pressure

Description

The pressure function object provides several methods to convert an input pressure field into derived forms, including:

• Static pressure, $p$:

$$p = \rho \, p_k$$

• Total pressure, $p_t$:

$$p_t = p_{\ref} + p + 0.5 \rho |\u|^2$$

• Pressure coefficient, static or total, $C_p$:

$$C_p = \frac{p}{0.5 \rho_{\infty} \mag{\u_{\infty}}^2}$$

• Isentropic pressure, $p_i$:

$$p_i = p \left[1 + \frac{\gamma-1}{2} M^2 \right]^{\frac{\gamma}{\gamma-1}}$$

where:

$\rho$ : Density [kg/m3]

$\u$ : Velocity [m/s]

$\rho_{\inf}$ : Freestream density [kg/m3]

$p_{\inf}$ : Freestream pressure [Pa]

$\u_{\inf}$ : Freestream velocity [m/s]

$p_k$ : Kinematic pressure (p/rho) [m2/s2]

$p_s$ : Static pressure [Pa]

$p_0$ : Total pressure [Pa]

$p_{\ref}$ : Reference pressure level [Pa]

$p_i$ : Total isentropic pressure

$C_p$ : Pressure coefficient

$C_{p_0}$ : Total pressure coefficient

$\gamma$ : Specific heat ratio

For incompressible cases, the kinematic pressure (pressure divided by density) can be converted to pressure via a user-specified reference density.

Operands

Operand	Type	Location
input	volScalarField	$FOAM_CASE/<time>/<inpField>
output file	-	-
output field	volScalarField	$FOAM_CASE/<time>/<outField>

Usage

Example of the pressure function object by using functions sub-dictionary in system/controlDict file:

pressure1
{
    // Mandatory entries (unmodifiable)
    type            pressure;
    libs            (fieldFunctionObjects);

    // Mandatory entries (runtime modifiable)
    mode            <option>;

    // Optional entries (runtime modifiable)
    p               <pName>;
    U               <UName>;
    rho             <rhoName>;
    rhoInf          1.0; // enabled if rho=rhoInf
    pRef            0.0;
    hydroStaticMode none;
    g               (0 -9.81 0); // enabled if hydroStaticMode != none
    hRef            0.0;         // enabled if hydroStaticMode != none
    pInf            0.0;
    UInf            (1 0 0);
}

where the entries mean:

Property	Description	Type	Required	Default
type	Type name: pressure	word	yes	-
libs	Library name: fieldFunctionObjects	word	yes	-
mode	Calculation mode (see below)	word	yes	-
p	Name of the pressure field	word	no	p
U	Name of the velocity field	word	no	U
rho	Name of the density field	word	no	rho
rhoInf	Freestream density for coefficient calculation	scalar	conditional	-
pRef	Reference pressure for total pressure	scalar	no	0
hydrostaticMode	Hydrostatic contributions (see below)	word	no	none
g	Gravity vector (see below)	vector	no	-
hRef	Reference height (see below)	scalar	no	-
pInf	Freestream pressure for coefficient calculation	scalar	no	-
UInf	Freestream velocity for coefficient calculation	vector	no	-

Options for the mode entry:

static      | static pressure
total       | total pressure
isentropic  | isentropic pressure
staticCoeff | static pressure coefficient
totalCoeff  | total pressure coefficient

The hydrostaticMode entry provides handling for the term $\rho (\vec{g} \cdot \vec{h})$ where options include

none     | not included
add      | add the term, e.g. to convert from p_rgh to p
subtract | subtract the term, e.g. to convert from p to p_rgh

If the hydrostaticMode is active, values are also required for gravity, g, and reference height, hRef. By default these will be retrieved from the database. When these values are not available the user must provide them, e.g.

g           (0 -9.81 0);
hRef        0;

Default behaviour assumes that the case is compressible. For incompressible cases, i.e. solved using the kinematic pressure:

$$p_k = \frac{p}{\rho} \qquad [\mathrm{m}^2 \mathrm{s}^{-2}]$$

The rho entry can be used to set the freestream density:

rho         rhoInf;
rhoInf      100000;

The inherited entries are elaborated in:

• functionObject
• fieldExpression

Usage by the postProcess utility is not available.

Stored properties

Fields

The derived pressure is stored on the mesh database, using the default name:

• static pressure: static(p)
• static pressure coefficient: static(p)_coeff
• total pressure: total(p)
• total pressure coefficient: total(p)_coeff

This can be overridden using the result entry.

Further information

Tutorial:

• cavity

Source code:

• pressure

API:

• Foam::functionObjects::pressure

History: Introduced in version 2.2.0