# Trouble Discretizing a ParametricRegion; Joukowsky Map and Wing Profiles

I'm trying to compue a mesh of full 2D region delimited by the a circle transformed with a Complex map.

{g, Ω} =
Module[{R = 1., a = 0.2, b = 0.3, JMC, JM, z0, r},
JMC = R \[Function] (z \[Function] z + R^2/z);
JM = JMC[R];
z0 = a + I b;
r = Abs[R - z0];
{ParametricPlot[{Re[#], Im[#]} &@
With[{z = z0 + r E^(I θ)}, JM[z]], {θ, 0, 2 π},
PlotRange -> All, Frame -> True],
ParametricRegion[{Re[#], Im[#]} &@
With[{z = z0 + r E^(I θ)}, JM[z]], {{θ, 0,
2 π}}]}
]


But I get some errors with differents methods:

Needs["NDSolveFEM"]
BoundaryDiscretizeRegion[Ω]
ToBoundaryMesh[Ω]


There is any way to accomplish this task or to handle such problematic regions?

You can use this:

NDSolveFEMToElementMesh[
DiscretizeGraphics[g, PlotRange -> All][
"MakeRepresentation"["ElementMesh"]]]["Wireframe"]


Let plot be the output of ParametricPlot.

{poly} = Cases[Normal@plot, _Line, Infinity] /. Line -> Polygon;
BoundaryDiscretizeRegion[poly]


(Since you're discretizing the region anyway, the symbolic parametric description must not be needed.)

@Michael E2 wrote one kind of answer I was expecting. I would eventually like to understand why things behave this way and how to get discretized regions of different "quality". So I wait some time to accept the previous answer.

For those interested, I'll add some background information and a way to discretize a "complement" of the previous region. This can be interesting to represent the region occupied by a fluid around an idealized, simple, parametrically described, wing.

This wing is built with the family (depending on parameter R) of the Joukowsky maps of the complex plane:

JoukowskiMapComplex = R \[Function] (z \[Function] z + R^2/z);


which in polar and cartesian coordinates become:

JoukowskiMapPolar =
R \[Function] ({ρ, θ} \[Function] {(ρ +
R^2/ρ) Cos[θ], (ρ -
R^2/ρ) Sin[θ]});
JoukowskiMapCartesian =
R \[Function] ({x, y} \[Function]
With[{q = R^2/(x^2 + y^2)}, {x + q x, y - q y}]);


For some good choice of the parameters, a circle centered in $X_0 + i Y_0$ and passing through $R + i0$ is mapped to a "wing" and a circle centered in $0+i0$ with radius $\rho$ is mapped to an ellipsis around the wing, with the distance between the two increasing as $\rho$ approaches to $0$.

Manipulate[
Module[{JMPC = JoukowskiMapPolar[R], JMCC = JoukowskiMapCartesian[R],
r = Abs[X0 + I Y0 - R]},
GraphicsRow[{
ParametricPlot[{Through[{Re, Im}[ρ E^(I θ)]],
Through[{Re, Im}[X0 + I Y0 + r E^(I θ)]]}, {θ, 0,
2 π}, Frame -> True, AspectRatio -> Automatic,
AxesStyle -> Dashed],
ParametricPlot[{JMPC[ρ, θ],
JMCC @@ Through[{Re, Im}[
X0 + I Y0 + r E^(I θ)]]}, {θ, 0, 2 π},
Frame -> True, AspectRatio -> Automatic, AxesStyle -> Dashed]
}, ImageSize -> Large]
],

Row[{Control[{{X0, 0.2, "\!$$\*SubscriptBox[\(X$$, $$0$$]\)"}, 0, 1,
0.05, Appearance -> "Labeled"}],
Control[{{Y0, 0.3, "\!$$\*SubscriptBox[\(Y$$, $$0$$]\)"}, -2, 2,
0.05, Appearance -> "Labeled"}]}],
Row[{Control[{{R, 1}, 0, 2, 0.05, Appearance -> "Labeled"}],
Control[{{ρ, 0.05}, 0, 1.5, 0.01, Appearance -> "Labeled"}]}],
ControlPlacement -> Bottom
]


With this in mind the fluid around this wing can be meshed by first meshing the the anular region between the two circles and then transforming the coordinates of the incidents.

Options[JoukowskiMesh] = Options[DiscretizeRegion];
JoukowskiMesh[R_, {X0_, Y0_}, ρ_, opts : OptionsPattern[]] :=
Module[{r, Ω, mesh},
r = Abs[X0 + I Y0 - R];
Ω =
RegionDifference[Disk[{X0, Y0}, r], Disk[{0, 0}, ρ]];
mesh = DiscretizeRegion[Ω, opts];
MeshRegion[JoukowskiMapCartesian[R] @@@ MeshCoordinates[mesh],
Join @@ MeshCells[mesh, All]]
]

mesh = JoukowskiMesh[1, {0.1, 0.05}, 0.05,
MaxCellMeasure -> {1 -> 0.02, 2 -> 0.02}]


As desired, the mesh refines near the wing. Because the Joukowsky map in the complex plane has very interesting properties, the mesh obtained with this method is of high quality, as a close-up can show.

• Do you have 10.0.2? (My employer's IT dept. is very slow to make upgrades available, and I have only 10.0.1.) The region functionality is still under development, and one might expect the situation to improve. Dec 22, 2014 at 1:59
• @MichaelE2 yes, I have 10.0.2, with an academic site licensing. I understand Region are both "first class Citizen in the WL" and "actively developed". Hope they become better documented and supported soon. Dec 22, 2014 at 8:26
• Well, it seems to be a bug to me. Perhaps you should report it to WRI. Dec 22, 2014 at 14:24