检索结果-南通市图书馆

On hairpin vortex generation from near-wall streamwise vortices

在线全文

学校读者我要写书评

暂无评论

Acta Mechanica Sinica 2015年第2期31卷 139-152页

作者： Yinshan Wang weixi huang Chunxiao Xu AML Department of Engineering MechanicsTsinghua University

The generation of a hairpin vortex from near-wall streamwise vortices is studied via the direct numerical simulation（DNS） of the streak transient growth in the minimal channel flow at Re_τ- *** streak profile is obtained by conditionally averaging the DNS data of the fully developed turbulent channel flow at the same Reynolds *** near-wall streamwise vortices are produced by the transient growth of the streak which is initially subjected to the sinuous perturbation of the spanwise *** is shown that the arch head of the hairpin vortex first grows from the downstream end of the stronger streamwise vortex and then connects with the weaker,opposite-signed streamwise vortex in their overlap region,forming a complete individual hairpin *** vorticity transport along the vortex lines indicates that the strength increase and the spatial expansion of the arch head are due to the stretching and the turning of the vorticity vector,*** hairpin packets could be further produced from the generated individual hairpin vortex following the parent-offspring process.

关键词： Hairpin vortex Streamwise vortex Streak transient growth

Linear optimal control of transient growth in turbulent channel flows

在线全文

学校读者我要写书评

暂无评论

Acta Mechanica Sinica 2019年第4期35卷 729-739页

作者： Yang Song Chunxiao Xu weixi huang Institute of Applied Physics and Computational Mathematics Beijing 100094 China AML Department of Engineering MechanicsTsinghua University Beijing 100084 China

The suppression of linear transient growth in turbulent channel flows via linear optimal controls is investigated. The control algorithms are the LQR control based on full information of flow fields and the LQG control based on the information measured at walls. The influence of these controls on the development of small-scale and large-scale perturbations are considered respectively. It is found that the energy amplification of large-scale perturbations is suppressed significantly by both LQR and LQG control, while small-scale perturbations can be only affect by LQR control. Effects of the weighting parameters and control price on the control performance of both controls are analysed. It turns out that the different weighting parameters in cost function do not qualitatively change the evalution of control performance. As control price raises, the effectiveness of both controls decreases distinctly. For small-scale perturbations, the upper limit of the effective range of control price is lower than that for large-scale perturbations. As Reynolds number increases, it indicates that both LQR and LQG control get more effective on suppressing the energy amplification of large-scale perturbations.

关键词： Linear optimal control Turbulent channel flow Optimal transient growth

Direct numerical simulation of turbulent flows through concentric annulus with circumferential oscillation of inner wall

维普期刊数据库评论

在线全文

维普期刊数据库

学校读者我要写书评

暂无评论

Applied Mathematics and Mechanics(English Edition) 2018年第9期39卷 1267-1276页

作者： Yichen YAO Chunxiao XU weixi huang Department of Engineering Mechanics Tsinghua University

Periodic wall oscillations in the spanwise or circumferential direction can greatly reduce the friction drag in turbulent channel and pipe flows. In a concentric annulus, the constant rotation of the inner cylinder can intensify turbulence fluctuations and enhance skin friction due to centrifugal instabilities. In the present study, the effects of the periodic oscillation of the inner wall on turbulent flows through concentric annulus are investigated by the direct numerical simulation(DNS). The radius ratio of the inner to the outer cylinders is 0.1, and the Reynolds number is 2 225 based on the bulk mean velocity Um and the half annulus gap H. The influence of oscillation period is considered. It is found that for short-period oscillations, the Stokes layer formed by the circumferential wall movement can effectively inhibit the near-wall coherent motions and lead to skin friction reduction, while for long-period oscillations, the centrifugal instability has enough time to develop and generate new vortices, resulting in the enhancement of turbulence intensity and skin friction.

关键词： turbulence drag reduction circumferential oscillation Taylor vortex

Swimming strategy of settling elongated micro-swimmers by reinforcement learning

在线全文

学校读者我要写书评

暂无评论

Science China(Physics,Mechanics & Astronomy) 2020年第8期63卷 86-94页

作者： JingRan Qiu weixi huang ChunXiao Xu LiHao Zhao Applied Mechanics Laboratory(AML) Department of Engineering MechanicsTsinghua University100084 BeijingChina

Particular types of plankton in aquatic ecosystems can coordinate their motion depending on the local flow environment to reach regions conducive to their growth or *** their swimming strategies with regard to the local environment is important to obtain in-depth understanding of their behavior in the aquatic *** the present research,to examine an impact of the shape and gravity on a swimming strategy,plankton is considered as settling swimming particles of ellipsoidal *** Q-learning approach is adopted to obtain swimming strategies for smart particles with a goal of efficiently moving upwards in a two-dimensional steady *** obtained from reinforcement learning are compared to those of naive gyrotactic particles that are modeled considering the behavior of realistic *** is found that the elongation of particles improves the performance of upward swimming by facilitating particles’resistance to the perturbation of *** the case when the settling velocity is included,the strategy obtained by reinforcement learning has similar performance to that of the naive gyrotactic one,and they both align swimmers in upward *** similarity between the strategy obtained from machine learning and the biological gyrotactic strategy indicates the relationship between the aspherical shape and settling effect of realistic plankton and their gyrotactic feature.

关键词： swimming particles ellipsoid settling reinforcement learning

Optimal transient growth in turbulent pipe flow

在线全文

学校读者我要写书评

暂无评论

Applied Mathematics and Mechanics(English Edition) 2015年第8期36卷 1057-1072页

作者： Yang SONG Chunxiao XU weixi huang Guixiang CUI Key Laboratory of Applied Mechanics Department of Engineering MechanicsTsinghua University

The optimal transient growth process of perturbations driven by the pressure gradient is studied in a turbulent pipe flow. A new computational method is proposed, based on the projection operators which project the governing equations onto the sub- space spanned by the radial vorticity and radial velocity. The method is validated by comparing with the previous studies. Two peaks of the maximum transient growth am- plification curve are found at different Reynolds numbers ranging from 20 000 to 250 000. The optimal flow structures are obtained and compared with the experiments and DNS results. The location of the outer peak is at the azimuthal wave number n = 1, while the location of the inner peak is varying with the Reynolds number. It is observed that the velocity streaks in the buffer layer with a spacing of 100δv are the most amplified flow structures. Finally, we consider the optimal transient growth time and its dependence on the azimuthal wave length. It shows a self-similar behavior for perturbations of different scales in the optimal transient growth process.

关键词： turbulent pipe flow optimal transient growth coherent structure

Assessment of force models on finite-sized particles at finite Reynolds numbers

在线全文

学校读者我要写书评

暂无评论

Applied Mathematics and Mechanics(English Edition) 2020年第6期41卷 953-966页

作者： Ruyang LI weixi huang LihaoZHAO ChunxiaoXU Applied Mechanics Laboratory(AML) Department of Engineering MechanicsTsinghua UniversityBeijing 100084China

Finite-sized inertial spherical particles are fully-resolved with the immersed boundary projection method(IBPM)in the turbulent open-channel flow by direct numerical simulation(DNS).The accuracy of the particle surface force models is investigated in comparison with the total force obtained via the fully-resolved *** results show that the steady-state resistance only performs well in the streamwise direction,while the fluid acceleration force,the added-mass force,and the shear-induced Saffman lift can effectively compensate for the large-amplitude and high-frequency characteristics of the particle surface forces,especially for the wall-normal and spanwise *** modified steady-state resistance with the correction effects of the acceleration and the fluid shear can better represent the overall forces imposed on the particles,and it is a preferable choice of the surface force model in the Lagrangian point-particle method.

关键词： finite-sized particle immersed boundary projection method(IBPM) force model Saffman lift

Transient growth in turbulent particle-laden channel flow

在线全文

学校读者我要写书评

暂无评论

Acta Mechanica Sinica 2020年第1期36卷 1-11页

作者： Yang Song Chunxiao Xu weixi huang Lili Wang Institute of Applied Physics and Computational Mathematics Beijing100094China AML Department of Engineering MechanicsTsinghua UniversityBeijing100084China

Linear transient growth of optimal perturbations in particle-laden turbulent channel flow is investigated in this *** problem is formulated in the framework of a Eulerian-Eulerian approach,employing two-way coupling between fine particles and fluid *** model is first validated in laminar cases,after which the transient growth of coherent perturbations in turbulent channel flow is investigated,where the mean particle concentration distribution is obtained by direct numerical *** is shown that the optimal small-scale structures for particles are streamwise streaks just below the optimal streamwise velocity streaks,as was previously found in numerical simulations of particle-laden channel *** indicates that the optimal growth of perturbations is a dominant mechanism for the distribution of particles in the near-wall *** current study also considers the transient growth of small-and large-scale perturbations at relatively high Reynolds numbers,which reveals that the optimal large-scale structures for particles are in the near-wall region while the optimal large-scale structures for fluid enter the outer region.

关键词： Optimal transient growth Wall turbulence Coherent structures Particle-laden turbulence

Relationship between wall shear stresses and streamwise vortices in turbulent flows over wavy boundaries

维普期刊数据库评论

在线全文

维普期刊数据库

学校读者我要写书评

暂无评论

Applied Mathematics and Mechanics(English Edition) 2019年第3期40卷 381-396页

作者： Lihao WANG weixi huang Chunxiao XU Lian SHEN Zhaoshun ZHANG Applied Mechanics Laboratory Department of Engineering MechanicsTsinghua University Department of Mechanical Engineering and Saint Anthony Falls Laboratory University of Minnesota

The relationship between wall shear stresses and near-wall streamwise vortices is investigated via a direct numerical simulation(DNS) of turbulent flows over a wavy boundary with traveling-wave motion. The results indicate that the wall shear stresses are still closely related to the near-wall streamwise vortices in the presence of a wave. The wave age and wave phase significantly affect the distribution of a two-point correlation coefficient between the wall shear stresses and streamwise vorticity. For the slow wave case of c/Um = 0.14, the correlation is attenuated above the leeward side while the distribution of correlation function is more elongated and also exhibits a larger vertical extent above the crest. With respect to the fast wave case of c/U_m=1.4, the distribution of the correlation function is recovered in a manner similar to that in the flat-wall case. In this case, the maximum correlation coefficient exhibits only slight differences at different wave phases while the vertical distribution of the correlation function depends on the wave phase.

关键词： direct numerical simulation(DNS) wall shear stress near-wall streamwise vortex two-point correlation

Coherent structures in wall turbulence and mechanism for drag reduction control

在线全文

学校读者我要写书评

暂无评论

Science China(Physics,Mechanics & Astronomy) 2013年第6期56卷 1053-1061页

作者： XU ChunXiao DENG BingQing huang weixi CUI GuiXiang Department of Engineering Mechanics Tsinghua University

Herein is introduced the mechanism for active control influencing the generation of the near-wall streamwise vortices,which are closely related to the production of high skin friction in wall-bounded turbulent flows.A new opposition control scheme with adjusting control amplitude is proposed and evaluated in turbulent channel flow by direct numerical *** maximum drag reduction rate can be greatly enhanced by the strengthened *** the effectiveness of the control to the coherent structures at high Reynolds numbers is investigated by using a linear transient growth model.

关键词：湍流相干结构主动控制减阻率机制直接数值模拟控制方案增长模型结构控制