基于红外热像的自由剪切湍流被动标量高阶谱分析

High-Order Spectral Analysis of Passive Scalar in Free Turbulent Jet Based on Infrared Thermography

利用红外热像仪对高压水射流进行了连续探测。对红外热像序列表征的标量脉动场进行双谱分析,检测湍射流的相干结构、射流不同截面内相干结构的尺度关联性以及射流不同发展阶段的特征尺度。研究表明,以红外热像为样本的全局自双谱表征了射流不同波段能分量对相干的贡献;按三列对图像采样的横断面局部互双谱给出了沿流向的大涡区、小涡区、以及各向同性湍流区的频率耦合特征;按三行对图像采样的纵向局部互双谱给出了射流的轴心线区、剪切层、空气湍流区大涡层和各向同性湍流层等各纵向断面的相干特征;通过红外热像序列的双谱分析,由流场相干结构的尺度关联性,得到了射流诱导的空气湍流场在不同发展阶段的特征尺度。

The jetting flow structure has long been the important subject for shearing turbulence. Infrared thermography was employed to detect high pressure water jet continuously. Variations of infrared radiation temperature field represented represent passive scalar fluctuating field in turbulent jetting flow. Bispectrum analysis was utilized for analyzing the coherent structure, scaling relevance of the temperature distribution in different cross-sections of the jetting flow, evolution of the characteristic length scales for sub-regions of flow field with jetting development. The research demonstrates that the global self-correlation bispectrum, with whole image matrix as an input, represents the energy intensity in variant wavebands and contributions to the frequency coherence as well; the sectional local cross-correlation bispectrum, sampling along three column of the pixel matrix, depicts features of the frequency coupling for segments of the large eddy, the small eddy, and the random turbulence downstream; the longitudinal local cross-correlation bispectrum, sampling along three lines of the image matrix, illustrates features of the frequency coupling for the jet axis, the shear layer, the discontinuous energy layer, the large eddy layer and random turbulence layer in the air turbulence region driven by the water jetting stream. Evolution of the characteristic lengths were also obtained for the air turbulence field induced by the shear action of the jet in different stages, and bispectrum analysis of the infrared image sequence according to the relevance of the coherent structure.