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
•  By 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 a 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), flow patterns over airplanes for different angles of attack, flow patterns over a cricket ball, and a shuttle cock.
• We shall discuss the following topics:

1. • Flow over a Car
   • Aerodynamics of car
   • Determination of Drag
   • Drag for Different Cars
   • The Geometry of Sedan Car in ANSYS Space Claim
   • The meshing of Car Models in ANSYS Workbench
   • CFD solver setup and Simulation in ANSYS FLUENT as presented in the post
   • 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

Aerodynamics of a Car

• The aerodynamics of a car refers to the study of how air flows around and interacts with the vehicle.
• It plays a crucial role in determining a car’s performance, fuel efficiency, stability, and overall design. Here are some key aspects of car aerodynamics:

Aerodynamic Forces:

1. When a car is in motion, it experiences various aerodynamic forces, including:
   • Drag:
     • Drag is the resistance a car encounters as it moves through the air.
     • It opposes the car’s forward motion and is influenced by its shape, size, and speed.
     • Minimizing drag is essential for fuel efficiency.
   • Lift:
     • A lift is an upward force that can affect a car’s stability.
     • It’s more commonly associated with aircraft, but some sports cars are designed to generate downforce to improve traction and handling.

Streamlining of Aerodynamic Components:

• Car designers work to streamline the vehicle’s shape to reduce drag.
• This involves making the car’s body as smooth and aerodynamic as possible.
• This can include sculpting the body to minimize air resistance, using features like spoilers, and ensuring that airflow is controlled efficiently.
• Some common aerodynamic components found in cars include:
  • Spoiler:
    • A spoiler is often found on the rear of a car and helps reduce lift by disrupting the airflow.
    • It can also improve the car’s stability at high speeds.
  • Diffuser:
    • A diffuser is often located on the rear of a car and helps manage the airflow underneath the vehicle.
    • It can create downforce, which enhances traction and stability.
  • Air Dams:
    • Air dams are found on the front of the car and are designed to reduce air from flowing underneath the vehicle, which can create lift.
    • They also help direct air around the car rather than under it.
  • Side Skirts:
    • Side skirts run along the sides of the car, reducing the airflow under the vehicle and preventing air from entering the wheel wells, which can cause drag.

Ground Clearance:

• • The height of a car from the ground is crucial for aerodynamics.
  • Lowering a car reduces the amount of air that flows under it, reducing drag.
  • However, too low a car can compromise ground clearance and make it impractical for normal road conditions.

Wheel Design:

• Even the design of a car’s wheels can affect its aerodynamics. Wheels with flat surfaces or open spokes can create less drag compared to intricate designs with many small openings.
• Testing and Simulation:
  • Car manufacturers use wind tunnels and computational fluid dynamics (CFD) simulations to test and optimize a car’s aerodynamics.
  • These tools help in refining the design for improved performance.
• Trade-Offs:
  • It’s important to note that while enhancing aerodynamics can improve fuel efficiency and high-speed stability, it may also affect other aspects like passenger space, comfort, and aesthetics.
  • Car designers must balance these trade-offs to create a well-rounded vehicle.

 

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 and viscous drag

Total Drag force = Pressure Drag force + Viscous Drag

• Pressure drag depends on the pressure difference across the body, and viscous drag depends on viscous forces acting on the body

Pressure drag

• due to pressure differences across the body. This is also called as the form drag. The shape of the body decides the pressure drag.
• The pressure drag force, also known as form drag or pressure resistance, is one of the components of aerodynamic drag experienced by an object moving through a fluid (such as air or water). It arises due to the pressure difference between the front and rear surfaces of the object.
• When an object moves through a fluid, the fluid exerts pressure on its surfaces. In the case of a streamlined object, such as an airfoil or a well-designed vehicle, the pressure on the front surface is higher than on the rear surface. This pressure difference creates a force that opposes the motion, known as a pressure drag force.
• Pressure drag force is influenced by several factors, including the shape and contour of the object, the velocity of the fluid, the fluid’s density, and the angle of attack (in the case of airfoils). An object with a blunter or less streamlined shape tends to experience higher pressure drag compared to a more streamlined object.
• To quantify pressure drag force, the drag coefficient (Cd) is used. The drag coefficient represents the ratio of the drag force acting on the object to the dynamic pressure of the fluid and the reference area of the object.
• The formula for calculating pressure drag force is:

Pressure Drag Force, Fd  = 0.5 * Cd * ρ * V^2 * A

where, Cd is the drag coefficient, ρ is the density of the fluid, V is the Inlet velocity of the fluid relative to the object, and A is the reference area of the object which is typically the frontal area of the object perpendicular to the direction of motion.

• Reducing pressure drag is a key consideration in vehicle design and aerodynamics optimization.
• Streamlining the shape, reducing frontal area, adding fairings, or incorporating features like spoilers or vortex generators can help in lowering of pressure drag and improve the overall efficiency of the vehicle.

Viscous (Shear)  Drag

• Viscous drag, also known as shear drag or skin friction drag, is another component of aerodynamic drag experienced by an object moving through a fluid.
• it is due to viscous shear stress acting on the surface of the body
• Unlike pressure drag, which is related to the pressure difference on the surfaces of an object, viscous drag arises from the friction between the fluid and the surface of the object.
• When a fluid flows over the surface of an object, there is a thin layer called the boundary layer in which the fluid velocity gradually changes from zero at the surface to the free-stream velocity.
• This velocity gradient within the boundary layer creates a shearing effect, resulting in viscous drag.
• Viscous drag is influenced by the viscosity of the fluid, the surface roughness of the object, and the flow conditions. The roughness of the surface can disrupt the smooth flow of the fluid, increasing the viscous drag.
• Additionally, factors like the Reynolds number (which relates the inertial forces to the viscous forces) and the boundary layer thickness affect the magnitude of the viscous drag.
• To quantify viscous drag, the drag coefficient (Cd) is again used. The drag coefficient for viscous drag is typically calculated separately from the pressure drag coefficient.
• The total drag coefficient is the sum of the pressure drag coefficient and the viscous drag coefficient.The formula for calculating viscous drag force is:

Viscous Drag Force, Fdv = 0.5 * Cd_v * ρ * V^2 * A

where,

• • Cd_v is the viscous drag coefficient,
  • ρ is the density of the fluid,
  • V is the inlet velocity of the fluid relative to the object, and
  • A is the reference area of the object.
• Reducing viscous drag is important in aerodynamic design to minimize energy losses and improve efficiency. Smoothing the surface, using streamlined shapes, employing laminar flow control techniques, and reducing surface roughness are some approaches used to decrease viscous drag.

Aerodynamics of Vehicle

• When air flows over the car’s 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 the viscous drag

• Aerodynamics of car – forces acting on a car

Determination of 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 Coefficients for Different Vehicles

• 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 to drag compared to other types.  The racing car can have more lowest drag to run at high speed.

 

• The coefficient of drag, often denoted as Cd or Cx, is a dimensionless value that represents the aerodynamic resistance or drags experienced by a vehicle as it moves through the air.
• It quantifies how streamlined or aerodynamically efficient a vehicle is. A lower coefficient of drag indicates better aerodynamics and reduced resistance.
• The specific coefficient of drag for a vehicle depends on its shape, design, and various factors such as the frontal area, body contours, and features like spoilers or air dams. Different vehicles have different coefficients of drag, and they can vary significantly depending on the type of vehicle.
• To give you an idea of the range of coefficients of drag for different vehicles, here are some approximate values:

Sports Cars

• • High-performance sports cars often have lower coefficients of drag due to their streamlined designs.
  • They typically range from 0.25 to 0.35.

Sedans and Coupes

• • Standard sedans and coupes generally have coefficients of drag ranging from 0.25 to 0.35.
  • However, some newer models are designed to be more aerodynamic and can achieve even lower values.
  • Flow over a sedan car. The coefficient of drag is around 0.3

• • The following figure shows the flow over a hatchback car. The coefficient of drag is around 0.45

• More smooth flow for sedan cars and less drag (resistance for airflow)

SUVs and Autoriksha

• • Due to their larger size and less streamlined shapes, SUVs and trucks typically have higher coefficients of drag compared to sports cars and sedans.
  • They often range from 0.35 to 0.5.
  • When a truck flows over a truck, a large wake region is observed behind the truck

 

Hatchbacks and Compact Cars

• • Hatchbacks and compact cars tend to have coefficients of drag similar to sedans, ranging from 0.25 to 0.35.
  • The flow pattern between a pickup car and a hatchback car is shown below.
  • A  low-pressure region is formed in the backside container. This is results in high-pressure drag compared

Commercial Trucks and Vans

• • Commercial trucks and vans generally have higher coefficients of drag due to their boxy shapes and larger frontal areas.
  • Their coefficients of drag can vary widely, typically ranging from 0.40 to 0.70 or higher.
  • It’s important to note that the values provided are approximate and can vary depending on the specific model, year, and other design considerations.
  • Manufacturers continuously work on improving the aerodynamic efficiency of vehicles to reduce drag and improve fuel

• A sudden jump in flow for the back side of the bus and more drag (resistance for airflow)

 

• The coefficient of drag for buses and trucks can vary depending on their specific design, shape, and aerodynamic features.
• However, on average, buses typically have a coefficient of drag ranging from 0.55 to 0.8, while trucks have a coefficient of drag ranging from 0.6 to 1.0. These values are approximate and can vary significantly based on the vehicle’s size, shape, and aerodynamic enhancements.

 

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, and momentum with turbulent flows, we can find out the coefficient of drag and lift based on mean velocities
• CFD (Computational Fluid Dynamics) modeling is a powerful tool used to simulate and analyze the aerodynamics of vehicles. It involves using numerical methods to solve the governing equations of fluid flow around a vehicle, taking into account factors such as air density, velocity, and pressure.

 

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
• Optimizing a vehicle’s aerodynamics is essential not only for improving fuel efficiency but also for enhancing safety and handling at high speeds.
• Vehicle aerodynamics is  a complex field of engineering that requires a combination of design, testing, and technology to achieve the desired results

References

1. K.H. Lo, K. Kontis, Flow around an articulated lorry model, Experimental Thermal Fluid Science,  82, 58-74 (2017) 
2. Basic of Aerodynamics Forces Acting on Aeroplane, NASA website