Influence of uniform momentum zones on frictional drag within the turbulent boundary layer

Based on a set of experimental databases of turbulent boundary layers obtained from particle image velocimetry in the streamwise-wall-normal plane at friction-velocity-based Reynolds number Reτ=612,the influence of uniform momentum zones(UMZs)on the skin-friction drag is investigated.The skin-friction drag is measured by the single-pixel ensemble correlation method.The results show that the velocity fields with the number of UMZs larger than the mean value have a relatively low skin-friction drag,while the velocity fields with the number of UMZs less than the mean value have a relatively high skin-friction drag.By analyzing the statistical characteristics of UMZs,the dynamic correlation between the UMZs and skin-friction drag is explored.The velocity fields with a low number of UMZs present a sweep event.These sweep motions intensify the small-scale Reynolds shear stress in the near-wall region by modulation effects.The enhancement of small-scale Reynolds shear stress is the direct reason for the high skin-friction drag.Increasing the proportion of velocity fields with high UMZs amount may be a direction to reduce the skin-friction drag within the TBL.     

奇妙的“波浪云”——浅谈开尔文-亥姆霍兹不稳定性现象

开尔文-亥姆霍兹不稳定性是两种流体作平行于水平界面的相对运动时的流动稳定性问题之一. 文章从自然现象"波浪云" 谈起,介绍了开尔文-亥姆霍兹不稳定性的理论定义、该不稳定性现象形成发展的过程及其流动可视化;并简单阐述了实验室和自然界中其他"类开尔文-亥姆霍兹不稳定" 现象,重点介绍了梵高名画《星月夜》和笔者在从事沟槽壁面湍流场可视化实验中观察到的一种展向"类开尔文-亥姆霍兹涡棍" 结构,指出它们的形成及发展演化是沟槽湍流减阻达到最优之后,减阻效应趋弱直至增阻的原因.     

那些年,北洋水利那些事儿 1)

本文基于部分历史史实,对北洋大学(今天津大学)水利系的发展进行了简要的梳理,并扼要地回顾了那些年,北洋大学水利那些事儿、那些人。重点从北洋大学-天津大学水利学科、水利系的历史延续及其在中国水利高等教育中的地位、中国第一水工试验所及 其发展过程、天津大学水利馆及知名教授和杰出校友的相关故事出发,以期较为全面地展示北洋大学-天津大学水利系125年的历 史传承和现代化发展成就,激励新一代天大水利人,续写北洋雄风,并深刻缅怀余元君等已故校友。     

镍磷物质的量比对负载型Ni2P/MCM-41催化剂结构及加氢脱硫性能的影响

以MCM-41为载体,采用一种简捷、温和法制备了负载型Ni₂P/MCM-41催化剂。用H₂程序升温还原(H₂-TPR)、X射线衍射(XRD)、N₂吸附比表面积测定(BET)和X射线光电子能谱(XPS)分析对催化剂进行了表征。以1%(质量分数)二苯并噻吩(DBT)的十氢萘溶液为原料,在连续固定床反应装置上,研究了初始Ni/P物质的量比对催化剂HDS活性的影响,并考察了催化剂的稳定性。结果表明,初始Ni/P物质的量比为1/2和1/3的前驱体,在390 ℃下还原时得到单一的Ni₂P相。初始Ni/P物质的量为1/2时,得到的催化剂活性最好。在反应温度340 ℃、压力3.0 MPa、氢/油体积比500、质量空速2.0 h^-1时,DBT的转化率接近100%。     

Effects of single synthetic jet on turbulent boundary layer

The turbulent boundary layer (TBL) is actively controlled by the synthetic jet generated from a circular hole. According to the datasets of velocity fields acquired by a time-resolved particle image velocimetry (TR-PIV) system, the average drag reduction rate of 6.2% in the downstream direction of the hole is obtained with control. The results of phase averaging show that the synthetic jet generates one vortex pair each period and the consequent vortex evolves into hairpin vortex in the environment with free-stream, while the reverse vortex decays rapidly. From the statistical average, it can be found that a low-speed streak is generated downstream. Induced by the two vortex legs, the fluid under them converges to the middle. The drag reduction effect produced by the synthetic jet is local, and it reaches a maximum value at x⁺=400, where the drag reduction rate reaches about 12.2%. After the extraction of coherent structure from the spatial two-point correlation analysis, it can be seen that the synthetic jet suppresses the streamwise scale and wall-normal scale of the large scale coherent structure, and slightly weakens the spanwise motion to achieve the effect of drag reduction.     

湍流边界层大尺度相干结构空间相位平均拓扑

湍流边界层大尺度相干结构(large-scale coherent structure,LSCS)是目前湍流边界层研究的热点之一.湍流边界层中存在大尺度相干结构均是通过各种谱的低波数峰值证实的,而对大尺度相干结构各种物理量的空间拓扑形态及其发展演化规律还缺乏研究.应用高时间分辨粒子图像测速多相机阵列系统,测量湍流边界层...     

Convection and correlation of coherent structure in turbulent boundary layer using tomographic particle image velocimetry

The present experimental work focuses on a new model for space–time correlation and the scale-dependencies of convection velocity and sweep velocity in turbulent boundary layer over a flat wall. A turbulent boundary layer flow at Reθ= 2460 is measured by tomographic particle image velocimetry(tomographic PIV). It is demonstrated that arch, cane,and hairpin vortices are dominant in the logarithmic layer. Hairpins and hairpin packets are responsible for the elongated low-momentum zones observed in the instantaneous flow field. The conditionally-averaged coherent structures systemically illustrate the key roles of hairpin vortice in the turbulence dynamic events, such as ejection and sweep events and energy transport. The space–time correlations of instantaneous streamwise fluctuation velocity are calculated and confirm the new elliptic model for the space–time correlation instead of Taylor hypothesis. The convection velocities derived from the space–time correlation and conditionally-averaged method both suggest the scaling with the local mean velocity in the logarithmic layer. Convection velocity result based on Fourier decomposition(FD) shows stronger scale- dependency in the spanwise direction than in streamwise direction. Compared with FD, the proper orthogonal decomposition(POD) has a distinct distribution of convection velocity for the large- and small-scales which are separated in light of their contributions of turbulent kinetic energy.     

SPME-GC-MS快速分析炼油厂废水中痕量VOCs

采用固相微萃取 -气相色谱 -质谱联用 (SPME- GC- MS)对炼油厂废水中痕量挥发性有机化合物(VOCs)进行了定性分析 ,简化了预处理过程 ,总分析时间为 1h左右。对比处理前后的废水样品的 MS定性结果 ,为废水处理工艺的评价以及有机物降解机理提供了依据。     

多脉冲激光对碳纳米管悬浮液光限幅特性影响

利用实验的方法研究了碳纳米管悬浮液对脉宽8 ns,波长532 nm多脉冲激光的光限幅效应.分析了直径分布为10~20 nm的多壁碳纳米管悬浮液对重复频率分别为1 Hz、 3 Hz、 5 Hz、 10 Hz情况下532 nm激光的光限幅效应,分析计算了不同重复频率下碳纳米管悬浮液的限幅阈值,比较了不同焦距的透镜会聚入射光束情况下对碳纳米管悬浮液光限幅效果的影响.实验结果表明：碳纳米管悬浮液对不同重复频率的532 nm 激光都具有较强的光限幅特性;碳纳米管悬浮液对激光在不同重复频率入射情况下的光限幅阈值变化很大,当入射激光的重复频率为5 Hz时,碳纳米管悬浮液的光限幅阈值比单脉冲激光入射时的限幅阈值低了2倍,重复频率为10 Hz时的限幅阈值比单脉冲时的限幅阈值低了近3倍;碳纳米管在紧焦系统中的光限幅效果更好.