Editing Chaplygin's equation (section)

==Derivation==
For two-dimensional [[potential flow]], the continuity equation and the [[Euler equations (fluid dynamics)|Euler equations]] (in fact, the [[Bernoulli's principle#Compressible flow equation|compressible Bernoulli's equation]] due to irrotationality) in Cartesian coordinates <math>(x,y)</math> involving the variables fluid velocity <math>(v_x,v_y)</math>, [[specific enthalpy]] <math>h</math> and density <math>\rho</math> are

:<math>
\begin{align}
\frac{\partial }{\partial x}(\rho v_x) + \frac{\partial }{\partial y}(\rho v_y) &=0,\\
h + \frac{1}{2}v^2 &= h_o.
\end{align}
</math>

with the [[equation of state]] <math>\rho=\rho(s,h)</math> acting as third equation. Here <math>h_o</math> is the stagnation enthalpy, <math>v^2 = v_x^2 + v_y^2</math> is the magnitude of the velocity vector and <math>s</math> is the entropy. For [[isentropic]] flow, density can be expressed as a function only of enthalpy <math>\rho=\rho(h)</math>, which in turn using Bernoulli's equation can be written as <math>\rho=\rho(v)</math>.

Since the flow is irrotational, a velocity potential <math>\phi</math> exists and its differential is simply <math>d\phi = v_x dx + v_y dy</math>. Instead of treating <math>v_x=v_x(x,y)</math> and <math>v_y=v_y(x,y)</math> as dependent variables, we use a coordinate transform such that <math>x=x(v_x,v_y)</math> and <math>y=y(v_x,v_y)</math> become new dependent variables. Similarly the [[velocity potential]] is replaced by a new function ([[Legendre transformation]])<ref>{{cite book|last1=Landau|first1=L. D.|authorlink1=Lev Landau|last2=Lifshitz|first2=E. M.|authorlink2=Evgeny Lifshitz|title=Fluid Mechanics|edition=2|year=1982|publisher=Pergamon Press|page=432}}</ref>

:<math>\Phi = xv_x + yv_y - \phi</math>

such then its differential is <math>d\Phi = xdv_x + y dv_y</math>, therefore

:<math>x = \frac{\partial \Phi}{\partial v_x}, \quad y = \frac{\partial \Phi}{\partial v_y}.</math>

Introducing another coordinate transformation  for the independent variables from <math>(v_x,v_y)</math> to <math>(v,\theta)</math> according to the relation <math>v_x = v\cos\theta</math> and <math>v_y = v\sin\theta</math>, where <math>v</math> is the magnitude of the velocity vector and <math>\theta</math> is the angle that the velocity vector makes with the <math>v_x</math>-axis, the dependent variables become

:<math>
\begin{align}
x &= \cos\theta \frac{\partial \Phi}{\partial v}-\frac{\sin\theta}{v}\frac{\partial \Phi}{\partial \theta},\\
y &= \sin\theta \frac{\partial \Phi}{\partial v}+\frac{\cos\theta}{v}\frac{\partial \Phi}{\partial \theta},\\
\phi & = -\Phi + v\frac{\partial \Phi}{\partial v}.
\end{align}
</math>

The continuity equation in the new coordinates become

:<math>\frac{d(\rho v)}{dv} \left(\frac{\partial \Phi}{\partial v} + \frac{1}{v} \frac{\partial^2 \Phi}{\partial \theta^2}\right) + \rho v \frac{\partial^2 \Phi}{\partial v^2} =0.</math>

For isentropic flow, <math>dh=\rho^{-1}c^2 d\rho</math>, where <math>c</math> is the speed of sound. Using the Bernoulli's equation we find

:<math>\frac{d(\rho v)}{d v} = \rho \left(1-\frac{v^2}{c^2}\right)</math>

where <math>c=c(v)</math>. Hence, we have

:<math>
\frac{\partial^2 \Phi}{\partial \theta^2} +
\frac{v^2}{1-\frac{v^2}{c^2}}\frac{\partial^2 \Phi}{\partial v^2}+v \frac{\partial \Phi}{\partial v}=0.</math>