A comparison study of conditional-sampling methods used to detect coherent structures in turbulent boundary layers

Abstract

The present work describes the results of an experimental study of coherent structures in a two-dimensional zero pressure gradient turbulent boundary layer by conditional sampling. The main aim of the work is to compare coherent events detected by various conditional sampling methods on a one-to-one basis and to investigate the degree of correspondence between them. The conditional-sampling techniques used include some existing ones, such as the uv-quadrant, the VITA, the u-level, and the pattern recognition methods, but also some suggested ones which are based on modifications of existing methods or ideas. The results are presented in two forms. First, the conditional averages of the streamwise and normal velocity fluctuations, and those of the Reynolds stress, are grouped into four classes based on the similarity of the pattern and then compared. Next, the correspondence between events detected by various methods are presented on a scale of zero to one, the latter indicating complete correspondence. The conditional-averaged patterns show very differing results for different methods. Also, very poor correspondences have been obtained between some pairs of methods. It is concluded that these methods may detect events of a different nature or different phases of the same event, and thus cause such discrepancies. The results are also discussed by relating them to the physics of the flow and coherent motions known as ejection and sweep.