Turbulence

turbulentturbulent flowatmospheric turbulenceturbulent mixingfluid turbulenceturbulent flowsturbulent airflowturbulent diffusionturbulentlyaerodynamic flowfields
In fluid dynamics, turbulence or turbulent flow is any pattern of fluid motion characterized by chaotic changes in pressure and flow velocity.wikipedia
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Drag (physics)

dragaerodynamic dragair resistance
In general terms, in turbulent flow, unsteady vortices appear of many sizes which interact with each other, consequently drag due to friction effects increases.
Drag force is proportional to the velocity for a laminar flow and the squared velocity for a turbulent flow.

Laminar flow

laminarlaminar-flowlaminar air flow
It is in contrast to a laminar flow regime, which occurs when a fluid flows in parallel layers, with no disruption between those layers. Smoke rising from a cigarette is mostly turbulent flow. However, for the first few centimeters the flow is laminar. The smoke plume becomes turbulent as its Reynolds number increases, due to its flow velocity and characteristic length increasing.
When a fluid is flowing through a closed channel such as a pipe or between two flat plates, either of two types of flow may occur depending on the velocity and viscosity of the fluid: laminar flow or turbulent flow.

Clear-air turbulence

clear air turbulenceturbulenceair turbulence
Clear-air turbulence experienced during airplane flight, as well as poor astronomical seeing (the blurring of images seen through the atmosphere.)
Clear-air turbulence (CAT) is the turbulent movement of air masses in the absence of any visual clues, such as clouds, and is caused when bodies of air moving at widely different speeds meet.

Reynolds number

ReynoldsRelow Reynolds
The onset of turbulence can be predicted by a dimensionless constant called the Reynolds number, which calculates the balance between kinetic energy and viscous damping in a fluid flow. Smoke rising from a cigarette is mostly turbulent flow. However, for the first few centimeters the flow is laminar. The smoke plume becomes turbulent as its Reynolds number increases, due to its flow velocity and characteristic length increasing.
With respect to laminar and turbulent flow regimes:

Astronomical seeing

seeingatmospheric seeingatmospheric turbulence
Clear-air turbulence experienced during airplane flight, as well as poor astronomical seeing (the blurring of images seen through the atmosphere.)
Astronomical seeing refers to the amount of apparent blurring and twinkling of astronomical objects like stars due to turbulent mixing in the atmosphere of Earth, causing variations of the optical refractive index.

Plume (fluid dynamics)

plumeplumesjets
Smoke rising from a cigarette is mostly turbulent flow. However, for the first few centimeters the flow is laminar. The smoke plume becomes turbulent as its Reynolds number increases, due to its flow velocity and characteristic length increasing.
A further phenomenon of importance is whether a plume has laminar flow or turbulent flow.

Mixed layer

mixed layer depthvertical mixingmixed surface layer
The oceanic and atmospheric mixed layers and intense oceanic currents.
Turbulence typically plays a role in the formation of fluid mixed layers.

Dynamic scraped surface heat exchanger

(dynamic) scraped surface heat exchanger
The flow conditions in many industrial equipment (such as pipes, ducts, precipitators, gas scrubbers, dynamic scraped surface heat exchangers, etc.) and machines (for instance, internal combustion engines and gas turbines).
removing the fouling layers, increasing turbulence in case of high viscosity flow, and avoiding the generation of ice and other process by-products.

Bruit

bruitsarterial bruitshumming noise
In the medical field of cardiology, a stethoscope is used to detect heart sounds and bruits, which are due to turbulent blood flow. In normal individuals, heart sounds are a product of turbulent flow as heart valves close. However, in some conditions turbulent flow can be audible due to other reasons, some of them pathological. For example, in advanced atherosclerosis, bruits (and therefore turbulent flow) can be heard in some vessels that have been narrowed by the disease process.
Bruit ( or, from French, 'noise'), also called vascular murmur, is the abnormal sound generated by turbulent flow of blood in an artery due to either an area of partial obstruction or a localized high rate of blood flow through an unobstructed artery.

Boundary layer

boundary layersboundary-layerboundary layer theory
Flow over a golf ball. (This can be best understood by considering the golf ball to be stationary, with air flowing over it.) If the golf ball were smooth, the boundary layer flow over the front of the sphere would be laminar at typical conditions. However, the boundary layer would separate early, as the pressure gradient switched from favorable (pressure decreasing in the flow direction) to unfavorable (pressure increasing in the flow direction), creating a large region of low pressure behind the ball that creates high form drag. To prevent this from happening, the surface is dimpled to perturb the boundary layer and promote transition to turbulence. This results in higher skin friction, but moves the point of boundary layer separation further along, resulting in lower form drag and lower overall drag.
However, the boundary layer inevitably thickens and becomes less stable as the flow develops along the body, and eventually becomes turbulent, the process known as boundary layer transition.

Kolmogorov microscales

Kolmogorov length scaleKolmogorov scale
The scale at which this happens is the Kolmogorov length scale.
Kolmogorov microscales are the smallest scales in turbulent flow.

Werner Heisenberg

HeisenbergW. HeisenbergHeisenberg, Werner
According to an apocryphal story, Werner Heisenberg was asked what he would ask God, given the opportunity.
He also made important contributions to the theories of the hydrodynamics of turbulent flows, the atomic nucleus, ferromagnetism, cosmic rays, and subatomic particles, and he was instrumental in planning the first West German nuclear reactor at Karlsruhe, together with a research reactor in Munich, in 1957.

Energy cascade

Via this energy cascade, turbulent flow can be realized as a superposition of a spectrum of flow velocity fluctuations and eddies upon a mean flow.
This concept plays an important role in the study of well-developed turbulence.

Lewis Fry Richardson

RichardsonLewis F. RichardsonLewis Fry Richardson Medal
Nevertheless, the turbulent diffusivity is the simplest approach for quantitative analysis of turbulent flows, and many models have been postulated to calculate it. For instance, in large bodies of water like oceans this coefficient can be found using Richardson's four-third power law and is governed by the random walk principle. The Russian mathematician Andrey Kolmogorov proposed the first statistical theory of turbulence, based on the aforementioned notion of the energy cascade (an idea originally introduced by Richardson) and the concept of self-similarity.
He was also interested in atmospheric turbulence and performed many terrestrial experiments.

Inviscid flow

inviscidinviscid fluiddissipation-less limit
The energy "cascades" from these large-scale structures to smaller scale structures by an inertial and essentially inviscid mechanism.
These "inviscid flow arrangements" are vortex-like and may play a key role in the formation of the tornado, the tropical cyclone, and turbulence.

Taylor microscale

Taylor microscales : The intermediate scales between the largest and the smallest scales which make the inertial subrange. Taylor microscales are not dissipative scale but pass down the energy from the largest to the smallest without dissipation. Some literatures do not consider Taylor microscales as a characteristic length scale and consider the energy cascade to contain only the largest and smallest scales; while the latter accommodate both the inertial subrange and the viscous sublayer. Nevertheless, Taylor microscales are often used in describing the term “turbulence” more conveniently as these Taylor microscales play a dominant role in energy and momentum transfer in the wavenumber space.
The Taylor microscale, which is sometimes called the turbulence length scale, is a length scale used to characterize a turbulent fluid flow.

Snow fence

snow fencingsnowfence
Snow fences work by inducing turbulence in the wind, forcing it to drop much of its snow load near the fence.
Snow fences work by causing turbulence in the wind, such that it drops much of its snow load on the lee side of the fence.

Vortex

vorticesvortex linesvortexes
In general terms, in turbulent flow, unsteady vortices appear of many sizes which interact with each other, consequently drag due to friction effects increases.
Vortices are a major component of turbulent flow.

Parasitic drag

form draginterference dragdrag
Flow over a golf ball. (This can be best understood by considering the golf ball to be stationary, with air flowing over it.) If the golf ball were smooth, the boundary layer flow over the front of the sphere would be laminar at typical conditions. However, the boundary layer would separate early, as the pressure gradient switched from favorable (pressure decreasing in the flow direction) to unfavorable (pressure increasing in the flow direction), creating a large region of low pressure behind the ball that creates high form drag. To prevent this from happening, the surface is dimpled to perturb the boundary layer and promote transition to turbulence. This results in higher skin friction, but moves the point of boundary layer separation further along, resulting in lower form drag and lower overall drag.
For comparison, the turbulent empirical relation known as the 1/7 Power Law (derived by Theodore von Kármán) is:

Fluid dynamics

hydrodynamicshydrodynamicfluid flow
In fluid dynamics, turbulence or turbulent flow is any pattern of fluid motion characterized by chaotic changes in pressure and flow velocity.
Turbulent flows are unsteady by definition.

Vortex stretching

stretchingstretching the vortexvortex tubes
;Rotationality : Turbulent flows have non-zero vorticity and are characterized by a strong three-dimensional vortex generation mechanism known as vortex stretching.
Vortex stretching is at the core of the description of the turbulence energy cascade from the large scales to the small scales in turbulence.

Andrey Kolmogorov

KolmogorovA. N. KolmogorovA.N. Kolmogorov
The Russian mathematician Andrey Kolmogorov proposed the first statistical theory of turbulence, based on the aforementioned notion of the energy cascade (an idea originally introduced by Richardson) and the concept of self-similarity.
Andrey Nikolaevich Kolmogorov (, 25 April 1903 – 20 October 1987) was a 20th-century Soviet mathematician who made significant contributions to the mathematics of probability theory, topology, intuitionistic logic, turbulence, classical mechanics, algorithmic information theory and computational complexity.

Dissipation

dissipateddissipativedissipate
;Dissipation : To sustain turbulent flow, a persistent source of energy supply is required because turbulence dissipates rapidly as the kinetic energy is converted into internal energy by viscous shear stress.
Waves or oscillations, lose energy over time, typically from friction or turbulence.

Large eddy simulation

LESlarge-eddy simulationLarge Eddy Simulation (LES)
Large eddy simulation
Large eddy simulation (LES) is a mathematical model for turbulence used in computational fluid dynamics.

Navier–Stokes equations

viscous flowNavier-Stokes equationsNavier-Stokes equation
Although it is possible to find some particular solutions of the Navier–Stokes equations governing fluid motion, all such solutions are unstable to finite perturbations at large Reynolds numbers.
The nonlinearity makes most problems difficult or impossible to solve and is the main contributor to the turbulence that the equations model.