A COMPARISON STUDY OF CONDITIONAL-SAMPLING METHODS USED TO DETECT COHERENT STRUCTURES IN TURBULENT BOUNDARY-LAYERS

The present work describes the results of an experimental study of coh erent structures in a two-dimensional zero pressure gradient turbulent boundary layer by conditional sampling. The main aim of the work is t o compare coherent events detected by various conditional sampling met hods on a one-to-one basis and to investigate the degree of correspond ence between them. The conditional-sampling techniques used include so me existing ones, such as the uv-quadrant, the VITA, the u-level, and the pattern recognition methods, but also some suggested ones which ar e based on modifications of existing methods or ideas. The results are presented in two forms. First, the conditional averages of the stream wise and normal velocity fluctuations, and those of the Reynolds stres s, are grouped into four classes based on the similarity of the patter n 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 co rrespondences have been obtained between some pairs of methods. It is concluded that these methods may detect events of a different nature o r different phases of the same event, and thus cause such discrepancie s. The results are also discussed by relating them to the physics of t he flow and coherent motions known as ejection and sweep.