Streak instability and onset of turbulence in a transitional boundary layer

2019 Fiscal Year Final Research Report

• Project/Area Number: 17K06139
• Research Category: Grant-in-Aid for Scientific Research (C)
• Allocation Type: Multi-year Fund
• Section: 一般
• Research Field: Fluid engineering
• Research Institution: Tohoku University
• Principal Investigator: Izawa Seiichiro 東北大学, 工学研究科, 准教授 (90333856)
• Co-Investigator(Kenkyū-buntansha): 福西 祐 東北大学, 工学研究科, 教授 (60189967) ; 西尾 悠 首都大学東京, システムデザイン研究科, 特任助教 (70712743)
• Project Period (FY): 2017-04-01 – 2020-03-31
• Keywords: 境界層遷移 / 局所不安定領域 / ストリーク / 乱流斑点

Outline of Final Research Achievements

A wind tunnel experiment was performed to investigate the requirements for providing a suitable environment for easily creating a localized turbulent region in a laminar boundary layer. A combination of a short-duration jet and suctions was used to prepare a potentially unstable region upstream, and another jet was ejected downstream against the region at various timings and different relative spanwise locations. High- and low-speed regions appeared alternately near the wall in both the streamwise and spanwise directions inside a target region. The turbulent spot generation was enhanced when the triggering jet was used at the timing and location that enlarged the low-velocity area in the convecting disturbed region. Moreover, a two-point simultaneous measurement, which was performed to investigate the evolution of the turbulent region, revealed that the turbulent spot grew in the region following the weakly disturbed region.

Free Research Field

流体工学

Academic Significance and Societal Importance of the Research Achievements

境界層遷移の後期過程においては非線形性が卓越するため，乱流がどこでどのようにして誕生するのかは未だ詳らかではない．例えばバイパス遷移では，ストリークが2次不安定性により崩壊して乱流斑点を生じることが知られているが，その場所やタイミングを予測することは困難である．本研究は，乱流の初生に求められる条件を明らかにすることを目的として，境界層内の流体構造のどこを刺激すれば遷移の促進あるいは遅延につながるのかを調べたものである．これらの研究成果は，乱流遷移現象についての新たな視点を提供するとともに，境界層内の速度変動が非線形成長する遷移末期を対象とする流れの制御戦略にも資するものである．