The wall turbulence in a horizontal channel flow is visualized by platelet type of tracer particles. The tracer is around 5-50 micrometer in size and has a glossy interfacial property reflecting light dependent on the orientation of local shear strain. The present study aims to find out the difference of the wall turbulence in the channel caused by inclusion of various sizes of bubbles. The visualization results newly confirm that streamwise vortices remain in the case of large bubbles mixed while it does not emerge clearly in the case of small bubbles, implying that frictional drag reduction occurs more effectively by use of small bubbles.
