# NAG CL Interfaced03pwc (dim1_​parab_​euler_​hll)

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## 1Purpose

d03pwc calculates a numerical flux function using a modified HLL (Harten–Lax–van Leer) Approximate Riemann Solver for the Euler equations in conservative form. It is designed primarily for use with the upwind discretization schemes d03pfc, d03plc or d03psc, but may also be applicable to other conservative upwind schemes requiring numerical flux functions.

## 2Specification

 #include
 void d03pwc (const double uleft[], const double uright[], double gamma, double flux[], Nag_D03_Save *saved, NagError *fail)
The function may be called by the names: d03pwc, nag_pde_dim1_parab_euler_hll or nag_pde_parab_1d_euler_hll.

## 3Description

d03pwc calculates a numerical flux function at a single spatial point using a modified HLL (Harten–Lax–van Leer) Approximate Riemann Solver (see Toro (1992), Toro (1996) and Toro et al. (1994)) 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 d03pfc, d03plc and d03psc, 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 d03pwc.
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 function calculates an 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.

## 4References

Toro E F (1992) The weighted average flux method applied to the Euler equations Phil. Trans. R. Soc. Lond. A341 499–530
Toro E F (1996) Riemann Solvers and Upwind Methods for Fluid Dynamics Springer–Verlag
Toro E F, Spruce M and Spears W (1994) Restoration of the contact surface in the HLL Riemann solver J. Shock Waves 4 25–34

## 5Arguments

1: $\mathbf{uleft}\left[3\right]$const double Input
On entry: ${\mathbf{uleft}}\left[\mathit{i}-1\right]$ must contain the left value of the component ${U}_{\mathit{i}}$, for $\mathit{i}=1,2,3$. That is, ${\mathbf{uleft}}\left[0\right]$ must contain the left value of $\rho$, ${\mathbf{uleft}}\left[1\right]$ must contain the left value of $m$ and ${\mathbf{uleft}}\left[2\right]$ must contain the left value of $e$.
Constraints:
• ${\mathbf{uleft}}\left[0\right]\ge 0.0$;
• Left pressure, $\mathit{pl}\ge 0.0$, where $\mathit{pl}$ is calculated using (3).
2: $\mathbf{uright}\left[3\right]$const double Input
On entry: ${\mathbf{uright}}\left[\mathit{i}-1\right]$ must contain the right value of the component ${U}_{\mathit{i}}$, for $\mathit{i}=1,2,3$. That is, ${\mathbf{uright}}\left[0\right]$ must contain the right value of $\rho$, ${\mathbf{uright}}\left[1\right]$ must contain the right value of $m$ and ${\mathbf{uright}}\left[2\right]$ must contain the right value of $e$.
Constraints:
• ${\mathbf{uright}}\left[0\right]\ge 0.0$;
• Right pressure, $\mathit{pr}\ge 0.0$, where $\mathit{pr}$ is calculated using (3).
3: $\mathbf{gamma}$double Input
On entry: the ratio of specific heats, $\gamma$.
Constraint: ${\mathbf{gamma}}>0.0$.
4: $\mathbf{flux}\left[3\right]$double Output
On exit: ${\mathbf{flux}}\left[\mathit{i}-1\right]$ contains the numerical flux component ${\stackrel{^}{F}}_{\mathit{i}}$, for $\mathit{i}=1,2,3$.
5: $\mathbf{saved}$Nag_D03_Save * Communication Structure
saved may contain data concerning the computation required by d03pwc as passed through to numflx from one of the integrator functions d03pfc, d03plc or d03psc. You should not change the components of saved.
6: $\mathbf{fail}$NagError * Input/Output
The NAG error argument (see Section 7 in the Introduction to the NAG Library CL Interface).

## 6Error Indicators and Warnings

NE_ALLOC_FAIL
Dynamic memory allocation failed.
See Section 3.1.2 in the Introduction to the NAG Library CL Interface for further information.
On entry, argument $⟨\mathit{\text{value}}⟩$ had an illegal value.
NE_INTERNAL_ERROR
An internal error has occurred in this function. Check the function call and any array sizes. If the call is correct then please contact NAG for assistance.
See Section 7.5 in the Introduction to the NAG Library CL Interface for further information.
NE_NO_LICENCE
Your licence key may have expired or may not have been installed correctly.
See Section 8 in the Introduction to the NAG Library CL Interface for further information.
NE_REAL
Left pressure value $\mathit{pl}<0.0$: $\mathit{pl}=⟨\mathit{\text{value}}⟩$.
On entry, ${\mathbf{gamma}}=⟨\mathit{\text{value}}⟩$.
Constraint: ${\mathbf{gamma}}>0.0$.
On entry, ${\mathbf{uleft}}\left[0\right]=⟨\mathit{\text{value}}⟩$.
Constraint: ${\mathbf{uleft}}\left[0\right]\ge 0.0$.
On entry, ${\mathbf{uright}}\left[0\right]=⟨\mathit{\text{value}}⟩$.
Constraint: ${\mathbf{uright}}\left[0\right]\ge 0.0$.
Right pressure value $\mathit{pr}<0.0$: $\mathit{pr}=⟨\mathit{\text{value}}⟩$.

## 7Accuracy

d03pwc performs an exact calculation of the HLL (Harten–Lax–van Leer) numerical flux function, and so the result will be accurate to machine precision.

## 8Parallelism and Performance

d03pwc is not threaded in any implementation.

d03pwc must only be used to calculate the numerical flux for the Euler equations in exactly the form given by (2), with ${\mathbf{uleft}}\left[\mathit{i}-1\right]$ and ${\mathbf{uright}}\left[\mathit{i}-1\right]$ containing the left and right values of $\rho ,m$ and $e$, for $\mathit{i}=1,2,3$, respectively. The time taken is independent of the input arguments.

## 10Example

This example uses d03plc and d03pwc to solve the Euler equations in the domain $0\le x\le 1$ for $0 with initial conditions for the primitive variables $\rho \left(x,t\right)$, $u\left(x,t\right)$ and $p\left(x,t\right)$ given by
 $ρ(x,0)=5.99924, u(x,0)=-19.5975, p(x,0)=460.894, for ​x<0.5, ρ(x,0)=5.99242, u(x,0)=-6.19633, p(x,0)=046.095, for ​x>0.5.$
This test problem is taken from Toro (1996) and its solution represents the collision of two strong shocks travelling in opposite directions, consisting of a left facing shock (travelling slowly to the right), a right travelling contact discontinuity and a right travelling shock wave. There is an exact solution to this problem (see Toro (1996)) but the calculation is lengthy and has, therefore, been omitted.

### 10.1Program Text

Program Text (d03pwce.c)

### 10.2Program Data

Program Data (d03pwce.d)

### 10.3Program Results

Program Results (d03pwce.r)