Aerodynamics of Vehicle and Determination of Drag from a CFD Analysis
Introduction to Vehicle Aerodynamics
what is aerodynamics
 The objective of aerodynamics is to reduce drag and avoid unwanted lift forces to maintain the stability of the vehicle (car, bus, and airplane)
 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 relatives’ motions decide the drag and lifer forces acting on a vehicle. The application of aerodynamics is given on the page of flow Technology which covers flying cars, flying boats, flow over vehicles (car, bus, and auto risk show), and flow pattern over airplanes for different angles of attack, flow pattern over a cricket ball and a shuttle cock .
 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
 When air flows over the car body, the viscous boundary layer is formed over the body
 The highpressure region is on the front part of the car and low pressure ( separated flow region) is observed back of the car body
 Due to the large pressure difference across the car, the pressure drag becomes significant compared to viscous drag
 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
F_{D} = 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.
 The coefficient of drag and lift is low for the streamlined body. A sedantype car has a lower resistance of drag compared to other types. The racing car can have more lowest drag to run at high speed.
 The flow pattern over a pickup car and hatchback car is shown below. A lowpressure region is formed in the backside container. This is results in highpressure drag compared
 When a truck flows over a truck, a large wake region is observed behind the truck
 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.
Conclusion
 Basics of Fluid Mechanics helps to understand aerodynamics using streamlines and forces acting on fluids
 Aerodynamics is an important topic for shape optimization vehicles to reduce fuel consumption with lower drag
 Computational fluid dynamics (CFD) plays a role in vehicle aerodynamics in many industries
References
 K.H. Lo, K. Kontis, Flow around an articulated lorry model, Experimental Thermal Fluid Science, 82, 5874 (2017)