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

Contact surface — where it appears

A surface separating two bodies of gas that move together at the same pressure but are otherwise different. Temperature, density and composition all jump across it, and it carries a permanent record of where each side came from.

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

Also named here as shock tube — the same set of essays touches all of them, so they are one junction rather than several.

Three waves out of one discontinuity. The x–t diagram of the burst diaphragm. A shock runs right at a speed of its own, a contact surface follows it more slowly, and an expansion fan spreads left as a family of rays that opens with time — the three wave families the Euler equations possess, produced at once by an initial condition with no waves in it at all. Every straight line here is a speed the solution computed, and the fan is drawn as the rays it actually consists of.

One diaphragm, every wave

Two states of the same gas at rest, separated by nothing, is the simplest initial condition compressible flow admits — and its answer contains all three waves the equations possess at once: a shock one way, an expansion fan the other, and between them a surface across which the density jumps and the pressure does not.

compressible · Shock tube The four waves, and the one that never goes away. The shock tube in space and time: a shock running right, an expansion fan running left, and the contact surface between them. The shock and the fan are travelling disturbances that leave; the contact is made of fluid, so it is carried along and is there for ever.

A surface that remembers the diaphragm

Between the shock and the expansion in a shock tube there is a surface across which the pressure and the velocity are identical and the temperature differs by a factor of two. It is made of fluid, so it never goes away, and nothing in the pressure field says it is there.

compressible · Shock tube

Named alongside it

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

EntropyExpansion fanRiemann problemShock tubeCharacteristicsConserved quantityMaterial lineMeasurementMemory kernelModel validityNormal shockRankine–Hugoniot conditions

All concepts