  This paper proposes a novel particle image velocimetry (PIV) technique to
generate an instantaneous two-dimensional velocity field for sediment-laden
fluid based on the optical flow algorithm of ultrasound imaging. In this paper,
an ultrasonic PIV (UPIV) system is constructed by integrating a medical
ultrasound instrument and an ultrasonic particle image velocimetry algorithm.
The medical ultrasound instrument with a phased sensor array is used to acquire
acoustic echo signals and generate two-dimensional underwater ultrasound
images. Based on the optical flow field, the instantaneous velocity of the
particles in water corresponding to the pixels in the ultrasonic particle
images is derived from the grayscale change between adjacent images under the
L-K local constraint, and finally, the two-dimensional flow field is obtained.
Through multiple sets of experiments, the proposed algorithm is verified. The
experimental results are compared with those of the conventional
cross-correlation algorithms. The results show that the L-K optical flow method
can not only obtain the underwater velocity field accurately but also has the
advantages of good smoothness and extensive suitability, especially for the
flow field measurement in sediment-laden fluid than conventional algorithms.
