The inviscid Burgers’ equation
The inviscid Burgers’ equation is a model for nonlinear wave propagation, especially in fluid mechanics. It takes the form
egin{displaymath}
fbox{$displaystyle
u_t + u u_x = 0
$}
%
end{displaymath} (3.5)
The characteristic equations are, according to (3.4),
egin{displaymath}
frac{{
m d}x}{{
m d}t} = u qquad frac{{
m d}u}{{
m d}t} = 0.
end{displaymath}
The second of these shows that $u$ is constant along the characteristics of the Burgers’ equation, and then the first equation shows that the characteristic lines are straight lines in the $x,t$-plane.
The solution of the two characteristic ordinary differential equations above is simple:
egin{displaymath}
x = u t + C_1 qquad u = C_2
end{displaymath}
The general solution of the partial differential equation may be found in terms of $x$ and $t$ by noting that $C_2$ must be a function of $C_1$, $C_2=C_2(C_1)$, and then substituting $x-ut$ for $C_1$:
egin{displaymath}
u = C_2(x-ut).
end{displaymath}
Some special cases are singular in those terms; they require that $C_1$ is written in terms of $C_2=u$:
egin{displaymath}
x = u t + C_1(u).
end{displaymath}
Normally, either expression may be taken to be the general solution of the ordinary differential equation. One-parameter function $C_2$, respectively $C_1$ remains to be identified from whatever initial or boundary conditions there are.
The inviscid Burgers’ equation
The inviscid Burgers’ equation is a model for nonlinear wave propagation, especially in fluid mechanics. It takes the form
egin{displaymath}
fbox{$displaystyle
u_t + u u_x = 0
$}
%
end{displaymath} (3.5)
The characteristic equations are, according to (3.4),
egin{displaymath}
frac{{
m d}x}{{
m d}t} = u qquad frac{{
m d}u}{{
m d}t} = 0.
end{displaymath}
The second of these shows that $u$ is constant along the characteristics of the Burgers’ equation, and then the first equation shows that the characteristic lines are straight lines in the $x,t$-plane.
The solution of the two characteristic ordinary differential equations above is simple:
egin{displaymath}
x = u t + C_1 qquad u = C_2
end{displaymath}
The general solution of the partial differential equation may be found in terms of $x$ and $t$ by noting that $C_2$ must be a function of $C_1$, $C_2=C_2(C_1)$, and then substituting $x-ut$ for $C_1$:
egin{displaymath}
u = C_2(x-ut).
end{displaymath}
Some special cases are singular in those terms; they require that $C_1$ is written in terms of $C_2=u$:
egin{displaymath}
x = u t + C_1(u).
end{displaymath}
Normally, either expression may be taken to be the general solution of the ordinary differential equation. One-parameter function $C_2$, respectively $C_1$ remains to be identified from whatever initial or boundary conditions there are.
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