Heat flow in semi inifinite wall: Dirichlet problem or separate variables?

In summary, this conversation discusses a problem involving a semi-infinite wall with a stationary heat flow. The problem can be solved in two ways: by treating it as a problem of separate variables using Fourier Series or as a differential geometry problem using conform transformations. The solution using conform transformations involves transforming the image and using polar coordinates to find T(x,y). However, the process of finding A and B using the contour conditions remains unclear.
  • #1
libelec
176
0

Homework Statement



The plane region defined by [tex]\[A = \left\{ {(x,y)/0 < x < 1,0 < y < \infty } \right\}\][/tex] defines the profile of a semi infinite wall, where there's a stationary regime heat flow. The left and right sides are isolated at 15º, and the base is isolated at 0º. Find T(x,y) in each point of the wall.

Homework Equations



Two-dimensional Heat equation: [tex]\[\frac{{{\partial ^2}T}}{{\partial {x^2}}} + \frac{{{\partial ^2}T}}{{\partial {y^2}}} = \frac{{\partial T}}{{\partial t}}\][/tex]

Image

ico65l.jpg


The Attempt at a Solution



This problem got me thinking, because it's done either one of two ways: either I treat it as a problem of separate variables and use Fourier Series, or I treat it as a differential geometry problem (Dirichlet problem, since in stationary regime the heat equation becomes Laplace's equation). I tried thinking of it as a Dirichlet problem.

If I treat it as a differential geometry problem, I have to use conform transformations. First I have to multiply the whole thing by [tex]\[\pi \][/tex], so that then the wall has its sides at x = 0 and x = [tex]\[\pi \][/tex] (the base remains at y = 0, and it remains semi infinite). Then I have to traslate it [tex]\[ - \frac{\pi }{2}\][/tex], so that now the sides are at x = [tex]\[ - \frac{\pi }{2}\][/tex] and x = [tex]\[\frac{\pi }{2}\][/tex].

So far, I transformed [tex]\[x + iy\][/tex] into [tex]\[\left( {\frac{\pi }{2}x - \frac{\pi }{2}} \right) + i\frac{\pi }{2}y\][/tex]. Now, I've only seen this kind of problem in which there are 3 contour conditions being solved by applying the transform sin(z). If I do that, then I transform the strip into Im(z) > 0, now I have transformed [tex]\[x + iy\][/tex] into [tex]\[\sin \left( {\frac{\pi }{2}x - \frac{\pi }{2}} \right)\cosh \left( {\frac{\pi }{2}y} \right) + i\cos \left( {\frac{\pi }{2}x - \frac{\pi }{2}} \right)\sinh \left( {\frac{\pi }{2}y} \right)\][/tex].

The image is then transformed into:

242y9mp.jpg


Now, using polar coordinates, T doesn't depend of the radius, [tex]\rho \[/tex], so [tex]\[{\nabla ^2}T = \frac{1}{\rho }\frac{{{\partial ^2}T}}{{\partial {\varphi ^2}}}\][/tex]. This means that [tex]\[T(x,y) = A\theta + B\][/tex].

But I can't go pass that. I don't know how to find A and B using the countour conditions.

Any ideas?
 
Physics news on Phys.org
  • #2
It compiled awfully! If you don't understand what it says, I repost it:

Homework Statement



The plane region defined by [tex]\[A = \left\{ {(x,y)/0 < x < 1,0 < y < \infty } \right\}\][/tex] defines the profile of a semi infinite wall, where there's a stationary regime heat flow. The left and right sides are isolated at 15º, and the base is isolated at 0º. Find T(x,y) in each point of the wall.

Homework Equations



Two-dimensional Heat equation: [tex]\[\frac{{{\partial ^2}T}}{{\partial {x^2}}} + \frac{{{\partial ^2}T}}{{\partial {y^2}}} = \frac{{\partial T}}{{\partial t}}\][/tex]

Image

ico65l.jpg


The Attempt at a Solution



This problem got me thinking, because it's done either one of two ways: either I treat it as a problem of separate variables and use Fourier Series, or I treat it as a differential geometry problem (Dirichlet problem, since in stationary regime the heat equation becomes Laplace's equation). I tried thinking of it as a Dirichlet problem.

If I treat it as a differential geometry problem, I have to use conform transformations. First I have to multiply the whole thing by [tex]\[\pi \][/tex], so that then the wall has its sides at x = 0 and x = [tex]\[\pi \][/tex] (the base remains at y = 0, and it remains semi infinite). Then I have to traslate it [tex]\[ - \frac{\pi }{2}\][/tex], so that now the sides are at x = [tex]\[ - \frac{\pi }{2}\][/tex] and x = [tex]\[\frac{\pi }{2}\][/tex].

So far, I transformed [tex]\[x + iy\][/tex] into [tex]\[\left( {\frac{\pi }{2}x - \frac{\pi }{2}} \right) + i\frac{\pi }{2}y\][/tex]. Now, I've only seen this kind of problem in which there are 3 contour conditions being solved by applying the transform sin(z). If I do that, then I transform the strip into Im(z) > 0, now I have transformed [tex]\[x + iy\][/tex] into [tex]\[\sin \left( {\frac{\pi }{2}x - \frac{\pi }{2}} \right)\cosh \left( {\frac{\pi }{2}y} \right) + i\cos \left( {\frac{\pi }{2}x - \frac{\pi }{2}} \right)\sinh \left( {\frac{\pi }{2}y} \right)\][/tex].

The image is then transformed into:

242y9mp.jpg


Now, using polar coordinates, T doesn't depend of [tex]\rho [/tex], so then [tex]\[{\nabla ^2}T = \frac{1}{\rho }\frac{{{\partial ^2}T}}{{\partial {\varphi ^2}}}\][/tex]. This means that [tex]\[T(x,y) = A\theta + B\][/tex].

But I can't go pass that. I don't know how to find A and B using the countour conditions.

Any ideas?
 

FAQ: Heat flow in semi inifinite wall: Dirichlet problem or separate variables?

What is the Dirichlet problem in heat flow in a semi-infinite wall?

The Dirichlet problem in heat flow refers to finding a solution for the temperature distribution in a semi-infinite wall, where the boundary conditions are specified at the surface of the wall and at infinity. In other words, the problem involves determining the temperature at each point in the wall based on the known surface temperature and the behavior of the temperature at infinity.

What are the separate variables in heat flow in a semi-infinite wall?

The separate variables in heat flow in a semi-infinite wall refer to the separation of the temperature distribution into two independent variables: one that depends on the position within the wall and one that depends on time. This allows for the solution to be expressed as a product of these two variables, making it easier to solve the problem.

How does the Dirichlet problem differ from the Neumann problem in heat flow?

The Dirichlet and Neumann problems in heat flow are both boundary value problems, but they differ in the types of boundary conditions they involve. The Dirichlet problem specifies the temperature at the boundary of the wall, while the Neumann problem specifies the heat flux at the boundary. In other words, the Dirichlet problem deals with prescribing the temperature, while the Neumann problem deals with prescribing the heat flow.

What are some real-life applications of heat flow in semi-infinite walls?

Heat flow in semi-infinite walls is a common phenomenon in many real-life applications. Some examples include heat transfer in building walls, thermal insulation materials, and heat dissipation in electronic devices. Understanding how heat flows in semi-infinite walls is crucial in designing efficient and safe structures or products.

What are some methods for solving the Dirichlet problem in heat flow?

There are several methods for solving the Dirichlet problem in heat flow, including separation of variables, integral transform methods, and numerical methods such as finite difference or finite element methods. The choice of method depends on the complexity of the problem and the desired accuracy of the solution.

Similar threads

Replies
3
Views
1K
Replies
3
Views
1K
Replies
16
Views
1K
Replies
34
Views
2K
Replies
5
Views
983
Replies
4
Views
753
Back
Top