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Heat equation/Solution to the 2-D Heat Equation in Cylindrical Coordinates

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Definition

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We start by changing the Laplacian operator in the 2-D heat equation from rectangular to cylindrical coordinates by the following definition:



By changing the coordinate system, we arrive at the following nonhomogeneous PDE for the heat equation:



We choose for the example the Robin boundary conditions and initial conditions as follows:

Solution

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Step 1: Solve Associated Homogeneous Equation

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Separate Variables

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This means that a separation constant can be found that both sides will equal. Let's define it to be This yields:



and multiplying the other side by yields:



After defining another separation constant , it yields:



Multiplying the other side by R yields:



We now have separate differential equations for each variable.

Translate Boundary Conditions

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Solve SLPs

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The SLP for is a singular Bessel type, whose eigenvalues depends on and are non-negative solutions to the following equation:



and the eigenfunction is:



where is the Bessel function of the first kind of order .

Solve Time Equation

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Step 2: Satisfy Initial Condition

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Let's define the solution as an infinite sum:



With the initial condition:



where

The weight function in the inner product in integrals involving the Bessel functions. The Bessel functions are orthogonal relative to the "weighted" scalar product

Step 3: Solve Non-homogeneous Equation

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Solving the non-homogeneous equation involves defining the following functions:





Substitute the new definitions into the non-homogeneous equations:



We will use the following substitutions in our equation above:



We can eliminate the derivatives by substituting:



From the linear independence of , it follows that:



This first-order ODE can be solved with the following integration factor:



Thus, the equation becomes:





We satisfy the initial condition: