Force

The force report is represented in the simulation tree by a node. It has its own properties but is subject to the report node pop-up menu. It also has an expert property for obtaining values using bands.

The force on a surface is computed as:

Figure 1. EQUATION_DISPLAY

f = \sum_{f}^{} {(f_{f}^{p r e s s u r e} + f_{f}^{s h e a r}) \cdot n}_{f}

(422)

where:

$f_{f}^{p r e s s u r e}$ and $f_{f}^{s h e a r}$ are the pressure and shear force vectors on the surface face $f$ .
$n_{f}$ is a user-specified direction vector that indicates the direction in which to compute the force.

The pressure force vector on surface face $f$ is computed as:

Figure 2. EQUATION_DISPLAY

f_{f}^{p r e s s u r e} = (p_{f} - p_{r e f}) a_{f}

(423)

where $p_{f}$ is the face static pressure, $a_{f}$ is the face area vector, and $p_{r e f}$ is the reference pressure. As defined, $f_{f}^{p r e s s u r e}$ is the force that the fluid exerts on the surface. The shear force on surface face $f$ is computed as:

Figure 3. EQUATION_DISPLAY

f_{f}^{s h e a r} = - T_{f} \cdot a_{f}

(424)

where $T_{f}$ is the stress tensor at face $f$ . This shear force is exerted on the surface by the fluid.

Note

The report reference pressure

p_{r e f}

is defined relative to the simulation reference pressure. This definition means that a

p_{r e f}

of 0 Pa does not indicate absolute zero pressure, but is rather the value of the simulation reference pressure. Further, when the surface on which the force is computed is a closed surface, the choice of

p_{r e f}

affects only the overall accuracy of the result: therefore set it to a value that is close to the average pressure on the surface. For an unclosed surface,

p_{r e f}

is effectively the pressure that is applied on the back-side of the surface, and therefore directly affects the result.

Note

The force report calculates the force on the boundaries (or boundary threshold parts). The boundary of a Porous Baffle Interface consists of an open portion and a closed portion: the force report calculates the total force on both portions. Whereas pressure acts on both the open and closed portions, wall shear acts only on the closed portion. Therefore, take care when interpreting the results of force reports on these interfaces.

Force in Harmonic Balance

When the Harmonic Balance model is selected, the force on a surface is computed as:

Figure 4. EQUATION_DISPLAY

R = \sum_{f}^{} {\tilde{f}}_{f} = \sum_{f}^{} ({\tilde{f}}_{f}^{pressure} + {\tilde{f}}_{f}^{shear}) \cdot n_{f}

(425)

where:

${\tilde{f}}_{f}^{pressure}$ is the pressure force vector on the surface face $f$ .
${\tilde{f}}_{f}^{shear}$ is the shear force vector on the surface face $f$ .
$n_{f}$ is a user-specified direction vector.

The value of ${\tilde{f}}_{f}$ depends on the selected Representation:

Volume Mesh Representation: ${\tilde{f}}_{f} = {\hat{f}}_{0 f}$ where ${\hat{f}}_{0 f}$ is the time-mean value of force, Force Fourier Mode 0.
HB Solution View Representation: ${\tilde{f}}_{f}$ refers to the type of force corresponding to the selection in the HB Solution View node. For example, Force at Physical Time of 5 s or Force Fourier Mode 1 Real.

Force Report Properties

In addition to constants, you can enter field function syntax directly.

Units

Units that are used for specifying the quantity.

Coordinate System

Sets the Cartesian coordinate system in which the report value is computed and in which the Direction vector is interpreted. Banded results are not computed in this coordinate system but only in the laboratory coordinate system.

Direction

A Cartesian vector that indicates the direction in which the force is computed. Also used for the banding direction.

Force Option

The forces (shear and pressure) that are considered in the computation of force.

Shear: only the force due to shear, $f_{f}^{s h e a r}$ in Eqn. (422) is considered.
Pressure: only the force due to pressure, $f_{f}^{p r e s s u r e}$ in Eqn. (422) is considered.
Pressure + Shear: both pressure and shear forces, $f_{f}^{p r e s s u r e}$ and $f_{f}^{s h e a r}$ in Eqn. (422) are considered.

Reference Pressure

The reference pressure for the computation of Eqn. (423).

Parts

Use this property to select the parts on which the report is computed.

Pressure Option

This property lets you exclude the hydrostatic pressure. For details, see Options for Excluding Hydrostatic Pressure from Fluid Loads Calculations.

Number of Bands

When set to a non-zero integer value, the reporting surface is divided into this number of intervals in the specified Direction and the report value is computed at the center of each interval (or band). To retrieve these results, you must use Java API calls in the ForceAndMomentBaseReport class. See Using Bands with Force and Moment Reports.

Representation

Selects a representation. Additional selections appear as representations that are added to the simulation.

Smooth Values

When this property is activated, the report is calculated using node (interpolated) values of the chosen quantity rather than cell values.

Account for Idealization

Applies idealizations related to the chosen parts before computing. For details, see Obtaining Whole Domain Reports through Idealizations.