Fig. 5 The time-frequency variation in the transient velocity of oil when the concentration is 0.97 ppm
Fig. 5 (b) shows the curve of the change in the instantaneous normal velocity over time. The instantaneous normal velocity of the point fluctuates near the value of 0.0299, which shows that the normal velocity distribution in the central region is balanced with respect to that of Fig. 4 (b). However, the magnitude of the change in the normal velocity is relatively large, ranging from -0.4759 to 0.4649, with a variation of 0.9408. In addition, Fig. 5 (d) shows that the normal velocity pulsation frequency mainly includes the subharmonic frequency of 0.09 Hz and is approximately twice the frequency of 0.21 Hz of the base frequency.
Fig. 6 shows the time-frequency variations in the instantaneous flow velocity and normal velocity when the particle concentration is 3.00 ppm. The instantaneous flow velocity changes over time, as shown in Fig. 6 (a), and the transient flow velocity of the point fluctuates near a value of 0.1635. Under the action of the flow field base pulsation frequency of 0.12 Hz (as shown in Fig. 6 (c)), the flow velocity amplitude in the first period has a large fluctuation phenomenon, and the reflow phenomenon occurs in the third cycle. As the particle concentration increases and the viscous shear force of the oil increases, the flow velocity pulsation degree of the oil shown in Fig. 6(a) relative to that of Fig. 4 (a) and Fig. 5 (a) decreases from -0.1292 to 0.8407, with a variation range of 0.9699. In addition, Fig. 6(c) shows that the flow velocity pulsation frequency is mainly 0.09 Hz, which is a subharmonic frequency of the base frequency.