# Shock Reduction through Opposing Jets--Aerodynamic Performance and Flight Stability Perspectives
## 1 Introduction  
The typical objectives of commercial aeronautics research during the twentieth century were centered around achieving long range, high altitude and speed. Efforts in the form of Concorde and Tupolev-144 were made for supersonic commercial travels but due to technical problems, these endeavors were abandoned. Therefore, the speed of commercial aviation was curtailed to subsonic and transonic flow regimes because of multi-faceted concerns [1]. Since 2003, there is no supersonic passenger aircraft in service [2]. Supersonic flight faces multiple problems such as undesirable acoustic signature, high drag, increased aerodynamic heating and flight instabilities. The wave drag is a specific problem of a supersonic flight, which contributes more than half of the total drag and increases rapidly with increasing free stream Mach number. This increase in drag value builds up high temperature and pressure that may damage the leading edge of the blunt body and it must be prevented [3]. This motivates researchers around the globe to explore technologies that may be used to minimize the drag for blunt bodies in supersonic flow regimes. In 1950s, the opposing jet gained immense popularity in the research community due to its durability and relatively economic implementation as compared to other methods [4, 5]. The jet pushes the shock-wave away from the stagnation point and develops an interface between the shock-wave and blunt body. The interface is composed of a complex flow structure that include formation of expansion and reflection waves within the jet, barrel shocks tocover these waves, and an enlarged vortical structure named re-circulation region [6]. The re-circulation region ahead of the blunt body plays a prominent role in the reduction of drag. The effective shock reduction through opposing jets is also dependent on flow conditions and geometric shapes [7; 8]. The parameters of opposing flow jets have been investigated experimentally, numerically, as well as analytically. Experimentally, researchers ascertained an encapsulation of body due to formation of bow and re-compression shocks, called as shock caps. The ejection of secondary flow field (opposing jet) opposes the free stream flow and forms a contact surface that separate these two shocks. The encapsulation due to shocks provide a protecting blanket to reduce the body temperature [9; 10]. Opposing jet also reduces the drag. The ejection of jet distributes the pressure over the surface and generate different low pressure zones (re-circulation or dead air regions and stagnation point or leading edge pressure after ejection of jet) as shown in Figure 1a,b [11; 12]. Jet to body diameter ratio, shape of jet orifice and pressure ratio also effect the performance of opposing jet. The exposed front side of blunt body helps in the strength and positioning of the re-circulation regions [13]. The change in pressure ratios between free stream flow and opposing jet develop long penetration mode (\(LPM\)) and short penetration mode (\(SPM\)) [14]. The under-expanded jet (\(LPM\)) attained at low pressure ratio penetrates the bow shock and displaces it to the maximum position for the formation of conical shape [4; 15]. An unstable diamond shaped pattern can be observed due to the formation of weak expansion and compression waves (reflected waves) which makes the \(LPM\) mode unstable in Figure 1b [16]. The size of re-circulation regions in \(LPM\) mode are thinner and remain at the leading edge of the blunt body to cover the jet boundary. The displaced shock-wave reattaches to the surface of blunt body after passing over the re-circulation region and shows changes in pressure [17; 18]. These changes are named turning shock-waves. A similar kind of trend is observed in \(SPM\) mode, which manifests at higher pressure ratios. The size and positioning of re-circulation regions are different than the \(LPM\) mode. Re-circulation regions are enlarged in width and placed at some distance from the vicinity of opposing jet.