22-08-2013, 02:21 PM
Shock wave–boundary-layer interactions (SBLIs)
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Introduction
Shock wave–boundary-layer interactions (SBLIs) occur when a shock wave and a boundary layer converge and, since both can be found in almost every supersonic flow, these interactions are commonplace. The most obvious way for them to arise is for an externally generated shock wave to impinge onto a surface on which there is a boundary layer. However, these interactions also can be produced if the slope of the body surface changes in such a way as to produce a sharp compression of the flow near the surface – as occurs, for example, at the beginning of a ramp or a flare, or in front of an isolated object attached to a surface such as a vertical fin.
If the flow is supersonic, a compression of this sort usually produces a shock wave that has its origin within the boundary layer. This has the same affect on the viscous flow as an impinging wave coming from an external source. In the transonic regime, shock waves are formed at the downstream edge of an embedded supersonic region; where these shocks come close to the surface, an SBLI is produced.
In any SBLI, the shock imposes an intense adverse pressure gradient on the boundary layer, which causes it to thicken and possibly also to separate. In either case, this increases the viscous dissipation within the flow. Frequently, SBLIs are also the cause of flow unsteadiness. Thus, the consequences of their occurrence almost invariably are detrimental in some respect.
On transonic wings, they increase the drag and they have the potential to cause flow unsteadiness and buffet. They increase blade losses in gas-turbine engines, and complicated boundary-layer control systems must be installed in supersonic intakes to minimize the losses that they cause either directly by reducing the intake efficiency or indirectly because of the disruption they cause to the flow entering the compressor. These systems add weight to an aircraft and absorb energy. In hypersonic flight, SBLIs can be disastrous because at high Mach numbers, they have the potential to cause intense localized heating that can be severe enough to destroy a vehicle.
Shock Wave–Boundary-Layer Interactions: Why They Are Important?
The shock wave–boundary layer interaction (SBLI) occurring within a flow are numerous and frequently can be a critical factor in determining the performance of a vehicle or a propulsion system. SBLIs occur on external or internal surfaces, and their structure is complex. Shock wave/boundary layer interactions (SWBLI) in hypersonic flows are characterised by extremely large pressure variations and intense wall heat transfer, especially when the shock is strong enough to separate the boundary layer.
Shock Wave-Boundary Layer Interaction of Scramjet Intake Flows
A scramjet propulsion system is a hypersonic air-breathing engine in which heat addition, due to combustion of fuel and air, occurs in the flow that is supersonic relative to the engine. In a conventional ramjet, engine the incoming supersonic airflow is decelerated to subsonic speeds by means of a multi-shock intake system and diffusion process. Fuel is added to the subsonic airflow, the mixture combusts and then re-accelerates through a mechanical choke to supersonic speeds. By contrast, the airflow in a pure scramjet remains supersonic throughout the combustion process and does not require a choking mechanism. Modern scramjet engines are able to seamlessly make the transition between ramjet and scramjet operation.
As flight Mach numbers increase beyond Mach 5, the use of supersonic combustion can provides higher performance (i.e. specific impulse) due to inlet efficiency offset by higher Rayleigh losses associated with combustion. Crossover points between ramjet and scramjet operation indicate the benefits of operating in ramjet until Mach 5-6. The process of decelerating airflow at flight Mach 6 to subsonic speeds for combustion results in near-stagnation conditions, with attendant high pressures and heat transfer rates.