Fig. 8 The time-frequency variation in the transient velocity of oil when the concentration is 14.40 ppm Tab. 1 shows the main frequencies of the instantaneous flow velocity and normal velocity for different particle concentrations. The base frequency of 0.12 Hz is reflected in the frequency domain of the flow velocity. As the particle concentration increases, the main frequency of the flow velocity decreases from twice the base frequency to the subharmonic frequency, which shows an attenuation trend. Similarly, with increasing particle concentration, the main frequency of the normal velocity of oil decreases from the base frequency to the subharmonic frequency, which also shows an attenuation trend.