Applications and significance of turbulent flow in nature and technology

From: veritasium

Turbulent flow, in contrast to laminar flow where fluid particles move in organized, parallel layers, is a complex phenomenon [00:00:17], [00:00:30]. While seemingly messy and lacking a universally agreed-upon formal definition [00:01:16], [00:01:20], its presence is unmistakable [00:01:26]. Turbulent flow is pervasive in both natural systems and technological applications, offering a richness and unpredictability that makes it profoundly significant [00:15:32].

Characteristics of Turbulent Flow

Despite the lack of a formal definition, turbulent flow can be understood through several key characteristics:

• Unpredictability and Chaos Turbulent flow is inherently unpredictable, messy, and definitionally chaotic [00:01:41], [00:01:46]. It exhibits a sensitive dependence on initial conditions, meaning a small change in the fluid can drastically alter its final state [00:01:52], [00:01:56]. This makes precise predictions impossible, limiting analysis to statistical approaches [00:02:02], [00:02:06]. Even the Navier-Stokes equations, which are meant to govern all fluid flow, including turbulence, are notoriously difficult to solve, with a million-dollar prize offered for significant progress [00:02:09], [00:02:18].
• Eddies and Vortices A defining characteristic of turbulent flow is its composition of numerous interacting swirls of fluid, known as eddies or vortices [00:03:10], [00:03:13]. These eddies span a vast range of sizes, from micrometers up to meters in diameter, as observed in the air within a room [00:03:19], [00:03:23].
• Diffusiveness Turbulent flow is diffusive, meaning it effectively mixes substances [00:05:01], [00:05:04]. It causes things like dye, heat, or momentum to spread out and become distributed throughout the fluid [00:05:08], [00:05:11], [00:05:14].
• Dissipativeness Turbulence is dissipative, consuming energy at its largest scales (big eddies) and transferring it down to progressively smaller eddies [00:07:07], [00:07:11], [00:07:15]. This energy is ultimately dissipated as heat at the smallest scales [00:07:19]. Consequently, maintaining turbulence requires a constant energy source to generate large eddies [00:07:25], [00:07:27].

Natural Occurrences and Significance

Turbulent flow is not merely an exception but the rule in our everyday lives, occurring more frequently than laminar flow [00:06:14], [00:06:18].

Macro-scale Phenomena

• Atmospheric and Cosmic Scales Turbulent motion is observed from the largest structures in the universe down to human scales [00:15:01]. Examples include:
  • The turbulent air in a typical room [00:02:58].
  • The surface of the Sun, where hot plasma rises in huge convection currents forming Texas-sized turbulent cells [00:03:38], [00:03:45].
  • The giant turbulent swirls on Jupiter, such as the Great Red Spot, a vortex larger than Earth, with the rest of the planet covered in eddies of various sizes [00:03:49], [00:03:53].
  • The turbulent dust between stars, which causes radio sources to twinkle, similar to how atmospheric turbulence makes stars twinkle [00:04:05], [00:04:10]. The Orion Nebula, spanning 24 light-years, is a stunning example of this turbulent cosmic dust [00:04:16].
  • The Earth’s atmosphere near the surface [00:06:28] and within cumulus and cumulonimbus clouds [00:06:31].
  • Turbulence plays an essential role in the formation of raindrops, effectively “making it rain” [00:06:36], [00:06:41].

Biological and Human Systems

• Internal Fluid Dynamics Turbulent flow is also present within the human body:
  • The air flowing in and out of your lungs [00:06:22].
  • The blood pumping through your aorta [00:06:25].
• Animal Adaptation Animals have adapted to live in a turbulent world. An experiment demonstrated that a dead fish placed in the wake of an object could actually swim upstream, suggesting that fish can exploit turbulent water for more efficient movement [00:14:39], [00:14:44], [00:14:48].

Technological Applications and Consequences

Fluid flow impacts aerodynamics and efficiency significantly, and understanding turbulence is crucial for engineering applications [00:07:32].

Boundary Layers and Drag

• Skin Friction When fluid flows over a surface, a boundary layer forms—a region where fluid velocity adjusts from zero at the surface (due to friction and adhesion) to the free stream velocity further away [00:07:44], [00:07:54], [00:08:04], [00:08:10], [00:08:19]. The surface applies a force to the fluid to form this layer, resulting in an equal and opposite force on the surface known as skin friction [00:08:27], [00:08:32], [00:08:37].
• Turbulent Boundary Layers and Increased Drag If fluid velocity is high or the surface is long, a laminar boundary layer can transition to turbulence [00:08:42], [00:08:46]. In a turbulent boundary layer, the fluid swirls and mixes, bringing faster-flowing fluid closer to the surface, which significantly increases skin friction and thus drag [00:08:51], [00:08:59], [00:09:02]. Boundary layers around airplanes and large ships are mostly turbulent, and skin friction accounts for the majority of the drag they experience [00:09:07], [00:09:12].
• Surface Smoothness Laminar boundary layers can be “tripped” into turbulence by small obstacles or rough surfaces [00:09:16]. Therefore, clean, smooth surfaces can significantly reduce drag, saving fuel costs for vehicles like cars and airplanes [00:09:24], [00:09:30], [00:09:38].

Engineering for Controlled Turbulence

While often associated with increased drag, turbulence can be advantageous when strategically induced.

• Airplane Wings and Stall Prevention

  • Although planes are designed to be as smooth as possible to reduce drag [00:09:43], [00:09:50], some features, like vortex generators, are deliberately added [00:10:03], [00:10:10].
  • At low speeds or high angles of attack, the airflow over a wing can separate from its surface, leading to a condition called stall, which dramatically decreases lift [00:10:23], [00:10:28], [00:10:32], [00:10:36], [00:10:49].
  • Vortex generators are small fins on the wing that induce turbulence [00:10:55], [00:11:02]. This turbulence mixes faster-flowing air from higher up down closer to the wing’s surface, “energizing” the fluid flow [00:11:06]. This energized flow can then follow the wing’s curve for longer, keeping the airflow attached and maintaining lift [00:11:15], [00:11:22], [00:11:25]. Thus, inducing turbulence on wings allows airplanes to fly more efficiently and effectively, enabling climbs at higher angles of attack [00:11:30], [00:11:32], [00:11:37].

• Golf Balls and Distance

  • Early golf balls were smooth, but golfers noticed that nicked and dirty balls flew further [00:11:44], [00:11:50], [00:11:54].
  • A smooth ball creates a laminar boundary layer with low skin friction [00:12:03], [00:12:08], [00:12:11]. However, this laminar flow separates easily, leaving a large wake of low-pressure turbulent air behind the ball, which causes significant pressure drag [00:12:15], [00:12:19], [00:12:21].
  • Dimples on golf balls force the boundary layer to become turbulent [00:13:01]. A turbulent boundary layer can cling to the ball’s surface for longer before separating, thereby reducing the size of the turbulent wake and significantly decreasing pressure drag [00:12:32], [00:12:36], [00:12:40], [00:12:45], [00:12:48]. This reduction in pressure drag outweighs the increase in skin friction, allowing golf balls to travel further [00:12:48], [00:13:17]. Dimples can reduce the drag coefficient by almost a factor of two [00:13:12], [00:13:14].

• Harnessing Wake Energy Scientists are also exploring ways to harness the energy contained within turbulent wakes [00:13:29], [00:13:32], [00:13:37]. One pattern observed in the wake of an object is periodic vortex shedding, forming a von Karman Vortex Street, which can be seen in laboratory experiments or even in cloud patterns around islands from space [00:13:48], [00:13:50], [00:13:53], [00:13:57], [00:14:00], [00:14:05], [00:14:12], [00:14:17], [00:14:21].

• Rocket Nozzles Turbulent flow is also studied in specialized areas like rocket nozzles [00:15:41], [00:15:46].

Conclusion

Turbulence is an omnipresent and vital aspect of fluid dynamics, essential to phenomena ranging from the formation of raindrops and the twinkling of stars to the efficient flight of airplanes and the trajectory of golf balls [00:15:07], [00:15:10]. Unlike the perceived simplicity of laminar flow, turbulent flow, with its inherent complexity and unpredictability, offers a richer and deeper understanding of the messy, yet beautiful, world around us [00:15:25], [00:15:32].