An experimental study of the unsteady nature of the viscous sublayer

Measurements of the fluctuations in the mass transfer rate to small circular electrodes mounted flush with a pipe wall are used to study the unsteady nature of the viscous sublayer. These fluctuations in the mass transfer rate reflect flow fluctuations at y+ca 0.5. From these measurements it is concluded that the flow disturbances are elongated in the direction of flow and are characterized by a time scale equal to the diameter of the pipe divided by the bulk average velocity. An attempt is made to relate the mass transfer fluctuations to the fluctuation in velocity gradient at the wall. It is concluded that the ratio of the root-mean-square fluctuating velocity gradient at the wall to the average velocity gradient at the wall is at least 0.11.