# Computation of plane wave scattering on semi infinite plane

I have attempted to code up the simple math required to plot the total field set up by an incident plane wave on a semi-infinite flat plate which can be found here.

To summarise:

$$\phi_s(r,\theta ) = e^{ikr}/2 \left( w\left[ e^{i\pi/4}\sqrt{2kr}\sin(\frac{\theta + \Theta}{2}) \right]+ w\left[ e^{i\pi/4}\sqrt{2kr}\sin(\frac{\theta - \Theta}{2}) \right]\right) \enspace,$$

s the scattered potential in polar coordinates according to the article. $\Theta$ denotes the angle of the incoming plane wave and $k$ is the wave number. Finally

$$w(z) = e^{-z^2}(1-\mathrm{erf}(z)) \enspace$$

If you run the code you will see that I do not get the expected wave field as in the pictures.

My question is as follows: Have I made a mistake in my implementation or is the formula given incorrect?

Their Calculation: My code is as follows:

import numpy as np
from scipy.special import erf
import matplotlib.pyplot as plt

"""Plane wave scattering by a plane wave incident on a seminfinite plane
"""

global T
global k
T = np.pi/4.0 # Incident plane wave angle
k = 10        # Wave number

def phi_scattered(r,t):

W = lambda z: np.exp(-z**2)*(1-erf(z))
term1 = W(np.exp(1j*np.pi/4.0)*np.sqrt(2*k*r)*np.sin((t+T)/2.0))
term2 = W(np.exp(1j*np.pi/4.0)*np.sqrt(2*k*r)*np.sin((t-T)/2.0))
res =  np.exp(1j*k*r)/2.0*(term1 + term2)

return res

def phi_incident(r,t):
return np.exp(1j*k*r*np.cos(t-T))

x1 = np.linspace(-50,50,100)
x2 = np.linspace(-50,50,100)
X1, X2 = np.meshgrid(x1,x2)

R = np.sqrt(X1**2+X2**2)
THETA = np.arctan2(X2,X1)

phi = phi_scattered(R,THETA) + phi_incident(R,THETA)
plt.figure()
plt.imshow(np.real(phi), vmin = np.min(np.real(phi)), vmax = np.max(np.real(phi)))
plt.colorbar()
plt.show()

• What did you already do to resolve the discrepancy? Are you just looking for people to debug your code here, or what is your question concretely? – Wolfgang Bangerth Aug 17 '15 at 19:48
• @WolfgangBangerth Hi, well Im just frustrated because as far as I can tell I have written the equations like in the reference. So I am wondering is the referenced equation correct or have I made a blatant mistake in my code? I guess I am looking for a fresh set of eyes as I cant seem to see what could be wrong. Maybe you can suggest what I would do to 'debug' it? I have tried all sorts of things, changing sines to cosines, different implementations of the error function etc. but nothing helps! – Dipole Aug 17 '15 at 19:58
• You can add some background in the question, what it is used for? What are the equations? That said, you should use the same parameters that the document, i.e, $k$, $\Theta$, $x,y$, colormap and visualization technique. If you just plot your scattered field it does not look like the reference. Check it here. – nicoguaro Aug 17 '15 at 21:34
• @nicoguaro Please see my update, I hope this helps! – Dipole Aug 18 '15 at 12:27
• You are not even plotting the same type of graphic and the parameters are not the same. That does not make the calculations right, but at least you can compare apples with apples. Check my code. Furthermore, the problem is the scattered field, so you need to check that term only (again, check my code). – nicoguaro Aug 18 '15 at 13:02

1) Use contour instead of imshow, otherwise your plot is x-flipped.

2) Use the same limits for axis (that is from -20 to 20) as in the paper.

3) As others mentioned, use the same parameters as in the paper (that is k=1).

4) Styling the plot in the same way as in the paper also makes plot comparing easier.

I've fixed these issues, here is the code:

import numpy as np
from scipy.special import erf
import matplotlib.pyplot as plt

"""Plane wave scattering by a plane wave incident on a semi-infinite plane"""

T = np.pi / 4.0  # Incident plane wave angle
k = 1.0  # 10  # Wave number

def phi_scattered(r,t):
W = lambda z: np.exp(-z**2)*(1-erf(z))
term1 = W(np.exp(1j*np.pi/4.0)*np.sqrt(2*k*r)*np.sin((t+T)/2.0))
term2 = W(np.exp(1j*np.pi/4.0)*np.sqrt(2*k*r)*np.sin((t-T)/2.0))
res =  np.exp(1j*k*r)/2.0*(term1 + term2)

return res

def phi_incident(r,t):
return np.exp(1j*k*r*np.cos(t - T))

x1 = np.linspace(-20, 20, 100)
x2 = np.linspace(-20, 20, 100)
X1, X2 = np.meshgrid(x1,x2)

R = np.sqrt(X1**2 + X2**2)
THETA = np.arctan2(X2, X1)

phi = phi_scattered(R, THETA) + phi_incident(R, THETA)

plt.figure()
plt.contourf(x1, x2, np.real(phi), cmap=plt.cm.gist_rainbow)
plt.colorbar()
plt.contour(x1, x2, np.real(phi), colors='k', linestyles='solid')
plt.axis('image')
plt.savefig('scattering.png', dpi=300)
plt.show()


You can see that it's flipped along x-axis.