Burgers' equation

There are 4 parameters in Burgers' equation: ${\displaystyle u,x,t}$ and ${\displaystyle \nu }$. In a system consisting of a moving viscous fluid with one spatial (${\displaystyle x}$) and one temporal (${\displaystyle t}$) dimension, e.g. a thin ideal pipe with fluid running through it, Burgers' equation describes the speed of the fluid at each location along the pipe as time progresses. The terms of the equation represent the following quantities:[8]

• ${\displaystyle x}$: spatial coordinate
• ${\displaystyle t}$: temporal coordinate
• ${\displaystyle u(x,t)}$: speed of fluid at the indicated spatial and temporal coordinates
• ${\displaystyle \nu }$: viscosity of fluid

The viscosity is a constant physical property of the fluid, and the other parameters represent the dynamics contingent on that viscosity.

This is a numerical simulation of the inviscid Burgers Equation in two space variables up until the time of shock formation.

The inviscid Burgers' equation is a conservation equation, more generally a first order quasilinear hyperbolic equation. The solution to the equation and along with the initial condition

$${\displaystyle {\frac {\partial u}{\partial t}}+u{\frac {\partial u}{\partial x}}=0,\quad u(x,0)=f(x)}$$

can be constructed by the method of characteristics. The characteristic equations are

$${\displaystyle {\frac {dx}{dt}}=u,\quad {\frac {du}{dt}}=0.}$$

Integration of the second equation tells us that ${\displaystyle u}$ is constant along the characteristic and integration of the first equation shows that the characteristics are straight lines, i.e.,

$${\displaystyle u=c,\quad x=ut+\xi }$$

where ${\displaystyle \xi }$ is the point (or parameter) on the x-axis (t = 0) of the x-t plane from which the characteristic curve is drawn. Since ${\displaystyle u}$ at ${\displaystyle x}$-axis is known from the initial condition and the fact that ${\displaystyle u}$ is unchanged as we move along the characteristic emanating from each point ${\displaystyle x=\xi }$, we write ${\displaystyle u=c=f(\xi )}$ on each characteristic. Therefore, the family of trajectories of characteristics parametrized by ${\displaystyle \xi }$ is

$${\displaystyle x=f(\xi )t+\xi .}$$

Thus, the solution is given by

$${\displaystyle u(x,t)=f(\xi )=f(x-ut),\quad \xi =x-f(\xi )t.}$$

This is an implicit relation that determines the solution of the inviscid Burgers' equation provided characteristics don't intersect. If the characteristics do intersect, then a classical solution to the PDE does not exist and leads to the formation of a shock wave. Whether characteristics can intersect or not depends on the initial condition. In fact, the breaking time before a shock wave can be formed is given by[9]

$${\displaystyle t_{b}=\inf _{x}\left({\frac {-1}{f'(x)}}\right)}$$

Inviscid Burgers' equation for linear initial condition

Subrahmanyan Chandrasekhar provided the explicit solution in 1943 when the initial condition is linear, i.e., ${\displaystyle f(x)=ax+b}$, where a and b are constants.[10] The explicit solution is

$${\displaystyle u(x,t)={\frac {ax+b}{at+1}}.}$$

This solution is also the complete integral of the inviscid Burgers' equation because it contains as many arbitrary constants as the number of independent variables appearing in the equation.[11] Using this complete integral, Chandrasekhar obtained the general solution described for arbitrary initial conditions from the envelope of the complete integral.

This is a numerical solution of the viscous two dimensional Burgers equation using an initial Gaussian profile. We see shock formation, and dissipation of the shock due to viscosity as it travels.

The viscous Burgers' equation can be converted to a linear equation by the Cole–Hopf transformation,[12][13][14]

$${\displaystyle u=-2\nu {\frac {1}{\phi }}{\frac {\partial \phi }{\partial x}},}$$

which turns it into the equation

$${\displaystyle {\frac {\partial }{\partial x}}\left({\frac {1}{\phi }}{\frac {\partial \phi }{\partial t}}\right)=\nu {\frac {\partial }{\partial x}}\left({\frac {1}{\phi }}{\frac {\partial ^{2}\phi }{\partial x^{2}}}\right)}$$

which can be integrated with respect to ${\displaystyle x}$ to obtain

$${\displaystyle {\frac {\partial \phi }{\partial t}}=\nu {\frac {\partial ^{2}\phi }{\partial x^{2}}}+g(t)\phi }$$

where ${\displaystyle g(t)}$ is a function that depends on boundary conditions. If ${\displaystyle g(t)=0}$ identically (e.g. if the problem is to be solved on a periodic domain), then we get the diffusion equation

$${\displaystyle {\frac {\partial \phi }{\partial t}}=\nu {\frac {\partial ^{2}\phi }{\partial x^{2}}}.}$$

The diffusion equation can be solved, and the Cole–Hopf transformation inverted, to obtain the solution to the Burgers' equation:

$${\displaystyle u(x,t)=-2\nu {\frac {\partial }{\partial x}}\ln \left\{(4\pi \nu t)^{-1/2}\int _{-\infty }^{\infty }\exp \left[-{\frac {(x-x')^{2}}{4\nu t}}-{\frac {1}{2\nu }}\int _{0}^{x'}f(x'')dx''\right]dx'\right\}.}$$

• Euler–Tricomi equation
• Chaplygin's equation
• Conservation equation
• Fokker–Planck equation

• Burgers' Equation at EqWorld: The World of Mathematical Equations.
• Burgers' Equation at NEQwiki, the nonlinear equations encyclopedia.