Wavelet spectrum analysis on energy transfer of multi-scale structures in wall turbulence

The streamwise velocity components at different vertical heights in wall turbulence were measured. Wavelet transform was used to study the turbulent energy spectra, indicating that the global spectrum results from the weighted average of Fourier spectrum based on wavelet scales. W＇avelet transform with more vanishing moments can express the declining of turbulent spectrum. The local wavelet spectrum shows that the physical phenomena such as deformation position in the boundary layer, and the or breakup of eddies are related to the vertical energy-containing eddies exist in a multi-scale form. Moreover, the size of these eddies increases with the measured points moving out of the wall. In the buffer region, the small scale energy-containing eddies with higher frequency are excited. In the outer region, the maximal energy is concentrated in the low-frequency large-scale eddies, and the frequency domain of energy-containing eddies becomes narrower.     

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.     

秋荷白露大如珠——从荷叶上的露珠谈超疏水现象

以科普文章的形式介绍超疏水现象及其物理机理和工程应用.从中国古代诗歌中描写荷叶上的露珠的名篇佳句引入,介绍了自然界的超疏水现象及其物理机理和超疏水材料在工程中的应用,超疏水材料是一种具有广泛应用前景和应用价值的新材料.号召公众热爱自然、观察自然、认识自然、学习自然,与自然和谐相处.     

圆柱尾流雷诺应力与平均运动变形率的空间弛豫效应

针对圆柱尾流中沿流向存在的Karman涡街周期性涡旋结构,对湍流雷诺应力与平均运动变形率之间的空间弛豫效应进行了实验研究.在回流式水槽中,放入不同直径的圆柱模型,获得不同雷诺数下的圆柱尾流,利用二维高时间分辨率粒子图像测速(TRPIV)技术测量圆柱尾流二维瞬时速度空间分布图像的时间序列.经过数字图像处理,获得二维雷诺应...     

用子波谱分析壁湍流多尺度结构的能量传递

测量了壁湍流不同法向位置的流向速度.用子波变换研究湍流能谱,表明子波全局谱是Fourier谱按子波尺度加权平均的结果,高阶消失矩的子波变换能够表征湍谱的衰减特性;子波局部谱显示边界层内涡的变形、破碎等现象与边界层法向位置有关,含能涡结构呈多尺度分布,随着测点远离壁面,含能涡的尺度增大;在缓冲区出现更高频的小尺度含能涡,在外区能量集中在低频大尺度涡结构中,含能涡的频带变窄.