- #1
meldraft
- 281
- 2
Hey all,
I am using the Joukowski transform to calculate the electrostatic potential and streamlines around an elliptical cylinder. The flow is caused by two point charges (one positive, one negative). My idea was, of course, to first solve the problem for a circular cylinder, and then transform everything using the Joukowski transform.
I am getting some weird results, so I would be grateful is anyone can point out the mistake in my calculations!
The equation for the complex potential of a point charge located at point z0=a is:
[tex]φ_1=\frac{Λ}{2 \pi}ln(z-a)[/tex]
where Λ is the strength of the source (or sink), and z,a are complex numbers.
Also, the potential to simulate a circular cylinder originating at (0,0), is that of a doublet:
[tex]φ_c=-\frac{μ}{z}[/tex]
where μ is a real constant. Therefore, the potential function for my problem should be:
[tex]Φ=\frac{Λ}{2 \pi}ln(z-a)-\frac{Λ}{2 \pi}ln(z+a)-\frac{μ}{z}[/tex]
The field looks realistic at this point.
Then, I do the Joukowski transform:
[tex]J=z+\frac{λ^2}{z}[/tex]
[tex]λ=\sqrt{a*R-R^2}[/tex]
[tex]R=(a+b)/2[/tex]
where a,b are the 2 semi-axes of the ellipse and R is the radius of the original circular cylinder. I know that this is correct, because i can see the ellipse in the plot. My problem is that the field around it is not what I would expect:
1. The potential lines are no longer always perpendicular to the boundary of the ellipse
2. The conformal map is causing the source and sink to come closer to point (0,0), because of the distortion caused by the growing ellipse.
I have no idea why either of them is happening, as I figured the transform should continue to satisfy the original boundary conditions.
The way that I plot the potential is the following:
contour(real(J),imag(J),-real(f))
so, basically, the original potential corresponds to the transformed mesh. I have also set the axes to always be equal to each other, so what I see is not distorted due to axis scaling.
I have been trying to figure out where the mistake is for a few days now, and I really am stuck. Can anyone spot what I have done wrong?
P.S. I am also attaching two pictures, one for the cylinder, where everything is normal, and one for the problematic transform. You can see how the source and sink have moved away from their origins at (-4,0) and (4,0), and that the potential lines are no longer perpendicular to the boundary of the ellipse.
I am using the Joukowski transform to calculate the electrostatic potential and streamlines around an elliptical cylinder. The flow is caused by two point charges (one positive, one negative). My idea was, of course, to first solve the problem for a circular cylinder, and then transform everything using the Joukowski transform.
I am getting some weird results, so I would be grateful is anyone can point out the mistake in my calculations!
The equation for the complex potential of a point charge located at point z0=a is:
[tex]φ_1=\frac{Λ}{2 \pi}ln(z-a)[/tex]
where Λ is the strength of the source (or sink), and z,a are complex numbers.
Also, the potential to simulate a circular cylinder originating at (0,0), is that of a doublet:
[tex]φ_c=-\frac{μ}{z}[/tex]
where μ is a real constant. Therefore, the potential function for my problem should be:
[tex]Φ=\frac{Λ}{2 \pi}ln(z-a)-\frac{Λ}{2 \pi}ln(z+a)-\frac{μ}{z}[/tex]
The field looks realistic at this point.
Then, I do the Joukowski transform:
[tex]J=z+\frac{λ^2}{z}[/tex]
[tex]λ=\sqrt{a*R-R^2}[/tex]
[tex]R=(a+b)/2[/tex]
where a,b are the 2 semi-axes of the ellipse and R is the radius of the original circular cylinder. I know that this is correct, because i can see the ellipse in the plot. My problem is that the field around it is not what I would expect:
1. The potential lines are no longer always perpendicular to the boundary of the ellipse
2. The conformal map is causing the source and sink to come closer to point (0,0), because of the distortion caused by the growing ellipse.
I have no idea why either of them is happening, as I figured the transform should continue to satisfy the original boundary conditions.
The way that I plot the potential is the following:
contour(real(J),imag(J),-real(f))
so, basically, the original potential corresponds to the transformed mesh. I have also set the axes to always be equal to each other, so what I see is not distorted due to axis scaling.
I have been trying to figure out where the mistake is for a few days now, and I really am stuck. Can anyone spot what I have done wrong?
P.S. I am also attaching two pictures, one for the cylinder, where everything is normal, and one for the problematic transform. You can see how the source and sink have moved away from their origins at (-4,0) and (4,0), and that the potential lines are no longer perpendicular to the boundary of the ellipse.