We proposed a new type of ultrasonic motor which consists of a fluid-f
illed ring stator and a rotor. The fluid is caused to flow in the circ
umferential direction by the ultrasonic vibration travelling along the
ring stator. The operation mechanism was previously analyzed. In this
paper, we present the results of experiments aimed at optimizing this
motor. The acoustic streaming velocity of the motor can be increased
by using the resonance of the fluid confined in the ring stator, emplo
ying a higher mode of stator vibration, and increasing the roughness o
f the stator wall. Also, it is clear that the nonlinearity of the acou
stic field in the fluid is the main cause of the saturation of the aco
ustic streaming velocity, and that the saturated acoustic streaming ve
locity increases when the attenuation of the fluid acoustic field is i
ncreased.