The characteristics around counterflow jets are difficult to predict due to the many variables. The effects of drag reduction using counterflow jets under various conditions have been investigated by a computational and experimental effort. In order to analyze the counterflow jets with different freestreams, a numerical simulation was conducted by solving a two-dimensional axisymmetric RANS equation. In a short penetration mode, the trends of drag reduction with the freestream conditions (altitude: 10 to 19 km, M∞: 2.0 to 4.0) were reviewed. The experiments were performed under two freestream conditions (M∞ = 3.8, M∞ = 7). The effects of drag reduction under different nose shape were investigated by using the blowdown supersonic wind tunnel of Mach number 3.8 at Chungnam national university, and the Mach 7 hypersonic shock tunnel was used to analyze the effect of drag reduction by plasma jet.
According to the results of the numerical analysis, the difference in the overall flow structure arises from the freestream Mach number. The recirculation zone is the key feature of the flow. From the mechanism of the flow, the recirculation zone is affected by the velocity and pressure at the boundary. And the position and pressure of the reattachment point are the main parameters representing the recirculation zone. The cause of the difference in the flow structure depending on the freestream Mach number can be explained by these parameters.
Plasma jets have a general tendency of drag reduction with nozzle thrust for freestream Mach number under conditions the flow satisfies the steady state. Plasma acts as a high thermal energy in a counterflow jets. Plasma generated by electrical energy causes the temperature of the cold gas to rise. If the cold gas and plasma are the same injection pressure, the plasma jets are injected at a relatively low mass flow rate. The plasma jets have higher nozzle thrust at the same mass flow rate and pressure. Therefore, the plasma jets can achieve a corresponding drag reduction at low flow rates when compared to gas at room temperature due to the mass flow rate decreasing with temperature.
When the injection condition and the freestream Mach number are determined in a counterflow jets in which the flow field satisfies the steady state, there is a maximum point of drag reduction depending on the injection condition. Derivation of the maximum point of drag reduction in stable flow is a meaningful result when considering practical implementation of counterflow jets. This tendency of the maximum drag reduction fully demonstrates the possibility of the application of a counterflow jets to a high-speed vehicle. In addition, it is advantageous to operate at a high altitude because the low injection pressure is effective for the application of a vehicle.