Case Study: CFD Simulation of External Aerodynamics of a Sedan Car

Project Overview

This case study presents a Computational Fluid Dynamics (CFD) simulation of the external aerodynamics of a sedan car. The analysis evaluates aerodynamic drag and lift, investigates flow separation and wake behavior, and assesses overall aerodynamic performance relevant to vehicle efficiency and stability.

The study demonstrates the application of steady-state, turbulence-resolved external flow analysis commonly used during automotive design and validation stages.

Objectives of the Study

The primary objectives of this CFD analysis are to:

• Predict aerodynamic drag (Cd) and lift (Cl) coefficients

• Analyze velocity and pressure distribution on vehicle surfaces

• Identify flow separation, recirculation, and wake regions

• Evaluate external aerodynamic behavior at highway operating conditions

• Establish a baseline aerodynamic model for future optimization studies

System Description

The sedan car geometry represents a mid-size passenger vehicle with realistic proportions including bumper, hood, windshield, roof, and trunk. The model also includes simplified representations of side mirrors and underbody as bluff features to capture dominant aerodynamic effects while maintaining mesh predictability.

To minimize boundary interference, the computational domain extends several vehicle lengths upstream, downstream, and laterally. A moving ground boundary condition replicates real road motion effects on external flow.

Geometry and Computational Domain

• Full 3D representation of the sedan car exterior

• Wind-tunnel–style rectangular domain

• Upstream and lateral distances set to mitigate artificial boundary effects

• Downstream outlet extended to capture wake development

Material Properties

• Working fluid: Air

• Fluid assumed incompressible

• Standard atmospheric density and viscosity properties used

Boundary Conditions

• Inlet: Uniform velocity corresponding to design speed (~90 km/h)

• Outlet: Fixed static pressure representing ambient conditions

• Vehicle surface: No-slip wall

• Ground: Moving wall at vehicle speed

• Domain sides & top: Slip boundaries

Governing Equations:

The CFD simulation is based on the three-dimensional, steady-state Reynolds-Averaged Navier–Stokes (RANS) equations for turbulent external flow. Turbulence effects and velocity–pressure coupling are resolved using established numerical models appropriate for bluff-body aerodynamic flows.

Solver Strategy

• Steady-state flow solution

• Iterative pressure–velocity coupling

• Convergence monitored via:

  • Residual reduction

  • Stabilization of aerodynamic force coefficients (Cd, Cl)

Post-Processing and Key Results

Velocity Distribution

The airflow field around the sedan illustrates:

• Acceleration of flow over the hood and roof

• High-velocity regions along the rooftop

• Strong velocity deficits in the rear wake zone, indicating significant flow separation

Pressure Distribution

Surface pressure contours reveal:

• High static pressure at the front stagnation region

• Low-pressure regions ahead of and below the rear deck

• Limited pressure recovery on the trunk, contributing to increased drag

Flow Separation and Wake

Flow separation is prominent at:

• Rear windshield–trunk junction

• Side mirror wake zones
  A large recirculation zone develops behind the vehicle, dominating the overall drag behavior.

Aerodynamic Performance

• Drag coefficient (Cd): ~0.30 – 0.33

• Lift coefficient (Cl): Slightly positive

• Dominant drag contribution arises from pressure drag rather than frictional drag

These values provide a baseline performance metric for the sedan body shape under steady flow conditions.

Engineering Insights Gained

• The rear geometry significantly influences wake size and drag production.

• Pressure drag dominates total aerodynamic resistance.

• Separation around bluff features (mirrors, A-pillars) contributes to localized vortex shedding.

• External aerodynamic CFD enables early-stage identification of critical design regions prior to physical testing, reducing development time and cost.

Industrial Applications

• Passenger car aerodynamic assessment and benchmarking

• Fuel consumption and range optimization studies

• Baseline aerodynamic performance evaluation before wind-tunnel testing

• Concept-level design iteration and comparison

Benefits to Industry

• Accurate prediction of drag reduction potential

• Insight into flow separation and wake mechanisms

• Reduced reliance on physical prototypes during early design

• Improved decision-making through visualization of complex flow physics

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