Microbubble-laden turbulent boundary layer investigation by Feature Tracking.

In this paper we present results concerning the velocity statistics of a microbubble-laden turbulent boundary layer over a flat plate, and compare them with the corresponding statistics obtained for the single-phase (water) flow. The present work is part of an overall project on drag reduction by microbubbles, which includes experiments on towed plate and turbulent channel flow. The experiment discussed here have been carried out at a Reynolds number based on the momentum thickness of Re? = 3700. The concentration of the microbubbles is approximately 0.1% at the measurement location. The histogram of the bubble diameter approaches a lognormal distribution with a peak at 0.2 mm, approximately twice the Kolmogorov microscale. Since the amount of the microbubbles is very small, as well as well their dimension, the measurements could be carried out by standard (2D) PIV technique. The images pairs have been analyzed with a Feature Tracking (FT) algorithm to obtain the velocity field, while classical image processing techniques have been used to characterize the bubble field in terms of bubble dimensions and centroid locations. The velocity statistics focus on the mean value and on turbulent fluctuations profiles. The ability of the used algorithm to directly provide the local velocity gradients is used to gather information also on the fluctuating spanwise vorticity. Bubbles are observed to accumulate in the buffer region independently on their dimension, resulting in a very inhomogeneous concentration distribution. This finding is to be correlated with the fact that the major changes in the mean streamwise velocity are observed only in the region 0 < y+ < 50.