Clarification of Generation Mechanism for Transient Fluid-induced Forces on Bluff Body due to Gusty Wind

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An experimental setup which can reproduce lateral wind was newly manufactured and was used for the measurement of transient lift force on a square prism running along a guide rail. Some relationship to Karman vortex shedding was implied. The difficulty to identify each force component was also pointed out.Transient aerodynamic forces on a square prism and a group of 5 circular cylinders were measured by use of another existing wind tunnel which can also generate sudden increase of longitudinal wind velocity. It was found that the asymmetric generation of separation bubbles on both side surfaces was important for the mechanism of transient lift on a square prism. And the complex flow pattern around the 5 circular cylinders was found to determine the transient forces. The transient forces were well reproduced by CFD simulation which showed an evidence of relationship with Karman vortex. A simulation of time history of gusty wind due to an ideal tornado was conducted to discuss safety evaluation of a train wagon during tornado passage nearby.