In this study, experimental study was conducted using counterflow jet devices, which have not been studied much in domestic research. To simulate the supersonic environment, a test stand using a open jet system was constructed. A fundamental test using a plasma jet model and an air jet model was carried out in a test stand constituted.
The test conditions were selected on the altitude and Mach number of the high-speed vehicle operation in supersonic environment. The flow visualization system consisted of a light source, a concave mirror, a knife edge, and a high-speed camera to observe the change of the flow structure through the Schlieren visualization method. The drag force measurement device used a load cell and a torque sensor to observe the possibility and tendency of drag reduction according to test conditions.
A fundamental test was conducted to determine the possibility of drag reduction of the counterflow jets in a supersonic flow. In the study, flow visualization and drag measurement were performed by using air and plasma gas emitted from a commercial plasma actuator. The test conditions were set at a supply pressure of 3.5 bara at which plasma jet were maintained for 2 seconds and more. A flow visualization result of the selected test conditions, the flow structure changes were less. As a result of the drag measurement, the drag reduction effect of the plasma jets were 1.5% larger than the air jets. This result showed the possibility of drag reduction through a counterflow jet in a supersonic environment.
Because the plasma jet model is difficult to control the jet pressure. The air jet model which can control the pressure is designed to observe the change of the flow structure and the drag reduction according to the pressure ratio condition. Test results using the air jet model were observed through visualization that the flow due to the jet was injected to the outside in the pressure ratio range of 0.60 to 0.65. Under the condition that the pressure ratio is 1.58 or less, an unsteady state flow was formed, and the penetration length and drag reduction effect of the flow jets were observed. In the pressure ratio range of 1.58 to 1.70, the flow structure and drag reduction tendency were changed, and unsteady state and steady state flow appeared. After that, the steady state flow structure was observed at pressure ratios above 1.70 and the change of drag reduction effect was showed to small at steady state. The drag reduction effect was the largest at 11% under the condition of the pressure ratio of 1.58 just before the flow structure changed. As the length of the flow characteristics were showed, the distance from the head of the model to the shock wave due to jets and the distance to the free stagnation point were measured. As a result of comparison, the distance to the shock wave showed a maximum difference of 19%, and the distance to the free stagnation point was similar in the range of 5%. In this study, the flow structure and the drag reduction effect according to the pressure ratio condition were observed through the fundamental test using the air jet model and the data were obtained by comparing with the previous research. It is expected that the test stand and test results will help to study the counterflow jet in the future. Based on this study, it will be necessary to carry out the research to get the better experimental data through the improvement study of the counterflow jet and the analysis for the application to the high-speed vehicles.