As tube banks are set in a duct in a boiler and a heat exchanger, the resonance phenomenon or the self sustained tone are generated due to the interference between vortex shedding and acoustic characteristics of the duct. It is necessary to know the resonance frequency of the duct, namely sound speed, for avoiding any trouble that may arise. In general, it is said that the sound speed decreases in the duct with tube banks and an evaluation formula is given. However, this formula is obtained to

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... pply to the perpendicular direction of the flow. We wanted to know whether this formula would be able to be applied to the flow direction and to various arrays of patterns or not. In this paper, the applicability limit of this expression is discussed by using FEM analysis and experiments.