Abstract

When travelling under high-thrust conditions, such as during take-off and climb, the substantial mass flow demand by a turbofan engine is sufficient to accelerate the flow over the intake lip to supersonic conditions. This faster-than sound flow pocket is terminated by a near-normal shock wave. The adverse pressure gradient imposed by this disturbance on the boundary layer can cause the latter to separate, introducing large scale unsteadiness and an increase in viscous losses. These losses have a direct negative repercussion on the overall engine efficiency as the total pressure reaching the fan face is reduced. Moreover, if the separated boundary layer does not reattach before the engine face, the unsteadiness, characteristic of separated flows, may increase the stress on the fan, which can ultimately reduce component lives.

Turbofan engines are designed to operate over a wide range, defined by inflow speed, incidence and engine demand. The consequences of increasing the last two parameters are considered in the experimental investigation currently being undertaken at the Cambridge University Engineering Department. In particular, the main interest is to assess the the onset and severity of any unsteady regimes.

In conclusion, come along to see pretty pictures of shock waves!