## 2 Material and Methods  
### Experiment Setup  
A small scale cylindrical silo was used for the experiments. The silos inside diameter was 400 mm and the height 1200 mm, as shown in Figure 2. Four air extractors were located on top of the silo with fitted flow meters and regulators (MR 3000). The air was extracted from the silo using a vacuum system. The airflow was controlled through needle valves. The air extracted was balanced with the air and material coming into the silo. This material created the circulating air currents which moved the dust particles and fines (particles smaller than 100 microns) away from the central feeding jet. The reference materials salt was fed centrally with a feeding rates of 14 g/s into the experimental silo. The dust particles circulating with the air currents in the silo were illuminated by a laser sheet. This laser sheet was created with a Coherent(r) DPSS 532 which projects a 532 nm green light laser beam through a 5 mm glass rod; as shown in Figure 3.  
Figure 3: Cross-section of the optical arrangement to create a flat laser sheet. The laser beam is perpendicular to the glass rod. Figure 2: Top and side elevation views of the experimental silo and camera set up. As particles circulated with the secondary air currents in the cylindrical silo, they reflected the laser light. The intensity of reflected light was correlated to the optical particle concentration. The optical particle concentration was captured by a BlackMagic URSA mini Pro 4.6k camera. The camera recorded the video footage (at 60 fps) of particles passing through the laser sheet. The zoom lens used on the camera was a Nikon (AF-S Nikkor 24-70 mm 1:2.8G ED). The post processing of the obtained video footage is shown in Figure 4. The frames from the video footage were cropped and particles were identified by the algorithm. An averaged filter was applied for every cropped frame and contour plots with brightness levels were plotted. Figure 4: Workflow for obtaining the optical particle concentration. Optical particle concentrations in the contour plots were plotted according to a \"colour bar\", as shown in Figure 5. The colours in the colour bar corresponded to brightness levels where the blue colour represented the minimum brightness level and the red colour represented the maximum brightness level. The brightness parameters a\({}_{1}\) to e\({}_{1}\) were obtained by using a computer programme. The programme structure is described as a flowchart in the Figure 6. After the brightness parameters a\({}_{1}\) to e\({}_{1}\) were identified the colour grading, a two-dimensional array of brightness values was generated and the pixel brightness in the contour maps were analysed. The brightness values from the contour plots were sampled in lines which allowed the plotting of optical particle concentrations over time, providing information on the highest optical particle concentration occurring in the experimental silo during the silo filling. In order to obtain meaningful optical particle concentrations for the experimental silo, the central particle jet was excluded from the image analysis and a large number of video frames were included in the analysis. The experiments were conducted to identify the region of highest particle concentrations in the bench scale silo as shown in Figure 7. The region of the highest optical particle concentration in the experimental silo was identified to be at a height of 220 mm from the silo bottom and 27.5 mm radially away from the centreline of the silo i.e., the central material feeding stream. The materials used in the experiments are described in the next section. Figure 5: Colours of “colour-bar” represents the corresponding brightness levels of the contour plot. Figure 3.0 Flowchart of the computer program that reads colour values in contour plots. When observing the particles optically in experiments it is difficult to distinguish between lumps of particles or single particles, hence, the averaging of a large number of frames is an important step to obtain meaningful contour plots.