Fluid model

Fluid model#

The Fluid model represents the primary medium for CFD simulations in Flow360. It integrates several components that together govern the fluid dynamics behavior, including the Navier-Stokes solver, turbulence modeling, transition effects, and initial conditions. The Fluid model is applied to volume entities within your simulation domain.

Note: The Fluid model itself does not contain fluid parameters such as density and temperature. These properties are instead specified in the Operating Condition section of your simulation setup. This separation allows the same Fluid model configuration to be used with different conditions.

Major Components#

The Fluid model consists of five primary components, each documented in detail in its own section:

  1. Navier-Stokes Solver: Controls the core flow equations that govern momentum, continuity, and energy in the fluid. This component determines how the simulation resolves velocity, pressure, and density fields.

  2. Turbulence Model: Handles the modeling of turbulent flow structures through various approaches such as Spalart-Allmaras or k-Omega SST. This significantly impacts flow separation prediction and overall solution accuracy.

  3. Transition Model: Determines how and when flow transitions from laminar to turbulent within the simulation, which is critical for correctly predicting aerodynamic performance, especially at moderate Reynolds numbers.

  4. Initial Condition: Defines the starting flow state for the simulation, which can significantly impact convergence rates and stability, especially for complex flows.

  5. Stopping Criteria: Allows automatic termination of the simulation when monitored output fields (forces, probe values, or surface probe data) reach specified tolerance thresholds, providing efficient convergence control.

Configuration Example#

Below is a representative example of a Fluid model configuration (shown for reference purposes):

Fluid:
  Navier-Stokes Solver:
    Absolute Tolerance: 1.0e-10
    Order of Accuracy: 2
    Low Mach Preconditioner: True
    
  Turbulence Model:
    Type: Spalart-Allmaras
    Absolute Tolerance: 1.0e-8
    
  Transition Model:
    Type: None
    
  Initial Condition:
    Type: NavierStokesInitialCondition
    Rho: "rho"
    U: "u"
    V: "v"
    W: "w"
    P: "p"

  Stopping Criteria:
      - Name: "Lift convergence"
        Output type: Force
        Output field: CL
        Tolerance: 0.01
        Tolerance window size: 100

Common Applications#

The Fluid model is used in virtually all Flow360 simulations, including:

  • External aerodynamics (aircraft, automobiles, sports equipment)

  • Internal flows (ducts, channels, pipes)

  • Turbomachinery (fans, compressors, turbines)

  • Propulsion systems (propellers, rotors, jets)

  • Heat transfer applications (when coupled with thermal models)

Best Practices#

  • Match the solver settings to your specific application requirements and flow regime

  • For most aerospace applications, the Spalart-Allmaras turbulence model provides a good balance of accuracy and efficiency

  • Consider enabling the transition model for flows at moderate Reynolds numbers where transition location significantly impacts results

  • For challenging simulations, start with more robust settings (lower CFL, higher gradient limiters) and then relax these constraints as the solution develops

  • Remember that fluid properties (viscosity, etc.) must be specified in the Operating Condition section, not in the Fluid model

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