Output Fields and Nondimensional Values

Output Fields and Nondimensional Values#

This section explains the various output fields available in Flow360 simulations and how they are nondimensionalized.

Note: Flow360 uses nondimensional values internally for numerical stability and consistency. When interpreting output data, it’s important to understand how these values are nondimensionalized.

Reference Values for Nondimensionalization#

Property

Reference Value for Nondimensionalization

Examples in Flow360

Length

Grid Unit Length

wallDistance in volume and surface outputs

Density

Freestream Density (\(ρ_∞\))

primitiveVars in all outputs

Velocity

Reference velocity scaling (\(U_{scale}\))

primitiveVars in all outputs

Pressure

\(ρ_∞ × U_{scale}^2\)

primitiveVars, nodeForcesPerUnitArea

Temperature

Freestream Temperature (\(T_∞\))

T in volume outputs

Heat Flux

\(ρ_∞ × U_{scale}^3\)

heatFlux in surface outputs

Force (BET/AD/Porous Media)

\(ρ_∞ × U_{scale}^2× (Grid Unit Length)^2\)

Forces in BET/AD/Porous Media outputs

Moment (BET/AD/Porous Media)

\(ρ_∞ × U_{scale}^2× (Grid Unit Length)^3\)

Moments in BET/AD/Porous Media outputs

Reference Velocity Scaling#

The reference velocity scaling (\(U_{scale}\)) is the reference velocity used for nondimensionalizing velocity-related variables (velocity fields, heat flux, angular speeds, etc.):

  • For AerospaceCondition: \(U_{scale} = C_∞\) (speed of sound)

  • For LiquidOperatingCondition: \(U_{scale}\) is the reference velocity (if set) or velocity magnitude (if reference velocity is not set)

Note: It is important to distinguish \(U_{scale}\) from \(U_{ref}\) (reference velocity used for force and moment coefficients). While both may have the same value in some cases, they serve different purposes:

  • \(U_{scale}\) is used for nondimensionalizing velocity-related variables (velocity fields, heat flux, volumetric heat sources, angular speeds, etc.)

  • \(U_{ref}\) is the user-specified reference velocity used specifically for force and moment coefficients (CL, CD, etc.), skin friction coefficient (Cf), pressure coefficient (Cp), and total pressure coefficient (Cpt)

Important Output Fields#

Skin Friction Coefficient (Cf, CfVec)#

The skin friction coefficient represents the wall shear stress nondimensionalized by the dynamic pressure:

  • CfVec is the skin friction coefficient vector, showing both magnitude and direction

  • Cf is the magnitude of that vector

To calculate the dimensional viscous stress on the wall:

\(\tau_{wall} [\frac{N}{m^2}] = C_f × \frac{1}{2}ρ_∞ × U_{ref}^2\)

where \(U_{ref}\) is the reference velocity used for force and moment coefficients (set via Mach or MachRef parameters in the operating condition).

Recommended Method: For convenience, Flow360 provides the wall shear stress directly in physical units through the wall_shear_stress_magnitude_pa field (in Pascals or N/m²). This is the recommended method to access wall shear stress in dimensional form without manual conversion. Available since Flow360 version 25.2.

CfVec is particularly useful for identifying boundary layer separation:

  • Fully attached flow follows the surface along the streamwise direction

  • Separated flow induces local recirculation

  • Negative values of the streamwise component (e.g., CfVecX for flow in the x-direction) indicate boundary layer separation

Pressure Coefficient (Cp)#

The pressure coefficient represents the difference between local and freestream static pressure, normalized by dynamic pressure:

To calculate the dimensional pressure:

\(p [\frac{N}{m^2}] = C_p × \frac{1}{2}ρ_∞ × U_{ref}^2 + p_∞\)

where \(U_{ref}\) is the reference velocity used for force and moment coefficients (set via Mach or MachRef parameters in the operating condition), and \(p_∞\) is the ambient pressure (pressure at the farfield).

Recommended Method: For convenience, Flow360 provides the pressure directly in physical units through the pressure_pa field (in Pascals or N/m²). This is the recommended method to access pressure in dimensional form without manual conversion. Available since Flow360 version 25.2.

Total Pressure Coefficient (Cpt)#

The total pressure coefficient is useful for identifying losses in the flow field:

Total pressure (or stagnation pressure) is the sum of static pressure, dynamic pressure, and gravitational head (often negligible). At stagnation points where velocity is zero, dynamic pressure becomes zero and total pressure equals static pressure.

To calculate the dimensional total pressure:

\(p_t [\frac{N}{m^2}] = {C_p}_t × \frac{1}{2}ρ_∞ × U_{ref}^2 + {p_\infty}_t\)

Total pressure coefficient is excellent for visualizing:

  • Boundary layer development

  • Regions of separation in the flow volume

  • Wakes behind objects

Q-Criterion#

Q-criterion is used to identify vortical structures in the flow field. It represents the balance between rotation rate and strain rate in the flow.

  • Positive values indicate areas where rotation dominates over strain (vortex cores)

  • Higher values indicate stronger vortices

  • The default isosurface value in Flow360 is calculated as:

    Q_default = RefMach² / (all wall's bounding box length)²

Visualization Tips#

Boundary Layer Separation#

  • Use the streamwise component of CfVec to identify separated regions

  • Set a visualization scale with 3 levels (e.g., -1e-6, 0, 1e-6) to easily distinguish between attached (positive values) and separated (negative values) flow regions

Surface Flow Patterns#

  • Use surface streamlines with CfVec components instead of velocity

  • This shows recirculation patterns on the surface

Vortex Visualization#

  • Use qcriterion isosurfaces to identify vortices

  • For airplane simulations: recommended isosurface value is approximately Mach²/WingSpan²

  • For rotor-dominated flows: recommended isosurface value is approximately TipMach²/RotorDiameter²

  • Larger isosurface values show only stronger vortices

  • Smaller values show more flow features but may clutter visualization

Boundary Layer Visualization#

  • Use Cpt (total pressure coefficient) to visualize boundary layer development

  • Lower values (typically shown in blue) highlight boundary layer regions

BET Visualization#

When using Blade Element Theory (BET) models, the volumeOutput can include additional betMetrics that provide visualization of:

  • Blade loading distributions

  • Inducted velocities

  • Local angle of attack

  • Other BET-specific quantities

These metrics are useful for analyzing rotor and propeller performance.

History Files#

Flow360 generates various history files that record time-series data during simulations:

Actuator Disk Output#

  • Records thrust, torque, and power for actuator disk models

  • Useful for tracking propulsion system performance

BET Loading Output#

  • Records sectional forces and moments for blade elements

  • Can be used to analyze blade loading distributions

Aeroacoustic Output#

  • Records acoustic data at observer locations

  • Used for noise prediction and analysis

Heat Transfer#

  • Records heat flux and temperature information

  • Important for thermal analysis applications

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