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

Mixing loss — where it appears

The loss of mechanical energy when two streams at different speeds are mixed to one speed in a duct. Momentum is conserved and kinetic energy is not, so the loss follows from the momentum balance alone, whatever the turbulence doing the mixing.

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

A pump curve out of the momentum theorem. The pressure an ejector delivers, against how much it is entraining, at a fixed nozzle. It has the shape of every pump characteristic ever measured — a shut-off pressure with no flow, falling to no pressure at free delivery — and it was obtained from a momentum balance on a tube with nothing in it. The shut-off value here is 36.0 kPa and the machine at its best power runs at 4.56 times its own motive flow.

Mixing is a pump

Two streams at different speeds mixing in a tube destroy energy — the same Borda–Carnot expression a handbook prints beside a sudden enlargement, with two streams in it instead of one. And while they destroy it the pressure rises, which makes the loss the mechanism of a machine with no moving parts.

applied · Ejector Six nozzles, six pump curves, and where each one is best. The head ratio a water jet pump delivers against the flow ratio it entrains, for six area ratios from a narrow nozzle to one filling four-fifths of the throat. A wide nozzle makes a tall, steep curve that is finished at a small flow; a narrow one makes a low, long curve. The dots are each curve's best-efficiency point. Nozzle, suction, throat-friction and diffuser losses are included at borrowed representative values, and the mixing loss is computed.

The nozzle that is best at one thing

Put the four losses back into a jet pump and three questions get three answers. The most head comes from a nozzle four-fifths of its throat, in closed form; the best efficiency from one a quarter of it; and the most flow from whichever nozzle is smallest, because flow has no optimum at all.

applied · Ejector

Named alongside it

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

The Borda–Carnot lossEfficiencyEjectorEntrainmentOptimisationPressure recoveryControl volumeDiffuserLoss coefficientMomentum fluxMomentum theoremPump characteristic

All concepts