報告書番号

U00073

タイトル（和文）

共存対流乱流境界層の乱流特性と遷移挙動の解明

タイトル（英文）

Study on Turbulent Characteristics and Transition Behavior of Combined-Convection Boundary Layer

概要 （図表や脚注は「報告書全文」に掲載しております）

浮力と同方向の主流を付加することで形成される共存対流乱流境界層における主流速度の増加に起因する境界層の安定化機構を実験および理論的に検討した．グラスホフ数によらず，層流化とともに，境界層内の乱流挙動には類似の変化が現れることを明らかにするとともに，主流が増加しGrx / Rexの値が 3×10^6 以下になると共存対流境界層が層流化することを示した．さらに，共存対流境界層の空間構造特性をPIVにより把握した．通常の壁乱流に見られる準組織構造が存在せず，かわりに外層全域にわたる大規模な流体運動が乱流エネルギの生成機構において，重要な役割を果たしていることを突き止めた．そして，主流の付加に伴う外層での大規模渦の抑制に起因して層流化が生じることを示した．

概要 （英文）

The stabilizing mechanism of the turbulent combined-convection boundary layer along an isothermally-heated flat plate in air aided by a weak freestream are investigated experimentally and theoretically. The turbulent statistics of the combined-convection boundary layer measured with hot- and cold wires at different Grashof numbers indicates that with an increase in the freestream velocity, a similar change in the turbulent quantities appears independently of local Grashof number. Then based on the such experimental results, it is verified that the laminarization of the boundary layer due to an increase in freestream velocity arises at Grx / Rex < 3×10^6. Then, through the experiments with a particle image velocimetry (PIV), the spatio-temporal structure of the turbulent combined-convection boundary layer is investigated. For instantaneous velocity vectors obtained with PIV, large-scale fluid motions, which play a predominant role in the generation of turbulence, are frequently observed in the outer layer, while quasi-coherent structures do not exist in the near-wall region. Thus, it is revealed that increasing freestream restricts large-scale fluid motions in the outer layer, and consequently the generation of turbulence is suppressed and the boundary layer becomes laminar.

報告者

キーワード