pressure

Table of Contents

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:

Usage by the postProcess utility is not available.

Stored properties

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:

Source code:

History

  • Introduced in version 2.2.0
  • Previously known as: