Kuramoto–Sivashinsky equation

The 1d version of the Kuramoto–Sivashinsky equation is

${\displaystyle u_{t}+u_{xx}+u_{xxxx}+{\frac {1}{2}}u_{x}^{2}=0}$

An alternate form is

${\displaystyle v_{t}+v_{xx}+v_{xxxx}+vv_{x}=0}$

obtained by differentiating with respect to ${\displaystyle x}$ and substituting ${\displaystyle v=u_{x}}$. This is the form used in fluid dynamics applications.[9]

The Kuramoto–Sivashinsky equation can also be generalized to higher dimensions. In spatially periodic domains, one possibility is

${\displaystyle u_{t}+\Delta u+\Delta ^{2}u+{\frac {1}{2}}|\nabla u|^{2}=0,}$

where ${\displaystyle \Delta }$ is the Laplace operator, and ${\displaystyle \Delta ^{2}}$ is the biharmonic operator.

The Cauchy problem for the 1d Kuramoto–Sivashinsky equation is well-posed in the sense of Hadamard—that is, for given initial data ${\displaystyle u(x,0)}$, there exists a unique solution ${\displaystyle u(x,0\leq t<\infty )}$ that depends continuously on the initial data.[10]

The 1d Kuramoto–Sivashinsky equation possesses Galilean invariance—that is, if ${\displaystyle u(x,t)}$ is a solution, then so is ${\displaystyle u(x-ct,t)-c}$, where ${\displaystyle c}$ is an arbitrary constant.[11] Physically, since ${\displaystyle u}$ is a velocity, this change of variable describes a transformation into a frame that is moving with constant relative velocity ${\displaystyle c}$. On a periodic domain, the equation also has a reflection symmetry: if ${\displaystyle u(x,t)}$ is a solution, then ${\displaystyle -u(-x,t)}$ is also a solution.[11]

Solutions of the Kuramoto–Sivashinsky equation possess rich dynamical characteristics.[11][12][13] Considered on a periodic domain ${\displaystyle 0\leq x\leq L}$, the dynamics undergoes a series of bifurcations as the domain size ${\displaystyle L}$ is increased, culminating in the onset of chaotic behavior. Depending on the value of ${\displaystyle L}$, solutions may include equilibria, relative equilibria, and traveling waves—all of which typically become dynamically unstable as ${\displaystyle L}$ is increased. In particular, the transition to chaos occurs by a cascade of period-doubling bifurcations.[13]

A converged relative periodic orbit for the KS equation with periodic boundary conditions for a domain size ${\displaystyle L=35}$. After some time the system returns to its initial state, only translated slightly (~4 units) to the left. This particular solution has three unstable directions and three marginal directions.

Applications of the Kuramoto–Sivashinsky equation extend beyond its original context of flame propagation and reaction–diffusion systems. These additional applications include flows in pipes and at interfaces, plasmas, chemical reaction dynamics, and models of ion-sputtered surfaces.[9][14]

• List of nonlinear partial differential equations
• List of chaotic maps
• Clarke's equation
• Laminar flame speed

• Weisstein, Eric W. "Kuramoto-Sivashinsky Equation". MathWorld.