Turbulent Structure in the Vicinity of a Roughness Element for the Boundary Layer over a k-Type Rough Wall in Low-Reynolds-Number. Measurement of the Flow Interaction in the Open-face between Roughness Elements and a Flow-Pattern in a Groove.

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  • 低レイノルズ数におけるｋ形粗面境界層の粗さ要素近傍の構造　　溝部界面における干渉と溝内部流のパターン
  • テイレイノルズスウ ニ オケル kケイソメン キョウカイソウ ノ アラサ ヨウソ キンボウ ノ コウゾウ コウブ カイメン ニ オケル カンショウ ト コウナイ ブリュウ ノ パターン

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Abstract

Measurements of the mean and the turbulent velocities have been performed in the vicinity of a roughness element for the turbulent boundary layer developing over a k-type rough wall, which consists of two-dimensional square ribs arrayed with pitch ratio of 4. The Reynolds-numbers based on momentum thickness θ and roughness height k are about 500 and 50 respectively, and relative roughness height k/δ6=0.156. LDV (Laser Doppler Velocitimetry) that it is capable to measure reverse flow was used in order to clarify the momentum-exchange process in the open-face (yT=0 mm) between roughness elements and the flow-pattern in a groove. The variation of the mean and the turbulent quantities profiles at YT=0 mm depends on the development of a free-shear flow formed with disappearance of the roughness element and the formation of eddies in the groove. Pressure drag coefficient acting on the roughness element in the local skin friction coefficient is 92% in the present k-type rough wall. The momentum exchange for the mean flow and turbulence contribute 6% and 59% respectively to the local skin friction coefficient. The existence of a large-scale eddy and two small corner-eddies is confirmed from the flow pattern in the groove.

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