2022 年 88 巻 909 号 p. 22-00010
The relaminarization process of a turbulent boundary layer under a sink flow was investigated experimentally. The boundary layer was accelerated because of the contraction of the flow area. The mean and fluctuating velocity components were measured using a hot-wire anemometer. With relaminarization, the fluctuating velocity, which was normalized by the free-stream velocity, decreased. The degree of decrease differed between the streamwise and wall-normal components, depending on the distance from the wall. The conditional probabilities that two hot-wire probes separated in the spanwise direction simultaneously observed turbulent and nonturbulent flows were obtained. A spanwise-wide turbulent region was spread near the wall, but its width decreased as the distance from the wall increased. When the layer was relaminarized, this decrease in the spanwise width started near the wall. The low-speed streak spacing of the coherent structure was obtained from the lateral correlation coefficient of the streamwise fluctuating velocity and average spanwise spectra, and both parameters were somewhat consistent. Both the spanwise spacing of the streaks and the vertical scale of the coherent structure increased because of relaminarization. The conditional lateral correlation coefficient of the streamwise fluctuating velocity based on turbulence or nonturbulence was calculated. The streak spacing was slightly wider under nonturbulent conditions. This trend is consistent with the spreading behavior of the streak spacing, owing to relaminarization. With relaminarization, coherent structures became less frequent.
