Deriving the derivative boundary conditions from natural formulation

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  • Thread starter maistral
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Summary:

How to derive the finite difference derivative formulation from the natural boundary formulation?

Main Question or Discussion Point

PS: This is not an assignment, this is more of a brain exercise.

I intend to apply a general derivative boundary condition f(x,y). While I know that the boxed formulation is correct, I have no idea how to acquire the same formulation if I come from the general natural boundary condition formulation. I honestly do not know what am I doing wrong. Can someone check where am I incorrect?
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Answers and Replies

19,541
3,917
Looks OK. So what is the problem?
 
208
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Looks OK. So what is the problem?
The left side and the right side have different results.
 
19,541
3,917
If ##q_x## is the heat flux in the x direction, then ##q_x## is always given by $$q_x=-k\frac{dT}{dx}$$irrespective of whether it is the left- or the right boundary. Of course, the sign of the flux in the x direction can be negative. So, at the fictitious point at the left boundary, you have:
$$T(-\Delta x)=T(+\Delta x)+q_x(0)\Delta x$$and, at the fictitious point at the right boundary, you have:
$$T(L+\Delta x)=T(L-\Delta x)-q_x(L)\Delta x$$
 
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that is a good answer
 

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