# Scaling parameters in central difference solution

• robby991
In summary, the individual is seeking to scale their Matlab code to run in a different space domain, from 0 to 1 instead of 0 to 1E-6. They propose to introduce a new variable, x2 = x/L, and are wondering how to scale the other parameters/variables in their code such as Length, Ds, Vmax, and Km. They have attempted to scale these parameters using a new variable L, and dividing or multiplying by Length as appropriate. They are seeking confirmation on whether this approach is correct.

#### robby991

Hi, I developed Matlab code to solve the diffusion equation using the central difference equation, with an added term at the end. The equation is the following:

Code:
 dS/dt=Ds*d^2S/dx^2-(Vmax*S/Km+S)

In my code, the length of the space domain is very small, 1E-6. I would like to scale my code to run in the space domain between x = 0 and 1, rather than x = 0 to 1E-6. I propose to introduce a new variable, x2 = x/L. My question is what parameters/variables in my code need to be scaled also. My code is:

Code:
clear all;

numx = 10;                      %number of grid points in space
numt = 1000;                    %number of time steps to be iterated over
tmax = .0045;
Length = 1E-6;                  %length of grid
Ds = .019E-9;                   %requirement Ds(dt)/dx^2 < .5, cm^2/sec
Vmax = 275E-6;                  %mol/cm^2sec
Km = 3E-3;                      %mol/cm^3

x = linspace(0,Length,numx);    %vector of x values, to be used for plotting
t = linspace(0,tmax,numt)';     %vector of t values, to be used for plotting
S = zeros(numt,numx);           %initialize everything to zero

dx = x(2)-x(1);                 %Define grid spacing in time
dt = t(2)-t(1);                 %Define grid spacing in time

%specify initial conditions%

t(1) = 0;      %1st t position = 0

S(1,:) = cos(x*pi/(2*Length));
S(:,numx) = 0;

S_exact = cos(x*pi/(2*Length))*exp(-(pi*sqrt(Ds)/(2*Length))^2*tmax);

%iterate central difference equation%

for j=1:numt-1

%2nd Derivative Central Difference Iteration%

for i=2:numx-1
S(j+1,i) = S(j,i) + (dt/dx^2)*Ds*(S(j,i+1) - 2*S(j,i) + S(j,i-1))-((Vmax*dt*S(j,i))/(Km+S(j,i)));
end

S(j+1,1)=S(j,1)+dt*Ds*2*(S(j,2)-S(j,1))./dx.^2-((Vmax*dt*S(j,i))/(Km+S(j,i)));      %Neumann Boundary Condition

end

plot(x,S(numt,:));
hold on
plot(x,S_exact,'r*')

error = max(S(numt,:)-S_exact)

To scale, the domain would now be:

Code:
x2=x/Length;

and now, dx = x2(2)-x2(1);

How do I change me code to accommodate this? How do my other variables change/scale? i.e. Km, Ds, Vmax etc. Thank you.

This is exactly the same as working in different units, for example changing your lengths from km to mm (as an example of units which happen to be a factor of 10^6 different)

If you change the values of ALL the constants in your code which have physical dimensions, you don't need to change anything else.

I have one last question regarding this. Do I have to scale the matrix vectors in the time domain also? I scaled all the parameters related to the space domain (x), but I also have the time vectors (y). If so, do I divide by the same constant I used to scale the space domain?

I ran into a little difficulty and was wondering if my scaling of parameters was correct in my code. Say I have a grid of length "Length", and instead of going from 0 to Length I want to scale from 0 to 1. In addition I have the following constants in my calculations:

Code:
Length = 1E-4;                  %cm
Ds = .019E-9;                   % cm^2/sec
Vmax = 275E-6;                  %mol/cm^2sec
Km = 3E-3;                      %mol/cm^3

To scale from 0 to 1, I would simply make a new variable L where:
Code:
L = Length/Length.

Next, to scale the other variables I will do the following:

Code:
D = Ds/Length^2
V = Vmax*Length^2
K = Km*Length^3

Is this correct? I would appreciate any input on the matter. Thank you.

I would recommend the following steps to scale your code:

1. Scale the space domain: As you have proposed, introduce a new variable x2 = x/L to scale the space domain from 0 to 1. This will require you to change the definition of dx as dx = x2(2)-x2(1).

2. Scale the diffusion coefficient (Ds): Since the diffusion coefficient is a function of space (Ds = Ds(x)), it will also need to be scaled accordingly. You can use the same scaling factor L to scale Ds, i.e. Ds2 = Ds(x2) = Ds(x/L).

3. Scale the reaction parameters (Vmax and Km): The reaction parameters, Vmax and Km, are not dependent on the space domain and thus do not need to be scaled. They can remain the same as before.

4. Scale the initial conditions: As the initial conditions are specified in terms of x, they will also need to be scaled using the same factor L. So, the initial condition for S should be S(1,:) = cos(x2*pi/(2*Length)).

5. Scale the boundary conditions: Since you have a Neumann boundary condition at x = 0, it will not be affected by the scaling. However, for the boundary condition at x = 1, you will need to use the scaled value of x, i.e. S(j+1,numx) = S(j,numx)+dt*Ds*2*(S(j,numx-1)-S(j,numx))./dx.^2-((Vmax*dt*S(j,numx))/(Km+S(j,numx))).

6. Scale the exact solution: The exact solution should also be scaled using the same factor L, i.e. S_exact = cos(x2*pi/(2*Length))*exp(-(pi*sqrt(Ds2)/(2*Length))^2*tmax).

By following these steps, your code should be able to accommodate the scaled space domain. However, it is always recommended to test your code for different values of L to ensure its accuracy.

## 1. What are scaling parameters in central difference solution?

Scaling parameters in central difference solution are numerical values that are used to adjust the magnitude of different variables in a mathematical model. These parameters are used to ensure that the model produces accurate and realistic results.

## 2. Why are scaling parameters important in central difference solution?

Scaling parameters are important in central difference solution because they help to maintain the stability and accuracy of the numerical algorithm. Without proper scaling, the model may produce inaccurate or unstable results.

## 3. How are scaling parameters determined in central difference solution?

The determination of scaling parameters in central difference solution is based on the characteristics of the specific problem being solved. These parameters can be chosen manually or calculated using mathematical techniques such as sensitivity analysis.

## 4. Can scaling parameters be applied to all types of central difference solutions?

Yes, scaling parameters can be applied to all types of central difference solutions. However, the specific parameters chosen may differ depending on the problem being solved and the numerical algorithm being used.

## 5. What are the consequences of not using scaling parameters in central difference solution?

Not using scaling parameters in central difference solution can lead to inaccurate and unstable results. This can result in incorrect conclusions and predictions, and may require additional computational effort to correct the errors.

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