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DrivAer Automotive Aerodynamics

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About DrivAer

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Reference Model

• Scale: Full-scale passenger car (4.6m length)
• Configurations: Fastback, Estate, and Notchback variants
• Features: Detailed underbody, wheels, mirrors, and aerodynamic elements
• Validation: Extensively validated against wind tunnel data

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Simulation Objectives

• Calculate drag coefficient (CD) for fuel efficiency assessment
• Analyze pressure distribution and flow separation
• Study wake characteristics and vortex formation
• Validate CFD setup against experimental data

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Geometry Preparation

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DrivAer Model Files

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Geometry Checklist

Geometry Checklist:
✓ Watertight STL mesh
✓ Manifold
✓ Proper scale (4.63m length)

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Platform Configuration

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1. Project Setup

1. Create new project: “DrivAer Aerodynamics Study”
2. Upload DrivAer STL file (typically 10-50MB)
3. Verify scaling and orientation in 3D viewer

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2. Domain Configuration

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Recommended Domain Size

• Upstream: 3× vehicle length (13.9m)
• Downstream: 10× vehicle length (46.3m)
• Width: 6× vehicle width (10.8m)
• Height: 4× vehicle height (6.0m)
• Ground clearance: 0.05m

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Flow Conditions

Standard Test Conditions:
- Velocity: 30 m/s (108 km/h, 67 mph)
- Reynolds Number: ~9 million (based on length)
- Turbulence Intensity: 0.1-0.5% (wind tunnel quality)
- Temperature: 20°C
- Pressure: 1 atm

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3. Advanced Meshing Strategy

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Base Mesh Resolution

• Domain Base: 0.2-0.3m cells
• Vehicle Region: 0.05-0.1m cells
• Total Cell Count: Target 8-15 million cells

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Critical Refinement Zones

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4. Solver Configuration

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Flow Physics

Solver Settings:
- Type: Incompressible
- Mode: Steady State
- Turbulence: k-ω SST (industry standard for automotive)
- Pressure-Velocity Coupling: SIMPLE
- Convergence Criteria: 1e-4 for all residuals

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Advanced Options

• Potential Flow Initialization: Recommended for faster convergence
• Parallel Processing: Use 4-8 cores for optimal performance
• Relaxation Factors: Conservative (0.7 for momentum, 0.8 for pressure)

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5. Boundary Conditions

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Domain Boundaries

Inlet:
  Type: Velocity Inlet
  Velocity: 30 m/s (X-direction)
  Turbulence Intensity: 0.1%
  Hydraulic Diameter: 2.9m (based on domain)

Outlet:
  Type: Pressure Outlet  
  Gauge Pressure: 0 Pa
  Backflow Direction: -X

Sides & Top:
  Type: Symmetry
  Zero normal gradient for all variables

Ground:
  Type: Moving Wall
  Velocity: 30 m/s (X-direction)
  Roughness: Smooth

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Vehicle Surfaces

Body Panels:
  Type: No-slip Wall
  Roughness: Smooth (painted surface)
  Heat Transfer: Adiabatic

Wheels (if rotating):
  Type: Rotating Wall
  Rotation Speed: 217 rad/s (30 m/s / 0.69m radius)
  Axis: Y-direction

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Simulation Execution

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Pre-simulation Validation

• Mesh Quality: Orthogonality > 0.1, Aspect Ratio < 1000
• Y+ Values: Check wall distance for turbulence model
• Domain Independence: Boundaries > 5× vehicle dimensions
• Mass Conservation: Verify inlet/outlet flow rates

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Convergence Monitoring

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Key Residuals

1. Continuity: Target < 1e-4
2. Momentum (UVW): Target < 1e-4
3. Turbulence (k, ω): Target < 1e-5
4. Energy (if thermal): Target < 1e-6

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Force Monitoring

Expected Convergence:
- Drag Force: ±0.1% over last 100 iterations
- Lift Force: ±0.5% over last 100 iterations  
- Side Force: Should approach zero for symmetric geometry

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Runtime Estimates

• 8M cells, 4 cores: 4-6 hours
• 15M cells, 8 cores: 6-10 hours
• Convergence: Typically 1500-3000 iterations

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Results Analysis

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Drag Coefficient Validation

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Expected Values (DrivAer Fastback)

Experimental Reference:
- CD = 0.243 ± 0.005 (wind tunnel)
- CL = 0.010 ± 0.003 (slight downforce)

CFD Target Accuracy:
- CD within ±3% of experimental
- CL within ±0.005 absolute

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Calculation Formula

Drag Coefficient: CD = FD / (0.5 × ρ × V² × A)

Where:
- FD = Drag force (N)
- ρ = Air density (1.225 kg/m³)
- V = Velocity (30 m/s)
- A = Frontal area (2.16 m²)

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Flow Visualization

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Performance Metrics

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Aerodynamic Efficiency

Key Performance Indicators:
- Drag Coefficient: Target < 0.25
- Lift Coefficient: Target ±0.05
- Pitching Moment: Stability assessment
- Pressure Base: Wake pressure recovery

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Design Optimization Insights

1. Drag Reduction: Focus on rear separation and underbody flow
2. Downforce: Analyze front/rear balance for stability
3. Side Force: Minimize for crosswind stability
4. Cooling: Assess radiator and brake cooling airflow

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Advanced Analysis

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Pressure Coefficient Distribution

CP = (P - P∞) / (0.5 × ρ × V²)

Critical Locations:
- Windshield: CP ≈ -0.8 to -1.2
- Roof Center: CP ≈ -0.6 to -0.8  
- Rear Window: CP ≈ -0.2 to -0.5
- Base: CP ≈ -0.3 to -0.4

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Benchmark Comparison

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Optimization Strategies

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Aerodynamic Improvements

1. Rear Spoiler: Optimize angle and position
2. Underbody Panels: Smooth underbody flow
3. Side Mirrors: Streamline or camera replacement
4. Wheel Design: Optimize wheel aerodynamics
5. Grille Blocking: Minimize cooling drag when possible

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Parametric Studies

Design Variables:
- Rear spoiler angle: 0-15 degrees
- Ride height: 0.05-0.15m
- Wheel rim design: Multiple configurations
- Front dam height: Optimize ground clearance

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Professional Validation

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Mesh Independence Study

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Turbulence Model Comparison

• k-ω SST: Most accurate for automotive (recommended)
• k-ε Realizable: Faster, less accurate rear separation
• Spalart-Allmaras: Good for attached flows only

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Industry Applications