スライド 1

東京大学グローバルＣＯＥプログラム機械システム・イノベーション国際拠点
2009.04.02 GMSI Seminar Series
Long and short-lived structures in turbulent and transitional flows:
modelling and simulation
Scientific Manager
Dr. Sylvain Lardeau
Department of Aeronautics, Imperial College London, UK
日時：平成21年4月2日（木） 15：00～16：00
会場：東京大学工学部2号館 2階 222講義室
要旨
Coherent Structures (CS) have been studied experimentally
and numerically for many years, although their formation
and the role they play in the dynamic of turbulent flows are
still unclear, while it remain crucial for several applications,
such as flow control. Short-lived near-wall structures (e.g.
hairpin vortices) are thought to be fundamental in the
momentum exchange in the near-wall layer, and are usually
of primary concern for effective flow control targeting, whilst
their control has proved to be remarkably short-lived. On
the other hand, long-lived, or large-scale fluctuations are
usually difficult to detect (experiments) or to simulate.
Present work at ICL is trying to address those issues by
simulating and analysing highly-resolved, purpose-made,
flow configurations. First, example of CS is given, at lowReynolds number for several examples of increasing
complexity: free-shear layers, wall-bounded flows at low
Reynolds numbers, and attached and separated flows of
practical configurations at high Reynolds numbers. Of
particular interest are the fundamental implications for
turbulence modelling.
写真
主催：東京大学グローバルＣＯＥプログラム「機械システム・イノベーション国際拠点」事務室
URLhttp://www.mechasys.jp/ 担当者:笠木伸英
TEL / FAX： 03-5841-7437 E-mail: [email protected]
東京大学グローバルＣＯＥプログラム機械システム・イノベーション国際拠点
2009.04.02 GMSI Seminar Series
Instances of transition to turbulence in boundary layers
Dr. Tamer Zaki
Lecturer
Department of Mechanical Engineering, Imperial College London, UK
日時：平成21年4月2日（木） 16：00～17：00
会場：東京大学工学部2号館 2階 222講義室
要旨
Transition to turbulence in boundary layers is often classified as
orderly or bypass. The early stages of orderly transition are
characterized by the amplification of two-dimensional TollmienSchlichting instability waves. These waves develop a threedimensional secondary instability, and subsequently breakdown to
turbulence. When the transition deviates from this description, the
breakdown mechanism is termed bypass transition.
Transition mechanisms are studied using direct numerical simulations
(DNS). The view that bypass transition is the complement of the
orderly route suggests that it can be studied starting from the
complement of Tollmien-Schlichting waves in Orr-Sommerfeld theory.
"Continuous mode transition" refers to studies of the interaction of
pairs of continuous modes. DNS of these bi-modal interactions can
emulate breakdown of boundary layers in response to a fully turbulent
free stream, but the two modes must be of low- and high-frequency.
The former penetrates the boundary layer and generates the highamplitude streaks, or Klebanoff distortions, which precede breakdown
in bypass transition. The high-frequency mode only exists in the free
stream, and is required in order to trigger the secondary instability of
the streaks and breakdown into turbulent spots. A hybrid transition
mechanism is also simulated, where both Tollmien-Schlichting and
continuous modes are present in the flow and interact,. A Floquet
analysis provides an explanation of the results observed in the DNS.
主催：東京大学グローバルＣＯＥプログラム「機械システム・イノベーション国際拠点」事務室
URLhttp://www.mechasys.jp/
TEL / FAX： 03-5841-7437
担当者:笠木伸英
E-mail: [email protected]

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