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Hi everyone!
We're given a three dimensional cone with perimeters d0 at the top and d1 at the bottom and a substance that diffuses through the cone with diffusion constant D from top to bottom. The concentration of the substance is held constant at the top plane of the cone. The z axis is chosen to go through the center of the cone from top to bottom.
I have already done some math.
We start with the Diffusion Equation
[itex]\frac{\partial \varphi}{\partial t}=D ( \frac{\partial^{2}}{\partial x^{2}}+\frac{\partial^{2}}{\partial y^{2}}+\frac{\partial^{2}}{\partial z^{2}})\varphi(x,y,z,t)[/itex]
Because of the symmetry of the cone
[itex]\frac{\partial^{2}\varphi(x,y,z,t)}{\partial x^{2}}=\frac{\partial^{2}\varphi(x,y,z,t)}{\partial y^{2}}[/itex]
So the PDE reduces to
[itex]\frac{\partial \varphi}{\partial t}=D ( \frac{2\partial^{2}}{\partial x^{2}}+\frac{\partial^{2}}{\partial z^{2}})\varphi(x,y,z,t)[/itex]
Now one can use separation of variables:
[itex]\varphi(x,y,z,t)=\alpha(x)\beta(z)\gamma(t)[/itex]
So the PDE becomes:
[itex]\frac{1}{\gamma(t)}\frac{\partial \gamma}{\partial t}=D(\frac{1}{\alpha(x)}\frac{2\partial^{2}\alpha(x)}{\partial x^{2}}+\frac{1}{\beta(z)}\frac{\partial^{2}\beta(z)}{\partial z^{2}})[/itex]
Now all three terms must be constants. So:
[itex]\frac{\partial \gamma(t)}{\partial t}=\lambda \gamma(t)[/itex]
[itex]\frac{\partial^{2} \alpha(x)}{\partial x^2}=\lambda_{x} \alpha(x)[/itex]
[itex]\frac{\partial^{2} \beta(z)}{\partial z^2}=\lambda_{z} \beta(z)[/itex]
And
[itex] \lambda=2\lambda_{x}+\lambda_{z}[/itex]
The time-dependant eigenvalue equation is easy to solve.
But I am not sure how to solve the second and third equation.
I tried to solve the second equation by using
[itex]\alpha(x)=A exp(-kx)+B exp(kx) [/itex]
and by applying boundary conditions:
[itex] \alpha(x=-r)=\alpha(x=r)=0 [/itex]
But then I get A=-B=0
Somebody please help me out
Homework Statement
We're given a three dimensional cone with perimeters d0 at the top and d1 at the bottom and a substance that diffuses through the cone with diffusion constant D from top to bottom. The concentration of the substance is held constant at the top plane of the cone. The z axis is chosen to go through the center of the cone from top to bottom.
Homework Equations
I have already done some math.
We start with the Diffusion Equation
[itex]\frac{\partial \varphi}{\partial t}=D ( \frac{\partial^{2}}{\partial x^{2}}+\frac{\partial^{2}}{\partial y^{2}}+\frac{\partial^{2}}{\partial z^{2}})\varphi(x,y,z,t)[/itex]
Because of the symmetry of the cone
[itex]\frac{\partial^{2}\varphi(x,y,z,t)}{\partial x^{2}}=\frac{\partial^{2}\varphi(x,y,z,t)}{\partial y^{2}}[/itex]
So the PDE reduces to
[itex]\frac{\partial \varphi}{\partial t}=D ( \frac{2\partial^{2}}{\partial x^{2}}+\frac{\partial^{2}}{\partial z^{2}})\varphi(x,y,z,t)[/itex]
The Attempt at a Solution
Now one can use separation of variables:
[itex]\varphi(x,y,z,t)=\alpha(x)\beta(z)\gamma(t)[/itex]
So the PDE becomes:
[itex]\frac{1}{\gamma(t)}\frac{\partial \gamma}{\partial t}=D(\frac{1}{\alpha(x)}\frac{2\partial^{2}\alpha(x)}{\partial x^{2}}+\frac{1}{\beta(z)}\frac{\partial^{2}\beta(z)}{\partial z^{2}})[/itex]
Now all three terms must be constants. So:
[itex]\frac{\partial \gamma(t)}{\partial t}=\lambda \gamma(t)[/itex]
[itex]\frac{\partial^{2} \alpha(x)}{\partial x^2}=\lambda_{x} \alpha(x)[/itex]
[itex]\frac{\partial^{2} \beta(z)}{\partial z^2}=\lambda_{z} \beta(z)[/itex]
And
[itex] \lambda=2\lambda_{x}+\lambda_{z}[/itex]
The time-dependant eigenvalue equation is easy to solve.
But I am not sure how to solve the second and third equation.
I tried to solve the second equation by using
[itex]\alpha(x)=A exp(-kx)+B exp(kx) [/itex]
and by applying boundary conditions:
[itex] \alpha(x=-r)=\alpha(x=r)=0 [/itex]
But then I get A=-B=0
Somebody please help me out