Doug McLean | Common Misconceptions in Aerodynamics
TLDRIn this seminar, the speaker, an expert in aerodynamics, discusses common misconceptions in the field, focusing on the physics of aerodynamic flows. They delve into the inaccuracies in explanations of lift, the misuse of Newton's third law in popular discourse, and the complexities of vorticity and the biot-savart law. The talk aims to debunk myths and enhance understanding by emphasizing the importance of considering both Bernoulli's principle and Newton's laws for a comprehensive explanation of aerodynamic phenomena.
Takeaways
- 🌟 The speaker expresses a personal fascination with the observable phenomena of aerodynamics, such as the Sun shadow graph on an airplane wing, which provides a visual confirmation of lift generation during flight.
- 📚 The speaker's academic background in physics underpins a lifelong interest in the basic physics underlying aerodynamics and a commitment to debunking misconceptions in the field.
- 💡 The talk focuses on misconceptions in aerodynamics, aiming to identify and correct erroneous ways of looking at aerodynamic flows that stem from misinterpretations of the basic physics.
- 🔍 The speaker emphasizes the importance of Newton's laws, particularly the second and third, in understanding the generation of lift and the interactions between a fluid and a solid body like an airplane wing.
- 🛫 The explanation of lift is highlighted as a particularly challenging and often misunderstood topic, with common but inadequate explanations including the Bernoulli principle and the idea of downward turning of airflow.
- 🔄 The speaker critiques the misuse of Newton's third law in explanations of lift, arguing that it is often misapplied to suggest motion where the law actually pertains to forces, and that lift requires understanding the dynamics of fluid flow as described by Newton's second law.
- 🌀 The talk touches on the complexities of vorticity and the common misunderstandings surrounding its role in aerodynamics, particularly the incorrect notion that vorticity induces velocity at a distance.
- 🌬️ The speaker discusses the global impact of lift on the atmosphere, challenging the classical view that there is a net downward momentum in the atmosphere due to an airplane's lift, and presenting a more nuanced picture involving pressure distributions.
- 🛠️ The script mentions various inventive attempts to reduce induced drag by manipulating vorticity, which the speaker argues are misguided because they misunderstand the fundamental physics of how lift and drag are generated.
- 📘 The speaker's book is referenced as a resource for a more detailed exploration of the topics discussed in the seminar, indicating that the talk is based on but not exhaustive of the content found within.
- ❓ The talk concludes with an invitation for questions, indicating the speaker's willingness to engage in further discussion and clarification on the complex subject of aerodynamics.
Q & A
What did the speaker find exciting about flying from Seattle to Detroit?
-The speaker found it exciting to visually witness the Sun shadow graph, which is a visualization of the shock on the upper surface of the wing, and to see the principles of aerodynamics in action during the flight.
What is the speaker's educational background related to the topic of the seminar?
-The speaker's undergraduate degree is in physics, which has influenced their interest in explaining things in intuitive physical terms and getting things right in terms of basic physics.
Why did the speaker choose to talk about misconceptions in aerodynamics?
-The speaker chose to discuss misconceptions in aerodynamics to highlight the erroneous ways of looking at aerodynamic flows and to correct these misunderstandings by focusing on the basic physics involved.
What is the common misconception about Newton's third law in the context of aerodynamics?
-A common misconception is that Newton's third law can be used to simplistically explain phenomena like lift and thrust without delving into the details of the fluid flow field and accelerations, which are actually governed by Newton's second law.
What is the issue with using the term 'induced drag' in aerodynamics?
-The term 'induced drag' can be misleading as it suggests that the drag is caused by the wake, whereas in reality, the wake is a result of the flow field and not the cause of the drag. The actual cause is the pressure field and the span-wise distribution of lift on the wing.
Why do some explanations of lift fail to be satisfactory?
-Some explanations of lift fail because they either rely on indirect reasons like stream tube pinching or incorrect reasons like equal transit time, without directly addressing the acceleration of the fluid and the forces involved, which are central to Newton's second law.
What is the role of the biot-savart law in understanding the vorticity field?
-The biot-savart law is a mathematical equation that relates the vorticity vector to the velocity field. It is used to calculate the velocity at a point based on the distribution of vorticity, but it does not imply a cause-and-effect relationship where vorticity induces velocity remotely.
Why is it incorrect to think that eliminating vorticity from a flow field would reduce induced drag?
-Eliminating vorticity from a flow field would require violating kinematic laws, as the net vorticity cannot be locally eliminated without changing the global flow field. The induced drag is related to the large spread out global flow field and the pressure field, not just the local vorticity.
What is the 'at rest plane slice' concept mentioned by the speaker?
-The 'at rest plane slice' concept refers to analyzing the momentum change in a slice of the atmosphere through which an airplane has flown. The classical approach suggests there should be a net downward momentum proportional to the lift over time, but a more rigorous analysis shows this is not the case.
What is the conclusion of the analysis regarding the downward momentum accumulated in the atmosphere due to an airplane's lift?
-The conclusion is that there is no downward momentum accumulated in the atmosphere due to an airplane's lift. The downward momentum is actually manifested as an overpressure on the ground plane, consistent with conservation of mass and Newton's third law.
Why is it a mistake to use a local criterion like the zero C_F (Coefficient of Friction) for deciding where separation happens in 3D flows?
-Using a local criterion like zero C_F for separation in 3D flows is a mistake because separation in 3D requires analyzing the global flow pattern. A local criterion does not account for the complexities of 3D flows, where separation is a line of near discontinuity in the region of origin of the skin friction lines.
Outlines
😀 Introduction and Personal Reflections
The speaker begins by expressing gratitude to Professor Martin and the department for the invitation, reflecting on the joy of air travel and the thrill of observing the Sun shadow graph from a window seat on an airplane. They introduce the topic of misconceptions in aerodynamics, emphasizing the importance of understanding basic physics and avoiding common misunderstandings. The speaker's background in physics and their commitment to clear, intuitive explanations are highlighted, setting the stage for a discussion on aerodynamic phenomena.
🔍 Misconceptions in Aerodynamics and Basic Physics
This paragraph delves into the subject of aerodynamic misconceptions, focusing on the misinterpretation of basic physics principles. The speaker outlines the plan to address these misconceptions, starting with a review of the fundamental physics involved in fluid dynamics, including the Navier-Stokes equations and Newton's laws. The paragraph establishes the importance of understanding the continuum nature of fluids and the application of classical physics to aerodynamics, setting the foundation for debunking common myths.
🚀 Misinterpretations of Newton's Third Law in Aerodynamics
The speaker critiques common misinterpretations of Newton's third law, particularly in the context of rocket propulsion and aerodynamic lift. They argue that while the law is often cited to explain thrust and lift, it is frequently misunderstood. The paragraph clarifies that Newton's third law is about forces, not motion, and that understanding the generation of aerodynamic forces requires delving into the details of fluid flow fields and Newton's second law, rather than relying on oversimplified explanations.
🌀 The Complexity of Explaining Aerodynamics: Lift and Flow
This paragraph discusses the challenges in explaining aerodynamic lift, highlighting the shortcomings of popular explanations based on Bernoulli's principle and the downward turning of airflow. The speaker points out that these explanations often fail to account for the complex interactions between fluid flow and pressure, emphasizing the need for a more comprehensive understanding that includes both the near-field effects of pressure differences and the far-field effects of flow direction.
📉 The Importance of Considering the Entire Flow Field
The speaker emphasizes the importance of considering the entire flow field in understanding aerodynamic phenomena. They argue against the misconception that the pressure difference between the upper and lower surfaces of an airfoil is confined to a region close to the surface, instead explaining that it is spread out over a wide area. The paragraph also addresses the reciprocal relationship between the velocity field and the pressure field, noting that the pressure field is sustained by the fluid's inertia and acceleration.
🔗 The Misunderstanding of Vorticity and Biot-Savart Law
This paragraph addresses misconceptions surrounding vorticity and the Biot-Savart law, which is often misused to imply that vorticity induces velocity at a distance. The speaker clarifies that vorticity is not a cause of velocity changes but rather a trace of the flow's rotation. They critique the notion that devices can be used to eliminate vorticity and reduce induced drag, explaining that such attempts misunderstand the fundamental principles of fluid dynamics.
🌪️ The Impact of Lift on the Atmosphere and Induced Drag
The speaker explores the impact of lift on the atmosphere, particularly the concept of induced drag and the mistaken belief that the atmosphere accumulates downward momentum as a result of an aircraft's lift. They discuss the errors in classical approaches to calculating the momentum in the atmosphere and present the correct understanding that the atmosphere does not accumulate downward momentum due to the lift generated by an aircraft.
📚 Conclusion and Invitation for Further Exploration
In the concluding paragraph, the speaker summarizes the key points discussed in the seminar, emphasizing the importance of understanding the complex interactions in fluid dynamics and the need to move beyond oversimplified explanations. They invite the audience to explore these concepts further, particularly through the lens of the speaker's book, which provides a more detailed exploration of the topics covered in the seminar.
Mindmap
Keywords
💡Aerodynamics
💡Lift
💡Newton's Third Law
💡Vorticity
💡Biot-Savart Law
💡Pressure Distribution
💡Continuum Fluid Mechanics
💡Navier-Stokes Equations
💡Bernoulli Principle
💡Downwash
💡Misconceptions
Highlights
The speaker expresses enthusiasm for the seminar's unusual title and thanks the department for the invitation.
A unique perspective on air travel as a joy rather than a chore, with a fascination for the Sun shadow graph visualization on airplane wings.
The importance of basic physics in understanding aerodynamics and the speaker's academic background in physics.
The goal of the seminar: debunking misconceptions in aerodynamics to strengthen understanding of correct explanations.
The common threads among misconceptions are identified as being thin, suggesting the complexity of aerodynamic phenomena.
Aerodynamic lift is a subject of ongoing discussion and misconception, despite its fundamental role in flight.
The speaker's book is mentioned as a resource, with a humorous aside about its promotion by Professor Martin.
The challenge of explaining aerodynamic lift, with a quote highlighting the difficulty and the distinction between engineers and scientists in aerospace.
Misunderstandings about Newton's third law in the context of lift and propulsion, emphasizing the need for a deeper explanation.
The role of Newton's second law in generating aerodynamic forces, contrasting with the common but incorrect reliance on the third law.
The limitations of Bernoulli's principle and stream tube theory in explaining lift, pointing out their shortcomings.
The complementary nature of Bernoulli and Newtonian explanations for lift, advocating for a comprehensive understanding.
The misuse of the Biot-Savart law and the misconception that vorticity induces velocity in the flow field.
The speaker critiques the idea of 'induced drag' as a misnomer and the flawed attempts to reduce it through manipulating vorticity.
The incorrect assumption in a 1984 AIAA paper about propellers reducing induced drag, debunked by Stokes's theorem.
The absence of accumulated downward momentum in the atmosphere due to lift, contrary to classical assumptions.
The conclusion that a global flow pattern analysis is necessary for understanding separation in 3D flows, rather than a local criterion.
The economic significance of aviation as a major export and contributor to the world's GDP, highlighting the importance of aerodynamics.
Transcripts
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