Non-Dimensionalisation of Two Differential Equations: Working Out and Struggles

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In summary, the speaker is looking for help with non-dimensionalizing two differential equations. They have successfully done one equation but are struggling with the other. They provide their working out and ask for assistance with the left hand side of the equation. They also suggest using a substitution to convert the equation to a desired form.
  • #1
mt91
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I've got two differential equations I need to non-dimensionalise

I've managed to do the \[ dc/dT=α- μc \] with the following working out:

By letting:
\[ N=N0n \]
\[ C=C0c \]
\[ t=t0T \]

1596461095620.png


However I'm struggling with the first equation.

1596461128387.png


I'm up to here, it's just the left hand side of the equation I'm struggling to work out. Any help would be great, cheers
 
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  • #2
Looking at $\frac{dN}{dt}= \frac{rN}{h+ rN}- bNC$ and comparing that to the desired $\frac{dr}{dt}= \frac{n}{n+1}- \beta nc$ my first thought would be to divide the numerator and denominator of $\frac{rN}{h+ rN}$ by h. That gives $\frac{\frac{rN}{h}}{1+ \frac{rN}{h}}$. So we should let $n= \frac{rN}{h}$. With that substitution, the first equation becomes $\frac{h}{r}\frac{dn}{dt}= \frac{n}{1+ n}- \left(\frac{br}{h}\right)nC$.
 

FAQ: Non-Dimensionalisation of Two Differential Equations: Working Out and Struggles

What is non-dimensionalisation and why is it important in solving differential equations?

Non-dimensionalisation is the process of removing units from a mathematical equation to make it dimensionless. This is important in solving differential equations because it simplifies the equations and allows for easier analysis and comparison between different systems.

How do you non-dimensionalise two differential equations?

To non-dimensionalise two differential equations, you need to choose appropriate non-dimensional variables and substitute them into the equations. Then, you can use scaling factors to remove the units and make the equations dimensionless.

What are some common struggles when non-dimensionalising two differential equations?

Some common struggles when non-dimensionalising two differential equations include choosing the right non-dimensional variables, determining the appropriate scaling factors, and ensuring that the resulting equations are still accurate and meaningful.

Can you provide an example of non-dimensionalising two differential equations?

Yes, for example, let's say we have two differential equations: dx/dt = ax and dy/dt = by, where a and b are constants with units of inverse time. To non-dimensionalise these equations, we can choose the non-dimensional variables x' = x/x0 and y' = y/y0, where x0 and y0 are characteristic values of x and y. Then, we can use the scaling factors t0 = 1/a and y0 = 1/b to make the equations dimensionless. The resulting equations would be dx'/dt' = x' and dy'/dt' = y', which are now simpler and easier to analyze.

What are the benefits of non-dimensionalising two differential equations?

The benefits of non-dimensionalising two differential equations include simplifying the equations, making them easier to analyze and compare, and providing insight into the underlying physical processes of the system. It also allows for the creation of dimensionless parameters that can be used to predict the behavior of the system under different conditions.

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