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 sampling studies of turbulent boundary layers have been studied for several conditional sampling techniques. It is shown that the results, such as event detection 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 acquisition are also found to affect the results. It is suggested that proper choice should make the event detection rate assuming either a constant or a maximum. Phase alignment is also shown to lead to improved conditional sampling results and is therefore recommended. The results suggest that discrepancies in previous conditional sampling studies could be at least partly due to the use of different operational parameters rather than due to different flow mechanisms and therefore proper care must be taken in the interpretation of the results and in comparison between the results of previous conditional sampling studies. (C) 1999 The Japan Society of Fluid Mechanics and Elsevier Science B.V. All rights reserved.