An isentropic (constant entropy) process is usually* one that is both internally reversible (can be reversed without leaving a trace on its surroundings) and adiabatic (no heat transfer occurs between the system and its surroundings).
Most devices (pumps, turbines, etc.) are essentially adiabatic, and all perform best when irreversibilities are minimized. Therefore, isentropic processes are often used to model real processes. Also, because an isentropic process represents the most efficient process possible, they are used to define efficiencies of real processes.
For an
ideal gas,
pressure,
density, and
temperature at two
states of an isentropic process are related by the equations:
p2/p1 = (ρ2/ρ1)^γ = (T2/T1)^(γ/(γ-1))
where p is pressure, ρ is density, T is temperature, γ is the specific heat ratio (1.4 for air), and 1 and 2 represent different points in the process. These relations are especially useful in aerodynamics, where most areas of flow around an airfoil (beyond the boundary layer) can be assumed to be isentropic.
From the
second law of thermodynamics, an isentropic process of
liquids and
solids is also an
isothermal (constant temperature) process.
Isentropic conditions also occur at stagnation points - a fluid condition defined by the point at which a flow is isentropically brought to zero velocity. (see: Pitot tube.)
The Carnot cycle consists of two isentropic and two isothermal processes.
*Note: Although an internally reversible, adiabatic process is always isentropic, an isentropic process is not necessarily both reversible and adiabatic; this can occur if, for example, an increase in entropy is offset by a decrease in entropy due to heat loss.