What Are the Subdisciplines of Fluid Dynamics

Aerodynamics of a Commercial Airplane – as predicted with Computational Fluid Dynamics (CFD)

An interesting, lesser-known aspect of aerodynamics is the study of the flight of birds and insects. This area, called bio-aerodynamics, investigates the unique ways that nature has solved the challenges of flight, providing inspiration for future innovations in engineering and aviation.  The conversion of metabolic and muscular power into aerodynamic power, known as the biomechanics of flight, is an intricate field of research with many practical applications. The University of California at Berkely has an indepth series of articles on this topic for those interested in reading more. 

(The force diagram of a bird taking off, in-flight, and landing – courtesy Nature – Credit D. Chin and D. Lentink)

This field is widely used in the aerospace industry, where the design and performance of aircrafts rely heavily on the principles of aerodynamics. Aerodynamics can also be applied to the design of other vehicles, such as cars and boats, to enhance their speed and efficiency. The study of aerodynamics has led to the development of various technologies, such as winglets, which improve the aerodynamic performance of aircraft.

Hydrodynamics

What is hydrodynamics? Hydrodynamics is the study of the motion of liquids, commonly water, and the forces that act on them. This branch of fluid dynamics investigates the forces and flow characteristics of liquids, such as pressure, viscosity, and turbulence, which play a crucial role in understanding various natural and man-made phenomena.

The history of hydrodynamics dates back to ancient times, with early civilizations like the Egyptians and Romans attempting to understand and control water flow for irrigation and transportation purposes. However, the modern understanding of hydrodynamics began to take shape in the 18th century, with the works of famous scientists like Leonhard Euler and Jean le Rond d'Alembert, who developed the fundamental equations of fluid motion.

Hydrodynamics has a wide range of practical applications. Engineers and scientists rely on hydrodynamic principles to design ships and submarines, develop wastewater treatment processes, and study the impact of climate change on sea levels and coastal environments.  Most of these industries have traditionally relied on experimental model testing and more recently begun to leverage computational fluid dynamics (CFD) in predicting performance. For more information on how this transitioning is making new product development easier, check out this Ebook from Siemens.

A lesser-known aspect of hydrodynamics is the study of superfluids, which are fluids that exhibit zero viscosity and therefore flow without any resistance. When stirred, superfluids will continue to flow indefinitely. How cool is that? Another interesting phenomenon seen in superfluids is the fountain effect. A tube with a nanoporous membrane plug at one end is suspended in a bath of helium superfluid. The superfluid can flow into the tube through the membrane. When heated, the helium flows up through the tube and squirts out like a fountain.  Superfluid characteristics, observed in certain substances like liquid helium at extremely low temperatures, have fascinated scientists due to their uniqueness in nature and potential applications in areas like quantum mechanics and advanced cooling systems.  The great mathematician and physicist Lev Landau received the 1962 Nobel Prize in Physics for his efforts in defining the mathematical theory of superfluidity. 

Hydrology

What is hydrology? Hydrology is the study of the movement, distribution, and properties of water on Earth, encompassing surface water, groundwater, and atmospheric moisture. This interdisciplinary field combines aspects of fluid dynamics, meteorology, geology, and ecology to understand the complex water cycle and its effect on the environment and human activities.

Hydrology has its roots in ancient civilizations, where the management of water resources played a critical role in the development of agriculture and urban planning. The field has evolved over time, with significant contributions by scientists like Pierre Perrault, Edmond Halley and Edme Mariotte in the 17th and 18th centuries, who laid the foundation for modern hydrological concepts like the water balance equation.

What is the Water Balance Equation in Hydrology?

The water balance equation is an accounting of the inputs and outputs of water in a given system, represented as precipitation minus evapotranspiration minus runoff equals change in storage.  The water balance equation can be expressed mathematically as:

P = ET + Q + ΔS

where P is precipitation, ET is evapotranspiration, Q is runoff, and ΔS is the change in storage of water in the system over a given period of time.

The importance of hydrology is hard to overstate, as it plays a vital role in the management of water resources, flood control, and environmental conservation. Hydrologists work closely with engineers and policymakers to ensure the sustainable use of water, protect ecosystems, and mitigate the effects of natural disasters like floods and droughts. Additionally, hydrology is crucial in understanding the impacts of climate change on water availability and quality.

(Hydrology)

An interesting and lesser-known aspect of hydrology is the study of ecohydrology, which focuses on the interactions between water and ecosystems. This sub-discipline aims to understand how water influences the structure and function of ecosystems, and how these, in turn, affect the hydrological cycle. Ecohydrology plays a critical role in the management and restoration of ecosystems, such as wetlands and forests, which provide essential services like water purification and flood mitigation.

Acoustics

Acoustics is the study of sound waves and their behavior in different mediums.

“Wait, what is sound?” you ask.

Sound is a compressive wave in a fluid, a vibration being transmitted by impact reactions between molecules in a liquid or gas.

(A sound wave of vibrating fluid molecules, visualized by spheres - courtesy Dan Russell)

Acousticians investigate the generation, transmission, and reception of sound waves, as well as their interaction with objects and the environment.

The study of acoustics dates back to ancient times, with philosophers like Pythagoras exploring the mathematical relationships between musical notes. However, the modern understanding of acoustics emerged in the 19th and 20th centuries, with the works of scientists like Lord Rayleigh and Hermann von Helmholtz, who made significant contributions to the theory of sound and its applications in areas like music, architecture, and underwater navigation.

Acoustics has a wide range of practical applications, from designing concert halls and recording studios with optimal sound quality to developing noise reduction technologies for aircraft and automobiles. Acoustics is also crucial in the field of underwater acoustics, where the study of sound propagation in water is used for tasks like submarine detection, underwater communication, and marine mammal research.  Today, Computational Fluid Dynamics (CFD) is being frequently used to assess the noise created by modern technologies, as demonstrated in the image below. 

(Acoustic Noise Being Created by Aeroacoustics of a moving vehicle – as predicted by CFD Simulation – Courtesy of Siemens Digital Industries Software)

A fascinating but lesser-known aspect of acoustics is the study of bioacoustics, which investigates the production, reception, and use of sound by animals. This interdisciplinary field combines biology, ecology, and acoustics to understand how animals use sound for communication, navigation, and hunting, providing valuable insights into animal behavior and the functioning of ecosystems.  A very interesting potential application for artificial intelligence is in the decoding of sounds that animal use to communicate. With this technology we may soon find out what whales, dogs and birds have been saying to one another and to us for all this time. 

Gas Dynamics

What is gas dynamics? Gas dynamics is the study of the behavior of gases in motion, particularly at high speeds and temperatures. Gas dynamics is sometimes also labeled as Compressible Fluid Dynamics. This branch of fluid dynamics focuses on the compressibility, thermodynamics, and shock waves associated with high-speed gas flows, which are critical for understanding phenomena like supersonic flight and combustion processes.

The field of gas dynamics emerged in the early 20th century, with the works of scientists like Ludwig Prandtl and Theodore von Kármán, who developed the fundamental theories and experimental methods for studying high-speed gas flows. The development of gas dynamics was closely tied to the evolution of aviation and space exploration, as engineers sought to understand and control the behavior of gases at high speeds.

(A US Navy F/A-F18E Super Hornet Going Supersonic with a Shock Wave Being Visible by Condensing Water Vapor - Courtesy US Department of Defense)

Gas dynamics has numerous practical applications, from designing supersonic aircraft and rocket engines to understanding the behavior of high-velocity gases in industrial processes and natural phenomena like volcanic eruptions and meteor impacts. The study of gas dynamics is also critical for the development of hypersonic vehicles, which travel at speeds greater than Mach 5 and hold promise for revolutionizing air and space travel.

An interesting and lesser-known aspect of gas dynamics is the study of rarefied gas dynamics, which deals with the behavior of gases in low-pressure environments, such as those encountered in the upper atmosphere or in outer space. In these conditions, the conventional continuum-based fluid dynamics approaches may not be applicable, and researchers must often rely on specialized techniques, like the kinetic theory of gases and numerical simulations, to understand and predict the behavior of rarefied gases.

Conclusions

Fluid dynamics encompasses a diverse range of fascinating subdisciplines within physics and engineering. Aerodynamics examines the motion of gases within the atmosphere and how forces act on objects moving through air, from airplanes to birds. Hydrodynamics studies the motion of liquids, especially water, and how forces act within and on liquid objects. Hydrology focuses on the distribution and movement of water on and beneath the Earth's surface, seeking to understand and predict things like groundwater flow and flood risks.  Acoustics analyzes how sound waves propagate and behave within fluids, both gases like air and liquids. Finally, gas dynamics considers the behavior, effects and properties of gases in motion, whether under high pressures or low densities.  

Taken together, these subdisciplines within fluid dynamics show us the remarkable, intricate ways that fluids behave and interact all around us and within our environment. They provide valuable insight for improving our designs, predicting natural phenomena, and gaining a deeper scientific understanding of the complex physical world that we inhabit. The fluids that envelop and sustain our planet and its living things continue to yield fascinating secrets and puzzles for contemporary and future generations of fluid dynamicists to explore