Fig. 6 The time-frequency variation in the transient velocity of oil when the concentration is 3.00 ppm
In Fig. 6 (b), the instantaneous normal velocity at this point fluctuates around -0.0387 over time, which indicates that the velocity distribution in the central region has a negative trend in the normal direction relative to that of Fig. 4 (b) and Fig. 5 (b). However, the magnitude of the change in the normal velocity is relatively small, from -0.3491 to 0.3944 with a variation range of 0.7435. Fig. 6 (d) shows that the normal velocity pulsation frequency is 0.06 Hz, which is a subharmonic frequency of the base frequency.
Fig. 7 shows the time-frequency variations in the instantaneous flow velocity and normal velocity when the particle concentration is 6.58 ppm. The instantaneous flow velocity of the point shown in Fig. 7 (a) fluctuates near 0.161 over time. Under the action of the flow field pulsation frequency of 0.12 Hz (as shown in Fig. 7 (c)), the amplitude of the flow velocity in the latter two periods has a large fluctuation phenomenon. As the particle concentration increases and the viscous shear force increases, the flow velocity pulsation degree is reduced from 0.0807 to 0.8113 relative to that of Figs. 4-6 (a), with a variation range of 0.8920. In addition, Fig. 7 (c) shows that the flow velocity pulsation frequency is mainly a subharmonic frequency of 0.03 Hz and a base frequency of 0.12 Hz.
In Fig. 7 (b), the instantaneous normal velocity of the point fluctuates near 0.032 over time, which shows that in the central region, the normal upward velocity distribution has a positive trend relative to the trend of Fig. 4-6 (b). However, the magnitude of the change in the normal velocity is relatively larger from -0.3905 to 0.6698, with a range of 1.0603. In addition, Fig. 7 (d) shows that the normal velocity pulsation frequency is mainly the subharmonic frequency of 0.09 Hz and is approximately twice the frequency of 0.26 Hz of the base frequency.