# Analysis of Cavity Corner Geometry Effect on Recirculation Zone Structure
## 2 Materials and Methods
### Experimental Facility and Instrumentation  
Velocity fields were experimentally measured using a micro-particle image velocimetry system (\(\mu\)PIV). The principal scheme of the \(\mu\)PIV system is given in Figure 3. During the experiments, laser illuminated fluorescent particles were captured by a CCD camera. The results were obtained as flow velocity vector fields calculated from changes in particle location over a specified time interval. As shown in Figure 3., laser beam was directed toward the microscope, where it was reflected by \(90^{\circ}\) due to the dichroic mirror installed inside the microscope. The laser beam was then directed through the microscope lens, which directly concentrated the light to the experimental section. A dichroic mirror also ensured the specific wavelength required to excite fluorescent particles and filter light travelling towards the CCD camera.  
Figure 2: Mesh independence test.  
The \(\upmu\)PIV facility consisted of an Elveflow OB1 MK3 (Elveflow, Paris, France) pressure controller, a Nd:YAG type dual-cavity laser and a 2048 \(\times\) 2048 pixel FlowSense EO CCD camera (Dantec Dynamics, Skovlunde, Denmark) coupled with an inverted Leica DM IIM microscope (Leica Microsystems, Wetzlar, Germany). The FlowSense EO CCD camera has a pixel size of 7.4 \(\upmu\)m and provides a field of view of approximately 1.5 \(\times\) 0.6 mm at 10\(\times\) image magnification. For device control and image analysis, DynamicStudio v6.11 (Dantec Dynamics, Skovlunde, Denmark) software was employed. The image pairs of fluid flow with tracer particles were captured at 15 Hz frequency with the time interval between consecutive images ranging from 1 \(\upmu\)s to 500 \(\upmu\)s depending on Re. Time-averaged results were obtained by averaging no less than 200 images.  
Water was chosen as the working fluid, with 1 \(\upmu\)m diameter fluorescent particles (Invitrogen, Waltham, MA, USA). Flow was created using a programmable pressure controller, ensuring steady channel inlet pressure. The specific gravity of fluorescent particles was 1.05; therefore, flow was considered single-phased, and Brownian motion was avoided. Laser excitation and emission wavelengths were 535 nm and 575 nm, respectively. Additionally, water temperature was ensured to be 21 \(\pm\) 1 \({}^{\circ}\)C during experiments. Experimental channels were manufactured in polycarbonate, with the preferred channel geometry engraved into them. Cavity geometry Ch4 (Figure 1) was chosen as an experimental object. Before entering the cavity, flow was ensured to be fully developed by making the inlet length 50 hydraulic diameters long.  
The uncertainty of velocity measurements was determined by assessing the contribution of expanded uncertainty of pressure-control-unit generated flow, channel cross-section dimensions, the uncertainty in the correlation calculation due to its dependence on the selected time-period between image pairs and the uncertainty of velocity measurement repeatability. Analysis revealed that the expanded uncertainty of velocity measurement by \(\upmu\)PIV did not exceed 7%.