Properties of a relaminarizing turbulent boundary layer under a favorable pressure gradient

Behavior of the turbulent boundary layer on a flat plate in a relaminarization process with flow acceleration due to a convergence of the sectional area is experimentally investigated. Statistical properties are measured and the turbulent structure is examined in detail. The bursting phenomena are also investigated with the VITA technique. The ensemble averages of the streamwise fluctuating velocities are plotted, and the characteristics in each process are investigated. In the relaminarization process, low-amplitude fluctuating portions appear intermittently in the velocity signal. In the retransition process an irregular velocity fluctuation appears intermittently; then the fluctuation grows and gradually becomes a turbulent condition. In the relaminarization process, vorticity increases in a large eddy and decreases in a small one. The small eddies become large, and the contribution from the high wave number decreases. In the ensemble averages of the fluctuating 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 mixing length concept. (C) 1998 Elsevier Science Inc. All rights reserved.