# Burgers equation

(Difference between revisions)
 Revision as of 22:10, 14 January 2006 (view source)← Older edit Latest revision as of 15:22, 14 July 2007 (view source)Jola (Talk | contribs) m (Reverted edits by Willi (Talk); changed back to last version by Jola) (6 intermediate revisions not shown) Line 1: Line 1: == Problem definition == == Problem definition == - :$\frac{\partial u}{\partial t}+ u \frac{\partial u}{\partial x}=\mu \frac{\partial^2 u}{\partial x^2}$ + :$\frac{\partial u}{\partial t}+ u \frac{\partial u}{\partial x}=\nu \frac{\partial^2 u}{\partial x^2}$ == Domain == == Domain == Line 6: Line 6: == Initial Condition == == Initial Condition == - :$u(x,0)=1-tanh(\frac{x-x_c}{2\mu})$ + :$u(x,0)=1-tanh(\frac{x-x_c}{2\nu})$ - + - + == Boundary condition == == Boundary condition == Line 15: Line 13: == Exact solution == == Exact solution == - :$u(x,t)=1-tanh(\frac{x-x_c-t}{2\mu})$ + :$u(x,t)=1-tanh(\frac{x-x_c-t}{2\nu})$ + == Numerical method == + :$dx=0.2,dt=0.2dx$ + :Long $\frac{dx}{\nu}=1$ + :Medium$\frac{dx}{\nu}=3$ + :Short $\frac{dx}{\nu}=10$ - == Numerical method == === Space === === Space === ==== Explicit Scheme (DRP)==== ==== Explicit Scheme (DRP)==== :${(\frac{\partial u}{\partial x})}_i=\frac{1.0}{dx}\sum_{k=-3}^3 a_k u_{i+k}$ :${(\frac{\partial u}{\partial x})}_i=\frac{1.0}{dx}\sum_{k=-3}^3 a_k u_{i+k}$ - The coefficients can be found in Tam(1993).At the right boundaries use fourth order central difference and fourth backward difference.At left boundaries use second order central difference for i=2 and fourth order central difference for i=3.The Dispersion relation preserving (DRP) finite volume scheme can be found in Popescu (2005).The second order derivative can be calculated from the first order derivative inside the domain.Near the boundaries use a second order central difference. + The coefficients can be found in Tam(1993).At the right boundaries use fourth order central difference and fourth backward difference.At left boundaries use second order central difference for i=2 and fourth order central difference for i=3.The Dispersion relation preserving (DRP) finite volume scheme can be found in Popescu (2005).The second order derivative can be calculated from the first order derivative inside the domain using the abouve defined discretization.Near the boundaries use a second order central difference. ====Implicit Scheme(Compact)==== ====Implicit Scheme(Compact)==== + =====First derivative ===== :Domain: $\alpha v_{i-1} + v_i + \alpha v_{i+1}=\frac{a}{2h}(u_{i+1}-u_{i-1})$ :Domain: $\alpha v_{i-1} + v_i + \alpha v_{i+1}=\frac{a}{2h}(u_{i+1}-u_{i-1})$ :Boundaries: $v_1+\alpha v_2=\frac{1}{h}(au_1+bu_2+cu_3+du_4)$ :Boundaries: $v_1+\alpha v_2=\frac{1}{h}(au_1+bu_2+cu_3+du_4)$ Line 30: Line 33: :$\mbox{Boundary:} \alpha=2 ,a=-(\frac{11+2\alpha}{6}),b=\frac{6-\alpha}{2},c=\frac{2\alpha-3}{2},d=\frac{2-\alpha}{6}$ :$\mbox{Boundary:} \alpha=2 ,a=-(\frac{11+2\alpha}{6}),b=\frac{6-\alpha}{2},c=\frac{2\alpha-3}{2},d=\frac{2-\alpha}{6}$ Both the schemes are 4th order accurate in the domain.The compact scheme has third order accuracy at the boundary. Both the schemes are 4th order accurate in the domain.The compact scheme has third order accuracy at the boundary. + ===== Second derivative ===== + :Domain: $\alpha v_{i-1} + v_i + \alpha v_{i+1}=\frac{a}{h^2}(u_{i+1}-2u_i+u_{i-1})$ + where v in here refers to the second derivative.Second order central difference is used at the boundaries.The coefficients used are + :$\alpha=1/10 ,a=6/5$ ===Time (4th Order Runga-Kutta)=== ===Time (4th Order Runga-Kutta)=== :$\frac{\partial u}{\partial t}=f$ :$\frac{\partial u}{\partial t}=f$ Line 38: Line 45: ,M=1,2..5 .The coefficients a and b can be found in Williamson(1980) ,M=1,2..5 .The coefficients a and b can be found in Williamson(1980) == Results == == Results == - [[Image:Nonlinear_1d.png]] + [[Image:Burger_long.png]] + [[Image:Burger_medium.png]] + [[Image:Burger_short.png]] == Reference == == Reference ==

## Problem definition

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

## Domain

$x \in \left[0,25\right]$

## Initial Condition

$u(x,0)=1-tanh(\frac{x-x_c}{2\nu})$

## Boundary condition

$u(0,t)=2$
$u_n=2u_{n-1}-u_{n-2}$

## Exact solution

$u(x,t)=1-tanh(\frac{x-x_c-t}{2\nu})$

## Numerical method

$dx=0.2,dt=0.2dx$
Long $\frac{dx}{\nu}=1$
Medium$\frac{dx}{\nu}=3$
Short $\frac{dx}{\nu}=10$

### Space

#### Explicit Scheme (DRP)

${(\frac{\partial u}{\partial x})}_i=\frac{1.0}{dx}\sum_{k=-3}^3 a_k u_{i+k}$

The coefficients can be found in Tam(1993).At the right boundaries use fourth order central difference and fourth backward difference.At left boundaries use second order central difference for i=2 and fourth order central difference for i=3.The Dispersion relation preserving (DRP) finite volume scheme can be found in Popescu (2005).The second order derivative can be calculated from the first order derivative inside the domain using the abouve defined discretization.Near the boundaries use a second order central difference.

#### Implicit Scheme(Compact)

##### First derivative
Domain: $\alpha v_{i-1} + v_i + \alpha v_{i+1}=\frac{a}{2h}(u_{i+1}-u_{i-1})$
Boundaries: $v_1+\alpha v_2=\frac{1}{h}(au_1+bu_2+cu_3+du_4)$

where v refers to the first derivative.For a general treatment of compact scheme refer to Lele (1992).In this test case the following values are used

$\mbox{Domain:} \alpha=0.25 , a=\frac{2}{3}(\alpha+2)$
$\mbox{Boundary:} \alpha=2 ,a=-(\frac{11+2\alpha}{6}),b=\frac{6-\alpha}{2},c=\frac{2\alpha-3}{2},d=\frac{2-\alpha}{6}$

Both the schemes are 4th order accurate in the domain.The compact scheme has third order accuracy at the boundary.

##### Second derivative
Domain: $\alpha v_{i-1} + v_i + \alpha v_{i+1}=\frac{a}{h^2}(u_{i+1}-2u_i+u_{i-1})$

where v in here refers to the second derivative.Second order central difference is used at the boundaries.The coefficients used are

$\alpha=1/10 ,a=6/5$

### Time (4th Order Runga-Kutta)

$\frac{\partial u}{\partial t}=f$
$u^{M+1} =u^M + b^{M+1}dtH^M$
$H^M=a^MH^{M-1}+f^M$

,M=1,2..5 .The coefficients a and b can be found in Williamson(1980)

## Reference

Mihaela Popescu, Wei Shyy , Marc Garbey (2005), "Finite volume treatment of dispersion-relation-preserving and optimized prefactored compact schemes for wave propagation", Journal of Computational Physics, Vol. 210, pp. 705-729.

Tam and Webb (1993), "Dispersion-relation-preserving finite difference schemes for computational acoustics", Journal of Computational Physics, Vol. 107, pp. 262-281.

SK Lele (1992), "Compact finite difference schemes with spectrum-like resolution", Journal of Computational Physics, Vol.103, pp.16-42.

Williamson (1980), "Low Storage Runge-Kutta Schemes", Journal of Computational Physics, Vol.35, pp.48–56.