T-I-1

Analytical functions: conformal map and applications to hydrodynamics

This homework deals with the application of conformal maps to the study of two-dimensional hydrodynamics. A conformal map is a geometrical transformation which preserves all (oriented) crossing angles between lines. In dimension $d\ge 3$ a conformal map is necessarily composed from the following limited number of transformations: translations, rotations, homothetic transformation and special conformal transformation (which is the composition of a reflection and an inversion in a sphere). However in two dimensions, $d=2$, the space of conformal mappings is much larger and one can show that, given an open set $\mathrm{\Omega }\in \mathbb{ℂ}$, any holomorphic function $f:\mathrm{\Omega }\to \mathbb{ℂ}$ such that $\mathrm{\$}{f}^{\prime }(z)\ne 0\mathrm{\$}$, $\mathrm{\forall }z\in \mathrm{\Omega }$ defines a conformal map from $\mathrm{\Omega }$ to $f(\mathrm{\Omega })$. The aim of this HW is to exploit this property to study some hydrodynamic flows in two spatial dimensions.

Joukovski's transformation

The Joukovski's transformation is defined by the following application

$J:\begin{array}{ccc}\mathbb{ℂ}\setminus \{0\}& \to & \mathbb{ℂ}\\ z& \mapsto & z+\frac{1}{z}\end{array}$

• Compute $J^{\prime }(z)$ and deduce from it the maximal ensemble on which $J$ is a conformal map. Show that $J$ is always surjective. Under which condition on the set $\mathrm{\Omega }$ the application $J$ on $\mathrm{\Omega }$ is surjective ? Give some examples of such (maximal) ensembles.

• Give the image by $J$ of the following sub-sets: (a) the half-line passing through the origin $O$ and making an angle $\alpha$ with the $x$-axis, (b) the circle centered at the origin of radius $R$ (analyse in particular the case $R=1$). What is the image, by $J$, of the outside of the unit circle $|z|>1$.

Hint: it might be useful to use polar coordinates, writing $z=r{e}^{i\theta }$.

• Get a better idea of this Joukowski's transformation using the following code in Mathematica:

Jouk[z_] := z + 1/z Jouk[1 - R Sin[\[Alpha]] + R Cos[u] + I (R Cos[\[Alpha]] + R Sin[u])]; ParametricPlot[{Re[%], Im[%]} /. {R -> 1.15, \[Alpha] -> 1.3}, {u, 0, 2 \[Pi]}, PlotRange -> {{-3, 3}, {-1, 1}}, AspectRatio -> 1/3]

• Study the conformal map $J$ in the vicinity of $z=1$: we consider a "smooth" curve $\gamma$ passing through $z=1$, with a well defined tangent. Show that the image of $\gamma$ exhibits a cusp in $J(1)$. In this purpose, we parametrize this curve $\gamma$ by $z(t)$ with $z(0)=1$ and $z^{\prime }(0)\ne 0$. Write then the Taylor expansion of $z$ in $t=0$ up to first order and the expansion of $J$ close to $1$ up to second order.

Harmonic functions

We recall that a function $\phi :\mathrm{\Omega }\to \mathbb{ℝ}$ or $\mathbb{ℂ}$ ($\mathrm{\Omega }$ being an open set of $\mathbb{ℂ}$) is called a "harmonic function" if satisfies the Laplace equation

$\mathrm{\Delta }\phi =0\text{où}\mathrm{\Delta }\phi \equiv \frac{{\partial }^2\phi }{\partial x^2}+\frac{{\partial }^2\phi }{\partial y^2}$

in all point $z=x+iy\in \mathrm{\Omega }$. Similarly to conformal maps, harmonic functions in two dimensions, are closely related to holomorphic functions.

• Let us consider $g:\mathrm{\Omega }\to \mathbb{ℂ}$ a holomorphic function. Show that $g,\phi =\mathrm{R}\mathrm{e}g,\psi =\mathrm{I}\mathrm{m}g$ are harmonic functions.

\bigskip \noindent B. Soit $\varphi$ une fonction harmonique à valeurs \emph{réelles} définie sur un ouvert $\Omega \subset \mathbb{C}$ $\emph{simplement connexe}$. Montrez qu'il existe $g: \Omega \to \mathbb{C}$ holomorphe telle que $\varphi = \mathrm{Re}\, g$. La fonction $\psi = \mathrm{Im}\, g$ est appelée \emph{conjuguée

 harmonique} de $\varphi$. (Indication: le gradient de $\psi$ est

connu.) Quelle pathologie peut-on avoir si $\Omega$ n'est pas simplement connexe?

\bigskip \noindent C. Interprétation géométrique: montrez que les lignes de courant de $\vec{\nabla} \varphi$ sont les lignes de niveau de $\psi$.