Fig. 7 The time-frequency variation in the transient velocity of oil when the concentration is 6.58 ppm
Fig. 8 shows the time-frequency variations in the instantaneous flow velocity and normal velocity when the particle concentration is 14.40 ppm. As shown in Fig. 8 (a), the instantaneous flow velocity at this point fluctuates around 0.166 over time. Under the action of the flow field pulsation frequency of 0.12 Hz (as shown in Fig. 8 (c)), the flow velocity amplitude appears to exhibit large fluctuations in the first two cycles. As the particle concentration increases and the viscous shear force increases, the flow velocity pulsation degree decreases with respect to that of Figs. 4-7 (a), from -0.1459 to 0.6922 with a variation range of 0.8381. In addition, Fig. 8 (c) shows that the flow pulsation frequency is mainly a subharmonic frequency of 0.03 Hz and a base frequency of 0.12 Hz.
As shown in Fig. 8 (b), the instantaneous normal velocity at this point fluctuates around -0.008, which indicates that the normal velocity distribution in the central region is relatively balanced with respect to that of Fig. 4-7 (b). However, the magnitude of the change in the normal velocity is relatively small, ranging from -0.4071 to 0.2982 with a variation range of 0.7053. In addition, Fig. 8 (d) shows that the normal velocity pulsation frequency is mainly the subharmonic frequency of 0.03 Hz and is approximately twice the frequency of 0.26 Hz of the base frequency.
The above results show that with increasing particle concentration in oil, the mean value of the magnitude of the instantaneous flow velocity pulsation is gradually reduced, and the varying amplitude simultaneously decreases. On the other hand, the changes in the mean and variance amplitude of the instantaneous normal velocity pulsation are not obvious due to the influence of the flow field structure.