基于小波及Hilbert-Huang变换提取壁湍流相干结构

为了研究壁湍流中的相干结构,在直流抽吸式低（变）湍流度风洞的实验段中,应用恒温热线风速仪IFA-300精确测量平板湍流边界层不同法向位置的流向速度信号.分析壁湍流脉动速度的概率密度分布,并与高斯分布比较,验证了壁湍流中相干结构引起的猝发现象的统计规律.提出了一种利用小波变换和Hilbert-Huang（HHT）变换相结合来提取相干结构的方法.对壁湍流脉动速度进行HHT变换,通过变换后各模态的能量分析,分离出主要含能模态对应的相干结构.利用小波重构法继续提取未分离的小尺度相干结构.对剩余湍流信号进行概率统计,并比较提取的全部相干结构与原始湍流信号的波形和HHT边际谱,验证了湍流信号既包含非随机的相干结构信号也包含随机的非相干信号.

Extraction of Coherent Structures in Wall Turbulence Based on Wavelet and Hilbert-Huang Transforms

In order to study the coherent structures in wall turbulence,the constant temperature hot-wire anemometer IFA-300 was used to measure the flow velocity signals at different normal locations of the turbulent boundary layer in the wind tunnel,which is of direct suction-type low（variable）turbulent intensity.The probability density distribution of wall turbulence fluctuating velocity was analyzed and compared with the Gaussian curves,and the results confirm the statistical laws of bursts caused by coherent structures in wall turbulence.A method for extracting coherent struc-tures was proposed,which is hybridized by wavelet transform and Hilbert-Huang transform（HHT）.The HHT was applied to analyze the turbulent fluctuation velocity,and the main coherent structures were extracted by analyzing the energy of each mode.The wavelet reconstruction method was used to extract small-scale coherent structures from the left data.The probability density distribution of the remained signals was analyzed.All of the coherent structures ex-tracted by the above method were compared with the original turbulent signal,and their HHT marginal spectrums were also compared,which verifies that turbulence signal contains both non-random coherent signal and random non-coherent signal.