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Help with PDE in circular annulus(poisson eq)

by smoger
Tags: circular annulus, pde, poisson eq
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Dec28-11, 10:25 PM
P: 3
what is the general solution of the poisson equation :

2A/∂r2 + 1/r ∂A/∂r + 1/r22A/∂θ2 = f(r,θ)

the function f(r,θ) is :
f(r,θ)=1/r (Ʃ Xncos(nθ) + Ynsin(nθ))

where the boundary is :

I(a<r<b, 0<θ<2pi)

the boundary condition is the netural boundary on (r=a) expressed as :

∂A/∂r=0 (r=a)

How can i find the A(r,θ)? i can not find any books related to this.
Most of them only consider laplace equation where f(r,θ)=0
someone help me.
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Dec29-11, 12:29 PM
P: 263
Use the Green function for Neumann boundary conditions.
Dec29-11, 01:28 PM
Sci Advisor
HW Helper
P: 7,105
Using the idea of separating the variables, you should be able to see from the PDE that

A(r,θ) = Cn(r) cos(nθ) + Sn(r) sin(nθ)

is a solution for the right hand side terms (1/r)(Xn cos(nθ) + Yn sin(nθ))

That will give you ordinary differential equations to solve for Cn(r) and Sn(r).

Feb28-12, 01:06 PM
P: 1
Help with PDE in circular annulus(poisson eq)

A(r,θ)=∫∫f(ρ,θ') g(r,θ,ρ,θ') dρ dθ' + cte' from eq. 5.0.19 Ref.1.
g(r,θ,ρ,θ') = -ln{[r^2 + ρ^2 - 2rρ cos(θ-θ')] [b^2 + (rρ/b)^2 - 2rρ cos(θ-θ')]}/4∏ + r^2/(4∏b^2)
from third line of page 68 of Ref.2.



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