Properties of a Relaminarizing Turbulent Boundary Layer under a Favorable Pressure Gradient.(Analysis of Bursting Structure with VITA Technique).

Abstract

The bursting phenomena of the relaminarizing turbulent boundary layer on a flat plate are experimentally investigated using the VITA technique. First, an attempt is made to obtain a normalized bursting frequency universally applicable to the development process of the turbulent boundary layer, the relaminarization process and the retransition process. The ensemble averages of the streamwise fluctuating velocities are drawn and the characteristics of each process are investigated. No time scales can render the non-dimensional bursting frequencies universally constant. The tendency of the distribution of the bursting frequency in the relaminarization process is different from that in the development and retransition process of the turbulent boundary layer. This presumably supports the previous visualization result, according to which the relaminarization process begins from the outer region. In the ensemble averages of the fluctating velocity, it is considered that the relaminarization changes the ejection and sweep, though it does not particularly attenuate the bursting in the inner layer. This phenomenon is qualitatively explained with the aid of the mean velocity profiles and the mixing length concept.