Digital Image Processing of Hydrogen Bubble Lines for Instantaneous VelocityProfiles

摘要

Boundary layer transition and turbulence have been described as being comprisedof smaller structures called turbulent spots. Many researchers have obtained ensemble averaged velocity measurements in the transition region and some attempts have been made to obtain instantaneous information. Ensemble averaging can predict the overall scales of the flow field but it fails to yield information on the flow physics of the substructures within the spot. To facilitate investigation of these flow features and to establish the spot's relationship with transition or turbulence, an experimental method to obtain instantaneous full velocity profiles has been developed. The qualitative hydrogen bubble technique of flow visualization has been augmented to yield quantitative unsteady boundary layer information. The technique has been advanced by incorporating digital image processing of videotaped boundary layer flow. The method was verified by direct comparison with a laser Doppler anemometer and an analytical uncertainty analysis was performed. The technique compared well against the LDA, in both steady and unsteady flows. Recommendations were made for effective use of this technique and for further development.