Influence of a Normal Slot Boundary Layer Suction System onto a Shock Train

The recompression of supersonic gas flow is a very common flow phenomenon in modern aerodynamics and occurs in a great number of applications for instance supersonic ramjet or scramjet inlets, internal diffusers and supersonic ejectors. Under certain conditions even one or more shocks can appear downstream of the first shock. This series of shocks is a so called shock train. In contrast to other shock systems the supersonic flow is decelerated at first through a shock system and followed by a mixing region as shown in Fig. 1. For the whole interaction region Crocco et al. [1] have coined the term pseudo-shock. The structure and length of the shock train depends very much on the so called confinement level which is the ratio of the boundary layer thickness

δ

to the half height of the nozzle h. This was investigated very thoroughly by Carroll [2] and Om et al. [3]. In case of a shock train, due to the occurrence of successive shocks the pressure recovery along the shock train extends and greatly deviates form the pressure gradient that would occur at a single normal shock, [4]. However, a short recompression region or at best a single normal shock leads to very high pressure and temperature gradients, which provides interesting opportunities for various gas dynamic applications. In order to reduce the shock train length and thereby increasing the pressure gradient the shock system must be exposed to a higher back pressure. However, under normal conditions this would lead to a relocation of the shock train farther upstream, because the boundary layer can only withstand a certain back pressure level. The ability of the boundary layer to overcome a strong back pressure is limited, because the flow velocity hence impulse drops to zero near the wall. In order to remove the parts of the boundary layer with small impulse a normal suction slot 1.5 mm wide is placed upstream of the shock train, see Fig. 1.