Moving Bodies

Moving body outputs include both local surface variables defined across the geometry and reduced statistical quantities derived from those surface variables. Surface variables retain spatially distributed information on the moving body surface, including fluid stresses, phase volume fraction, surface velocity, and other kinematic and transport quantities defined at the body-fluid interface. Reduced statistical quantities include integrated force, time-averaged force, wetted area, and surface-averaged transport metrics. Together, these datasets enable detailed analysis of hydrodynamic loading, momentum exchange, and multiphase interaction at moving solid boundaries.

All moving body visualization outputs are written to binary VTK surface datasets and are synchronized with the Plane/Probe Write Interval as well as the Volume Write Interval, ensuring frame-consistent visualization alongside fluid, particle, and scalar fields. The names of these output files are SliceMovingBody_{MovingBody}.pvd and SliceMovingBody_{MovingBody}.pvd, where {MovingBody} is the Dynamic Name of the moving body family. Each moving body family is written to its own output dataset. Data are written at each face of the moving body output surface. The resolution of these spatially varying output data is directly correlated with the moving body mesh quality.

In parallel with spatial outputs, the moving body output system generates time-dependent statistics written to the standard ASCII statistics files. These include integrated and time-averaged quantities such as total force, wetted area, mean surface velocity, and phase-resolved surface metrics.

Output Panel

General

▢ Output with Slices

When this is enabled, static body output is written with the Plane/Interface (or slice) datasets, allowing static body surface data to be visualized and analyzed alongside fluid and scalar fields within the same output frames. When disabled, static body data are written only with the volume outputs. This option reduces disk storage requirements.

Output Control

Local Properties

These quantities describe the local hydrodynamic state at the moving body surface.

▢ Fluid Stresses

Pa | This writes the instantaneous fluid stress tensor evaluated at the moving body surface. This includes both pressure and viscous contributions and represents the instantaneous hydrodynamic load acting on the body.

For an incompressible fluid, the stress tensor is defined as

\[\boldsymbol{\sigma} = -p\,\mathbf{I} + 2\mu\,\mathbf{S}\]

where

\[S_{ij} = \frac{1}{2} \left( \frac{\partial u_i}{\partial x_j} + \frac{\partial u_j}{\partial x_i} \right)\]

is the resolved strain-rate tensor, \(p\) is the static pressure, and \(\mu\) is the effective dynamic viscosity.

For moving bodies, all stress quantities are evaluated using the relative velocity between the fluid and the surface:

Dynamic Pressure

The dynamic pressure represents the inertial contribution to surface loading,

\[p_{\mathrm{dyn}} = \frac{1}{2}\rho \left| \mathbf{u}_{\mathrm{rel}} \right|^{2}.\]

This is computed using the relative velocity between the fluid and the moving surface, and it is useful for identifying stagnation regions and impact loading.

Dynamic Stress

The dynamic stress acting on the surface is the full traction vector obtained from the stress tensor,

\[\mathbf{t} = \boldsymbol{\sigma} \cdot \mathbf{n} = -p\,\mathbf{n} + 2\mu\,\mathbf{S} \cdot \mathbf{n}.\]

This quantity includes both pressure (normal) and viscous (shear) contributions and represents the total instantaneous force per unit area acting on the moving body.

Shear Stress

The shear stress is the tangential component of the viscous traction acting along the surface,

\[ \begin{align}\begin{aligned}\boldsymbol{\tau}_w = \mathbf{t}_{\mathrm{visc}} - \left( \mathbf{t}_{\mathrm{visc}} \cdot \mathbf{n} \right)\mathbf{n},\\\quad \text{where} \quad \mathbf{t}_{\mathrm{visc}} = 2\mu\,\mathbf{S} \cdot \mathbf{n}.\end{aligned}\end{align} \]

This represents the shear force per unit area acting parallel to the moving surface.

▢ Time-Averaged Fluid Stresses

Pa | This writes the fluid stresses associated with the time-averaged surface stress tensor, providing a smoothed representation of the hydrodynamic loading on the moving body. The time-averaged stress tensor \(\bar{\sigma}\) is computed from the instantaneous stress tensor \(\sigma\), using the averaging scheme specified Time Averaging within Simulation Parameters.

▢ Fluid Volume Fractions

dimensionless | This exports the local phase volume fraction(s) at the moving body surface for each fluid phase.

▢ Surface Velocity Vector

m/s | This writes the instantaneous velocity of the moving body surface.

Moving Body Variables

▢ {Moving Body Variable}

This describes the current value of the local Moving Body Variable. The selection name is the dynamic name given to the variable. This option only appears if a Moving Body Variable is defined on one or more moving bodies. The Moving Body Variable will calculated and printed along all surfaces of all Children Geometry associated with the family. The smoothness of the output is directly linked to the child geometry mesh resolution.