Drag force is one of the most important forces studied in fluid mechanics and hydrodynamics. This force is a crucial factor in the design of aerodynamic objects. In this article, drag force is introduced, its types are explained, the related equations are described, and examples of drag calculations are provided.
What is Drag Force in Aerodynamics?
Drag force is the resistance force caused by the movement of an object through a fluid, such as water or air. This force acts against the direction of the incoming flow velocity, which is the relative speed between the object and the fluid.
In aerodynamics, the force that acts against the direction of an aircraft’s movement is called drag force. This force is created by the interaction between the surface of the aircraft and the air. Drag is one of the main topics in aerodynamics and is crucial in the design of any object that moves through water or air. Proper design of an aircraft can reduce its fuel consumption by minimizing drag.
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Kinds of Drag Force
Drag force in aerodynamics can be categorized into several types, each arising from different physical phenomena. Here are the primary kinds of drag forces:
Skin Friction Drag (Viscous Drag): This drag is caused by the friction between the fluid and the surface of the object. The rougher the surface, the greater the frictional drag.
Pressure Drag (Form Drag): This drag is caused by the shape of the object. The shape affects the fluid flow, creating a pressure difference between the front and back of the object. The object’s shape, the properties and speed of the fluid, and the roughness of the surface all strongly influence this type of drag.
All drag force models can be categorized into the above two types. However, in some sources, additional drag forces have been introduced, some of which are:
Profile Drag: Refers to the total drag experienced by streamlined objects such as airfoils, combining both pressure drag and skin friction drag.
Wave Drag: It occurs in objects moving close to or faster than the speed of sound due to the formation of shock waves.
Induced Drag: It arises from the generation of lift. It happens due to the pressure difference between the upper and lower surfaces of a wing, which results in vortices forming at the wingtips. These vortices contribute to an aerodynamic force that acts in a direction opposing the aircraft’s forward motion.
Interference Drag: It occurs when multiple airflow streams move at different speeds, leading to interactions that increase aerodynamic resistance. This drag is caused by the varying velocities of airflow around an object, contributing to increased drag force.
Parasite Drag: This is the sum of form drag, skin friction drag, and interference drag.
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What is Aerodynamic Drag in a Vehicle?
A car experiences air resistance, also called drag force, as it moves. The drag force significantly impacts fuel consumption. Therefore, designing a car with low drag is crucial for better fuel economy. Take buses, for example. Their boxy shape with a flat back creates a zone of low pressure behind them, increasing drag. You might feel this low pressure if you stand near the back of a moving bus. In contrast, private cars have tapered rear ends that reduce their cross-section gradually. This tapering helps smooth airflow and minimize pressure drag drag.
Have you ever driven a hatchback on a rainy day? You will likely notice the rear window gets bombarded with raindrops compared to the front. This is because of the reduced pressure at the back of the car. As the car moves, air swirls around the back creating a vortex that pulls rain toward the rear window. That is why many hatchbacks have rear wipers to maintain visibility.
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What are the Aerodynamic Drag Formula and The Drag Coefficient?
The aerodynamic drag force acting on an object moving through a fluid can be approximated by the following formula:
![\[ F_D=\frac{1}{2}C_D \rho U^2A \]](https://cfdland.com/wp-content/ql-cache/quicklatex.com-a060b3297cf244b907a6c7755371c08a_l3.png "Rendered by QuickLaTeX.com")
Where ρ is the density of the fluid (air in most cases), U is the velocity of the object relative to the fluid, CD is the drag coefficient, which depends on the shape and surface characteristics of the object, A is the reference area of the object perpendicular to the direction of motion.
The drag coefficient can be determined through experimental results or analytical solutions. It is also possible to calculate the drag coefficient using Computational Fluid Dynamics (CFD) simulations, although this typically requires validation against experimental data. One example of a drag coefficient obtained analytically is the Stokes drag, which applies to spherical objects floating in a fluid with a Reynolds number (Re) less than 1. For this situation:
![\[ A=\pi r^2 \]](https://cfdland.com/wp-content/ql-cache/quicklatex.com-32040d3717fe39beb22bcad34d0beaf1_l3.png "Rendered by QuickLaTeX.com")
![\[ C_D=\frac{24}{\mathrm{Re}} \]](https://cfdland.com/wp-content/ql-cache/quicklatex.com-c33a643b97998f62b82f3bb868fd6484_l3.png "Rendered by QuickLaTeX.com")
![\[ F_D=6\pi r \mu U \]](https://cfdland.com/wp-content/ql-cache/quicklatex.com-9a14260ae044d0ad882943ed0e140508_l3.png "Rendered by QuickLaTeX.com")
Where r is sphere radius and Re is Reynolds number. For more information on the Reynolds number look at “What is Reynolds Number (Re)?”
In many cases, the drag coefficient CD is determined using experimental results and is presented in figures, such as Figure 1, as a function of the Reynolds number.
Figure 1. Drag coefficients for a smooth circular cylinder in cross flow and for a sphere. Boundary layer separation angles are for a cylinder, from Incropera et al., Fundamentals of Heat and Mass Transfer, 7th edition, Wiley.
What is The Separation Phenomenon?
The separation phenomenon in fluid dynamics describes the detachment of fluid flow from a surface due to adverse pressure gradients or other flow conditions, leading to turbulent eddies and increased drag. This occurs when the boundary layer of fluid adjacent to the surface slows down and may even reverse direction, creating areas of low pressure behind the object. This low-pressure area generates a significant pressure difference between the two sides of the object, resulting in high drag forces.
The flow separation phenomenon is associated with an adverse pressure gradient. This issue can be seen in Figure 2.
Figure 2. Velocity profile associated with separation on a circular cylinder in cross flow. From Incropera et al., Fundamentals of Heat and Mass Transfer, 7th edition, Wiley.
Figure 3. Boundary layer formation and separation on a circular cylinder in cross flow. From Incropera et al., Fundamentals of Heat and Mass Transfer, 7th edition, Wiley.
Since the momentum of the fluid in a turbulent boundary layer is greater than in a laminar boundary layer, it is reasonable to expect that transition to turbulence delays the occurrence of separation. For this reason, efforts are made to disturb the airflow over the surface of the object. One method is to increase surface roughness. For example, a golf ball has dimples on its surface that induce turbulent flow and delay separation. This modification reduces pressure drag and increases frictional drag, resulting in an overall reduction in drag.
Figure 4. The effect of turbulence on separation. From Incropera et al., Fundamentals of Heat and Mass Transfer, 7th edition, Wiley.
The Drag Force in ANSYS Fluent Simulations
ANSYS Fluent software has a very high capability to simulate aerodynamics, including drag force calculations. It does not matter whether the type of drag is frictional or pressure drag; Fluent calculates the drag force accurately. These calculations have many applications, such as the design of wind and steam turbines, HVAC systems, airplanes, helicopters, the design of cars, submarines, ships, bicycles, and wherever an object like an airfoil is used and moves, Fluent’s simulations are crucial. Understanding and reducing drag force is important.
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Conclusion
In conclusion, understanding and accurately calculating drag force is essential in the design and optimization of aerodynamic objects across various industries. By exploring the different types of drag forces, their underlying equations, and practical examples, this article highlights the significance of drag in fluid mechanics and hydrodynamics. Advanced tools like ANSYS Fluent enable precise simulations, aiding in the development of efficient designs for vehicles, aircraft, turbines, and more. Proper management of drag force not only enhances performance but also contributes to significant fuel savings and overall operational efficiency.