Unstart

Schlieren flow visualization showing unstart of axisymmetric inlet at Mach 2. Unstarted shock structure evident on left, started inlet on right.

In supersonic aerodynamics, unstart refers to a generally violent breakdown of the supersonic airflow. The phenomenon occurs when mass flow rate changes significantly within a duct. Avoiding unstarts is a key objective in design of the engine air intakes of supersonic aircraft.

Etymology

The term originated during the use of early supersonic wind tunnels. “Starting” the supersonic wind tunnel is the process in which the air becomes supersonic; unstart of the wind tunnel is the reverse process.^[1] The shock waves that develop during the starting or unstart process may be visualized with schlieren or shadowgraph optical techniques.

Unstart in aircraft engines

The design air intakes for supersonic aircraft can be compared to that of supersonic wind tunnels, and requires careful analysis in order to avoid unstarts.^[2] At high supersonic speeds (usually between Mach 2 to 3), the inlet is designed to have supersonic flow downstream of the air inlet's capture plane. If the mass flow across the inlet's capture plane does not match the downstream mass flow at the engine, the inlet will unstart. Depending on the aircraft and inlet design, this can have catastrophic consequences on the aircraft’s flight dynamics.^[3]

Aircraft notable for featuring unstart problems were the Lockheed SR-71 Blackbird, B-70 Valkyrie, and the F-107 fighter. On the Concorde supersonic airliner, air intakes were controlled by a FADEC system in order to prevent unstarts.

Theoretical basis

Using a more theoretical definition, unstart is the supersonic choking phenomenon that occurs in ducts with an upstream mass flow greater than the downstream mass flow. Unsteady flow results as the mismatch in massflow can not gradually propagate upstream in contrast to subsonic flow. Instead, in supersonic flow, the mismatch is carried forward behind a 'normal' or terminal shock wave that abruptly causes the gas flow to become subsonic. The resulting normal shock wave then propagates upstream at an effective acoustic velocity until the flow mismatch reaches equilibrium.

There are other ways of conceptualizing unstart which can be helpful. Unstart can be alternatively thought of in terms of a decreasing stagnation pressure inside of a supersonic duct; whereby the upstream stagnation pressure is greater than the downstream stagnation pressure. Unstart is also the result of a decreasing throat size in supersonic ducts. That is the entrance throat is larger than the diffusing throat. This change in throat size gives rise to the decreasing mass flow which defines unstart.^[4]

The choking reaction of unstart results in the formation of a shock wave inside of the duct.

Shock instability or buzz

Under certain conditions, the shock wave in front or within a duct may be unstable, and oscillate upstream and downstream. This phenomenon is known as buzz. Stronger shock waves interacting with low momentum fluid or boundary layer tend to be unsteady and buzz. Buzz conditions can cause structural dynamics-induced failure if adequate margins are not incorporated into design.

References

1. Liepmann, H.W. and Roshko, A. (1957). "Flow in Ducts and Wind Tunnels". Elements of Gasdynamics. John Wiley. ISBN 0-471-53460-9.
2. NASA's supersonic controls research
3. SR71 pilot's unstart experience
4. Anderson, John D. (2009). Fundamentals of Aerodynamics (5th ed.). McGraw-Hill. ISBN 978-0-07-339810-5.