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Next: Schwarz-Christoffel Theorem Up: Two-Dimensional Potential Flow Previous: Method of Images


Conformal Maps

Let $ \zeta=\xi+{\rm i}\,\eta$ and $ z=x +{\rm i}\,y$ , where $ \xi$ , $ \eta$ , $ x$ , and $ y$ are real. Suppose that $ \zeta=f(z)$ , where $ f$ is a well-behaved (i.e., single-valued, non-singular, and differentiable) function. We can think of $ \zeta=f(z)$ as a map from the complex $ z$ -plane to the complex $ \zeta $ -plane. In other words, every point $ (x$ , $ y)$ in the complex $ z$ -plane maps to a corresponding point $ (\xi$ , $ \eta)$ in the complex $ \zeta $ -plane. Moreover, if $ f(z)$ is indeed a well-behaved function then this mapping is unique, and also has a unique inverse. Suppose that the point $ z=z_0$ in the $ z$ -plane maps to the point $ \zeta=\zeta_0$ in the $ \zeta $ -plane. Let us investigate how neighboring points map. We have

$\displaystyle \zeta_0+d\zeta'$ $\displaystyle =f(z_0+dz'),$ (6.53)
$\displaystyle \zeta_0+d\zeta''$ $\displaystyle =f(z_0+dz'').$ (6.54)

In other words, the points $ z_0+dz'$ and $ z_0+dz''$ in the complex $ z$ -plane map to the points $ \zeta_0+d\zeta'$ and $ \zeta_0+d\zeta''$ in the complex $ \zeta $ -plane, respectively. If $ \vert dz'\vert$ , $ \vert dz''\vert\ll 1$ then

$\displaystyle d\zeta'$ $\displaystyle = f'(z_0)\,dz',$ (6.55)
$\displaystyle d\zeta''&= f'(z_0)\,dz'',$ (6.56)

where $ f'(z)= df/dz$ . Hence,

$\displaystyle \frac{d\zeta''}{d\zeta'} = \frac{dz''}{dz'}.$ (6.57)

Thus, it follows that

$\displaystyle \frac{\vert d\zeta''\vert}{\vert d\zeta'\vert} = \frac{\vert dz''\vert}{\vert dz'\vert},$ (6.58)

and

$\displaystyle {\rm arg}(d\zeta'')-{\rm arg}(d\zeta') = {\rm arg}(dz'')-{\rm arg}(dz').$ (6.59)

We can think of $ dz'$ and $ dz''$ as infinitesimal vectors connecting neighboring points in the complex $ z$ -plane to the point $ z=z_0$ . Likewise, $ d\zeta'$ and $ d\zeta''$ are infinitesimal vectors connecting the corresponding points in the complex $ \zeta $ -plane. It is clear, from the previous two equations, that, in the vicinity of $ z=z_0$ , the mapping from the complex $ z$ -plane to the complex $ \zeta $ -plane is such that the lengths of $ dz'$ and $ dz''$ expand or contract by the same factor, and the angle subtended between these two vectors remains the same. (See Figure 6.7.) This type of mapping is termed conformal.

Figure 6.7: A conformal map.
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Suppose that $ F(\zeta) = \phi(\xi,\eta)+{\rm i}\,\psi(\xi,\eta)$ is a well-behaved function of the complex variable $ \zeta $ . It follows that $ \nabla^{\,2}\phi=\nabla^{\,2}\psi=\nabla\phi\cdot\nabla\psi=0$ . Hence, the functions $ \phi(\xi,\eta)$ and $ \psi(\xi,\eta)$ can be interpreted as the velocity potential and stream function, respectively, of some two-dimensional, incompressible, irrotational flow pattern, where $ \xi$ and $ \eta$ are Cartesian coordinates. However, if $ \zeta=f(z)$ , where $ f(z)$ is well-behaved, then $ F(\zeta)=F[f(z)] = G(z) = \widetilde{\phi}(x,y) + {\rm i}\,\widetilde{\psi}(x,y)$ , where $ G(z)$ is also well-behaved. It follows that $ \nabla^{\,2}\widetilde{\phi}=\nabla^{\,2}\widetilde{\psi}=\nabla\widetilde{\phi}\cdot\nabla\widetilde{\psi}=0$ . In other words, the functions $ \widetilde{\phi}(x,y)$ and $ \widetilde{\psi}(x,y)$ can be interpreted as the velocity potential and stream function, respectively, of some new, two-dimensional, incompressible, irrotational flow pattern, where $ x$ and $ y$ are Cartesian coordinates. In other words, we can use a conformal map to convert a given two-dimensional, incompressible, irrotational flow pattern into another, quite different, pattern. Incidentally, a conformal map converts a line source into a line source of the same strength, and a vortex filament into a vortex filament of the same intensity. (See Exercise 12.)

As an example, consider the conformal map

$\displaystyle \zeta = {\rm i}\,{\rm e}^{\,\pi\,z/a}.$ (6.60)

Writing $ \zeta=r\,{\rm e}^{\,{\rm i}\,\theta}$ , it is easily demonstrated that $ x=a\,\ln r/\pi$ and $ y=a\,(\theta/\pi-1/2)$ . Hence, the positive $ \xi$ -axis ($ \theta=0$ ) maps to the line $ y=-a/2$ , the negative $ \xi$ -axis ( $ \theta=\pi$ ) maps to the line $ y=a/2$ , and the region $ \eta>0$ ( $ 0\leq \theta\leq \pi$ ) maps to the region $ -a/2<y<a/2$ . Moreover, the points $ \zeta = (0$ , $ \pm 1)$ map to the points $ z=a\,(0$ , $ -1/2\pm1/2)$ . (See Figure 6.8.) As we saw in Section 6.6, in the region $ \eta>0$ , the velocity potential

$\displaystyle F(\zeta) = {\rm i}\,\frac{\mit\Gamma}{2\pi}\,\ln\left(\frac{\zeta-{\rm i}}{\zeta+{\rm i}}\right)$ (6.61)

corresponds to the flow pattern generated by a vortex filament of intensity $ {\mit\Gamma}$ , located at the point $ \zeta = (0$ , $ 1)$ , in the presence of a rigid plane at $ \eta=0$ . Hence,

$\displaystyle G(z)=F({\rm i}\,{\rm e}^{\,\pi\,z/a}) = {\rm i}\,\frac{\mit\Gamma}{2\pi} \ln\,\tanh\left(\frac{\pi\,z}{2\,a}\right),$ (6.62)

corresponds to the flow pattern generated by a vortex filament of intensity $ {\mit\Gamma}$ , located at the origin, in the presence of two rigid planes at $ y=\pm a/2$ . This follows because the line $ \eta=0$ is mapped to the lines $ y=\pm a/2$ , and the point $ \zeta = (0$ , $ 1)$ is mapped to the origin. Moreover, if the line $ \eta=0$ is a streamline in the $ \eta$ -plane then the lines $ y=\pm a/2$ are also streamlines in the $ z$ -plane. Thus, these lines could all correspond to rigid boundaries. The stream function associated with the previous complex velocity potential,

$\displaystyle \psi(x,y)= \frac{{\mit\Gamma}}{\pi}\,\ln\left[\frac{\cosh(\pi\,x\,a)-\cos(\pi\,y/a) }{\cosh(\pi\,x/a)+\cos(\pi\,y/a)}\right],$ (6.63)

is shown in Figure 6.9.

Figure: The conformal map $ \zeta={\rm i}\,{\rm e}^{\,\pi\,z/a}$ .
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Figure: Stream lines of the two-dimensional flow pattern due to a vortex filament at the origin in the presence of two rigid planes at $ y=\pm a/2$ .
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As a second example, consider the map

$\displaystyle \zeta = z^{\,2}.$ (6.64)

This maps the positive $ \xi$ -axis to the positive $ x$ -axis, the negative $ \xi$ -axis to the positive $ y$ -axis, the region $ \eta>0$ to the region $ x>0$ , $ y>0$ , and the point $ \zeta = (0$ , $ 2\,a^{\,2})$ to the point $ z=(a$ , $ a)$ . As we saw in Section 6.6, in the region $ \eta>0$ , the velocity potential

$\displaystyle F(\zeta) = \frac{Q}{2\pi}\,\ln(\zeta^{\,2}+4\,a^{\,4}),$ (6.65)

corresponds to the flow pattern generated by a line source of strength $ Q$ , located at the point $ \zeta = (0$ , $ 2\,a^{\,2})$ , in the presence of a rigid plane at $ \eta=0$ . Thus, the complex velocity potential

$\displaystyle G(z)=F(z^{\,2}) = \frac{Q}{2\pi}\,\ln(z^{\,4}+4\,a^{\,4}),$ (6.66)

corresponds to the flow pattern generated by a line source of strength $ Q$ , located at the point $ z=(a$ , $ a)$ , in the presence of two orthogonal rigid planes at $ y=0$ and $ x=0$ . The stream function associated with the previous complex potential,

$\displaystyle \psi(x,y) = -\frac{Q}{2\pi}\,\tan^{-1}\left[\frac{4\,x\,y\,(x^{\,2}-y^{\,2})}{x^{\,4}-6\,x^{\,2}\,y^{\,2}+y^{\,4}+4\,a^{\,4}}\right],$ (6.67)

is shown in Figure 6.10.

Figure: Stream lines of the two-dimensional flow pattern due to a line source at $ (a$ , $ a)$ in the presence of two rigid planes at $ x=0$ and $ y=0$ .
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As a final example, consider the map

$\displaystyle z = \zeta+ \frac{l^{\,2}}{\zeta},$ (6.68)

where $ l$ is real and positive. Writing $ \zeta=r\,{\rm e}^{\,{\rm i}\,\theta}$ , we find that

$\displaystyle x$ $\displaystyle = 2\,l\,\cosh[\ln(r/l)]\,\cos\theta = (r+l^{\,2}/r)\,\cos\theta,$ (6.69)
$\displaystyle y$ $\displaystyle =2\,l\,\sinh[\ln(r/l)]\,\sin\theta = (r-l^{\,2}/r)\,\sin\theta.$ (6.70)

Thus, the map converts the circle $ \xi^{\,2}+\eta^{\,2}=c^{\,2}$ in the $ \zeta $ -plane, where $ c>l$ , into the ellipse

$\displaystyle \left(\frac{x}{a}\right)^2+\left(\frac{y}{b}\right)^2 = 1$ (6.71)

in the $ z$ -plane, where

$\displaystyle a$ $\displaystyle = 2\,l\,\cosh[\ln(c/l)] = c + l^{\,2}/c,$ (6.72)
$\displaystyle b$ $\displaystyle = 2\,l\,\sinh[\ln(c/l)] = c - l^{\,2}/c.$ (6.73)

Note that the center of the ellipse lies at the origin, and its major and minor axes run parallel to the $ x$ - and the $ y$ -axes, respectively. As we saw in Section 6.4, in the $ \zeta $ -plane, the complex velocity potential

$\displaystyle F = - V_0\left(\zeta+\frac{c^{\,2}}{\zeta}\right),$ (6.74)

represents uniform flow of unperturbed speed $ V_0$ , running parallel to the $ \xi$ -axis, around a circular cylinder of radius $ c$ , centered on the origin. Thus, assuming that $ c>l$ , in the $ z$ -plane, the potential represents uniform flow of unperturbed speed $ V_0$ , running parallel to the $ x$ -axis [which follows because at large $ \vert z\vert$ the map (6.68) reduces to $ z=\zeta$ , and so the flow at large distances from the origin is the same in the complex $ z$ - and $ \zeta $ -planes], around an elliptical cylinder of major radius $ a$ , aligned along the $ x$ -axis, and minor radius $ b$ , aligned along the $ y$ -axis. Note that $ l=(a^{\,2}-b^{\,2})^{1/2}/2$ and $ c=(a+b)/2$ . The corresponding stream function in the $ z$ -plane is

$\displaystyle \psi(x,y) = - V_0\left(r -\frac{c^{\,2}}{r}\right)\sin\theta,$ (6.75)

where

$\displaystyle r$ $\displaystyle = l\,\exp(\cosh^{-1}p),$ (6.76)
$\displaystyle \theta$ $\displaystyle = \tan^{-1}\left(\frac{y}{x}\,\frac{p}{[p^{\,2}-1]^{1/2}}\right),$ (6.77)
$\displaystyle p$ $\displaystyle =\left[\frac{x^{\,2}/l^{\,2}+y^{\,2}/l^{\,2}+4+\left([x^{\,2}/l^{\,2}+y^{\,2}/l^{\,2}+4]^2-16\,x^{\,2}/l^{\,2}\right)^{1/2}}{8}\right]^{1/2}.$ (6.78)

Figure 6.11 shows the streamlines of the flow pattern calculated for $ c=1.5\,l$ .

Figure: Stream lines of the two-dimensional flow pattern due to uniform flow parallel to the $ x$ -axis around an elliptical cylinder.
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Next: Schwarz-Christoffel Theorem Up: Two-Dimensional Potential Flow Previous: Method of Images
Richard Fitzpatrick 2016-01-22