| Co-Investigator(Kenkyū-buntansha) |
KIWATA Takahiro Kanazawa University, Faculty of Engineering, Associate Professor, 工学部, 助教授 (40225107)
KIMURA Sigeo Kanazawa University, Faculty of Engineering, Professor, 工学部, 教授 (70272953)
UENO Hisanori Kanazawa University, Faculty of Engineering, Professor, 工学部, 教授 (80019752)
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| Research Abstract |
The flow around a bluff body submerged in oscillatory flow consists of the envelopment of separating flow, which has flow-characteristics of a starting flow and an unsteady wake with a limited length because of flow of the oscillation in the alternate directions. The studies of forces acting upon and flow patterns around the bluff body are fundamental to such fluid engineering constructions as piers, submarine pipelines and other ocean structures. There are many aspects that remain indistinct yet about fluid forces induced by oscillating flow such as waves. Some experimental studies on fluid forces acting on a circular cylinder under oscillating flow were already reported. In this field of fluid dynamics, a Keulegan-Carpenter number and a viscous parameter beta, where KC=UmT/D (Um : a maximum fluid velocity, T : a period, D : a body diameter), beta=Re/KC-D^2IvT (v= kinematic viscosity) are the most important parameters.
Experiments by using a U-tube water tank :
(1) The fluid dynamic for
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ce coefficients of a circular cylinder were measured and compared with those of other studies in a U-tube water tank to verify the experimental set-up. Then, measurements of both in-line and transverse forces of a square cylinder and flat plates with various angles of attack were made in a wide range of KC numbers from 1 to 90. Flow visualization was also employed to confirm the correspondence between flow patterns and force coefficients. It is found that several classes of flow patterns appear within the measured KC number range and the vortex motions closely corresponds to variations of the in-line and transverse forces.
(2) The forces acting upon and the flow patterns around a rectangular cylinder with a cross-section of width/height ratio of 0.2-3.0 submerged in oscillatory flow were studied by using a U-tube water tank. In particular, we found that a drag coefficient CD of the d/h=0.6 cylinder becomes the biggest only beyond KC=120, compared with other case of the d/h cylinder in oscillatory flow. The good relationships between KC number and the flow patterns, and CD and wake width were shown.
Numerical Simulations by a finite difference method
An ALE (Arbitrary Lagrangian-Eulerian) finite difference method has been employed to simulate the flow around an oscillating bluff body, e.g., a circular cylinder and a square one, submerged in a still water. Simulations have been carried out under the assumption of 3-dimensional, unsteady, incompressible and viscous flow. Predictions are compared with the results of measurements of fluid-dynamic forces and flow-visualization. There is found to be good agreement between the computed and measured in-line forces, i.e., the values of inertia and drag coefficients, CM, CD represented by the well-known Morison equation, across a range of KC values. Several types of flow patterns are successfully reproduced and identified by the numerical simulations also, such as 3-D flow-structure of "longitudinal vortices or Honji's Vortex", "asymmetric pattern", "transverse street" and "double pair" for a circular cylinder. It is confirmed that the computed flow patterns agree well with the flow-visualized ones for both the circular cylinder and the square cylinder. Less
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