2. EXPERIMENTAL METHODS AND APPARATUS

2. 1 Shock Tube
Figure 2 shows the schematic view of the experimental apparatus. The experiments are done by using the non-diaphragm type shock tube. It has the quick-action piston valve^{5 )} in the high pressure chamber. In the present study, the high pressure chamber is charged up to 392-686 kPa (4-7 kgf/cm² ) . When the piston valve is opened, the normal shock wave is generated in the circular duct and it goes into the square duct. The bore and length of the circular duct are 52.7 mm and 5700 mm, respectively.In the square duct, the size of the cross section is 46.7 mm X 46.7 mm. The area of the cross section is kept constant through the ducts. The test section, which has the same cross section shape as the square duct, is connected to the end of the square duct. The test section is open to the atmosphere. Hence, the initial pressure ratio of the shock tube (P4/P1) is varied from 4 to 7. At the interface between the circular duct and the square duct, the cross sectional shape is discontinuously changed from circle to square. The abrupt change in the cross section of the duct may cause the disturbances on the shock wave surface. In order to damp such disturbance before the shock wave reaches the test section, the length of the square duct should be long enough. In the present case, the length-to-height ratio of the square duct is set as about 12.8.