ロッドの固有横振動数におよぼす周囲の水の影響

Abstract

The previous report indicated that the theoretical natural frequencies of lateral vibration of rod in off-shore boring obtained without any hydrodynamic considerations on the effect of surrounding water were always higher by about 1-20% than the corresponding frequencies obtained by experiments. This discrepancy between the theoretical and experimental values was studied in this report by the virtual mass theory applied to the surrounding water.As a result of this study, the above-mentioned discrepancy was considered to be caused mainly by the hydrodynamic force from surrounding water acting on the rod, and well explained by applying the virtual mass theory to the surrounding water.The correction factor based on this theory was proposed here for the correction of the natural frequency derived from no hydrodynamic consideration on the effect of surrounding water on the lateral vibration of rod. This factor, in actual case, could be reduced to the simple form, (1+ρw/ρ0) -0.5, where ρw was the density of surrounding water and po was the apparent density of rod. The value of this factor varied from 0.77 to 0.90.When the natural frequencies obtained in the previous report were multiplied by the correction factor, the corrected frequencies well coincided with the corresponding experimental frequencies.