Influences of boundary condition of pipe on acoustic wave deformation

Ken GOTO, Takahiro ITOH, Takahiko NAGAYAMA, Mio KASAI and Tomomi MARUTANI

A Japanese pipe hydrophone has been used for bedload measurements in mountain torrents. This measurement sensor is an indirect method using an acoustic signal. Bedload discharge rate can be estimated using the Japanese pipe hydrophone with a linear relationship between a maximum amplitude of eigen frequency of circumferential component of acoustic wave and a momentum of colliding sediment particle with a pipe. Acoustic wave is influenced by several factors, for example, one of factors is an installation condition of a pipe, however there were few studies concerning the acoustic wave deformation. We conducted flume tests on acoustic wave deformation focusing on the pipe fixed condition and re�]collision of sediment particles to a pipe. Results show as follows:(1) The maximum amplitude of circumferential component of acoustic wave is changed by the pipe fixed condition, but an inclination of the linear relationship did not changed. We proposed that the linear relationship should be checked every installation location. (2) A re�]collision of sediment particle occurred by wake. It influences a number of count sediment particles with the Japanese pipe hydrophone at the beginning and the end of flooding stage, and there is a little influence at high bedload discharge which is exceeded the limitation of a number of countble sediment particles. We proposed the method not to re�]collide focusing on the relationship between a step height and a reattachment point length from an experimental data in the condition of high Reynolds number and supercritical flow.