Use the product method to find the solution of the following Laplace equation in a cylinder subject to the given boundary conditions. Assume the solution u(r, z) is radially symmetric. Hint: The solution of xy" + y' – a²xy = 0 which is bounded at x = 0 is y(x) = Io(ax) where I, is the modified Bessel function of order 0. %3D 1 Upp +U, + Uzz = 0, 0

Advanced Engineering Mathematics
10th Edition
ISBN:9780470458365
Author:Erwin Kreyszig
Publisher:Erwin Kreyszig
Chapter2: Second-order Linear Odes
Section: Chapter Questions
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**Transcription and Explanation for Educational Use**

---

**Problem Statement:**

Use the product method to find the solution of the following Laplace equation in a cylinder, subject to the given boundary conditions. Assume the solution \( u(r,z) \) is radially symmetric.

**Hint:** The solution of \( xy'' + y' - \alpha^2 xy = 0 \) which is bounded at \( x = 0 \) is \( y(x) = I_0(\alpha x) \) where \( I_0 \) is the modified Bessel function of order 0.

**Equation and Boundary Conditions:**

\[
u_{rr} + \frac{1}{r}u_r + u_{zz} = 0, \quad 0 < r < 1, \, 0 < z < 2
\]

\[
u(r,0) = 0, \quad u(r,2) = 0, \quad 0 < r < 1
\]

\[
u(1,z) = z(2-z), \quad 0 < z < 2
\]

---

**Explanation:**

This problem involves utilizing the method of separation of variables to solve a Laplace equation in cylindrical coordinates. The radial symmetry implies simplifying the Laplace equation in polar coordinates. Understanding this requires familiarity with differential equations and boundary value problems, particularly those involving Bessel functions, which are common solutions to problems with cylindrical symmetry.

**Key Concepts:**

- **Laplace Equation:** A second-order partial differential equation often used to describe phenomena in electrostatics, heat flow, and fluid dynamics.
  
- **Radial Symmetry:** Simplifies the problem by reducing dependencies on angular coordinates, focusing only on radial and axial components.
  
- **Bessel Functions:** Special functions that frequently appear in solutions to differential equations with cylindrical or spherical symmetry.

- **Boundary Conditions:** These are constraints that the solution must satisfy, reflecting the physical setup, such as \( u(r,0) = 0 \) indicating that the function is zero along one boundary.

**Application:**

Through the product method, seek a solution of the form \( u(r,z) = R(r)Z(z) \) and use the hint involving the Bessel function \( I_0 \), which aids in solving the radial part of the equation. The boundary conditions help determine the form and coefficients of the solution.
Transcribed Image Text:**Transcription and Explanation for Educational Use** --- **Problem Statement:** Use the product method to find the solution of the following Laplace equation in a cylinder, subject to the given boundary conditions. Assume the solution \( u(r,z) \) is radially symmetric. **Hint:** The solution of \( xy'' + y' - \alpha^2 xy = 0 \) which is bounded at \( x = 0 \) is \( y(x) = I_0(\alpha x) \) where \( I_0 \) is the modified Bessel function of order 0. **Equation and Boundary Conditions:** \[ u_{rr} + \frac{1}{r}u_r + u_{zz} = 0, \quad 0 < r < 1, \, 0 < z < 2 \] \[ u(r,0) = 0, \quad u(r,2) = 0, \quad 0 < r < 1 \] \[ u(1,z) = z(2-z), \quad 0 < z < 2 \] --- **Explanation:** This problem involves utilizing the method of separation of variables to solve a Laplace equation in cylindrical coordinates. The radial symmetry implies simplifying the Laplace equation in polar coordinates. Understanding this requires familiarity with differential equations and boundary value problems, particularly those involving Bessel functions, which are common solutions to problems with cylindrical symmetry. **Key Concepts:** - **Laplace Equation:** A second-order partial differential equation often used to describe phenomena in electrostatics, heat flow, and fluid dynamics. - **Radial Symmetry:** Simplifies the problem by reducing dependencies on angular coordinates, focusing only on radial and axial components. - **Bessel Functions:** Special functions that frequently appear in solutions to differential equations with cylindrical or spherical symmetry. - **Boundary Conditions:** These are constraints that the solution must satisfy, reflecting the physical setup, such as \( u(r,0) = 0 \) indicating that the function is zero along one boundary. **Application:** Through the product method, seek a solution of the form \( u(r,z) = R(r)Z(z) \) and use the hint involving the Bessel function \( I_0 \), which aids in solving the radial part of the equation. The boundary conditions help determine the form and coefficients of the solution.
Expert Solution
Strategy

As per the question, we will assume : u(r,z) = f(r)g(z)

Then we will separate the r-dependent part from the z-dependent part, and create two different ODEs.

Next we will solve those ODEs for f(r) g(z) and apply boundary conditions to obtain : f(r)g(z)

Then applying superposition theorem we get the complete solution u(r,z)

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