**3.2 Manual time series analysis**

The structure velocity can be obtained manually by estimating the time displacement of output signals from both planes 1 and 2 via the peaks of the time series plots for both planes. Since the distance between the centres of the measurement electrodes for both planes are known, the structure velocity is hence computed using Eq. (22).

The time displacement from the time series data of both planes 1 and 2 used in the calculation of the structure velocity alongside with distance between centres of

**Figure 7.** *Cross-sectionally averaged void fraction time series for plane 1 and 2 at a superficial gas velocity of 0.02 m/s.*

**Figure 8.** *Camera control software window used to estimate velocity.*

measurement electrode is indicated in **Figure 7**. This is compared with that obtained via cross correlation later in this paper.
