The Lamb–Chaplygin dipole model is a mathematical description for a particular inviscid and steady dipolar vortex flow. It is a non-trivial solution to the two-dimensional Euler equations. The model is named after Horace Lamb and Sergey Alexeyevich Chaplygin, who independently discovered this flow structure.^[1] This dipole is the two-dimensional analogue of Hill's spherical vortex.

A two-dimensional (2D), solenoidal vector field ${\displaystyle \mathbf {u} }$ may be described by a scalar stream function ${\displaystyle \psi }$, via ${\displaystyle \mathbf {u} =-\mathbf {e_{z}} \times \mathbf {\nabla } \psi }$, where ${\displaystyle \mathbf {e_{z}} }$ is the right-handed unit vector perpendicular to the 2D plane. By definition, the stream function is related to the vorticity ${\displaystyle \omega }$ via a Poisson equation: ${\displaystyle -\nabla ^{2}\psi =\omega }$. The Lamb–Chaplygin model follows from demanding the following characteristics: ^{[citation needed]}

• The dipole has a circular atmosphere/separatrix with radius ${\displaystyle R}$: ${\displaystyle \psi \left(r=R\right)=0}$.
• The dipole propages through an otherwise irrorational fluid (${\displaystyle \omega (r>R)=0)}$ at translation velocity ${\displaystyle U}$.
• The flow is steady in the co-moving frame of reference: ${\displaystyle \omega (r<R)=f\left(\psi \right)}$.
• Inside the atmosphere, there is a linear relation between the vorticity and the stream function ${\displaystyle \omega =k^{2}\psi }$

The solution ${\displaystyle \psi }$ in cylindrical coordinates (${\displaystyle r,\theta }$), in the co-moving frame of reference reads:

${\displaystyle {\begin{aligned}\psi ={\begin{cases}{\frac {-2UJ_{1}(kr)}{kJ_{0}(kR)}}\mathrm {sin} (\theta ),&{\text{for }}r<R,\\U\left({\frac {R^{2}}{r}}-r\right)\mathrm {sin} (\theta ),&{\text{for }}r\geq R,\end{cases}}\end{aligned}}}$

where ${\displaystyle J_{0}{\text{ and }}J_{1}}$ are the zeroth and first Bessel functions of the first kind, respectively. Further, the value of ${\displaystyle k}$ is such that ${\displaystyle kR=3.8317...}$, the first non-trivial zero of the first Bessel function of the first kind.^{[citation needed]}

Since the seminal work of P. Orlandi,^[2] the Lamb–Chaplygin vortex model has been a popular choice for numerical studies on vortex-environment interactions. The fact that it does not deform make it a prime candidate for consistent flow initialization. A less favorable property is that the second derivative of the flow field at the dipole's edge is not continuous.^[3] Further, it serves a framework for stability analysis on dipolar-vortex structures.^[4]