Fluid Flow & Aerodynamics
why things fly, and why the usual explanation is wrong
Concept

Detonation — where it appears

A combustion wave in which a shock compresses the mixture and the reaction it starts drives the shock. Its speed is set by the thermodynamics rather than by transport, and its structure has a length given by the induction time behind the shock.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

The Hugoniot of a gas that can burn, and the gap in the middle of it. Pressure against specific volume, both scaled on the unburnt gas. The lower curve is the ordinary shock Hugoniot, which passes through the initial state because a jump of zero strength satisfies mass, momentum and energy. Adding a heat release lifts it away, and the initial state now sits in a region no wave can reach: between the two branches a Rayleigh line would need a positive slope, and its slope is minus the square of the mass flux. A burning gas has no weak waves available to it at all.

The other branch of the same curve

Put heat into the jump conditions and the Hugoniot lifts away from the initial state, leaving a gap no wave can occupy. A burning gas has no weak waves: it must run supersonically or subsonically, and the conservation laws pick the first speed exactly and say nothing at all about the second.

compressible · Detonation The structure the sensitivity produces. A detonation front, schematically: a leading shock, an induction zone in which nothing measurable happens, and a reaction zone behind it. The induction zone's length is set by the shock's own strength, and because that dependence is exponential the front is unstable and breaks into cells.

A gas that has not decided to react yet

Behind a detonation's leading shock there is a zone in which nothing measurable happens. Its length is set by the temperature the shock produced, exponentially — a one per cent change in the shock shortens it by fifteen per cent — and that sensitivity is why a detonation front cannot stay flat.

compressible · Detonation

Named alongside it

The objects these essays reach for when they reach for this one.

Chapman–JouguetCombustionConservationConstitutive lawDiscontinuityEntropyInduction timeInstabilityMeasurementMemory kernelModel limitModel validity

All concepts