- #1
ajptech
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Homework Statement
This is a research problem that I imagine is very similar to a homework problem. I am a PhD student in biology, and I lack the mathematical background needed to make sense of the topic.
I would like to find the solution to the fractional differential equation that describes the behavior of a viscoelastic solid. The behavior can be described by a spring in series with a fractional-order dashpot (a "spring-pot").
I have the input strain ([itex]\varepsilon[/itex] below) defined for each of my time points, t. I calculate the fractional derivative of the strain ([itex]D^{\alpha}\varepsilon[/itex] below) using a numerical method according to the Grünwald-Letnikov definition.
I want to solve for the stress ([itex]\sigma[/itex] below).
Homework Equations
The differential equation that describes the relationshp between strain ([itex]\varepsilon[/itex]) and stress ([itex]\sigma[/itex]) is:
[tex]D^{\alpha}\varepsilon=\frac{\sigma}{\eta}+\frac{1}{E}D^{\alpha}\sigma[/tex]
Where
- [itex]\sigma[/itex] is the viscoelastic stress, which is what I want to solve for.
- [itex]\alpha[/itex] is the order of the differentiation (I have this value).
- [itex]\varepsilon[/itex] is the viscoelastic strain (I have these value).
- [itex]\eta[/itex] is the viscosity of the spring-pot (I have this value).
- [itex]E[/itex] is the spring constant (I have this value).
The Attempt at a Solution
My naive approach was to rearrange the equation to yield:
[tex]D^{\alpha}\sigma=E\left(D^{\alpha}\varepsilon-\frac{\sigma}{\eta}\right)[/tex]
Then, for each time point, I calculate the change in the stress by:
[tex]D^{\alpha}\sigma\left(t\right)=E\left(D^{\alpha} \varepsilon \left(t\right)-\frac{\sigma\left(t-1\right)}{\eta}\right)[/tex]
And integrate with:
[tex]\sigma\left(t\right)=\sigma\left(t-1\right)+D^{\alpha}\sigma\left(t\right)[/tex]
I suspect there may be two problems with this approach, but I am not certain:
- First, the calculation of the fractional derivative uses [itex]\sigma\left(t-1\right)[/itex] instead of [itex]\sigma\left(t\right)[/itex]. Is this a valid approach for a numerical approximation, or is this a total no-no? If this is invalid, how can I approach the problem differently?
- Second, does it make sense to integrate the fractional derivative [itex]D^{\alpha}\sigma[/itex] to yield the resulting stress [itex]\sigma[/itex]? I am not sure if this is a correct way to interpret a fractional derivative.
I would really appreciate some assistance! If it would make more sense to solve the problem analytically, that would suit me just fine... I just don't have the slightest clue how to do this, unfortunately.