Session: 04-02 Cavitation and Bubbly Flows

Paper Number: 87600

87600 - Investigation of Drag Reduction by Bubbles Using Simultaneous Measurement of Wall Shear Stress, Liquid Phase Velocity and Bubble Distribution 

The phenomenon of drag reduction by bubbles has been put to practical use in ships as an air lubrication system, on the other hand the detailed mechanism of drag reduction has not yet been clarified. Recently, it has been reported that high drag reduction effect is caused by the fluctuation of void fraction. Therefore, it is necessary to evaluate the unsteady bubble fluctuation and the change of shear stress and flow field with it in order to clarify the phenomenon. In this study, simultaneous measurements of wall shear stress, liquid phase velocity and bubble distribution are carried out in bubbly channel flow. For simultaneous measurements, shear stress sensor using FBG (Fiber Bragg Grating) is developed. This shear stress sensor is direct type, and wall shear stress is measured by the strain obtained from FBG on the cantilever connected to the floating element. The diameter of floating element is 10 mm. It was confirmed that the present sensor has sufficient performance to evaluate the phenomenon in comparison with the previous research and the measurement result of the conventional wall shear stress sensor using semiconductor strain gauge. PTV (Particle Tracking Velocimetry) using the fluorescent particle and Shadowgraphy is adopted to investigate the liquid phase velocity and bubble distribution. The images under floating element of shear stress sensor are acquired by high-speed camera in synchronization with shear stress sensor. The results of simultaneous measurement verify that the drag reduction effect is further increased by the decrease of the mean streamwise velocity and the Reynolds shear stress when the void fraction turns from maximum to decreasing.

Presenting Author: Takamichi Hiroi National Maritime Research Institute