# NAG FL Interfaced03puf (dim1_​parab_​euler_​roe)

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## 1Purpose

d03puf 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 d03pff, d03plf or d03psf, but may also be applicable to other conservative upwind schemes requiring numerical flux functions.

## 2Specification

Fortran Interface
 Subroutine d03puf ( flux,
 Integer, Intent (Inout) :: ifail Real (Kind=nag_wp), Intent (In) :: uleft(3), uright(3), gamma Real (Kind=nag_wp), Intent (Out) :: flux(3)
#include <nag.h>
 void d03puf_ (const double uleft[], const double uright[], const double *gamma, double flux[], Integer *ifail)
The routine may be called by the names d03puf or nagf_pde_dim1_parab_euler_roe.

## 3Description

d03puf 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 routines d03pff, d03plf and d03psf, the left and right solution values are derived automatically from the solution values at adjacent spatial points and supplied to the subroutine argument numflx from which you may call d03puf.
The Euler equations for a perfect gas in conservative form are:
 $∂U ∂t + ∂F ∂x = 0 ,$ (1)
with
 (2)
where $\rho$ is the density, $m$ is the momentum, $e$ is the specific total energy, and $\gamma$ is the (constant) ratio of specific heats. The pressure $p$ is given by
 $p=(γ-1) (e-ρu22) ,$ (3)
where $u=m/\rho$ is the velocity.
The routine calculates the Roe approximation to the numerical flux function $F\left({U}_{L},{U}_{R}\right)=F\left({U}^{*}\left({U}_{L},{U}_{R}\right)\right)$, where $U={U}_{L}$ and $U={U}_{R}$ are the left and right solution values, and ${U}^{*}\left({U}_{L},{U}_{R}\right)$ is the intermediate state $\omega \left(0\right)$ arising from the similarity solution $U\left(y,t\right)=\omega \left(y/t\right)$ of the Riemann problem defined by
 $∂U ∂t + ∂F ∂y =0,$ (4)
with $U$ and $F$ as in (2), and initial piecewise constant values $U={U}_{L}$ for $y<0$ and $U={U}_{R}$ for $y>0$. The spatial domain is $-\infty , where $y=0$ is the point at which the numerical flux is required. This implementation of Roe's scheme for the Euler equations uses the so-called argument-vector method described in Roe (1981).

## 4References

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

## 5Arguments

1: $\mathbf{uleft}\left(3\right)$Real (Kind=nag_wp) array Input
On entry: ${\mathbf{uleft}}\left(\mathit{i}\right)$ must contain the left value of the component ${U}_{\mathit{i}}$, for $\mathit{i}=1,2,3$. That is, ${\mathbf{uleft}}\left(1\right)$ must contain the left value of $\rho$, ${\mathbf{uleft}}\left(2\right)$ must contain the left value of $m$ and ${\mathbf{uleft}}\left(3\right)$ must contain the left value of $e$.
Constraints:
• ${\mathbf{uleft}}\left(1\right)\ge 0.0$;
• Left pressure, $\mathit{pl}\ge 0.0$, where $\mathit{pl}$ is calculated using (3).
2: $\mathbf{uright}\left(3\right)$Real (Kind=nag_wp) array Input
On entry: ${\mathbf{uright}}\left(\mathit{i}\right)$ must contain the right value of the component ${U}_{\mathit{i}}$, for $\mathit{i}=1,2,3$. That is, ${\mathbf{uright}}\left(1\right)$ must contain the right value of $\rho$, ${\mathbf{uright}}\left(2\right)$ must contain the right value of $m$ and ${\mathbf{uright}}\left(3\right)$ must contain the right value of $e$.
Constraints:
• ${\mathbf{uright}}\left(1\right)\ge 0.0$;
• Right pressure, $\mathit{pr}\ge 0.0$, where $\mathit{pr}$ is calculated using (3).
3: $\mathbf{gamma}$Real (Kind=nag_wp) Input
On entry: the ratio of specific heats, $\gamma$.
Constraint: ${\mathbf{gamma}}>0.0$.
4: $\mathbf{flux}\left(3\right)$Real (Kind=nag_wp) array Output
On exit: ${\mathbf{flux}}\left(\mathit{i}\right)$ contains the numerical flux component ${\stackrel{^}{F}}_{\mathit{i}}$, for $\mathit{i}=1,2,3$.
5: $\mathbf{ifail}$Integer Input/Output
On entry: ifail must be set to $0$, $-1$ or $1$ to set behaviour on detection of an error; these values have no effect when no error is detected.
A value of $0$ causes the printing of an error message and program execution will be halted; otherwise program execution continues. A value of $-1$ means that an error message is printed while a value of $1$ means that it is not.
If halting is not appropriate, the value $-1$ or $1$ is recommended. If message printing is undesirable, then the value $1$ is recommended. Otherwise, the value $0$ is recommended. When the value $-\mathbf{1}$ or $\mathbf{1}$ is used it is essential to test the value of ifail on exit.
On exit: ${\mathbf{ifail}}={\mathbf{0}}$ unless the routine detects an error or a warning has been flagged (see Section 6).
Note: if the left and/or right values of $\rho$ or $p$ (from (3)) are found to be negative, then the routine will terminate with an error exit (${\mathbf{ifail}}={\mathbf{2}}$). If the routine is being called from the numflx etc., then a soft fail option (${\mathbf{ifail}}={\mathbf{1}}$ or $-1$) is recommended so that a recalculation of the current time step can be forced using the numflx argument ires (see d03pff or d03plf).

## 6Error Indicators and Warnings

If on entry ${\mathbf{ifail}}=0$ or $-1$, explanatory error messages are output on the current error message unit (as defined by x04aaf).
Errors or warnings detected by the routine:
${\mathbf{ifail}}=1$
On entry, ${\mathbf{gamma}}=⟨\mathit{\text{value}}⟩$.
Constraint: ${\mathbf{gamma}}>0.0$.
${\mathbf{ifail}}=2$
Left pressure value $\mathit{pl}<0.0$: $\mathit{pl}=⟨\mathit{\text{value}}⟩$.
On entry, ${\mathbf{uleft}}\left(1\right)=⟨\mathit{\text{value}}⟩$.
Constraint: ${\mathbf{uleft}}\left(1\right)\ge 0.0$.
On entry, ${\mathbf{uright}}\left(1\right)=⟨\mathit{\text{value}}⟩$.
Constraint: ${\mathbf{uright}}\left(1\right)\ge 0.0$.
Right pressure value $\mathit{pr}<0.0$: $\mathit{pr}=⟨\mathit{\text{value}}⟩$.
${\mathbf{ifail}}=-99$
See Section 7 in the Introduction to the NAG Library FL Interface for further information.
${\mathbf{ifail}}=-399$
Your licence key may have expired or may not have been installed correctly.
See Section 8 in the Introduction to the NAG Library FL Interface for further information.
${\mathbf{ifail}}=-999$
Dynamic memory allocation failed.
See Section 9 in the Introduction to the NAG Library FL Interface for further information.

## 7Accuracy

d03puf performs an exact calculation of the Roe numerical flux function, and so the result will be accurate to machine precision.

## 8Parallelism and Performance

d03puf must only be used to calculate the numerical flux for the Euler equations in exactly the form given by (2), with ${\mathbf{uleft}}\left(\mathit{i}\right)$ and ${\mathbf{uright}}\left(\mathit{i}\right)$ containing the left and right values of $\rho ,m$ and $e$, for $\mathit{i}=1,2,3$, 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 routine does not detect or correct any entropy violation. The time taken is independent of the input arguments.