse and spanwise RMS velocities in the vicinity of the wall by inducing smaller-scale vortices. The effects of the particles on the PDFs of the fluid fluctuating velocities normalized with the RMS velocities are small, regardless of the particle size, the particle volume fraction, and the Reynolds number.