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:

1. • 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 high-pressure 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 sedan-type 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  low-pressure region is formed in the backside container. This is results in high-pressure 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

1. K.H. Lo, K. Kontis, Flow around an articulated lorry model, Experimental Thermal Fluid Science,  82, 58-74 (2017)