a5c-ai/cfd-fluids

CFD Analysis Skill

Purpose

The CFD Analysis skill provides deep integration with computational fluid dynamics tools for internal and external flow analysis, enabling systematic setup, execution, and post-processing of fluid simulations.

Capabilities

• ANSYS Fluent, CFX, OpenFOAM workflow automation
• Mesh generation for complex geometries (structured, unstructured)
• Turbulence model selection (k-epsilon, k-omega, SST, LES)
• Boundary condition specification (inlet, outlet, wall, symmetry)
• Steady-state and transient flow simulations
• Post-processing for pressure, velocity, and flow visualization
• Mesh independence studies and validation
• Pressure drop and flow coefficient calculations

Usage Guidelines

Pre-Processing

Geometry Preparation

1. CAD Cleanup

   • Remove small features (< 3 cells)
   • Fill gaps and holes
   • Create smooth transitions
   • Define fluid domain boundaries

2. Domain Definition

   • Internal flow: Extract fluid volume
   • External flow: Create far-field boundary
   • Symmetry: Identify planes of symmetry
   • Periodic: Define periodic pairs

Mesh Generation

1. Mesh Types | Type | Application | Pros/Cons | |------|-------------|-----------| | Structured hex | Simple geometries | High quality, more effort | | Unstructured tet | Complex geometries | Flexible, more cells | | Polyhedral | Complex internal | Good quality, moderate count | | Hybrid | Mixed regions | Optimized for accuracy |

2. Boundary Layer Mesh

   First cell height: y+ = 1 (wall-resolved)
                     y+ = 30-300 (wall functions)
   
   y = y+ * mu / (rho * u_tau)
   u_tau = sqrt(tau_w / rho)
   

3. Mesh Quality Criteria

   Orthogonality: > 0.1 (> 0.3 preferred)
   Skewness: < 0.95 (< 0.8 preferred)
   Aspect ratio: < 100 (< 20 near walls)
   

Solver Configuration

Turbulence Models

| Model | Application | Wall Treatment | |-------|-------------|----------------| | k-epsilon Standard | General industrial | Wall functions | | k-epsilon Realizable | Rotation, separation | Wall functions | | k-omega SST | Aerospace, separation | Low-Re or wall functions | | Spalart-Allmaras | External aero | Low-Re | | LES/DES | Unsteady, vortex shedding | Wall-resolved |

Boundary Conditions

1. Inlet Conditions

   • Mass flow rate or velocity
   • Turbulence intensity (1-5% typical)
   • Hydraulic diameter or length scale
   • Temperature (if energy equation)

2. Outlet Conditions

   • Pressure outlet (most common)
   • Outflow (fully developed)
   • Mass flow outlet (specified)

3. Wall Conditions

   • No-slip (default)
   • Roughness (if significant)
   • Thermal (adiabatic, fixed T, heat flux)

Solution Settings

1. Discretization Schemes

   Convection: Second-order upwind (accuracy)
               First-order (stability)
   Pressure: PRESTO (complex geometry)
             Standard (simple geometry)
   

2. Convergence Criteria

   Residuals: < 1e-4 (typical)
              < 1e-6 (high accuracy)
   
   Monitor: Mass imbalance < 0.1%
            Force convergence
   

Post-Processing

1. Flow Visualization

   • Streamlines and pathlines
   • Velocity vectors
   • Contour plots (P, V, T)
   • Surface integral reports

2. Quantitative Results

   • Pressure drop
   • Flow coefficient (Cv)
   • Heat transfer coefficient
   • Force and moment

Process Integration

• ME-010: Computational Fluid Dynamics (CFD) Analysis

Input Schema

{
  "geometry": "CAD file path",
  "flow_type": "internal|external",
  "fluid": {
    "name": "string",
    "density": "number (kg/m3)",
    "viscosity": "number (Pa.s)",
    "specific_heat": "number (J/kg.K, if thermal)"
  },
  "inlet": {
    "type": "velocity|mass_flow|pressure",
    "value": "number",
    "temperature": "number (K, if thermal)"
  },
  "outlet": {
    "type": "pressure|outflow",
    "value": "number (if pressure)"
  },
  "analysis_type": "steady|transient",
  "turbulence_model": "k-epsilon|k-omega-sst|spalart-allmaras|laminar"
}

Output Schema

{
  "flow_results": {
    "pressure_drop": "number (Pa)",
    "flow_coefficient": "number (Cv)",
    "max_velocity": "number (m/s)",
    "reynolds_number": "number"
  },
  "forces": {
    "drag": "number (N)",
    "lift": "number (N)",
    "moment": "array [Mx, My, Mz]"
  },
  "thermal_results": {
    "heat_transfer_rate": "number (W)",
    "average_htc": "number (W/m2.K)",
    "outlet_temperature": "number (K)"
  },
  "mesh_statistics": {
    "cell_count": "number",
    "y_plus_range": [min, max],
    "orthogonality_min": "number"
  },
  "convergence": {
    "iterations": "number",
    "residuals": "object",
    "mass_imbalance": "number"
  }
}

Best Practices

1. Always perform mesh independence study
2. Verify y+ values match turbulence model requirements
3. Monitor mass and energy imbalance
4. Validate with experimental data when available
5. Start with steady-state before transient
6. Use appropriate turbulence model for flow physics

Integration Points

• Connects with CAD Modeling for geometry
• Feeds into Thermal Analysis for conjugate heat transfer
• Supports Heat Exchanger Design for performance prediction
• Integrates with Test Correlation for validation