# Burgers' equation

Burgers' equation is a fundamental partial differential equation from fluid mechanics. It occurs in various areas of applied mathematics, such as modeling of gas dynamics and traffic flow. It is named for Johannes Martinus Burgers (1895–1981). It relates to the Navier-Stokes equation for incompressible flow with the pressure term removed.[1]

For a given velocity $u(x,t)$ and viscosity coefficient $\nu$, the general form of Burgers' equation (also known as viscous Burgers' equation) in one space dimension is the dissipative system:

$\frac{\partial u}{\partial t} + u \frac{\partial u}{\partial x} = \nu \frac{\partial^2 u}{\partial x^2}$.

Added space-time noise $\eta(x,t)$ forms a stochastic Burgers' equation[2]

$\frac{\partial u}{\partial t} + u \frac{\partial u}{\partial x} = \nu \frac{\partial^2 u}{\partial x^2}-\lambda\frac{\partial\eta}{\partial x}$

This stochastic PDE is equivalent to the Kardar-Parisi-Zhang equation in a field $h(x,t)$ upon substituting $u(x,t)=-\lambda\partial h/\partial x$. But whereas Burgers' equation only applies in one spatial dimension, the Kardar-Parisi-Zhang equation generalises to multiple dimensions.

When the viscosity $\nu = 0$, Burgers' equation becomes the inviscid Burgers' equation:

$\frac{\partial u}{\partial t} + u \frac{\partial u}{\partial x} = 0,$

which is a prototype for conservation equations that can develop discontinuities (shock waves). The previous equation is the 'advection form' of the Burgers' equation. The 'conservation form' is

$\frac{\partial u}{\partial t} + \frac{1}{2}\frac{\partial}{\partial x}\big(u^2\big) = 0.$

## Solution

### Inviscid Burgers' equation

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. In fact by defining its current density as the kinetic energy density:

$j(u)= \frac 1 2 u^2$

it can be put into the current density homogeneous form:

$u_t + j_x (u) = 0$.

The solution of conservation equations can be constructed by the method of characteristics. This method yields that if $X(t)$ is a solution of the ordinary differential equation

$\frac{dX(t)}{dt} = u[X(t),t]$

then $U(t) := u[X(t),t]$ is constant as a function of $t$. For Burgers equation in particular $[X(t),U(t)]$ is a solution of the system of ordinary equations:

$\frac{dX}{dt}=U,\quad\mbox{and}\quad \frac{dU}{dt}=0.$

The solutions of this system are given in terms of the initial values by

$X(t)=X(0)+tU(0),\quad\mbox{and}\quad U(t)=U(0).$

Substitute $X(0)= \eta$, then $U(0)=u[X(0),0]=u(\eta,0)$. Now the system becomes

$X(t)=\eta+tu(\eta,0)\quad\mbox{and}\quad U(t)=U(0).$

Conclusion:

$u(\eta,0)=U(0)=U(t)=u[X(t),t]=u[\eta+tu(\eta,0),t].$

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.

### Viscous Burgers' equation

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 linearized by the Cole–Hopf transformation [3]

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

which turns it into the equation

$\frac{\partial}{\partial x} \Bigl( \frac{1}{\phi}\frac{\partial\phi}{\partial t}\Bigr) = \nu \frac{\partial}{\partial x} \Bigl( \frac{1}{\phi}\frac{\partial^2\phi}{\partial x^2}\Bigr)$

which can be rewritten as

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

with f(t) an arbitrary function. Assuming it vanishes, we get the diffusion equation

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

This allows one to solve an initial value problem:

$u(x,t)=-2\nu\frac{\partial}{\partial x}\ln\Bigl\{(4\pi\nu t)^{-1/2}\int_{-\infty}^\infty\exp\Bigl[-\frac{(x-x')^2}{4\nu t} -\frac{1}{2\nu}\int_0^{x'}u(x'',0)dx''\Bigr]dx'\Bigr\}.$

## References

1. ^ Burgers Equation (PDF)
2. ^ W. Wang and A. J. Roberts. Diffusion approximation for self-similarity of stochastic advection in Burgers’ equation. Communications in Mathematical Physics, July 2014.
3. ^ Eberhard Hopf (September 1950). "The partial differential equation ut + uux = μxx". Communications on Pure and Applied Mathematics 3 (3): 201–230. doi:10.1002/cpa.3160030302.