# Shock Reduction through Opposing Jets--Aerodynamic Performance and Flight Stability Perspectives
## 1 Introduction
The reattachment of shockwave also triggers similar turning shock-waves due to change in pressure regions (upstream and downstream pressure differences due to shock-wave) as shown in Figure 1a [19]. Analytical models provide the theoretical basis of opposing jet for the mitigation of shock-wave adverse effects. [PERSON] [20] developed an analytical model of jet flow-field, which primarily predicted the opposing force coefficient (\(C_{F}\)) of jet. Later on, researchers called this force coefficient static pressure of jet. The pressure ratio of jet and body was controlled by using the concept of jet exit Mach number (Mj). Numerically it was found that the interface at front of Mach disc is blunt for large PR values  
Figure 1: Schematic configuration of opposing jet with two distinct modes. (due to decrease in Mach of jet) as shown in Figure 1a. The simulated result suggested that the Mach disk of unstable mode is not clearly visible (due to absence of barrel shocks). The increase in free stream Mach (due to increase in pressure ratio) convert an unstable LPM into stable SPM. The development of opposing jet technology led researchers to study various other aspects such as effect of different jet ejection materials (water, gases and plasma). By injection of cold vaporized water in airflow, drag reduced by up to 50% at the cost of very high pressure ratio [21]. Therefore, instead of using vaporized water, different gases (\(He\), \(H_{2}\), \(CO_{2}\) and \(N_{2}\)) were used to investigate the reduction of drag [10; 22]. Helium (\(He\)) is found to have better performance for drag reduction as it is lighter and inert gas (as compared to \(CO_{2}\) and \(N_{2}\)) [23]. Although \(H_{2}\) is lightest, its reactive property can cause increase in heat transfer to the body [24]. A similar methodology was adopted by [PERSON] [25], in which plasma materials were used to reduce the shock. To get much better results for drag reduction [26], combination of passive (spike) and active (opposing jet) were tested [27; 28; 29]. The active and passive combination of flow control technologies on blunt bodies attain some additional factors (separated re-circulation regions) which produce unsteadiness in the flow and effects the stability of body. The ejection of jet reduces the unsteadiness in the flow by displacing the separation region and vortices on the surface of blunt body and stabilize the flow. Previously, researchers used wind tunnel tests [30; 31; 32; 33; 34; 35; 36], empirical and computational fluid dynamics (CFD) tools for the prediction of stability behavior. Empirical methods like DATCOM and ESDU were developed for the correct prediction of stability derivatives [37]. These methods were fast enough for the wind tunnel tests. However, these methodologies do not model the active flow control features. Therefore, a numerical approach like CFD can be used for the capturing the contributions of flow control techniques to stability characteristics [38; 39]. Studies conducted to date indicate that extensive investigations have been performed on the use of the opposing jet technique for the reduction of drag and heat transfer from the surface of body. However, nearly all work includes blunt bodies (like sphere, cylinder, truncated cone or aeroshells) for the ejection of jet. A limited work exists in which elliptical bodies (airfoil shaped) were used for the reduction of drag using flow control techniques, especially opposing jet [5]. For a supersonic configuration, such as Unmanned Combat Air Vehicle, a long spike or fuselage ahead of the wings will reduce its stealth features. Therefore, usage of opposing jets should be tested on the wing geometries as well. In this research, the effect of opposing jet on overall aerodynamic performance and the flight stability characteristics of a two-dimensional wing is explored. Specifically, the interaction of opposing jet with the supersonic free-stream flow is investigated using a numerical approach.