Chaplygin's equation
In gas dynamics, Chaplygin's equation, named after Sergei Alekseevich Chaplygin, is a partial differential equation useful in the study of transonic flow. It is
Here, is the speed of sound, determined by the equation of state of the fluid and conservation of energy.Derivation
For two-dimensional potential flow, the continuity equation and the Euler equations in Cartesian coordinates involving the variables fluid velocity, specific enthalpy and density are
with the equation of state acting as third equation. Here is the stagnation enthalpy, is the magnitude of the velocity vector and is the entropy. For isentropic flow, density can be expressed as a function only of enthalpy, which in turn using Bernoulli's equation can be written as.
Since the flow is irrotational, a velocity potential exists and its differential is simply. Instead of treating and as dependent variables, we use a coordinate transform such that and become new dependent variables. Similarly the velocity potential is replaced by a new function
such then its differential is, therefore
Introducing another coordinate transformation for the independent variables from to according to the relation and, where is the magnitude of the velocity vector and is the angle that the velocity vector makes with the -axis, the dependent variables become
The continuity equation in the new coordinates become
For isentropic flow,, where is the speed of sound. Using the Bernoulli's equation we find
where. Hence, we have
