Can a brick 'fly' (glide)? enhanced with AI

 This is a second version of the original article: Can a brick 'fly' (glide)?

This time, Google's AI Gemini (Flash v2.5) enhanced (or not...) this article by simply adding "Improve this:" to each original paragraph. This was done as an exercise to provide an alternative explanation and formatting while maintaining the original ideas. No major changes have been made to the AI-generated text, and the additional explanations (marked with asterisks in the original article) have been removed.

Redefining Aerodynamics: Beyond the Smooth Surface

Yes, it can—but perhaps not in the way most people understand. The common perception of "aerodynamics" often fixates on curvatures, rounded leading edges, smooth surfaces, and sharp trailing edges. These features, while visually appealing and often associated with modern design, are largely just cosmetic embellishments when we consider the fundamental meaning of the term.

Etymologically, "aerodynamics" simply means "air in motion." From a purely kinematic perspective, this implies that a turbulent fluid is, in fact, more "aerodynamic" than a laminar one. This might seem counterintuitive, but if we strictly adhere to the definition of air in motion, a more chaotic flow represents more "aero-dynamic" activity.

Consider this: based on this strict interpretation, a car or plane designed in the 1950s or 1960s, with its often less-streamlined forms, could be considered more aerodynamic than a contemporary vehicle. This isn't just wordplay or a philosophical debate; it's about precision in language. To truly understand and explain a phenomenon, we must name it as precisely as possible, avoiding vague terms and oversimplifications that provide quick answers without formal justification.

What do you think about this distinction? Does it change your perception of what "aerodynamic" truly means?

Fig. 1 AI-generated image.

The True Driver of Flight: Velocity, Not "Lift Mechanisms"

Following this line of thought, it becomes clear that anything can fly or glide with sufficient velocity! This critical factor—velocity—should be recognized as the true underlying cause for everything traditionally attributed to various "lift generation" mechanisms. We're talking about the reacting forces, air downwash, pressure differences, or circulation-vorticity often cited in hypotheses, theories, and even pseudo-theories.

Aerodynamics, then, isn't the "pretty girl" of fluid dynamics; it's a much ruder, more chaotic phenomenon. This is precisely why attempting to understand it from a smooth-flow perspective—for instance, through the lens of an "aerodynamically optimized" surface designer—isn't the best way to grasp its deeper truths.

The Myth of Natural Laminar Flow

Strictly speaking, laminar flow (defined as perfect layers sliding over each other) does not naturally exist in aerodynamics. There's always some degree of non-symmetrical interaction between air molecules and the surface, or even between fluid molecules themselves. One key reason is simply that perfectly smooth surfaces don't exist in reality.

Consequently, micro-vortices of various scales (down to molecular size!) are always generated from the very first "contact" between the fluid and the solid surface. These vortices are then mixed and amplified (think Kelvin-Helmholtz instability) and only barely diffused by the air's relatively low viscosity. Even on a theoretically "perfectly" smooth surface, curvatures inevitably lead to flow separation with its associated vorticity, especially since viscous effects become less relevant under high Reynolds number conditions.

At this juncture, the traditional, oversimplified explanation of aerodynamics—which often relies on the idea of a purely laminar flow—becomes incompatible with the omnipresence of vorticity. But perhaps, now it makes a lot more sense, right?

Fig. 2 AI-generated image.

The Boundary Layer: A Damped Detached Flow, Not a Laminar Ideal

So, where am I going with such seemingly exaggerated precision? In essence, the boundary layer—whether described as laminar, transitional, or turbulent—is fundamentally a region of detached flow. Its apparent "attachment" to the surface is merely a consequence of the fluid's viscosity, which acts tangentially to the surface, effectively damping the inherent detachment.

Therefore, the boundary layer, and indeed aerodynamics (and fluid dynamics in general!), should be conceived and analyzed primarily from a detached flow perspective. We shouldn't presuppose a level of attachment; instead, a robust viscosity model should intrinsically regulate it. Recall: aerodynamics is crude, not a delicate, smooth process.

This brings me to a crucial question: Why do most aerodynamicists continue to speak of "laminar/attached fluids" or "laminar boundary layers" under high Reynolds number conditions? Where is the empirical evidence for this in their experimental tests? This widespread notion of a stable, perfectly smooth laminar boundary layer at high speeds seems to fly in the face of physical reality.

Consider this: even on a microscopically "smooth" surface, the slightest curvature or imperfection, combined with the inherent instability of shear flows (as seen in the Kelvin-Helmholtz instability), leads to the rapid generation, mixing, and amplification of micro-vortices. Viscosity, while present, is often insufficient to fully suppress this turbulence at high Reynolds numbers. The notion of a laminar fluid coexisting with significant vorticity is a contradiction within the oversimplified explanations prevalent in aerodynamics. Yet, when viewed through the lens of inherent detachment and chaotic interaction, it becomes entirely consistent.

Video: Visualization of airflow past a symmetrical airfoil at different angles of attack. Source: Youtube

The Ubiquitous Cloud of Vorticity: Beyond Attached Flow

In the previous example, we observed that a macro-scale "attached" fluid appears only at the bottom of the object. Crucially, from nearly the first instant (perhaps at an angle of attack as low as two degrees), the fluid in the upper region is already partially detached, presenting as a diffused gray cloud of vorticity.

This leads to a profound realization: all objects immersed in a fluid inevitably generate a cloud of multi-scale vorticity in their surrounding area. This cloud can readily become turbulent under specific conditions, a phenomenon that occurs even around a flat plate without any incidence!

This is precisely the foundational approach of the work in [1]: a velocity-vorticity method that treats circulation as fundamentally detached from the entire surface. This methodology is uniquely capable of accurately capturing not only lift but also drag and pitching moment with remarkable precision, even for simplified geometries.

Ultimately, extending this approach to complex three-dimensional bodies will unveil the true genesis of hydro/aerodynamic forces: the intricate velocity field induced by the full, evolving cloud of detached vorticity. This represents a brute force, reality-driven approach that is indispensable for truly comprehending a phenomenon as fundamentally crude and chaotic as aerodynamics. It pushes us beyond the comfortable but often misleading ideal of smooth, attached flow, compelling us to embrace the inherent complexity of air in motion.

The Uncompromising Logic of Vorticity: Why Mathematics Is Non-Negotiable

Among all the theories attempting to explain lift generation, the circulation/vorticity approach stands alone with a truly solid mathematical foundation. It is rooted in a well-established (though arguably partially misinterpreted [2]) Potential Flow Theory. Compared to the often-cited Bernoulli (which is essentially a subtraction of pressures) or Newton's (a simplification based on momentum change) approaches, the circulation perspective might appear formidably complex or abstract to the general public. Perhaps this is precisely why it hasn't achieved wider popular appeal.

However, the perceived complexity of a rigorous explanation does not invalidate the profound contributions of scientific giants such as:

  • Lanchester, with his pioneering work on circulation and wing-tip vortices.

  • Prandtl, whose lifting-line theory revolutionized wing design.

  • Helmholtz and Kelvin, whose fundamental theorems on vortex dynamics laid crucial groundwork.

  • Kutta and Zhukovski, who formalized the lift-circulation relationship.

  • Falkner and Weissinger, who advanced methods for analyzing complex flows.

  • Batchelor, Chorin, Belotserkovski, Lewis, Katz, Cottet, Leonard, Winckelmans, Kamemoto, and Gharakhani, along with other contemporary researchers who have made invaluable contributions to theoretical hydrodynamics, potential flow, and vortex methods.

Their monumental work must not be overlooked simply because some prefer to understand complex phenomena through simplistic "physical" explanations or overly simplified models riddled with unstated assumptions. The very purpose of mathematics, in science, is to explain physics formally and sufficiently, without compromise or embellishment. In this sense, mathematics, too, is crude—unyielding, precise, and brutally honest in its depiction of physical reality. It demands rigor, and it is through this rigor that we genuinely unravel the complexities of fluid motion.

[1] Journal article: The Full Non-linear Vortex Tube-Vorton Method: the pre-stall condition | Advances in Aerodynamics | Full Text (springeropen.com)

[2] Journal article: The Full Multi-wake Vortex Lattice Method: a detached flow model based on Potential Flow Theory | Advances in Aerodynamics | Full Text (springeropen.com)






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