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LES of the Flow Around a Circular Cylinder in the Critical Reynolds Number Region-Study on Asymmetric Characteristics of Flow and Lift

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Obayashi Corporation Tokyo Japan

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The recent advancement of numerical techniques has made it possible to usually simulate the flow around bluff body. Some LES methods succeed in simulating the large scale wake structures associated with the flow separation from a circular cylinder at the sub-critical region of the Reynolds number Re.On the other hand, in the critical and the super critical Re regions, there appears a bubble which represents an intricate combination of laminar separation, transition, reattachment and turbulent separation of the boundary layers.According to the previous researches, the asymmetric flow pattern coupled with steadily existing lift is recognized around a circular cylinder in the critical Re region. Bearman 1969 found that the asymmetric situation is caused by a laminar separation bubble forming on only one side of the cylinder in the critical Re region, while the symmetric situation appears as a consequence of a two-sided separation bubble in the super critical Re. Kamiya et al. 1979 also recognized the steady lift, and investigated the asymmetric pressure distributions accompanied by the shift of the stagnation point away from the bubble. Schewe 1986 measured repeatedly the sign of the steady lifts in the cases that the asymmetric flow pattern was observed, and showed that the probability for the occurrences of each sign was nearly equal. This finding showed that asymmetric phenomena were basic behavior, namely not induced by asymmetric test condition. Schewe 1983 explained the asymmetric flow was originated from local perturbations or fluctuation of the oncoming flow. The immediate formation of one side bubble results in acceleration of the fluid and in deceleration on the other side. Deceleration of the fluid delays the transitions to the turbulence and the formation of the bubble.

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