Newsletter  2018.3  Index

Theme : "The Ninth JSME-KSME Thermal and Fluids Engineering Conference (TFEC9)" Preface T. HASHIMOTO・郡. MATSUDA・粂.J. Park How Turbulence Begins in a Transitional Flat-Plate Boundary Layer Yu FUKUNISHI, Yu NISIO, Seiichiro IZAWA (Tohoku University), Joe YOSHIKAWA (Industrial Technology Institute, Miyagi Prefectural Government) Numerical Simulation of Solidification and Deposition of Single Molten Droplet by Means of E-MPS Method S. KONDO, H. MAMORI (Tokyo University of Science), N. FUKUSHIMA (Tokai University), M. YAMAMOTO (Tokyo University of Science) Numerical simulation of a parallel plate particle separator for measurement of charging state of PM2.5 Takuto YONEMICHI, Koji FUKAGATA, Kentaro FUJIOKA, Tomoaki OKUDA・・eio University・・/li> Effect of viscoelastic Mach numbers on propulsive forces of a model helical flagellum in a viscoelastic fluid Kazuya TAJIMA, Fumihiko MIKAMI・・hiba University・・/li> Measurement of Ionic Current Fields by Using Variable Microgap Electrodes Takashi Fukuda, Kentaro Doi, Satoyuki Kawano (Osaka University)

 

How Turbulence Begins in a Transitional Flat-Plate Boundary Layer

Yu FUKUNISHI, Yu NISHIO, Seiichiro IZAWA Tohoku University, Joe YOSHIKAWA Industrial Technology Institute, Miyagi Prefectural Government

Abstract

Laminar-turbulent transitions of boundary layers are numerically simulated in order to investigate key vortical structures which directly trigger the onset of turbulence in the transition process. A short duration jet is ejected from a square hole into the boundary layer as a localized disturbance. Pairs of flow fields with slightly different jet velocities are prepared. In one of each pair, the flow field becomes turbulent while in the other, the flow field returns back to a laminar state. The key structures are searched by comparing the pairs. As the first flow field, streaky structures are formed using the cuboids installed at wall and a short duration jet is issued beside a low-speed streak changing its velocity. At both jet velocities, primary streamwise vortices are generated triggered by the jet ejection, which do not lead to a boundary layer transition. However, only in the stronger jet case, a secondary streamwise vortex, which is the key structure, appears upstream of the primary streamwise vortex. Following the appearance of the secondary streamwise vortex, new other vortices emerge one after another around the vortex and the flow field soon becomes turbulent. The difference in shapes between the primary streamwise vortex and the secondary one is focused. The secondary vortex has more angle to the flat plate, making it likely to be stretched by the mean velocity gradient of the boundary layer. The angle of the secondary vortex is created because of the swirling flow induced by the primary vortex. In other flow fields, namely when the jet location symmetric to the low speed streak and when there is no streak in the boundary layer, the key vortical structures which share the same feature as the secondary streamwise vortex are found. It is suggested that the streamwise vortex with an angle is playing the key role in starting the laminar-turbulent transition process.

 

Key words

Boundary layer transition, Key structure in transition, Streaky structures, Short duration jet, Numerical simulation

 

Figures

Fig. 1  Time variations of vortical structures. ν_jet = 0.08 and 0.10 cases compared.

Fig. 2  Side views of vortex A at t = 60 and vortex B at t = 120.( vjet = 0.10 case)

Fig. 3  Velocity vector field with color map of streamwise vorticity in y-z plane at t = 80.

Fig. 4  Key structures in laminar-turbulent transitions.
Symmetric blowing case and no streak case.