Turbulent flow is a type of fluid flow characterised by chaotic and irregular movements of the fluid. It occurs when the fluid velocity at a point changes continuously and rapidly in both magnitude and direction, leading to the formation of eddies and vortices. This is in contrast to laminar flow, which is characterised by smooth and orderly fluid motion. Turbulent flow is common in many real-world applications, such as the flow of wind, rivers, blood in arteries, and the flow of fluids through pipelines, turbines, and pumps.
The transition from laminar to turbulent flow often occurs when the Reynolds number, a dimensionless quantity that characterises the fluid flow, exceeds a critical value. Understanding and modelling turbulent flow is an important area of research in fluid mechanics due to its many practical applications.
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Turbulent flow structure
Lewis Fry Richardson’s famous rhyme about the structure of turbulence, published in “Weather Prediction by Numerical Process” in 1920, is still remembered today for its clarity and insight. Richardson’s principle states that “big whirls have little whirls that feed on their velocity, and little whirls have lesser whirls and so on to viscosity”. This principle was motivated by the energetics of turbulence and highlighted the unique feature of turbulent flows: they are energy-demanding.
Turbulent flows are characterised by the transfer of kinetic energy from larger to smaller eddies through a process known as the “energy cascade.” This transfer of energy is mainly inertial, and as the eddies become smaller, the viscosity of the fluid eventually dissipates the kinetic energy. The result is that a turbulent flow will eventually decay to a laminar flow at the smallest scales unless it is fed by an external source of energy.
Richardson’s studies emphasise the importance of understanding the energetics of turbulent flows. The energy cascade and the dissipation of energy at small scales play a crucial role in many applications, from meteorology to chemical engineering. By better understanding the behaviour of turbulent flows, scientists and engineers can optimise processes and design systems that involve fluid flow more efficiently.
Turbulent flow application
Turbulent flow refers to an irregular flow of fluid characterised by the presence of eddies, swirls, and flow instabilities. Unlike laminar flow, where fluid flows in parallel layers with no disruptions between them, turbulence is governed by both high-momentum convection and low-momentum diffusion.
Turbulence is a common phenomenon in many natural and engineering applications, such as the promotion of cigarette smoke, falls, blood flow in arteries, and atmospheric recirculation. It even plays an important role in the aerodynamics of vehicles like cars, planes, and ships, as well as in industrial processes like heat exchangers, quenching, and continuous casting of steel.
FAQs
How does turbulence differ from laminar flow?
Laminar flow is characterised by parallel fluid layers with no disruptions, while turbulence is characterised by irregular fluid motion and flow instabilities.
What are Kolmogorov microscales?
Kolmogorov microscales are the smallest turbulence scales that are characterised by a set of length, time, and velocity scales, as described by Kolmogorov's theory.
What is the purpose of simulating turbulent flow?
The purpose of simulating turbulent flow is to understand fluid behaviour, predict fluid performance, and optimise fluid systems in engineering and industrial applications.