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USING THE CENTRAL DIFFERENCE METHOD: IN central difference method numerical stencil will be (i-2) (i-1) (i) (i+1) (i+2) for fourth order approximation at point (i) we have to take information from both side right& left and the distance between them will be ∆x. our hypothesis…
Arun Reddy
updated on 01 Jan 2022
USING THE CENTRAL DIFFERENCE METHOD:
IN central difference method numerical stencil will be
(i-2) (i-1) (i) (i+1) (i+2)
for fourth order approximation at point (i) we have to take information from both side right& left and the distance between them will be ∆x.
our hypothesis equation will be :
∂²u/∂x² = af(i-2)+bf(i-1)+cf(i)+df(i+1)+ef(i+2)...............(1)
to find the value of (a,b,c,d,e) we are gone a use taylor table series method.
(a)f(i-2)= (a)f(i)-(a)f'(i)(2∆x)/1! +(a)f''(i)(2∆x)^2/2! - (a)f'''(i)(2∆x)^3/3! +(a)f''''(i)(2∆x)^4/4! - (a)f'''''(i)(2∆x)^5/5! + (a)f''''''(i)(2∆x)^6/6!+.............
(b)f(i-1)= (b)f(i)-(b)f'(i)(∆x)/1! + (b)f''(i)(∆x)^2/2! - (b)f'''(i)(∆x)^3/3! + (b)f''''(i)(∆x)^4/4! - (b)f'''''(i)(∆x)^5/5! + (b)f''''''(i)(∆x)^6/6!+ ..............
(c)f(i)=(c)f(i)
(d)f(i+1)=(d)f(i)+(d)f'(i)(∆x)/1! - (d)f''(i)(∆x)^2/2! + (d)f'''(i)(∆x)^3/3! - (d)f''''(i)(∆x)^4/4! + (d)f'''''(i)(∆x)^5/5! - (d)f''''''(i)(∆x)^6/6!+..........
(e)f(i+2)= (e)f(i)+(e)f'(i)(2∆x)/1! - (e)f''(i)(2∆x)^2/2! + (e)f'''(i)(2∆x)^3/3! - (e)f''''(i)(2∆x)^4/4! + (e)f'''''(i)(2∆x)^5/5! - (e)f''''''(i)(2∆x)^6/6!+........
arranging above terms in taylor tyabel:
f(i) | ∆xf(i) | ∆x^2f''(i) | ∆xf^3'''(i) | ∆x^4f''''(i) | ∆x^5f'''''(i) | ∆x^6f''''''(i) | |
(a)f(i-2) | a | -2a | 4a/2 | -8a/6 | 16a/24 | -32a/120 | 64a/240 |
(b)f(i-1) |
b | -b | b/2 | -b/6 | b/24 | -b/120 | b/240 |
(c)f(i) | c | 0 | 0 | 0 | 0 | 0 | 0 |
(d)f(i+1) | d | d | d/2 | d/6 | d/24 | d/120 | d/240 |
(e)f(i+2) | e | 2e | 4e/2 | 8e/6 | 16e/24 | -32e/120 | 64e/240 |
sum of above all | 0 | 0 | 1 | 0 | 0 | ? | ? |
from above this we can write in matrix form AX=B
[1 1 1 1 1 ; -2 -1 0 1 2 ; 2 1/2 0 1/2 2 ; -8/6 -1/6 0 1/6 8/6 ; 16/24 1/24 0 1/24 16/24] [a b c d e ]=[0 0 1 0 0 ]
after solving above matrix the values we get
a=-0.083333
b=1.333333
c=-2.500000
d=1.333333
e=-0.83333
dividing both the side by equation (1) by (∆x^2) we get
∂²u/∂x² = af(i-2)+bf(i-1)+cf(i)+df(i+1)+ef(i+2) / (∆x^2) %we can put the above vlues(a b c d e) in it
after this to get the second order deravative the coeficient of f''(i) has to be 1 so as to get 1 we have to ignore others or set it as 0.
here we can also see that a is equal to e and b is equal to d we can aslso deravie that and solve the equation and get to the final equation.
USING THE SKEWED RIGHT SIDED DIFFERENCE:
In this skewed right hand side difference method we have to define the five points on the right of (i).
the numerical stencil will be
(i) (i+1) (i+2) (i+3) (i+4) (i+5)
the hypothesis eqquation will be
∂²u/∂x² = af(i)+bf(i+1)+cf(i+2)+df(i+3)+ef(i+4)+gf(i+5)
writing taylor series expansion for each term
af(i)=af(i)
(b)f(i+1)= (b)f(i)+(b)f'(i)(∆x)/1! + (b)f''(i)(∆x)^2/2! + (b)f'''(i)(∆x)^3/3! + (b)f''''(i)(∆x)^4/4! + (b)f'''''(i)(∆x)^5/5! + (b)f''''''(i)(∆x)^6/6!+.........
(c)f(i+2)=(c)f(i)+(c)f'(i)(2∆x)/1! + (c)f''(i)(2∆x)^2/2! + (c)f'''(i)(2∆x)^3/3! + (c)f''''(i)(2∆x)^4/4! + (c)f'''''(i)(2∆x)^5/5! + (c)f''''''(i)(2∆x)^6/6!+.......
(d)f(i+3)=(d)f(i)+(d)f'(i)(3∆x)/1! + (d)f''(i)(3∆x)^2/2! + (d)f'''(i)(3∆x)^3/3! + (d)f''''(i)(3∆x)^4/4! + (d)f'''''(i)(3∆x)^5/5! + (d)f''''''(i)(3∆x)^6/6!+..........
(e)f(i+4)= (e)f(i)+(e)f'(i)(4∆x)/1! + (e)f''(i)(4∆x)^2/2! + (e)f'''(i)(4∆x)^3/3! + (e)f''''(i)(4∆x)^4/4! + (e)f'''''(i)(4∆x)^5/5! + (e)f''''''(i)(4∆x)^6/6!+........
(g)f(i+5)= (g)f(i)+(g)f'(i)(5∆x)/1! + (g)f''(i)(5∆x)^2/2! + (g)f'''(i)(5∆x)^3/3! - (g)f''''(i)(5∆x)^4/4! + (g)f'''''(i)(5∆x)^5/5! + (g)f''''''(i)(5∆x)^6/6!+........
arranging taylor series expansion in taylor term:
f(i) | ∆xf(i) | ∆x^2f''(i) | ∆x^3f'''(i) | ∆x^4f''''(i) | ∆x^5f'''''(i) | ∆x^6f''''''(i) | |
af(i) | a | 0 | 0 | 0 | 0 | 0 | 0 |
(b)f(i+1) | b | b | b/2 | b/6 | b/24 | b/120 | b/720 |
(c)f(i+2) | c | 2c | 4c/2 | 8c/6 | 16c/24 | 32c/20 | 64c/720 |
(d)f(i+3) | d | 3d | 9d/2 | 27d/6 | 81d/24 | 243d/120 | 729d/720 |
(e)f(i+4) | e | 4e | 16e/2 | 64e/6 | 256e/24 | 1024e/120 | 4096e/720 |
(g)f(i+5) | g | 5g | 25g/2 | 125g/6 | 625g/24 | 3125g/120 | 15625g/720 |
sum of above all | 0 | 0 | 1 | 0 | 0 | 0 | ? |
from above this we can write in matrix form AX=B
[1 1 1 1 1 1 ; 0 1 2 3 4 5 ; 0 1/2 2 9/2 8 25/2 ; 0 1/6 8/6 27/6 64/6 125/6 ; 0 1/24 16/24 81/24 256/24 1024/120 ; 0 1/120 32/120 243/120 1024/120 3125/120 ] [a b c d e g] = [ 0 0 1 0 0 0]
after solving the matrix the value we get:
a=3.75000
b=-12.83333
c= 17.83333
d=-13.00000
e=5.08333
g=-0.83333
dividing both the side by equation (1) by (∆x^2) we get
∂²u/∂x² = af(i)+bf(i+1)+cf(i+2)+df(i+3)+ef(i+4)+gf(i+5) %we can put the above vlues(a b c d e g) in it
putting the above all values we can get the equation.
USING SKEWED LEFT SIDED DIFFERENCE:
In this skewed left hand side difference method we have to define the five points on the left of (i).
the numerical stencil will be:
(i-5) (i-4) (i-3) (i-2) (i-1) (i)
the hypothesis eqquation will be
∂²u/∂x² = af(i-5)+bf(i-4)+cf(i-3)+df(i-2)+ef(i-1)+gf(i)
writing taylor series expansion for each term
(a)f(i-5)= (a)f(i)-(a)f'(i)(5∆x)/1! + (a)f''(i)(5∆x)^2/2! - (a)f'''(i)(5∆x)^3/3! + (a)f''''(i)(5∆x)^4/4! - (a)f'''''(i)(5∆x)^5/5! + (a)f''''''(i)(5∆x)^6/6!+.........
(b)f(i-4)=(b)f(i)-(b)f'(i)(4∆x)/1! + (b)f''(i)(4∆x)^2/2! - (b)f'''(i)(4∆x)^3/3! + (b)f''''(i)(4∆x)^4/4! - (b)f'''''(i)(4∆x)^5/5! + (b)f''''''(i)(4∆x)^6/6!+.......
(c)f(i-3)=(c)f(i)-(c)f'(i)(3∆x)/1! + (c)f''(i)(3∆x)^2/2! - (c)f'''(i)(3∆x)^3/3! + (c)f''''(i)(3∆x)^4/4! - (c)f'''''(i)(3∆x)^5/5! + (c)f''''''(i)(3∆x)^6/6!+..........
(d)f(i-2)= (d)f(i)-(d)f'(i)(2∆x)/1! + (d)f''(i)(2∆x)^2/2! - (d)f'''(i)(2∆x)^3/3! + (d)f''''(i)(2∆x)^4/4! - (d)f'''''(i)(2∆x)^5/5! + (d)f''''''(i)(2∆x)^6/6!+........
(e)f(i-1)= (e)f(i)-(e)f'(i)(∆x)/1! + (e)f''(i)(∆x)^2/2! - (e)f'''(i)(∆x)^3/3! + (e)f''''(i)(∆x)^4/4! - (e)f'''''(i)(∆x)^5/5! + (e)f''''''(i)(∆x)^6/6!+........
(g)f(i)=(g)f(i)
arranging taylor series expansion in taylor term:
f(i) | ∆xf(i) | ∆x^2f''(i) | ∆x^3f'''(i) | ∆x^4f''''(i) | ∆x^5f'''''(i) | ∆x^6f''''''(i) | |
af(i-5) | a | -5a | 25/a | -125a/6 | 625a/24 | -3125a/120 | 15625a/720 |
(b)f(i-4) | b | -4b | 8b | -64b/6 | 256b/24 | -1024b/120 | 4096b/720 |
(c)f(i-3) | c | -3c | 9c/2 | -27c/6 | 81c/24 | -243c/120 | 729c/120 |
(d)f(i-2) | d | -2d | 2d | -8d/6 | 16d/24 | 32d/120 | 64d/720 |
(e)f(i-1) | e | -e | e/2 | -e/6 | e/24 | -e/120 | e/720 |
(g)f(i) | g | 0 | 0 | 0 | 0 | 0 | 0 |
sum of above all | 0 | 0 | 1 | 0 | 0 | 0 | ? |
from above this we can write in matrix form AX=B
[1 1 1 1 1 1 ; -5 -4 -3 -2 -1 0 ; 25/2 8/2 9/2 2 1/2 0 ; -125/6 -64/6 -27/6 -8/6 -1/6 0 ; 625/24 256/24 81/24 16/24 1/24 0 ; -3125/120 -1024/120 -243/120 -32/120 -1/120 0 ] [ a b c d e g]=[ 0 0 1 0 0 0]
solving the above matrix we get the values
a= -0.83333
b= 5.08333
c=-13.00000
d=17.83333
e=-12.83333
g=3.75000
dividing both the side by equation (1) by (∆x^2) we get
∂²u/∂x² = af(i-5)+bf(i-4)+cf(i-3)+df(i-2)+ef(i-1)+gf(i) %we can put the above vlues(a b c d e g) in it
putting the above all values we can get the equation.
% function for central difference error
% creating the fuction for 4order approximation
function difference_central_order =central_difference(x,dx)
%analytical function f(x)= exp(x)*cos(x);
% f''(x)=-2*exp(x)*sin(x);
analytical_deravative= -2*exp(x)*sin(x);
% values of coefficients from taylor table
a=-0.083333;
b=1.333333;
c=-2.500000;
d=1.333333;
e=-0.83333;
%central differencing
%((a*f(x-2*dx))+(b*f(x-dx))+(c*f(x))+(d*f(x+dx))+(e*f(x+2*dx)))/dx^2;
central_differencing_fourth_order=((a*exp(x-2*dx)*cos(x-2*dx))+(b*exp(x-dx)*cos(x-dx))+(c*exp(x)*cos(x))+(d*exp(x+dx)*cos(x+dx))+(e*exp(x+2*dx)*cos(x+2*dx)))/dx^2;
difference_central_order= abs(central_differencing_fourth_order-analytical_deravative)
end
% function for skewed right side
% creating 4 order approximation using 2 order deravative by skewed right side difference
function error_skewed_right_side=skewed_right_side(x,dx)
%analytical function f(x)= exp(x)*cos(x);
% f''(x)=-2*exp(x)*sin(x);
analytical_deravative= -2*exp(x)*sin(x);
% values of coefficients from taylor table
a=3.75000;
b=-12.83333;
c= 17.83333;
d=-13.00000;
e=5.08333;
g=-0.83333;
skewed_right_side_fourth_order=((a*exp(x)*cos(x))+(b*exp(x+dx)*cos(x+dx))+(c*exp(x+2*dx)*cos(x+2*dx))+(d*exp(x+3*dx)*cos(x+3*dx))+(e*exp(x+4*dx)*cos(x+4*dx))+(g*exp(x+5*dx)*cos(x+5*dx)))/dx^2;
error_skewed_right_side=abs(skewed_right_side_fourth_order-analytical_deravative)
end
%function for skewed left side
% creating 4 order approximation using 2 order deravative by skewed left side difference
function error_skewed_left_sided= skewed_left_side(x,dx)
%analytical function f(x)= exp(x)*cos(x);
% f''(x)=-2*exp(x)*sin(x);
analytical_deravative= -2*exp(x)*sin(x);
% values of coefficients from taylor table
a= -0.83333;
b= 5.08333;
c=-13.00000;
d=17.83333;
e=-12.83333;
g=3.75000;
skewed_left_side_fourth_order=((a*exp(x-5*dx)*cos(x-5*dx))+(b*exp(x-4*dx)*cos(x-4*dx))+(c*exp(x-3*dx)*cos(x-3*dx))+(d*exp(x-2*dx)*cos(x-2*dx))+(e*exp(x-dx)*cos(x-dx))+(g*exp(x)*cos(x)))/dx^2;
error_skewed_left_sided=abs(skewed_left_side_fourth_order-analytical_deravative);
end
%programing
clear all
clc
close all
x=pi/3;
dx=linspace(pi/4,pi/4000,30);
for i=1:length(dx)
central_difference_error (i) =central_difference(x,dx(i));
skewed_right_side_error (i) =skewed_right_side(x,dx(i));
skewed_left_side_error (i) =skewed_left_side(x,dx(i));
end
% plotting graph of dx vs error
figure(1)
plot(dx,central_difference_error,'LineWidth',2)
hold on
plot(dx,skewed_right_side_error,"Color",'r','LineWidth',2)
hold on
grid on
plot(dx,skewed_left_side_error,"Color",'g','LineWidth',2)
xlabel('dx')
ylabel('error')
legend('central difference error','skewed right sided','skewed left side')
figure(2)
loglog(dx,central_difference_error,'LineWidth',2)
hold on
loglog(dx,skewed_right_side_error,"Color",'r','LineWidth',2)
hold on
grid on
loglog(dx,skewed_left_side_error,"Color",'g','LineWidth',2)
xlabel('dx')
ylabel('error')
legend('central difference error','skewed right sided','skewed left side')
%result graph
here we can easily notice central difference approximation produce more accurate solution.
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