This blog post explains why cigarette smoke rises straight up before swirling and dispersing from a fluid dynamics perspective in an easy and interesting way.
Fluid Dynamics Seen Through Cigarette Smoke
Everyone probably has memories from childhood of staring blankly at the smoke from their father’s cigarette or the incense smoke rising from the ancestral altar. At first, it clings to surfaces, then suddenly breaks free and shoots straight up into the sky. Eventually, a swirling vortex forms, and the smoke finally disperses. You’ve probably wondered at least once about the flow of this smoke. Why does this smoke exhibit such flow patterns? Couldn’t it simply swirl like a vortex from the start and then disperse? The academic discipline that studies these reasons is called fluid mechanics.
The term “fluid mechanics” itself might feel unfamiliar. However, the proportion of fluids like air and water in the environment we live in is too significant to ignore. Therefore, numerically analyzing the relationship between fluids and other objects is extremely important. While it’s difficult to cover every example and calculation in this article, I will discuss fluid motion through several representative cases.
Basic Concepts of Fluid Mechanics
Fluid mechanics is the study of fluids in motion or at rest. It also investigates the properties that arise when fluids come into contact with solids or other fluids. Generally, matter is divided into solids and fluids. The criterion distinguishing them is their response to shear stress. Solids may deform under shear stress but do not flow. Fluids, however, cannot resist shear stress and undergo motion. We describe this motion as the fluid flowing.
The no-slip condition (adhesion condition) refers to the property where fluid particles in contact with a wall remain fixed and do not move relative to it. For fluids with low viscosity, solving problems by treating regions sufficiently distant from objects as ideal, non-viscous fluids does not result in significant errors. However, near the surface of an object, viscosity cannot be ignored. This region is called the boundary layer, and within it, the effects of viscosity must be considered.
Laminar Flow and Turbulent Flow
Laminar flow refers to fluid motion where particles move in a highly orderly manner parallel to the wall. In this case, no particles enter perpendicular to the flow direction, and no vortices form. A good example illustrating laminar flow is observing the flow of water in a thin pipe, or adding ink to the flow for observation. When the Reynolds number is low, the ink flows in a straight line, and each part of the water moves parallel to the pipe wall.
In contrast, turbulent flow is highly irregular and complex, continuously changing over time. Turbulent flow contains numerous small vortices called eddies, and larger vortices gradually break up into smaller ones before dissipating. Turbulent flow exhibits complex patterns that are difficult to predict. Due to this irregularity, the study of turbulence remains one of the major unsolved problems in physics.
Applications of Fluid Mechanics
Despite its complex principles, fluid mechanics is closely connected to our daily lives. It plays a crucial role not only in the flow of smoke or water but also in the design of the cars and airplanes we use. For example, an airplane wing’s design must optimize interaction with air to generate lift and minimize drag. This design is entirely based on fluid dynamics principles. In aerospace, studying airflow and controlling it effectively is essential. Fluid dynamics principles apply not only to wing shape but also to various components like engine air intakes and exhausts.
Furthermore, the application of fluid dynamics has become increasingly prominent in sports science recently. Minimizing fluid resistance is particularly crucial in the design of sports equipment like swimsuits. The full-body swimsuits worn by famous swimmers reduce resistance in water, helping to shorten times. The design of such sports equipment is entirely based on fluid dynamics.
Separation refers to the phenomenon where a fluid detaches from a boundary layer. This occurs when a fluid passes over a solid surface, such as an airplane wing. When separation happens, the pressure drag—which contributes to total drag—increases sharply, while lift decreases. Separation is a critical issue in aircraft design, and research to reduce it continues.
Conclusion
Fluid mechanics deals with the invisible mechanical relationships of fluids, which can make it feel somewhat abstract. Nevertheless, fluid mechanics is practically applied in diverse fields such as automobiles, aircraft, and sports equipment, contributing to making our daily lives more convenient and safe. Even at this very moment, many scientists and engineers are striving to solve unsolved problems in fluid mechanics, and the results will enrich our lives further.
