Shock Reduction Through Active Flow Control: Aerodynamic and Flight Stability Considerations

Abstract

Typical challenges of supersonic flight include wave drag, acoustic signature, and aerodynamic heating due to the formation of shock waves ahead of the vehicle. Efforts in the form of sleek aerodynamic designs, better propulsion systems, and the implementation of passive and active techniques are generally adopted to achieve a weak shock wave system. Shock reduction can improve flight range, reduce fuel consumption and provide thermal protection of the forebody region. This research explores the effectiveness of a counter flow jet (active technique) to tailor the shock wave characteristics in supersonic flow regime. The discussions include the principle of operation, physics of fluid behavior, and overall contribution to flight stability characteristics. However, the present investigation comprises of three phases that manifest the importance of counterflow jet for reducing drag. In the first phase, the investigation of counterflow air jet on the aerodynamic performance and flight stability characteristics of an airfoil (NACA 0012 ) with blunt leadingedge in the supersonic regime is performed. Unsteady Reynolds-Averaged Navier-Stokes (URANS) based solver is used to model the flow field. The parametric study of opposing jet includes the effect of angles of attack (α), freestream Mach number (M∞), and pressure ratio (PR) on the aerodynamic performance of airfoil. The results indicate that the opposing jet reduces drag from 30% to 70%, improves the maximum lift-to-drag ratio from 2.5 to 4.0, and increases shock stand-off distance from 15% to 35% depending on flow conditions. The effect of opposing jet on longitudinal flight stability characteristics, studied for the first time, indicate improvement in dynamic stability coefficients (Cmq + Cmα˙ ) at low angles of attack. It is concluded that the opposing jet can help to mitigate flight disturbances in supersonic regime. Although the air opposing jet is useful to reduce drag ahead of an airfoil, the performance of the opposing jet can be further advanced by ejecting plasma ahead of an airfoil. So, in the second stage, the study is performed ii to compare air and plasma counterflow jets for drag reduction to investigate the effect of different jet ejection materials. The computational results for the ejection of counterflow plasma jet from the leading edge of an airfoil (NACA 0012 ) depicts more drag reduction (almost twice) as compared to conventional jet (non-plasma). The freestream conditions and the computational model remain the same as that of the counter flow air jet. Before the jet’s ejection, the weakly ionized argon plasma is generated by a plasma torch with constant stagnation pressure and temperature of 303, 975Pa and 3000K. Effect of Mach number and angles of attack variation on plasma-jet effectiveness are also analyzed. The gravitational, magnetic field effect, and chemical processes in the plasma formation are considered negligible. It is inferred that the efficacy of the counterflow plasma jet strongly depends upon the jet stagnation temperature. The third phase of investigation extends to the complete 3D picture of the counterflow air jet. The air opposing jet is executed from a 3D slender forebody. In literature, very little related work is present in which a jet is ejected from a slender forebody. However, most of the work is related to blunt shape bodies and their aerodynamic analysis. The 3D shockwave and its displacement from the body’s surface are discussed with constant freestream conditions. The analysis illustrates the parametric study of pressure ratio (PR = 3 to PR = 15 ) with increasing angles of attack (α = 0o to α = 4o). The effect of jet ejection on the longitudinal flight stability characteristic of the body indicates an improvement in the static stability coefficients (Cmα) at low angles of attack. The dynamic stability coefficients (Cmq + Cmα˙ ) are also improved with the implementation of the jet. By the aid of investigations, it is concluded that opposing jet helps to mitigate the adverse effects of shock and improve the vehicle’s performance. Moreover, the counter flow plasma jet depicts better performance as compared to the air jet. The implementation of the jet also enhanced the stability of the vehicle. Hence, the counterflow jet technique improves the overall efficiency of the body.