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.     

Extended self-similar scaling law of multi-scale eddy structure in wall turbulence

ＩｎｔｒｏｄｕｃｔｉｏｎＭｕｃｈｗｏｒｋｈａｓｂｅｅｎｄｅｖｏｔｅｄｉｎｔｈｅｌａｓｔｆｅｗｄｅｃａｄｅｓｔｏｔｈｅｍｅａｓｕｒｅｍｅｎｔａｎｄｍｏｄｅｌｉｎｇｏｆｔｈｅｓｃａｌｉｎｇｌａｗｏｆｓｔｒｕｃｔｕｒｅｆｕｎｃｔｉｏｎｏｆｔｕｒｂｕｌｅｎｔｆｌｏｗｓ.Ｔｈｅｓｏ＿ｃａｌｌｅｄ“ｖｅｌｏｃｉｔｙｓｔｒｕｃｔｕｒｅｆｕｎｃｔｉｏｎｏｆｏｒｄｅｒｎ”ｆｏｒｔｕｒｂｕｌｅｎｔｆｌｏｗｓｉｓｄｅｆｉｎｅｄａｓ〈ΔＶ(ｒ) ｎ〉 ,ｗｈｅｒｅΔＶ(ｒ) =Ｖ(ｘ ｒ) -Ｖ(ｘ)ｉｓｔｈｅｖｅｌｏｃｉｔｙｃｏｍｐ…     

用能量最大准则确定VITA法的平均周期

引入了子波变换的方法来确定检测壁湍流猝发事件的VITA法的短时间平均周期T。为了确定VITA法的短时间平均周期T,用子波变换对热膜测速仪测量得到的湍流边界层近壁区域的流向脉动速度的时间序列进行了分解,分解在时域和频域同时进行,根据每一个尺度的子波系数的模的平方在时域的积分得到壁湍流每一个尺度的脉动动能随尺度的分布,用能量最大准则确定与壁湍流猝发事件的时间尺度对应的能量最大的尺度,该尺度也就是VITA法的短时间平均周期T。     

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.     

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.     

壁面湍流发卡涡包空间模态的TRPIV实验研究

运用高时间分辨率粒子图像测速(Time-resolved PIV简称TRPIV),测量得到平板湍流边界层流向/法向平面内瞬时速度矢量空间分布的时间序列;采用空间局部平均速度结构函数的概念,识别和提取湍流边界层中大尺度发卡涡包结构的空间特征。发现在湍流边界层中不同法向位置多个正负发卡涡包结构同时交替存在。这些分布在不同法向高度的发卡涡包结构之间通过倾斜的涡量剪切层相联系,构成了湍流边界层中内、外区紧密相连、相互作用的一种稳态的分布方式。     

奇思妙想的泳衣与流体减阻

介绍了流体力学、仿生学、材料科学、人工智能科学等学科的科学技术 进步对游泳衣更新换代和提高游泳成绩的推动作用. 从游泳衣的变革中可 以看出,力学与其它学科的交叉与融合在体育科技进步中已经显示出越来越重要的 作用.     

湍流边界层对数律区相干结构的Tomo-TRPIV实验研究

应用层析高时间分辨率粒子图像测速技术（tomographic time-resolved particle image velocimetry，Tomo-TRPIV），测量了Re_θ=2 460的平板湍流边界层三维3分量瞬时速度场的时间序列.提出湍流空间多尺度局部平均速度结构函数的概念，应用流向脉动速度沿流向的空间多尺度局部平均速度结构函数的正负过零点法，从瞬时脉动速度场中检测相干结构的喷射和扫掠事件，对检测到的喷射和扫掠事件的瞬态局部速度场、速度梯度场、涡量场、速度变形率场进行空间相位对齐叠加平均，获得喷射和扫掠事件的局部速度场、速度梯度场、涡量场、速度变形率场的典型特征.研究发现，相干结构喷射和扫掠时，速度梯度、速度变形率、涡量均表现为空间反对称分布的4极子结构.特别是流向涡量是沿流向、法向、展向均为反对称分布的法向多层4极子结构，表明法向各层相干结构是紧密联系，互相关联的.这种法向多层的4极子反对称结构导致强烈的动量、质量和能量交换，维持了相干结构的演化和发展过程.      

《力学与实践》伴随我成长

<正>我1990年从天津大学数学系保送力学系攻读硕士研究生,师从王振东先生。因为我本科学的应用数学专业,对力学是零基础,阅读流体力学英文期刊文献更是如读天书。为了尽快入门,王振东先生就推荐我阅读《力学与实践》,从那时起我就与《力学与实践》结下了不解之缘,弹指一挥间,至今已经28年了。1997年我跟随舒玮先生和王振东先生到北京西