振動する円筒の周りの流れについて(その2)

Abstract

For several years we have been studying about the vibration of an elastically restrained circularcylinder in wind. To control the oscillation of smoke stacks induced by wind, this kind of theproblem is very important and in these years some papers on it have been published but theproblem is complicated and the limited data can be obtained at present.Although the oscillation of stacks in wind is on the problem in the range of supercriticalReynolds' number, it is interesting also to carry Out the tests in the subcritical range if the ex-citing force is only due to the alternative vortex shedding. So in this range we have investigatedthe alternative lift forces to the circular cylinders which would be affected by the amplitudes andthe frequencies of the vrbration.From the experiment we got the results as follows;1. Through the all range of wind speed 7 in/sec to 20 in/sec. The vibration of the cylinderwas caused by the natural frequency.2. We found one peak point of the amplitude in the range of the wind speed, though theremight be more than one over this range. The Strouhal number was about 0.15.3. The amlitude was nearly sinusoidal and stationary.4. In the low damping system as in our tests (logarithmic decrement a-0001-003), smallchange of wind speed near ｶhe critical speed influenced is amplitudes sensitively.We can see that the alternative lift force is approximately sincsoidal from the time variationof the amplhude and if the vibration of the cylinder is ~I linear sysem, it is regarded as like asthe resonant vibration, because the fre uency of external force always coincides with the naturalfrequency. On the detail of these phenomena we s':ill develoj further experiments in near future.For several years we have been studying about the vibration of an elastically restrained circularcylinder in wind. To control the oscillation of smoke stacks induced by wind, this kind of theproblem is very important and in these years some papers on it have been published but theproblem is complicated and the limited data can be obtained at present.Although the oscillation of stacks in wind is on the problem in the range of supercriticalReynolds' number, it is interesting also to carry Out the tests in the subcritical range if the ex-citing force is only due to the alternative vortex shedding. So in this range we have investigatedthe alternative lift forces to the circular cylinders which would be affected by the amplitudes andthe frequencies of the vrbration.From the experiment we got the results as follows;1. Through the all range of wind speed 7 in/sec to 20 in/sec. The vibration of the cylinderwas caused by the natural frequency.2. We found one peak point of the amplitude in the range of the wind speed, though theremight be more than one over this range. The Strouhal number was about 0.15.3. The amlitude was nearly sinusoidal and stationary.4. In the low damping system as in our tests (logarithmic decrement a-0001-003), smallchange of wind speed near ｶhe critical speed influenced is amplitudes sensitively.We can see that the alternative lift force is approximately sincsoidal from the time variationof the amplhude and if the vibration of the cylinder is ~I linear sysem, it is regarded as like asthe resonant vibration, because the fre uency of external force always coincides with the naturalfrequency. On the detail of these phenomena we s':ill develoj further experiments in near future