## 2 Experiment Setup  
The experiment is conducted on a low-speed large-scale axial compressor (LSLSAC) test facility at Beihang University, which is composed of a single-stage axial compressor with inlet guide vanes (IGV), as shown in Figure 1. The casing diameter was 1 m, and the hub-to-tip ratio was 0.6. The rotor and stator blades were nearly radially stacked by controlled diffusion airfoil (CDA). The rotor was rotated at 1100 RPM, while the stator hub was stationary. At the design condition, the load coefficient based on the rotor speed at the middle span was approximately 0.5, and the diffusion factor was about 0.5 for both the rotor and the stator at the middle span. The nominal rotor tip clearance was kept to be 1% blade height, while the nominal stator hub clearance is varied at 0.0%, 0.25%, 0.5%, and 1.0% blade height, which will be denoted as 0.00\(\tau\), 0.25\(\tau\), 0.50\(\tau\) and 1.00\(\tau\), respectively. More detailed design parameters are summarized in Table 1, which were also introduced in [28; 29; 30]. In order to measure the performance of the compressor, static pressure taps are set at varies axial locations on the casing wall, as shown in Figure 2. At plane 0, four circumferential static pressure taps were used to measure the mass flow coefficient. The static pressure rise of the compressor was measured by four circumferential static pressure taps at plane 1 and six circumferential static pressure taps at plane 5. The inlet total pressure was the ambient pressure, which was measured by an atmospheric pressure gauge. The outlet total pressure was measured by the pitot probes at plane 5. A torque meter was used to measure the input shaft power to the compressor, and combining with the inlet and outlet total pressure, the compressor efficiency can be calculated consequently. An L-shaped five-hole probe was used to measure the 3D velocity and pressure profiles at the inlet and outlet of the stator, i.e., at planes 3 and 4. At the radial direction, the nearest measured position to the shroud and the hub wall was 2.5% and 2% blade height, respectively. The detailed arrangement of the five-hole probe's measurement points is shown in [31]. In order to measure the 3D flow field of the stator outlet when the open corner separation occurs, the measured pressure was processed with a zone method like the one used for the seven-hole probe [32], and the measurable flow angle range was extended to 60\({}^{\circ}\). The pressure was acquired by the Rosemount 3051S1 differential pressure transducers, the measuring range, and uncertainty of which were \(\pm\) 6.22 kPa and 0.025% FS. After detailed analysis, the measurement uncertainties of five-hole probe were 0.5\({}^{\circ}\) for the flow angles, 1% (normalized by the \begin{table}
\begin{tabular}{c c} \hline Outer diameter (mm) & 1000 \\ Hub-to-tip ratio & 0.6 \\ Number of blades & 36 (IGV); 17(rotor); 20 (stator) \\ Blade camber angle (\({}^{\circ}\)) & 7(IGV); 34.3(rotor); 45.3 (stator) \\ Blade stagger angle (\({}^{\circ}\)) & 5(IGV); 37.7(rotor); 18.1 (stator) \\ Blade nominal height (mm) & 200(IGV); 200 (rotor); 200 (stator) \\ Blade chord (mm) & 95(IGV); 172(tor); 183(stator) \\ Rotor tip nominal clearance (mm) & 2.0 \\ Stator tip nominal clearance (mm) & 0.0/0.5/1.0/2.0 \\ \hline \end{tabular}
\end{table} 
Table 1: Design parameters of the compressor test facility. Figure 1: Low-speed large-scale axial compressor (LSLSAC) test facility. Figure 2: Schematics of the measurement locations on LSLSAC. flow dynamic pressure) for the total pressure, 2% (normalized by the flow dynamic pressure) for the static pressure, and 1% for the flow velocity, respectively. Oil-flow visualization tests are conducted on the suction surface and end wall of the stator passage, to reveal the evolution of the flow pattern at various flow coefficient when stator hub clearance size is changed. Detailed introduction of the oil-flow visualization can be found in [28].