# Computational Investigations and Control of Shock Interference +
## 1 Introduction  
A scramjet engine provides high-speed airbreathing propulsion for hypersonic vehicles. Major segments of a scramjet engine internal flowpath consist of an inlet, an isolator, a combustor, and a nozzle. A notable differentiating feature of scramjets from gas-turbines is that air compression is achieved by successive shocks in the inlet without the use of any rotating parts, such as a multi-stage compressor [1, 2, 3, 4]. One of the primary issues involving scramjet engines is the unstart and the resulting limitations of operability, which have been discussed in several papers [5, 6, 7, 8, 9, 10]. Unstart can occur in the inlet, isolator, or combustor. The isolator is a conduit of either rectangular or circular cross-section that separates (i.e., isolates) the inlet from the combustor. Throughout the combustion process, heat is released, leading to a pressure increase in the combustion chamber. The pressure rise in the combustor induces an adverse pressure gradient in the isolator, which resultsin thickening of the boundary layer, formation of the pseudo-shock-train in the isolator, and could also result in separation of the boundary layer, especially if the boundary layer is laminar [10]. In addition, high-speed flow entering the isolator from the inlet interacts with the pseudo-shock-train and separated or thickened boundary layer, leading to the generation of shock interference as well as flow instability in the isolator. The role of the isolator in the scramjet engine is shown in Figure 1.  
Shock-shock interference is defined as the phenomenon that occurs when two or more shock waves of different types and strengths intersect with each other. The shock interference can manifest in terms of planar and curved shock interaction and/or curved-curved shock interaction. However, each type of interference can result in a sudden increase in surface pressure and heat transfer and can induce boundary-layer transition [11; 12; 13; 14]. [PERSON] [15] performed a comprehensive investigation of curved-curved shock interactions. This was carried out through experimental and theoretical studies of the impingement of an oblique shock on a bow shock formed in front of a blunt body. From this work, [PERSON] coined the six types of distinct shock interference, titled Type I to Type VI, each type with a unique flow structure, which depend upon the intensity and direction of the intersecting shock waves. Most of the studies conducted on the investigation of shock interference have focused on external flows over conical shapes in the form of a blunt nose or round leading edges. However, shock interference can also occur in the scramjet flow path due to shocks generated by different parts in the flow field, leading to increased pressure and heating, which can affect the onset of unstart in the scramjet flow path. Investigation of shock interference in a supersonic internal flow path has not been reported in the open literature. The research presented in this paper addresses this technical gap. Shock-interference and interaction of shocks with boundary layer can lead to sudden unsteady disruption of the flow, resulting in the onset of \"unstart\" of the engine [8; 10], depending upon a combination of Reynolds number, magnitude of adverse pressure gradient, extent of boundary-layer separation, flow unsteadiness, turbulence intensity, and the boundary-layer thickness, and types of shocks interacting with each other and the boundary-layer. Due to the detrimental effects of unstart, the topic has been widely studied, but many challenging aspects remain open, including the root cause of the onset of unstart and how to prevent the onset of unstart, so that the operability envelope of modern scramjets can be maintained and expanded.