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.