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Isentropic nozzle flow
Isentropic nozzle flow describes the movement of a gas or fluid through a narrowing opening without an increase or decrease in entropy.
== Overview ==
Whenever a gas is forced through the tube, the gaseous molecules are deflected by the tube’s walls. If the speed of the gas is much less than the speed of sound, the gas density will remain constant and the velocity of the flow will increase. However, as the speed of the flow approximates the speed of sound, compressibility effects on the gas are to be considered. The density of the gas becomes position dependent. While considering flow through a tube, if the flow is very gradually compressed (i.e. area decreases) and then gradually expanded (i.e. area increases), the flow conditions are restored (i.e. return to its initial position). So, such a process is a reversible process. According to the Second Law of Thermodynamics, whenever there is a reversible flow, constant value of entropy is maintained. Engineers classify this type of flow as an ''isentropic'' flow of fluids. Isentropic is the combination of the Greek word "iso" (which means - same) and entropy.
When the change in flow variables is small and gradual, isentropic flows occur. The generation of sound waves is an isentropic process. A supersonic flow that is turned while there is an increase in flow area is also isentropic. Since there is an increase in area, therefore we call this an ''isentropic expansion''. If a supersonic flow is turned abruptly and the flow area decreases, the flow is irreversible due to the generation of shock waves. The isentropic relations are no longer valid and the flow is governed by the oblique or normal shock relations.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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