(a) t=t_{0}+0s  (b) t=t_{0}+0.1s  (c) t=t_{0}+0.2s 
Figure 4 Contours of second invariant of velocity gradient tensor and fluctuation velocity vectors on xy plane in active state of dragreducing flow at x=2400mm 
Newsletter 2020.3 Index
Theme : "The Conference of Fluid Engineering Division (March issue)”

Study of identification of largescale turbulence structures in dragreducing turbulent boundary layer flow by means of stereoscopic PIV measurements
Makoto HIRANO,

Abstract
We performed stereoscopic PIV measurements for the dragreducing turbulent boundary layer flows by injecting surfactant aqueous solution. For the case of the drag reduction ratio more than 60%, largescale turbulence structures were often observed in the fluctuating velocity field. In this study, we discussed the identification of such largescale turbulence structures by using the second invariant of the velocity gradient tensor and the imaginary part of the eigenvalues. We used a circulating water tank as experimental device. The circulating water tank has Acrylic test section. An acrylic test plate with a total length of 2796 mm, a width of 387 mm, and a thickness of 20 mm is installed perpendicular to the bottom of the test section. First we checked the reliability of stereoscopic PIV measurement. As can be seen from statics, the stereoscopic PIV measurement was sufficiently reliable except for near the wall. Therefore we judged that we can consider the instantaneous turbulence structures. Next from the instantaneous flow field colored by second invariant of velocity gradient tensor and the instantaneous flow field colored by imaginary part of the eigenvalues, we considered the largescale turbulence structures. Also the vector represents the velocity fluctuation. Looking at instantaneous flow field, large value of second invariant of velocity gradient tensor is close to largescale turbulence structures that can be judged from velocity fluctuation. But there is no clear correspondence between largescale turbulence structures and second invariant of velocity gradient tensor. And instantaneous flow field of imaginary part of the eigenvalues is same. Therefore we revealed the largescale turbulence structures is structure without strong turning.
Key words
Surfactant, Stereoscopic PIV measurement, Identification of turbulence structures, Drag reduction
Figures
Figure 1 Schematic of experimental equipment
Figure 2 Test plate and stereoscopic PIV measurement system
（a）Mean velocity
（b）Streamwise turbulence intensity
（c）Wallnormal turbulence intensity
（d）Spanwise turbulence intensity
Figure 3 Profiles of mean velocity and turbulence intensities with outer scaling in dragreducing flow
(a) t=t_{0}+0s  (b) t=t_{0}+0.1s  (c) t=t_{0}+0.2s 
Figure 5 Contours of magnitude of imaginary part of eigenvalue and fluctuation velocity vectors on xy plan in dragreducing flow at x=2400mm 