Design of a Near-Isentropic Supersonic Inlet Using Active Control

At wo-dimensional Mach 2.2 internal compression inlet with 97% total pressure recovery has been designed using viscid-inviscid computational tools. Losses are minimized by careful boundary-layer management combined with shape design for weak shocks. The resulting inlet has reduced stability to unstart in the face of atmospheric and engine-borne disturbances, necessitating the use of an active stabilization bleed system that recovers the disturbance-rejection capabilities required of modern inlets. Atmospheric disturbances that the inlet may en- counter during supersonic flight are characterized. Two separate physical mechanisms for unstart are identified, and active control algorithms to prevent these forms of unstart are designed and demonstrated using one- and two-dimensional unsteady Euler simulations. The resulting actively stabilized inlet can withstand flight velocity, temperature, and angle-of-attack perturbations consistent with atmospheric flight. N inlet acts as an interface between the freestream and an air- craft propulsion system. The flow is decelerated to a Mach number required at the compressor face, and energy is recovered in the form of increased pressure. Although it is theoretically possible to design an inlet to achieve virtually isentropic compression of the supersonic flow to sonic conditions, in practice this may be impos- sible to realize due to flow disturbances and geometric variations in the inlet. Because shock waves contribute to loss in pressure recov- ery, a more realistic design is one that can achieve near-isentropic internal compression with a number of weak oblique shock waves and a weak terminal normal shock just behind the throat to decel- erate the flow to subsonic conditions. Such a design is shown in Fig. 1. The potential payoff of such a high-recovery inlet is significant. In the case of a typical 100,000-lb (45,300 kg) supersonic aircraft, such as the quiet supersonic platform, improving the inlet recovery to 97% in cruise could increase the range by approximately 500 n miles. 1 In addition, because most supersonic inlets require bleed on the order of 8% of the inlet flow to stabilize the terminal shock and control boundary-layer separation for the worst-case atmospheric disturbances, the reduced bleed requirement of the present design, primarily due to the weaker shock waves, may lead to additional gains at the system level. The price of achieving high recovery is reduced stability to flow disturbances. These include atmospheric disturbances entering the inlet and engine-borne disturbances traveling upstream from the compressor face. These disturbances can cause a normal shock blowout event known as "inlet unstart." 2 Inlet unstart results in a severe increase in drag due to flow spillage and formation of a strong shock wave at the inlet lip. In addition, the accompanying decrease in mass flow through the inlet may also result in engine surge. We can define the unstart tolerance of the inlet as the magnitude of the disturbance that can be tolerated without unstart. The tradeoff between inlet stability and pressure recovery is primarily a conse- quence of the strength of the terminal normal shock. 3 Thus, an inlet