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Chapter Introduction
NAG Toolbox

# NAG Toolbox: nag_pde_1d_parab_euler_roe (d03pu)

## Purpose

nag_pde_1d_parab_euler_roe (d03pu) calculates a numerical flux function using Roe's Approximate Riemann Solver for the Euler equations in conservative form. It is designed primarily for use with the upwind discretization schemes nag_pde_1d_parab_convdiff (d03pf), nag_pde_1d_parab_convdiff_dae (d03pl) or nag_pde_1d_parab_convdiff_remesh (d03ps), but may also be applicable to other conservative upwind schemes requiring numerical flux functions.

## Syntax

[flux, ifail] = d03pu(uleft, uright, gamma)
[flux, ifail] = nag_pde_1d_parab_euler_roe(uleft, uright, gamma)

## Description

nag_pde_1d_parab_euler_roe (d03pu) calculates a numerical flux function at a single spatial point using Roe's Approximate Riemann Solver (see Roe (1981)) for the Euler equations (for a perfect gas) in conservative form. You must supply the left and right solution values at the point where the numerical flux is required, i.e., the initial left and right states of the Riemann problem defined below.
In the functions nag_pde_1d_parab_convdiff (d03pf), nag_pde_1d_parab_convdiff_dae (d03pl) and nag_pde_1d_parab_convdiff_remesh (d03ps), the left and right solution values are derived automatically from the solution values at adjacent spatial points and supplied to the function argument numflx from which you may call nag_pde_1d_parab_euler_roe (d03pu).
The Euler equations for a perfect gas in conservative form are:
 ( ∂ U)/( ∂ t) + ( ∂ F)/( ∂ x) = 0 , $∂U ∂t + ∂F ∂x = 0 ,$ (1)
with
U =
 [ ρ m e ]
and   F =
 [ m (m2)/ρ + (γ − 1) (e − (m2)/(2ρ)) (me)/ρ + m/ρ (γ − 1) (e − (m2)/(2ρ)) ]
,
(2)
where ρ$\rho$ is the density, m$m$ is the momentum, e$e$ is the specific total energy, and γ$\gamma$ is the (constant) ratio of specific heats. The pressure p$p$ is given by
 p = (γ − 1) (e − (ρu2)/2) , $p=(γ-1) (e-ρu22) ,$ (3)
where u = m / ρ$u=m/\rho$ is the velocity.
The function calculates the Roe approximation to the numerical flux function F(UL,UR) = F(U*(UL,UR))$F\left({U}_{L},{U}_{R}\right)=F\left({U}^{*}\left({U}_{L},{U}_{R}\right)\right)$, where U = UL$U={U}_{L}$ and U = UR$U={U}_{R}$ are the left and right solution values, and U*(UL,UR)${U}^{*}\left({U}_{L},{U}_{R}\right)$ is the intermediate state ω(0)$\omega \left(0\right)$ arising from the similarity solution U(y,t) = ω(y / t)$U\left(y,t\right)=\omega \left(y/t\right)$ of the Riemann problem defined by
 ( ∂ U)/( ∂ t) + ( ∂ F)/( ∂ y) = 0, $∂U ∂t + ∂F ∂y =0,$ (4)
with U$U$ and F$F$ as in (2), and initial piecewise constant values U = UL$U={U}_{L}$ for y < 0$y<0$ and U = UR$U={U}_{R}$ for y > 0$y>0$. The spatial domain is < y < $-\infty , where y = 0$y=0$ is the point at which the numerical flux is required. This implementation of Roe's scheme for the Euler equations uses the so-called parameter-vector method described in Roe (1981).

## References

LeVeque R J (1990) Numerical Methods for Conservation Laws Birkhäuser Verlag
Quirk J J (1994) A contribution to the great Riemann solver debate Internat. J. Numer. Methods Fluids 18 555–574
Roe P L (1981) Approximate Riemann solvers, parameter vectors, and difference schemes J. Comput. Phys. 43 357–372

## Parameters

### Compulsory Input Parameters

1:     uleft(3$3$) – double array
uleft(i)${\mathbf{uleft}}\left(\mathit{i}\right)$ must contain the left value of the component Ui${U}_{\mathit{i}}$, for i = 1,2,3$\mathit{i}=1,2,3$. That is, uleft(1)${\mathbf{uleft}}\left(1\right)$ must contain the left value of ρ$\rho$, uleft(2)${\mathbf{uleft}}\left(2\right)$ must contain the left value of m$m$ and uleft(3)${\mathbf{uleft}}\left(3\right)$ must contain the left value of e$e$.
Constraints:
• uleft(1)0.0${\mathbf{uleft}}\left(1\right)\ge 0.0$;
• Left pressure, pl0.0$\mathit{pl}\ge 0.0$, where pl$\mathit{pl}$ is calculated using (3).
2:     uright(3$3$) – double array
uright(i)${\mathbf{uright}}\left(\mathit{i}\right)$ must contain the right value of the component Ui${U}_{\mathit{i}}$, for i = 1,2,3$\mathit{i}=1,2,3$. That is, uright(1)${\mathbf{uright}}\left(1\right)$ must contain the right value of ρ$\rho$, uright(2)${\mathbf{uright}}\left(2\right)$ must contain the right value of m$m$ and uright(3)${\mathbf{uright}}\left(3\right)$ must contain the right value of e$e$.
Constraints:
• uright(1)0.0${\mathbf{uright}}\left(1\right)\ge 0.0$;
• Right pressure, pr0.0$\mathit{pr}\ge 0.0$, where pr$\mathit{pr}$ is calculated using (3).
3:     gamma – double scalar
The ratio of specific heats, γ$\gamma$.
Constraint: gamma > 0.0${\mathbf{gamma}}>0.0$.

None.

None.

### Output Parameters

1:     flux(3$3$) – double array
flux(i)${\mathbf{flux}}\left(\mathit{i}\right)$ contains the numerical flux component i${\stackrel{^}{F}}_{\mathit{i}}$, for i = 1,2,3$\mathit{i}=1,2,3$.
2:     ifail – int64int32nag_int scalar
${\mathrm{ifail}}={\mathbf{0}}$ unless the function detects an error (see [Error Indicators and Warnings]).

## Error Indicators and Warnings

Errors or warnings detected by the function:
ifail = 1${\mathbf{ifail}}=1$
 On entry, gamma ≤ 0.0${\mathbf{gamma}}\le 0.0$.
ifail = 2${\mathbf{ifail}}=2$
 On entry, the left and/or right density or pressure value is less than 0.0$0.0$.

## Accuracy

nag_pde_1d_parab_euler_roe (d03pu) performs an exact calculation of the Roe numerical flux function, and so the result will be accurate to machine precision.

nag_pde_1d_parab_euler_roe (d03pu) must only be used to calculate the numerical flux for the Euler equations in exactly the form given by (2), with uleft(i)${\mathbf{uleft}}\left(\mathit{i}\right)$ and uright(i)${\mathbf{uright}}\left(\mathit{i}\right)$ containing the left and right values of ρ,m$\rho ,m$ and e$e$, for i = 1,2,3$\mathit{i}=1,2,3$, respectively. It should be noted that Roe's scheme, in common with all Riemann solvers, may be unsuitable for some problems (see Quirk (1994) for examples). In particular Roe's scheme does not satisfy an ‘entropy condition’ which guarantees that the approximate solution of the PDE converges to the correct physical solution, and hence it may admit non-physical solutions such as expansion shocks. The algorithm used in this function does not detect or correct any entropy violation. The time taken is independent of the input parameters.

## Example

```function nag_pde_1d_parab_euler_roe_example
uleft = [1;
0;
2.5];
uright = [1;
0;
2.5];
gamma = 1.4;
[flux, ifail] = nag_pde_1d_parab_euler_roe(uleft, uright, gamma)
```
```

flux =

0
1.0000
0

ifail =

0

```
```function d03pu_example
uleft = [1;
0;
2.5];
uright = [1;
0;
2.5];
gamma = 1.4;
[flux, ifail] = d03pu(uleft, uright, gamma)
```
```

flux =

0
1.0000
0

ifail =

0

```