# Aerodynamics of Vehicle and Determination of Drag from a CFD Analysis

## Introduction to Vehicle Aerodynamics

• Aerodynamics is part of fluid dynamics to study various forces and relative motions between objects and air
•  Understanding the motion of air around an object (like a vehicle or airplane)  we can measure the forces of lift, drag, and gravity
• The lift force acts in the direction normal to airflow and the drag force is opposite to the direction of the vehicle
• Fluid Mechanics is an important subject to study the dynamics of fluid flow around the moving object like a car
• The shape of objects and relative decides the drag
• We shall discuss the following topics:
• Flow over a Car
• Determination of Drag
• Drag for Different Cars
• The geometry of Sedan Car in ANSYS Space Claim
• The meshing of Car Model in ANSYS Workbench
• CFD solver set up and Simulation in ANSYS FLUENT
• Determination of Drag Coefficients in FLUENT

## Flow Over a Car

• Flow over a car is turbulent.
• A boundary layer is observed for flow over a car  • The following figure shows streamline of airflow over a Lamborghini car ## Types of Drag and Lift Forces

• There are two types of dynamic forces acting due to relative motion between fluid and vehicles
• Drag force has two types:
• Pressure drag due to pressure difference across the body. This is also called as the form drag. The shape of body decides the pressure drag.
• Shear Drag: it is due to viscous shear stress acting on the surface of body Aerodynamics of Vehicle • Aerodynamics of car – forces acting on a car ### Coefficients of Drag for Car

• By integrating pressure difference and shear stress over the surface, we can find total drag
• Total drag force = Pressure drag + Viscous Drag
• Find coefficient drag separately due to pressure and viscous stress with respect to dynamic fluid pressure FD = Drag force
ρ = Air density

V = Airspeed
A = Frontal area ## Drag for Different Cars

• Drag force is higher for bluff bodies of old generation vehicles
• Due to improvements in the aerodynamics of vehicles in the last 50 years, the shape of the car is optimized to reduce drag and fuel consumption.   • Flow Over a sedan car. The coefficient of drag is around 0.3 • The following figure shows flow over a hatchback car. The coefficient of drag is around 0.45 ## CFD Modeling of  Flow Over A Car

• CFD modeling helps to determine the aerodynamics of the vehicle
• The testing of vehicles in wind tunnels can be replaced by CFD analysis of flow
• By numerically solving governing equations of mass, momentum with turbulent flows, we can find out the coefficient of drag and lift based on mean velocities
• Air flows over a car at speed of 60 kmph ## YouTube Video for Geometry, meshing and simulation

• Watch this video for a complete tutorial for geometry, meshing, and simulation using ANSYS tools.