Parameter effects on detection of coherent structures in turbulent boundary layers using conditional sampling methods

The effects of operational parameters on the results of conditional samplin g studies of turbulent boundary layers have been studied for several condit ional sampling techniques. It is shown that the results, such as event dete ction rate and phase-averaged event patterns, depend strongly on the choice of the values of these parameters. Other factors such as the existence of phase jitter in the event detections and sampling frequency of data acquisi tion are also found to affect the results. It is suggested that proper choi ce should make the event detection rate assuming either a constant or a max imum. Phase alignment is also shown to lead to improved conditional samplin g results and is therefore recommended. The results suggest that discrepanc ies in previous conditional sampling studies could be at least partly due t o the use of different operational parameters rather than due to different flow mechanisms and therefore proper care must be taken in the interpretati on of the results and in comparison between the results of previous conditi onal sampling studies. (C) 1999 The Japan Society of Fluid Mechanics and El sevier Science B.V. All rights reserved.