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# Es272 ch7

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### Es272 ch7

1. 1. Part 7: Ordinary Differential Equations – – – – – Runge-Kutta Methods Euler’s Method Heun’s Method Midpoint method Systems of Equations
2. 2. Runge-Kutta Methods  We want to solve an ordinary differential equation of the form: y ' dy f ( x, y ) dx  The general approach of the solution is in the form of: (new value)=(old value) + (slope) x (step size)  All of the one-step methods under this general category are called RungeKutta methods. > Euler’s method > Heun’s method > Midpoint method  The differences in these methods are in the way (slope) is defined. Slope= yi xi xi 1 1 yi h
3. 3. Euler’s Method  Simplest way of approximating the slope is in the form of first derivative. Apply forward scheme finite difference for the starting point: y yi ' yi 1 h predicted f ( xi , yi ) yi Then, yi 1 yi f ( xi , y i ) h Euler’s method formula where f(xi, yi) is calculated from the initial condition. 1 true yi xi xi 1  That is the slope at the beginning is approximated as the slope over the entire interval.  As in the forward scheme finite difference method, error for this approximation is O(h).
4. 4. EX: Use Euler’s method to solve dy 2x 3 12 x 2 20 x 8 .5 dx from x=0 to x=2 with a step size of 0.5. The initial condition at x=0 is y=1. Differential equation: f ( x , y ) Using Euler’s formula: y ( 0 .5 ) y (0) f ( 0 ,1) 0 . 5 2x 3 12 x 5 . 25 2 20 x t 8 .5 63 . 1 % Second step: y (1) y ( 0 .5 ) f ( 0 . 5,5 . 25 ) 0 . 5 5 . 875 t 95 . 8 % Other steps: y (1 . 5 ) 5 . 1250 y ( 2 .0 ) 4 . 5000 t t 131 % 125 . 0 % Error can be reduced by using a smaller step size (h). Considering that Euler’s method has an error O(h), halving the step size will halve the error.
5. 5. Heun’s Method  It is an improvement of Euler’s method.  Improve the slope of the interval by averaging derivatives at the starting and end points. Slope=f(xi+1’ yi+10) Slope=[f(xi’ yi)+f(xi+1’ yi+10)]/2 Slope=f(xi’ yi) xi Predictor xi 1 xi Corrector xi 1
6. 6. Approximate the slope at the starting point using Euler’s method y ' f ( x, y ) Extrapolate linearly to find y at the end point: 0 yi yi 1 f ( xi , y i ) h standard Euler method stops here. This intermediate solution (predictor) is used to calculate the slope at the end point: ' 0 yi f ( xi 1 , yi 1 ) 1 Calculate average of the two slopes at the starting and end points: ' ' yi ' y yi 1 2 Use this slope to re-calculate y value at the end point (corrector): ' yi 1 yi yh
7. 7. 0 Predictor: Corrector: yi yi 1 yi 1 yi f ( xi , y i ) h f ( xi , y i ) 0 f ( xi 1 , y i 1 ) h Heun’s method 2  Corrector equation involves yi+1 on both sides, which means that it can be applied iteratively for the end point (i.e., older approximation can be used iteratively to calculate a new approximation).  The iteration does not necessarily converge to the true solution but approaches a solution with a finite truncation error.  The method is accurate to O(h2).(compare to O(h) accuracy of Euler’s method).
8. 8. EX: Use Heun’s method to numerically solve the following ODE y ' 4e 0 .8 x 0 .5 y from x=0 to x=4 with a step size of h=1. Initial condition is (0,2). Calculate the slope at the starting point (x=0): ' y0 4e 0 0 .5 ( 2 ) 3 Apply Euler’s (predictor) formula to evaluate y10 0 y1 2 3 (1) 5 t 25 . 3 % Calculate slope at the end point (x=1) ' 0 y1 f ( x1 , y 1 ) 4e 0 . 8 (1 ) 0 .5 (5 ) Calculate average of the two slopes: ' y 3 6 . 401164 2 4 . 701082 6 . 402164 Euler’s method
9. 9. Use the corrector formula to calculate the new prediction at x=1: y1 2 4 . 701082 (1) 6 . 701082 t 8 . 18 % Above is based on a single iteration at y1. Apply another iteration to refine the predictor: y1 2 [3 4e 0 . 8 (1 ) 0 . 5 ( 6 . 701082 )] 6 . 275811 t 1 . 31 % 6 . 382129 t 3 . 03 % 2 Further correction: y1 2 [3 4e 0 . 8 (1 ) Results for all the points from x=0 to x=4: (15 iterations at each point) 0 . 5 ( 6 . 275811 )] 2 x y (true) y (Heun) 0 2.0000000 2.0000000 0.00 1 6.1946314 6.360866 2.68 2 14.843922 15.302237 3.09 3 33.677172 34.743276 3.17 4 75.338963 77.735096 3.18 t(%)
10. 10. Midpoint Method  It is another improvement of Euler’s method.  Approximate y at the midpoint of the interval, and use it for the average slope of the entire interval. Slope=f(xi+1/2’ yi+1/2) Slope=f(xi+1/2’ yi+1/2) xi xi yi 1/ 2 yi f ( xi , yi ) Predictor xi 1/ 2 h 2 yi 1 yi f ( xi 1/ 2 Corrector xi , yi 1 1/ 2 )h
11. 11.  No iteration can be made for the corrector.  Just as the centered difference is superior to forward or backward difference schemes, midpoint is superior to Euler’s method in the same manner. Euler’s method Midpoint method O(h) O(h2) truncation error Summary:  All the three methods, Euler’s, Heun’s (without iteration), and midpoint methods, mentioned so far are under the general category of one-step approaches called Runge-Kutta methods.  Heun’s and midpoint methods are generally superior to Euler’s method, even though they require somewhat more calculations.
12. 12. Systems of Equations  Problems requiring the solution of a system of ODE’s are very common in engineering applications. dy 1 dx dy 2 dx f 1 ( x , y 1 , y 2 ,..., y n ) f 2 ( x , y 1 , y 2 ,..., y n ) .... dy n dx n- initial conditions need to be known at the starting value of x f n ( x , y 1 , y 2 ,..., y n ) Euler’s Method:  Simply apply Euler’s method for each equation at each step.  Strcictly apply the one-step Euler’s formula for every equation before proceeding to the next-step (i.e., do not update any variable during a single step).
13. 13. EX: Solve the following system of ODEs using Euler’s method. dy 1 dy 2 0 .5 y1 dx dx 4 0 .3 y 2 0 .1 y1 Assume that for x=0, y1=4, and y2=6. Integrate to x=2 with a step size h=0.5. Apply Euler’s method: y1 ( 0 . 5 ) 4 y 2 ( 0 .5 ) 6 0 .5 ( 4 ) 0 .5 4 0 .3( 6 ) 3 0 . 1( 4 ) 0 . 5 6 .9 Other points are calculated in the same fashion. Results to x=2 are below: x y1 y2 0 4 6 0.5 3 6.9 1.0 2.25 7.715 1.5 1.6875 8.44525 2.0 1.265625 9.094087