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Master1022
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- TL;DR Summary
- Question about change of variables for the Diffusion PDE
Hi,
I understand the underlying concept of changing variables in PDEs (so that we can reduce it to a simpler form), however, I am just not completely clear on the mathematics of it so I have a quick question about it.
For example, if we have the transmission line equation [tex] \frac{\partial V}{\partial t} = \frac{L^2}{RC} \frac{\partial ^2 V}{\partial x^2} [/tex]
with initial condition: [itex] V(0,x) = 0 [/itex] and boundary conditions: [itex] V(t,0) = V_0 [/itex] and [itex] V(t,L) = 0 [/itex]. Now we want to change variables to reduce the problem into a simpler form without any free parameters.
The Method:
1) Rescale the length variable so that it ranges over one
I let [itex] \xi = \frac{x}{L} [/itex]
2) Rescale the other independent variable to remove free parameters from the general expression
[tex] \frac{\partial V}{\partial t} = \frac{L^2}{RC} \frac{\partial ^2 V}{\partial x^2} [/tex] [tex] \frac{\partial V}{\partial t} = \frac{L^2}{RC} \frac{\partial ^2 V}{\partial \left( L \xi \right)^2} [/tex] [tex] RC \frac{\partial V}{\partial t} = \frac{\partial ^2 V}{\partial \xi^2} [/tex]
then I let [itex] \tau = \frac{t}{RC} [/itex], therefore [tex] \frac{\partial V}{\partial \tau} = \frac{\partial ^2 V}{\partial \xi^2} [/tex]
So far, I am happy with this. Now this is where I start to have some questions
3) Redefine the independent variable to non-dimensionalize and simplify B/ICs
We want to simplify the boundary condition [itex] V(t, 0) = V_0 [/itex] and we therefore let [itex] U = \frac{V(t, 0)}{V_0} [/itex]. So now that boundary condition is = 1.
However, does making this change not introduce a new free parameter back into the geral expression?
[tex] \frac{\partial \left(U V_0 \right)}{\partial \tau} = \frac{\partial ^2 \left(U V_0 \right)}{\partial \xi^2} [/tex]
The source I am using seems to suggest that step 3) does not change the GE in the way I suggested, but instead changes V to U directly, but I cannot see why.
I see that we are non-dimensionalising it, but we did the same thing with both of the other variables and we subsituted their new values into the GE.
Thank you, any help is appreciated.
I understand the underlying concept of changing variables in PDEs (so that we can reduce it to a simpler form), however, I am just not completely clear on the mathematics of it so I have a quick question about it.
For example, if we have the transmission line equation [tex] \frac{\partial V}{\partial t} = \frac{L^2}{RC} \frac{\partial ^2 V}{\partial x^2} [/tex]
with initial condition: [itex] V(0,x) = 0 [/itex] and boundary conditions: [itex] V(t,0) = V_0 [/itex] and [itex] V(t,L) = 0 [/itex]. Now we want to change variables to reduce the problem into a simpler form without any free parameters.
The Method:
1) Rescale the length variable so that it ranges over one
I let [itex] \xi = \frac{x}{L} [/itex]
2) Rescale the other independent variable to remove free parameters from the general expression
[tex] \frac{\partial V}{\partial t} = \frac{L^2}{RC} \frac{\partial ^2 V}{\partial x^2} [/tex] [tex] \frac{\partial V}{\partial t} = \frac{L^2}{RC} \frac{\partial ^2 V}{\partial \left( L \xi \right)^2} [/tex] [tex] RC \frac{\partial V}{\partial t} = \frac{\partial ^2 V}{\partial \xi^2} [/tex]
then I let [itex] \tau = \frac{t}{RC} [/itex], therefore [tex] \frac{\partial V}{\partial \tau} = \frac{\partial ^2 V}{\partial \xi^2} [/tex]
So far, I am happy with this. Now this is where I start to have some questions
3) Redefine the independent variable to non-dimensionalize and simplify B/ICs
We want to simplify the boundary condition [itex] V(t, 0) = V_0 [/itex] and we therefore let [itex] U = \frac{V(t, 0)}{V_0} [/itex]. So now that boundary condition is = 1.
However, does making this change not introduce a new free parameter back into the geral expression?
[tex] \frac{\partial \left(U V_0 \right)}{\partial \tau} = \frac{\partial ^2 \left(U V_0 \right)}{\partial \xi^2} [/tex]
The source I am using seems to suggest that step 3) does not change the GE in the way I suggested, but instead changes V to U directly, but I cannot see why.
I see that we are non-dimensionalising it, but we did the same thing with both of the other variables and we subsituted their new values into the GE.
Thank you, any help is appreciated.