# The Duffing Oscillator

The previous blog post described the motion of a quartic oscillator: a particle moving in the potential $V(x) = \frac{1}{4}x^4 - \frac{1}{2}x^2$. In this case, the motion was always periodic (since the particle's energy is conserved).

If the equation of motion includes a friction term and a periodic driving force: $$\ddot{x} = -\frac{\mathrm{d}V}{\mathrm{d}x} - \delta \dot{x} + \gamma \cos(\omega t)$$ where $\gamma$, $\delta$ and $\omega$ are positive constants, the dynamics can become chaotic: this is an example of a Duffing oscillator.

The code generating this animation is available on my github page as a Jupyter Notebook, and given below as a standalone Python program which generates a movie. As written, you will need to have installed the Seaborn library in addition to NumPy, SciPy and Matplotlib. You will also need ffmpeg installed on your system to write the mp4 file.

The main change from the code for the quartic oscillator, apart from the new deriv function representing the above differential equation, is that the Poincaré section is plotted for a relatively long period of time in advance, and takes the form of a strange attractor, indicative of chaotic behaviour.

import numpy as np
from scipy.integrate import odeint, quad
from scipy.optimize import brentq
import matplotlib.pyplot as plt
from matplotlib import animation, rc
import seaborn as sbs
rc('font', **{'family': 'serif', 'serif': ['Computer Modern'], 'size': 20})
rc('text', usetex=True)
rc('animation', html='html5')

# The potential and its first derivative, as callables.
V = lambda x: 0.5 * x**2 * (0.5 * x**2 - 1)
dVdx = lambda x: x**3 - x

# The potential energy function on a grid of x-points.
xgrid = np.linspace(-1.5, 1.5, 100)
Vgrid = V(xgrid)

plt.plot(xgrid, Vgrid)
plt.xlabel('$x$')
plt.ylabel('$V(x)$')

def deriv(X, t, gamma, delta, omega):
"""Return the derivatives dx/dt and d2x/dt2."""

x, xdot = X
xdotdot = -dVdx(x) -delta * xdot + gamma * np.cos(omega*t)
return xdot, xdotdot

def solve_duffing(tmax, dt_per_period, t_trans, x0, v0, gamma, delta, omega):
"""Solve the Duffing equation for parameters gamma, delta, omega.

Find the numerical solution to the Duffing equation using a suitable
time grid: tmax is the maximum time (s) to integrate to; t_trans is
the initial time period of transient behaviour until the solution
settles down (if it does) to some kind of periodic motion (these data
points are dropped) and dt_per_period is the number of time samples
(of duration dt) to include per period of the driving motion (frequency
omega).

Returns the time grid, t (after t_trans), position, x, and velocity,
xdot, dt, and step, the number of array points per period of the driving
motion.

"""
# Time point spacings and the time grid

period = 2*np.pi/omega
dt = 2*np.pi/omega / dt_per_period
step = int(period / dt)
t = np.arange(0, tmax, dt)
# Initial conditions: x, xdot
X0 = [x0, v0]
X = odeint(deriv, X0, t, args=(gamma, delta, omega))
idx = int(t_trans / dt)
return t[idx:], X[idx:], dt, step

# Set up the motion for a oscillator with initial position
# x0 and initially at rest.
x0, v0 = 0, 0
tmax, t_trans = 18000, 300
omega = 1.4
gamma, delta = 0.39, 0.1
dt_per_period = 100

# Solve the equation of motion.
t, X, dt, pstep = solve_duffing(tmax, dt_per_period, t_trans, x0, v0, gamma, delta, omega)
x, xdot = X.T

# The animation
fig, ax = plt.subplots(nrows=2,ncols=2)
ax1 = ax[0,0]
ax1.plot(xgrid, Vgrid)
ax1.set_ylim(-0.3, 0.15)
ln1, = ax1.plot([], [], 'mo')
ax1.set_xlabel(r'$x / \mathrm{m}$')
ax1.set_ylabel(r'$V(x) / \mathrm{J}$')

# Position as a function of time
ax2 = ax[1,0]
ax2.set_xlabel(r'$t / \mathrm{s}$')
ax2.set_ylabel(r'$x / \mathrm{m}$')
ln2, = ax2.plot(t[:100], x[:100])
ax2.set_ylim(np.min(x), np.max(x))

# Phase space plot
ax3 = ax[1,1]
ax3.set_xlabel(r'$x / \mathrm{m}$')
ax3.set_ylabel(r'$\dot{x} / \mathrm{m\,s^{-1}}$')
ln3, = ax3.plot([], [])
ax3.set_xlim(np.min(x), np.max(x))
ax3.set_ylim(np.min(xdot), np.max(xdot))

# Poincaré section plot
ax4 = ax[0,1]
ax4.set_xlabel(r'$x / \mathrm{m}$')
ax4.set_ylabel(r'$\dot{x} / \mathrm{m\,s^{-1}}$')
ax4.scatter(x[::pstep], xdot[::pstep], s=2, lw=0, c=sbs.color_palette())
scat1 = ax4.scatter([x0], [v0], lw=0, c='m')
plt.tight_layout()

def animate(i):
"""Update the image for iteration i of the Matplotlib animation."""

ln1.set_data(x[i], V(x[i]))
ln2.set_data(t[:i+1], x[:i+1])
ax2.set_xlim(t_trans, t[i])
ln3.set_data(x[:i+1], xdot[:i+1])
if not i % pstep:
scat1.set_offsets(X[i])
return

anim = animation.FuncAnimation(fig, animate, frames=len(x), interval=1)

Writer = animation.writers['ffmpeg']
writer = Writer(fps=15, metadata=dict(artist='Me'), bitrate=1800)

anim.save('duffing.mp4', writer=writer)

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