# Output Fields and Nondimensional Values

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

# Nondimensional Outputs

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 Speed of Sound (C∞) primitiveVars in all outputs
Pressure ρ∞ × C∞² primitiveVars, nodeForcesPerUnitArea
Temperature Freestream Temperature (T∞) T in volume outputs
Heat Flux ρ∞ × C∞³ heatFlux in surface outputs
Force ρ∞ × C∞² × (Grid Unit Length)² Forces in various outputs
Moment ρ∞ × C∞² × (Grid Unit Length)³ Moments in various outputs

Note: There are also many dimensionless coefficients commonly used in CFD, such as pressure coefficient (Cp), skin friction coefficient (Cf), lift coefficient (CL), and drag coefficient (CD). These coefficients use a reference velocity that can be set via Mach or MachRef parameters and might differ from the speed of sound (C∞) used for basic nondimensionalization.

# 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:

τ_wall (N/m²) = Cf × (1/2)ρ∞ × U_ref²

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 (N/m²) = Cp × (1/2)ρ∞ × U_ref² + p∞

where 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 (N/m²) = Cpt × (1/2)ρ∞ × U_ref² + p_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

← Volume Output →