Concept

Real gas — where it appears

A gas whose behaviour departs from the perfect-gas law, because its molecules have size and attract one another or because its internal modes take up energy. Near the critical point or at high temperature the departure decides what its waves and shocks do.

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

The temperature behind a shock, which is not the jump condition's. The static temperature along the flow behind a Mach 6 normal shock, with the frozen value the jump conditions give and the equilibrium value they give a long way behind. The gas arrives at 2382 K and settles at 2059, over about four tenths of a millimetre.

A gas that has not finished being shocked

The jump conditions give the state a long way behind a shock. Immediately behind it the molecules have not started vibrating yet, so the temperature is 2,382 K where the equilibrium answer is 2,059 — and the gas takes four tenths of a millimetre to get from one to the other.

compressible · Real gas

In the island the signs are reversed: a weak expansion jump gains entropy and a weak compression loses it. The entropy change along the Hugoniot, against the volume ratio across the jump, from a state at Γ = -1.5 and from an ordinary state at T = 1.1, v = 3. From the ordinary state compression (a ratio below one) has entropy rising, the textbook case. From the island it falls for weak compressions, down to a ratio near 0.75, and rises for every expansion.

A heavy vapour has shocks that expand

Every gas-dynamics course proves that shocks compress and only compress, since an expansion shock would destroy entropy. That proof contains an assumption nobody mentions, because air always satisfies it. In a dense enough vapour of large molecules, close above its critical point, the sound speed falls as the fluid is squeezed, and the whole argument changes sign. There, a compression spreads out into a smooth fan, and an expansion steepens into a shock that gains entropy. Bethe saw the possibility in 1942. The arithmetic needs a heat capacity above 16.7 R, and a handful of industrial fluids have it.

compressible · Real gas

Named alongside it

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

Shock Characteristics Entropy Equilibrium Heat transfer Hypersonic Internal energy Isentropic Measurement Memory kernel Model limit Model validity

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