Understanding Aerodynamic Drag

The Efficient Engineer
12 Jan 202116:43
EducationalLearning
32 Likes 10 Comments

TLDRThis video delves into the concept of drag force, a critical factor in fluid dynamics affecting everything from vehicle fuel consumption to aircraft design. It distinguishes between friction and pressure drag, explains the impact of flow separation and turbulence, and highlights innovative strategies to minimize drag, such as artificial shark skin technology. The script also explores the drag coefficient, Reynolds number, and Stokes' Law, offering insights into optimizing object design for reduced drag and improved performance.

Takeaways
  • 🌊 Drag force is the resistance a fluid exerts on an object moving through it, and it can be split into two components: drag (parallel to flow) and lift (perpendicular to flow).
  • 🛠️ Engineers work to minimize drag forces to improve fuel efficiency and performance in vehicles, with innovations like artificial shark skin potentially saving millions in fuel costs.
  • 🔍 Understanding the origin of drag forces is crucial for optimizing object design, and these forces arise from wall shear stresses due to fluid viscosity and pressure stresses due to pressure distribution around the object.
  • 🔄 Drag is the resultant of wall shear and pressure stresses, with friction drag caused by shear stresses and pressure drag or form drag caused by pressure stresses.
  • 💨 Flow separation, where the fluid boundary layer detaches from the body, significantly increases pressure drag and can cause vortex shedding, leading to vibrations and instability.
  • 🏐 Golf balls have dimples to generate turbulence, which delays flow separation and reduces drag, allowing the ball to travel further.
  • 🚀 Airplane wings and submarines are designed to be streamlined to minimize the effect of flow separation, with some wings using vortex generators to delay separation and reduce drag.
  • 💧 Friction drag is influenced by the fluid's viscosity and is most significant for bodies with large surface areas facing the flow direction, with laminar flow delaying transition to turbulent flow to reduce friction drag.
  • 📉 The drag coefficient (C-D) in the drag equation captures various parameters affecting drag, such as object geometry and flow regime, and can be determined experimentally or through simulations.
  • 📚 Stokes' Law provides an exact solution for calculating the drag force on a sphere at low Reynolds numbers, which is useful for calculating terminal velocity and creating viscometers.
  • 🛩️ In aviation, specific components of drag like induced drag, wave drag, and interference drag are named for their causes, and understanding these can further optimize aircraft design.
Q & A

  • What are the two main components of the force exerted by a fluid on an object?

    -The two main components are drag, which acts in the direction of the fluid flow, and lift, which acts perpendicular to the flow direction.

  • What is the term used for the force exerted by a gas like air on an object?

    -The term used is 'aerodynamic forces'.

  • Why are drag forces often considered undesirable?

    -Drag forces are often undesirable because they can significantly affect the fuel consumption and performance of vehicles.

  • What are wall shear stresses and how are they related to drag?

    -Wall shear stresses act tangential to an object's surface and are caused by frictional forces due to a fluid's viscosity. They contribute to the component of drag known as friction drag.

  • What is pressure drag and how does it relate to flow separation?

    -Pressure drag is the component of drag caused by pressure stresses acting perpendicular to the object's surface. It is most significant for blunt bodies and increases when flow separation occurs, creating a wake of recirculating flow and a low-pressure separation region behind the body.

  • Why is it important to minimize flow separation when trying to reduce drag forces?

    -Minimizing flow separation is important because it reduces the pressure drag, which is significantly affected by the creation of a low-pressure separation region behind the body.

  • How do golf balls use turbulence to reduce drag?

    -Golf balls have dimples that generate turbulence, which delays flow separation, reduces pressure drag, and allows the ball to travel further.

  • What is the relationship between the angle of attack of an airfoil and drag force?

    -For airfoils, the angle of attack greatly influences the drag force. At high angles of attack, flow separation occurs, which significantly increases the drag force.

  • What is the drag coefficient and how is it used in the drag equation?

    -The drag coefficient (C-D) is a term in the drag equation that captures all the hard-to-measure parameters affecting drag, such as the object's geometry or flow regime. It can be determined experimentally or through numerical simulations and is used to represent the total drag force.

  • What is Stokes' Law and how is it applied?

    -Stokes' Law is an exact solution for the drag force acting on a sphere for Reynolds numbers less than 1. It states that the drag force is directly proportional to the velocity of the sphere and can be used to calculate the terminal velocity of a sphere falling in a fluid.

  • How can the concept of artificial shark skin be applied to reduce drag in the airline industry?

    -Artificial shark skin, which mimics the microstructure of real shark skin, can be used to modify the turbulent boundary layer near the wall, reducing friction drag. Research indicates that applying this to a commercial airliner could reduce total drag force by 2%, leading to significant fuel savings.

  • Why is maintaining laminar flow over the wings and fin of commercial aircraft challenging?

    -Maintaining laminar flow is challenging because it requires delaying the transition to the turbulent regime. Turbulence produces larger shear stresses, which increase friction drag. Techniques like Hybrid Laminar Flow Control, which uses suction to delay turbulence, have had some success.

Outlines
00:00
🌊 Understanding Drag Force

This paragraph introduces the concept of drag force, which is the resistance a fluid exerts on an object moving through it or vice versa. It distinguishes between two types of forces: drag, which acts parallel to the flow, and lift, which acts perpendicular. The paragraph focuses on drag, explaining its sources as wall shear stresses due to fluid viscosity and pressure stresses due to pressure distribution around the object. It also differentiates between friction drag and pressure drag, and discusses the significance of flow separation and how it leads to increased pressure drag and vortex shedding, which can cause instability.

05:02
🛫 Streamlining and Reducing Drag

The second paragraph delves into strategies to reduce drag, such as delaying flow separation and maintaining laminar flow to decrease pressure drag. It highlights the role of turbulence in delaying separation and reducing pressure drag, exemplified by the dimples on golf balls. The paragraph also discusses the importance of body shape in reducing drag, mentioning the use of vortex generators on airplane wings and the streamlined teardrop shape of bodies like airfoils. It touches on the concept of friction drag and how turbulence can increase it due to larger shear stresses, and ends with the idea of looking to nature, specifically sharks, for inspiration in reducing drag.

10:02
🔍 The Science of Drag Coefficient and Drag Force Equation

This paragraph explains the drag equation and the role of the drag coefficient (C-D), which accounts for various parameters affecting drag that are difficult to measure directly. It describes how the drag coefficient can be determined experimentally or through numerical simulations and how it varies with the Reynolds number for different shapes. The paragraph provides specific examples of how the drag coefficient behaves for flat plates, disks, and spheres, and introduces Stokes' Law, which gives an exact solution for the drag force on a sphere at low Reynolds numbers, and its application in calculating terminal velocity and creating a viscometer.

15:03
🎥 Exploring Advanced Topics and Streaming Services

The final paragraph shifts focus to advanced topics in drag, such as induced drag, wave drag, and interference drag, which are covered in an extended version of the video available on Nebula. It promotes Nebula as a platform for educational creators and introduces CuriosityStream, a documentary streaming service, offering a discount and a bundle deal that includes access to both platforms. The paragraph concludes by encouraging viewers to support the channel and the educational community by taking advantage of the streaming service offer.

Mindmap
Keywords
💡Drag Force
Drag force is the resistance that an object experiences when it moves through a fluid or a fluid flows past it. In the context of the video, drag force is a critical concept as it significantly impacts the fuel consumption and performance of vehicles. The script discusses how engineers aim to minimize this force to improve efficiency, and it delves into the two components of drag: friction drag and pressure drag.
💡Lift
Lift is the force that acts perpendicular to the flow direction and is responsible for keeping an object afloat or airborne. Although the video script mentions lift, it is noted that the focus of the video is on drag force, with lift to be covered in a separate video. Lift is crucial for understanding aerodynamics and hydrodynamics, complementing the discussion on drag.
💡Aerodynamic Forces
Aerodynamic forces are the forces exerted by air on an object moving through it. The script specifies that when dealing with air as the fluid, the forces are termed 'aerodynamic,' highlighting the importance of these forces in the context of vehicle design and flight.
💡Hydrodynamic Forces
Hydrodynamic forces are the forces exerted by a liquid on an object moving through it. The script contrasts these with aerodynamic forces, emphasizing that the type of fluid (gas or liquid) affects the nature of the forces acting on an object.
💡Wall Shear Stresses
Wall shear stresses are the tangential forces acting on an object's surface due to the friction caused by a fluid's viscosity. The script explains that these stresses are a component of the drag force and are integral to understanding how to minimize drag by reducing friction.
💡Pressure Stresses
Pressure stresses are the forces that act perpendicular to an object's surface, resulting from the distribution of pressure around the object. In the script, these stresses are identified as contributors to the drag force, particularly in the form of pressure drag.
💡Flow Separation
Flow separation occurs when the fluid boundary layer detaches from the body, leading to a wake of recirculating flow. The script discusses how flow separation can cause a significant increase in pressure drag and is undesirable as it can lead to instability and increased drag forces.
💡Vortex Shedding
Vortex shedding is a phenomenon where vortices are shed from the back of an object causing unwanted vibrations and instability. The script mentions this in the context of flow separation, indicating that it is another consequence of the fluid dynamics that engineers must consider when designing for reduced drag.
💡Adverse Pressure Gradient
An adverse pressure gradient is when the pressure in the flow direction increases, which can cause the flow to decelerate and potentially separate from the surface of an object. The script explains that this gradient is a key factor in the occurrence of flow separation.
💡Turbulence
Turbulence refers to the chaotic and irregular movement of fluid layers. The script discusses how turbulence can delay flow separation, thus reducing pressure drag. It also contrasts the effects of turbulence on pressure drag versus friction drag, showing its dual role in drag reduction and increase.
💡Streamlined
Streamlined shapes are designed to minimize drag by reducing the resistance of a fluid as it flows around an object. The script uses the term to describe the ideal shape for bodies moving through fluids, such as airplane wings or submarines, and explains how such shapes can delay flow separation and reduce drag.
💡Drag Coefficient
The drag coefficient is a dimensionless quantity that accounts for the effects of an object's shape and flow conditions on its drag force. The script explains that it encapsulates various parameters that are difficult to measure directly and is used in the drag equation to calculate the total drag force acting on an object.
💡Reynolds Number
Reynolds number is a dimensionless quantity used to predict flow patterns in fluid dynamics. The script discusses how the drag coefficient varies with the Reynolds number, indicating its importance in determining whether the flow is laminar or turbulent and thus its impact on drag forces.
💡Stokes' Law
Stokes' Law is an exact solution for the drag force acting on a sphere for low Reynolds numbers. The script mentions this law in the context of calculating the terminal velocity of a sphere falling in a fluid and its application in creating a viscometer to measure fluid viscosity.
Highlights

Sponsorship by CuriosityStream for access to documentaries and Nebula.

Drag force is split into drag and lift, with focus on drag in this video.

Drag forces are often undesirable, impacting vehicle fuel consumption and performance.

Innovative methods like artificial shark skin could save millions in airline fuel costs.

Understanding drag force origins involves wall shear and pressure stresses.

Friction drag and pressure drag are components of total drag force.

Flow separation and its impact on pressure drag, especially for blunt bodies.

Turbulence's role in delaying flow separation and reducing pressure drag.

Golf balls' dimples and their effect on drag reduction through turbulence.

Streamlined bodies like airfoils minimize drag by delaying flow separation.

Friction drag's dependency on fluid viscosity and body surface area.

Turbulence's contrasting effects on friction and pressure drag.

Nature-inspired engineering, such as shark skin's impact on reducing friction drag.

Potential fuel savings in aviation through applying shark skin-like textures.

Drag force magnitude's dependency on body geometry relative to flow direction.

The drag equation and its components, including the drag coefficient.

Reynolds number's influence on drag coefficient variation for different shapes.

Stokes' Law and its application in calculating terminal velocity and viscosity.

Specific drag components in aviation like induced drag, wave drag, and interference drag.

Nebula and CuriosityStream partnership offering educational content and discounts.

The educational impact and support for creators through the CuriosityStream and Nebula bundle.

Final thoughts on aerodynamic drag and its practical applications.

Transcripts

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Related Tags
Drag ForcesAerodynamicsFluid DynamicsEngineeringInnovationFuel EfficiencyShark SkinLaminar FlowTurbulenceVortex SheddingViscosity Measurement