# 湍流

In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to a laminar flow, which occurs when a fluid flows in parallel layers, with no disruption between those layers.

Turbulence is commonly observed in everyday phenomena such as surf, fast flowing rivers, billowing storm clouds, or smoke from a chimney, and most fluid flows occurring in nature or created in engineering applications are turbulent.:2 Turbulence is caused by excessive kinetic energy in parts of a fluid flow, which overcomes the damping effect of the fluid's viscosity. For this reason turbulence is commonly realized in low viscosity fluids. In general terms, in turbulent flow, unsteady vortices appear of many sizes which interact with each other, consequently drag due to friction effects increases. This increases the energy needed to pump fluid through a pipe. Turbulence can be exploited, for example, by devices such as aerodynamic spoilers on aircraft that "spoil" the laminar flow to increase drag and reduce lift.

The onset of turbulence can be predicted by the dimensionless Reynolds number, the ratio of kinetic energy to viscous damping in a fluid flow. However, turbulence has long resisted detailed physical analysis, and the interactions within turbulence create a very complex phenomenon. Richard Feynman has described turbulence as the most important unsolved problem in classical physics.

## Examples of turbulence

Laminar and turbulent water flow over the hull of a submarine. As the relative velocity of the water increases turbulence occurs.

【图1：Laminar and turbulent water flow over the hull of a submarine. As the relative velocity of the water increases turbulence occurs. 潜艇船体外，水的层流和紊流。湍流随着水的相对速度增加而出现。】

【图2：tip vortex from an airplane wing飞机机翼上的翼尖涡流】

• Smoke rising from a cigarette. For the first few centimeters, the smoke is laminar. The smoke plume becomes turbulent as its Reynolds number increases with increases in flow velocity and characteristic lengthscale.
• 烟雾从一支香烟上升起。最初的几厘米，烟雾是层流。 雷诺数（Reynolds number ）随着流速和特征长度增加，烟雾羽流（plume）变成了湍流。

• 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, the surface is dimpled to perturb the boundary layer and promote turbulence. This results in higher skin friction, but it moves the point of boundary layer separation further along, resulting in lower drag.
• 空气流过高尔夫球。 （假设高尔夫球静止而空气在上面流动最容易理解。）如果高尔夫球是光滑的，那么在典型条件下，球体前部的 边界层（ boundary layer会出现层流。 但是，因为压力梯度会从顺压（压力沿流动方向减小）切换到逆压（压力沿流动方向增大），边界层会提前分离，从而在球后形成一个大低压区，产生较高的型阻（form drag）。 在球的表面制造凹槽以扰动边界层并促进湍流可以避免这种情况。 这会产生较高的表面摩擦，但会进一步移动边界层分离点，从而导致阻力减少。
• 飞机飞行时经历晴空湍流（Clear-air turbulence），以及 天文视宁度（astronomical seeing）不佳（通过大气看到的图像模糊）。
• 大部分陆地的大气环流
• The oceanic and atmospheric mixed layers and intense oceanic currents.
• 海洋和大气混合层和强烈的洋流
• 许多工业设备(如管道、管道、除尘器、气体洗涤器、动态刮面热交换器等)和机器(如 内燃机internal combustion engine 燃气轮机gas turbine )中的流动状况。
• The external flow over all kinds of vehicles such as cars, airplanes, ships, and submarines.
• The external flow over all kinds of vehicles such as cars, airplanes, ships, and submarines.
• 各种交通工具，如汽车、飞机、船舶和潜艇的外部流。
• The motions of matter in stellar atmospheres.
• The motions of matter in stellar atmospheres.
• 恒星大气中物质的运动。
• A jet exhausting from a nozzle into a quiescent fluid. As the flow emerges into this external fluid, shear layers originating at the lips of the nozzle are created. These layers separate the fast moving jet from the external fluid, and at a certain critical Reynolds number they become unstable and break down to turbulence.
• A jet exhausting from a nozzle into a quiescent fluid. As the flow emerges into this external fluid, shear layers originating at the lips of the nozzle are created. These layers separate the fast moving jet from the external fluid, and at a certain critical Reynolds number they become unstable and break down to turbulence.
• 从喷嘴排放到静止流体中的射流。 当流体流入该外部流体中时，会在喷嘴的边缘产生剪切层。 这些层将快速移动的射流与外部流体分开，并且在某个临界雷诺数时，它们变得不稳定并分解为湍流。
• Biologically generated turbulence resulting from swimming animals affects ocean mixing. 
• Biologically generated turbulence resulting from swimming animals affects ocean mixing.
• 游泳动物引起的生物湍流会影响海洋混合。

• 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 inducing turbulence in the wind, forcing it to drop much of its snow load near the fence.
• 防雪栅栏的工作原理是在风中产生湍流，迫使其将大部分雪荷载降到栅栏附近。
• Bridge supports (piers) in water. In the late summer and fall, when river flow is slow, water flows smoothly around the support legs. In the spring, when the flow is faster, a higher Reynolds number is associated with the flow. The flow may start off laminar but is quickly separated from the leg and becomes turbulent.
• Bridge supports (piers) in water. In the late summer and fall, when river flow is slow, water flows smoothly around the support legs. In the spring, when the flow is faster, a higher Reynolds number is associated with the flow. The flow may start off laminar but is quickly separated from the leg and becomes turbulent.
• 桥在水中支撑码头。 在夏末和秋季，当河流流量缓慢时，水在支柱周围顺畅流动。 在春季，水流更快时，较高的雷诺数与流动相关联。 流动可能从层流开始，但很快与支柱分离并变得湍流。

• In many geophysical flows (rivers, atmospheric boundary layer), the flow turbulence is dominated by the coherent structures and turbulent events. A turbulent event is a series of turbulent fluctuations that contain more energy than the average flow turbulence. The turbulent events are associated with coherent flow structures such as eddies and turbulent bursting, and they play a critical role in terms of sediment scour, accretion and transport in rivers as well as contaminant mixing and dispersion in rivers and estuaries, and in the atmosphere.
• In many geophysical flows (rivers, atmospheric boundary layer), the flow turbulence is dominated by the coherent structures and turbulent events. A turbulent event is a series of turbulent fluctuations that contain more energy than the average flow turbulence.
• 在许多地球物理流动(河流、大气边界层)中，湍流主要由凝聚结构和湍流事件所控制。湍流事件是一系列包含比平均湍流更多能量的湍流波动。
• 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.
• 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.
• 在心脏病学的医学领域中，听诊器被用于检测心音和杂音，这些都是由于血液湍流引起的。 在正常个体中，心音是心脏瓣膜关闭时湍流的产物。 但是，在某些情况下，由于其他原因，某些是病理性原因，也可以听到心音或杂音。 例如，在 晚期动脉粥样硬化atherosclerosis中，在某些因疾病过程而变窄的血管中会产生湍流，因此会听到杂音。
• Recently, turbulence in porous media became a highly debated subject.
• Recently, turbulence in porous media became a highly debated subject.
• 最近，多孔介质中的湍流成为一个备受争议的话题

## Features

Flow visualization of a turbulent jet, made by laser-induced fluorescence. The jet exhibits a wide range of length scales, an important characteristic of turbulent flows.

laser-induced fluorescence. The jet exhibits a wide range of length scales, an important characteristic of turbulent flows.]]

【图3: False color image of the far field of a submerged turbulent jet. 紊乱淹没射流远场的伪彩色图像。射流的长度范围很广，这是湍流的一个重要特征激光诱导荧光。】

Turbulence is characterized by the following features:

Turbulence is characterized by the following features:

Irregularity
Turbulent flows are always highly irregular. For this reason, turbulence problems are normally treated statistically rather than deterministically. Turbulent flow is chaotic. However, not all chaotic flows are turbulent.
Irregularity : Turbulent flows are always highly irregular. For this reason, turbulence problems are normally treated statistically rather than deterministically. Turbulent flow is chaotic. However, not all chaotic flows are turbulent.


Diffusivity
The readily available supply of energy in turbulent flows tends to accelerate the homogenization (mixing) of fluid mixtures. The characteristic which is responsible for the enhanced mixing and increased rates of mass, momentum and energy transports in a flow is called "diffusivity".
Diffusivity :The readily available supply of energy in turbulent flows tends to accelerate the homogenization (mixing) of fluid mixtures.  The characteristic which is responsible for the enhanced mixing and increased rates of mass, momentum and energy transports in a flow is called "diffusivity".


Turbulent diffusion is usually described by a turbulent diffusion coefficient. This turbulent diffusion coefficient is defined in a phenomenological sense, by analogy with the molecular diffusivities, but it does not have a true physical meaning, being dependent on the flow conditions, and not a property of the fluid itself. In addition, the turbulent diffusivity concept assumes a constitutive relation between a turbulent flux and the gradient of a mean variable similar to the relation between flux and gradient that exists for molecular transport. In the best case, this assumption is only an approximation. 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. In rivers and large ocean currents, the diffusion coefficient is given by variations of Elder's formula.

Turbulent diffusion is usually described by a turbulent diffusion coefficient. This turbulent diffusion coefficient is defined in a phenomenological sense, by analogy with the molecular diffusivities, but it does not have a true physical meaning, being dependent on the flow conditions, and not a property of the fluid itself. In addition, the turbulent diffusivity concept assumes a constitutive relation between a turbulent flux and the gradient of a mean variable similar to the relation between flux and gradient that exists for molecular transport. In the best case, this assumption is only an approximation. 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. In rivers and large ocean currents, the diffusion coefficient is given by variations of Elder's formula.

Rotationality
Turbulent flows have non-zero vorticity and are characterized by a strong three-dimensional vortex generation mechanism known as vortex stretching. In fluid dynamics, they are essentially vortices subjected to stretching associated with a corresponding increase of the component of vorticity in the stretching direction—due to the conservation of angular momentum. On the other hand, vortex stretching is the core mechanism on which the turbulence energy cascade relies to establish and maintain identifiable structure function. In general, the stretching mechanism implies thinning of the vortices in the direction perpendicular to the stretching direction due to volume conservation of fluid elements. As a result, the radial length scale of the vortices decreases and the larger flow structures break down into smaller structures. The process continues until the small scale structures are small enough that their kinetic energy can be transformed by the fluid's molecular viscosity into heat. Turbulent flow is always rotational and three dimensional. For example, atmospheric cyclones are rotational but their substantially two-dimensional shapes do not allow vortex generation and so are not turbulent. On the other hand, oceanic flows are dispersive but essentially non rotational and therefore are not turbulent.

Rotationality :Turbulent flows have non-zero vorticity and are characterized by a strong three-dimensional vortex generation mechanism known as vortex stretching. In fluid dynamics, they are essentially vortices subjected to stretching associated with a corresponding increase of the component of vorticity in the stretching direction—due to the conservation of angular momentum. On the other hand, vortex stretching is the core mechanism on which the turbulence energy cascade relies to establish and maintain identifiable structure function. In general, the stretching mechanism implies thinning of the vortices in the direction perpendicular to the stretching direction due to volume conservation of fluid elements. As a result, the radial length scale of the vortices decreases and the larger flow structures break down into smaller structures. The process continues until the small scale structures are small enough that their kinetic energy can be transformed by the fluid's molecular viscosity into heat. Turbulent flow is always rotational and three dimensional. For example, atmospheric cyclones are rotational but their substantially two-dimensional shapes do not allow vortex generation and so are not turbulent. On the other hand, oceanic flows are dispersive but essentially non rotational and therefore are not turbulent.

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. Turbulence causes the formation of eddies of many different length scales. Most of the kinetic energy of the turbulent motion is contained in the large-scale structures. The energy "cascades" from these large-scale structures to smaller scale structures by an inertial and essentially inviscid mechanism. This process continues, creating smaller and smaller structures which produces a hierarchy of eddies. Eventually this process creates structures that are small enough that molecular diffusion becomes important and viscous dissipation of energy finally takes place. The scale at which this happens is the Kolmogorov length scale.

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. Turbulence causes the formation of eddies of many different length scales. Most of the kinetic energy of the turbulent motion is contained in the large-scale structures. The energy "cascades" from these large-scale structures to smaller scale structures by an inertial and essentially inviscid mechanism. This process continues, creating smaller and smaller structures which produces a hierarchy of eddies. Eventually this process creates structures that are small enough that molecular diffusion becomes important and viscous dissipation of energy finally takes place. The scale at which this happens is the Kolmogorov length scale.

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. The eddies are loosely defined as coherent patterns of flow velocity, vorticity and pressure. Turbulent flows may be viewed as made of an entire hierarchy of eddies over a wide range of length scales and the hierarchy can be described by the energy spectrum that measures the energy in flow velocity fluctuations for each length scale (wavenumber). The scales in the energy cascade are generally uncontrollable and highly non-symmetric. Nevertheless, based on these length scales these eddies can be divided into three categories.

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. The eddies are loosely defined as coherent patterns of flow velocity, vorticity and pressure. Turbulent flows may be viewed as made of an entire hierarchy of eddies over a wide range of length scales and the hierarchy can be described by the energy spectrum that measures the energy in flow velocity fluctuations for each length scale (wavenumber). The scales in the energy cascade are generally uncontrollable and highly non-symmetric. Nevertheless, based on these length scales these eddies can be divided into three categories.

Integral time scale
Integral time scale


The integral time scale for a Lagrangian flow can be defined as:

The integral time scale for a Lagrangian flow can be defined as:

$\displaystyle{ T = \left ( \frac{1}{\langle u'u'\rangle} \right )\int_0^\infty \langle u'u'(\tau)\rangle \, d\tau }$
$\displaystyle{ T = \left ( \frac{1}{\langle u'u'\rangle} \right )\int_0^\infty \langle u'u'(\tau)\rangle \, d\tau }$


T = \left ( \frac{1}{\langle u'u'\rangle} \right )\int_0^\infty \langle u'u'(\tau)\rangle \, d\tau

where u′ is the velocity fluctuation, and $\displaystyle{ \tau }$ is the time lag between measurements.

where u′ is the velocity fluctuation, and $\displaystyle{ \tau }$ is the time lag between measurements.

Integral length scales
Integral length scales


Large eddies obtain energy from the mean flow and also from each other. Thus, these are the energy production eddies which contain most of the energy. They have the large flow velocity fluctuation and are low in frequency. Integral scales are highly anisotropic and are defined in terms of the normalized two-point flow velocity correlations. The maximum length of these scales is constrained by the characteristic length of the apparatus. For example, the largest integral length scale of pipe flow is equal to the pipe diameter. In the case of atmospheric turbulence, this length can reach up to the order of several hundreds kilometers.: The integral length scale can be defined as
Large eddies obtain energy from the mean flow and also from each other. Thus, these are the energy production eddies which contain most of the energy. They have the large flow velocity fluctuation and are low in frequency. Integral scales are highly anisotropic and are defined in terms of the normalized two-point flow velocity correlations. The maximum length of these scales is constrained by the characteristic length of the apparatus. For example, the largest integral length scale of pipe flow is equal to the pipe diameter. In the case of atmospheric turbulence, this length can reach up to the order of several hundreds kilometers.: The integral length scale can be defined as


$\displaystyle{ L = \left ( \frac{1}{\langle u'u'\rangle} \right ) \int_0^\infty \langle u'u'(r)\rangle \, dr }$
$\displaystyle{ L = \left ( \frac{1}{\langle u'u'\rangle} \right ) \int_0^\infty \langle u'u'(r)\rangle \, dr }$


L = \left ( \frac{1}{\langle u'u'\rangle} \right ) \int_0^\infty \langle u'u'(r)\rangle \, dr

where r is the distance between two measurement locations, and u′ is the velocity fluctuation in that same direction.
where r is the distance between two measurement locations, and u′ is the velocity fluctuation in that same direction.


Kolmogorov length scales
Smallest scales in the spectrum that form the viscous sub-layer range. In this range, the energy input from nonlinear interactions and the energy drain from viscous dissipation are in exact balance. The small scales have high frequency, causing turbulence to be locally isotropic and homogeneous.
Kolmogorov length scales : Smallest scales in the spectrum that form the viscous sub-layer range. In this range, the energy input from nonlinear interactions and the energy drain from viscous dissipation are in exact balance. The small scales have high frequency, causing turbulence to be locally isotropic and homogeneous.


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.
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.


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. Sensitive dependence on the initial and boundary conditions makes fluid flow irregular both in time and in space so that a statistical description is needed. 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. As a result, the Kolmogorov microscales were named after him. It is now known that the self-similarity is broken so the statistical description is presently modified.

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. Sensitive dependence on the initial and boundary conditions makes fluid flow irregular both in time and in space so that a statistical description is needed. 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. As a result, the Kolmogorov microscales were named after him. It is now known that the self-similarity is broken so the statistical description is presently modified.

A complete description of turbulence is one of the unsolved problems in physics. According to an apocryphal story, Werner Heisenberg was asked what he would ask God, given the opportunity. His reply was: "When I meet God, I am going to ask him two questions: Why relativity? And why turbulence? I really believe he will have an answer for the first." A similar witticism has been attributed to Horace Lamb in a speech to the British Association for the Advancement of Science: "I am an old man now, and when I die and go to heaven there are two matters on which I hope for enlightenment. One is quantum electrodynamics, and the other is the turbulent motion of fluids. And about the former I am rather optimistic."

A complete description of turbulence is one of the unsolved problems in physics. According to an apocryphal story, Werner Heisenberg was asked what he would ask God, given the opportunity. His reply was: "When I meet God, I am going to ask him two questions: Why relativity? And why turbulence? I really believe he will have an answer for the first." A similar witticism has been attributed to Horace Lamb in a speech to the British Association for the Advancement of Science: "I am an old man now, and when I die and go to heaven there are two matters on which I hope for enlightenment. One is quantum electrodynamics, and the other is the turbulent motion of fluids. And about the former I am rather optimistic."

## Onset of turbulence

The plume from this candle flame goes from laminar to turbulent. The Reynolds number can be used to predict where this transition will take place

The plume from this candle flame goes from laminar to turbulent. The Reynolds number can be used to predict where this transition will take place

【图4：Laminar-turbulent transition层流-湍流转变。蜡烛火焰中的烟羽从层流变为湍流。使用雷诺数可以预测这种转变将在哪里发生】

The onset of turbulence can be, to some extent, predicted by the Reynolds number, which is the ratio of inertial forces to viscous forces within a fluid which is subject to relative internal movement due to different fluid velocities, in what is known as a boundary layer in the case of a bounding surface such as the interior of a pipe. A similar effect is created by the introduction of a stream of higher velocity fluid, such as the hot gases from a flame in air. This relative movement generates fluid friction, which is a factor in developing turbulent flow. Counteracting this effect is the viscosity of the fluid, which as it increases, progressively inhibits turbulence, as more kinetic energy is absorbed by a more viscous fluid. The Reynolds number quantifies the relative importance of these two types of forces for given flow conditions, and is a guide to when turbulent flow will occur in a particular situation.

The onset of turbulence can be, to some extent, predicted by the Reynolds number, which is the ratio of inertial forces to viscous forces within a fluid which is subject to relative internal movement due to different fluid velocities, in what is known as a boundary layer in the case of a bounding surface such as the interior of a pipe. A similar effect is created by the introduction of a stream of higher velocity fluid, such as the hot gases from a flame in air. This relative movement generates fluid friction, which is a factor in developing turbulent flow. Counteracting this effect is the viscosity of the fluid, which as it increases, progressively inhibits turbulence, as more kinetic energy is absorbed by a more viscous fluid. The Reynolds number quantifies the relative importance of these two types of forces for given flow conditions, and is a guide to when turbulent flow will occur in a particular situation.

This ability to predict the onset of turbulent flow is an important design tool for equipment such as piping systems or aircraft wings, but the Reynolds number is also used in scaling of fluid dynamics problems, and is used to determine dynamic similitude between two different cases of fluid flow, such as between a model aircraft, and its full size version. Such scaling is not always linear and the application of Reynolds numbers to both situations allows scaling factors to be developed.

This ability to predict the onset of turbulent flow is an important design tool for equipment such as piping systems or aircraft wings, but the Reynolds number is also used in scaling of fluid dynamics problems, and is used to determine dynamic similitude between two different cases of fluid flow, such as between a model aircraft, and its full size version. Such scaling is not always linear and the application of Reynolds numbers to both situations allows scaling factors to be developed.

A flow situation in which the kinetic energy is significantly absorbed due to the action of fluid molecular viscosity gives rise to a laminar flow regime. For this the dimensionless quantity the Reynolds number (Re) is used as a guide.

A flow situation in which the kinetic energy is significantly absorbed due to the action of fluid molecular viscosity gives rise to a laminar flow regime. For this the dimensionless quantity the Reynolds number () is used as a guide.

With respect to laminar and turbulent flow regimes:

With respect to laminar and turbulent flow regimes:

• laminar flow occurs at low Reynolds numbers, where viscous forces are dominant, and is characterized by smooth, constant fluid motion;
• laminar flow occurs at low Reynolds numbers, where viscous forces are dominant, and is characterized by smooth, constant fluid motion;
• 层流在雷诺数较小时发生，此时粘滞力占主导地位，其特征是平稳、恒定的流体运动
• turbulent flow occurs at high Reynolds numbers and is dominated by inertial forces, which tend to produce chaotic eddies, vortices and other flow instabilities.
• turbulent flow occurs at high Reynolds numbers and is dominated by inertial forces, which tend to produce chaotic eddies, vortices and other flow instabilities.
• 湍流在雷诺数较大时发生，并受惯性力支配，惯性力往往会产生混乱的紊流和其他流体不稳定性。

The Reynolds number is defined as

The Reynolds number is defined as

$\displaystyle{ \mathrm{Re} = \frac{\rho v L}{\mu} \,, }$

$\displaystyle{ \mathrm{Re} = \frac{\rho v L}{\mu} \,, }$

mathrm{Re} = \frac{\rho v L}{\mu}

where:

where:

• Rho|ρ(kg/m)是流体的密度
• v is a characteristic velocity of the fluid with respect to the object (m/s)
• v is a characteristic velocity of the fluid with respect to the object (m/s)
• v(m/s)是流体相对于物体的特征速度
• L is a characteristic linear dimension (m)
• L is a characteristic linear dimension (m)
• L(m)是线性尺寸特征
• Mu|μ(Pa·s或N·s/m或kg/(m·s))是流体动态速度

While there is no theorem directly relating the non-dimensional Reynolds number to turbulence, flows at Reynolds numbers larger than 5000 are typically (but not necessarily) turbulent, while those at low Reynolds numbers usually remain laminar. In Poiseuille flow, for example, turbulence can first be sustained if the Reynolds number is larger than a critical value of about 2040; moreover, the turbulence is generally interspersed with laminar flow until a larger Reynolds number of about 4000.

While there is no theorem directly relating the non-dimensional Reynolds number to turbulence, flows at Reynolds numbers larger than 5000 are typically (but not necessarily) turbulent, while those at low Reynolds numbers usually remain laminar. In Poiseuille flow, for example, turbulence can first be sustained if the Reynolds number is larger than a critical value of about 2040; moreover, the turbulence is generally interspersed with laminar flow until a larger Reynolds number of about 4000.

The transition occurs if the size of the object is gradually increased, or the viscosity of the fluid is decreased, or if the density of the fluid is increased.

The transition occurs if the size of the object is gradually increased, or the viscosity of the fluid is decreased, or if the density of the fluid is increased.

## Heat and momentum transfer

When flow is turbulent, particles exhibit additional transverse motion which enhances the rate of energy and momentum exchange between them thus increasing the heat transfer and the friction coefficient.

When flow is turbulent, particles exhibit additional transverse motion which enhances the rate of energy and momentum exchange between them thus increasing the heat transfer and the friction coefficient.

Assume for a two-dimensional turbulent flow that one was able to locate a specific point in the fluid and measure the actual flow velocity v = (vx,vy) of every particle that passed through that point at any given time. Then one would find the actual flow velocity fluctuating about a mean value:

Assume for a two-dimensional turbulent flow that one was able to locate a specific point in the fluid and measure the actual flow velocity (vx,vy)}} of every particle that passed through that point at any given time. Then one would find the actual flow velocity fluctuating about a mean value:

$\displaystyle{ v_x = \underbrace{\overline{v}_x}_\text{mean value} + \underbrace{v'_x}_\text{fluctuation} \quad \text{and} \quad v_y=\overline{v}_y + v'_y \,; }$

$\displaystyle{ v_x = \underbrace{\overline{v}_x}_\text{mean value} + \underbrace{v'_x}_\text{fluctuation} \quad \text{and} \quad v_y=\overline{v}_y + v'_y \,; }$

v_x = \underbrace{\overline{v}_x}_\text{mean value} + \underbrace{v'_x}_\text{fluctuation} \quad \text{and} \quad v_y=\overline{v}_y + v'_y \,

and similarly for temperature (T = 模板:Overline + T′) and pressure (P = 模板:Overline + P′), where the primed quantities denote fluctuations superposed to the mean. This decomposition of a flow variable into a mean value and a turbulent fluctuation was originally proposed by Osborne Reynolds in 1895, and is considered to be the beginning of the systematic mathematical analysis of turbulent flow, as a sub-field of fluid dynamics. While the mean values are taken as predictable variables determined by dynamics laws, the turbulent fluctuations are regarded as stochastic variables.

and similarly for temperature ( + T′}}) and pressure ( + P′}}), where the primed quantities denote fluctuations superposed to the mean. This decomposition of a flow variable into a mean value and a turbulent fluctuation was originally proposed by Osborne Reynolds in 1895, and is considered to be the beginning of the systematic mathematical analysis of turbulent flow, as a sub-field of fluid dynamics. While the mean values are taken as predictable variables determined by dynamics laws, the turbulent fluctuations are regarded as stochastic variables.

The heat flux and momentum transfer (represented by the shear stress τ) in the direction normal to the flow for a given time are

The heat flux and momentum transfer (represented by the shear stress ) in the direction normal to the flow for a given time are

\displaystyle{ \begin{align} \lt math\gt \begin{align} 开始 q&=\underbrace{v'_y \rho c_P T'}_\text{experimental value} = -k_\text{turb}\frac{\partial \overline{T}}{\partial y} \,; \\ q&=\underbrace{v'_y \rho c_P T'}_\text{experimental value} = -k_\text{turb}\frac{\partial \overline{T}}{\partial y} \,; \\ q&=\underbrace{v'_y \rho c_P T'}_\text{experimental value} = -k_\text{turb}\frac{\partial \overline{T}}{\partial y} \ Q & underbrace { v’ y rho c p t’} text { experimental value }-k { turb } frac { partial overline { t } ，; Q & underbrace { v’ y rho c p t’} text { experimental value }-k { turb } frac { partial overline { t } ， Q & underbrace { v’ y rho c p t’} text { experimental value }-k { turb } frac { partial overline { t } ， \tau &=\underbrace{-\rho \overline{v'_y v'_x}}_\text{experimental value} = \mu_\text{turb}\frac{\partial \overline{v}_x}{\partial y} \,; \tau &=\underbrace{-\rho \overline{v'_y v'_x}}_\text{experimental value} = \mu_\text{turb}\frac{\partial \overline{v}_x}{\partial y} \,; \tau &=\underbrace{-\rho \overline{v'_y v'_x}}_\text{experimental value} = \mu_\text{turb}\frac{\partial \overline{v}_x}{\partial y} \,; \end{align} }

\end{align}[/itex]

where cP is the heat capacity at constant pressure, ρ is the density of the fluid, μturb is the coefficient of turbulent viscosity and kturb is the turbulent thermal conductivity.

where is the heat capacity at constant pressure, is the density of the fluid, is the coefficient of turbulent viscosity and is the turbulent thermal conductivity.

## Kolmogorov's theory of 1941模板:Anchor

Richardson's notion of turbulence was that a turbulent flow is composed by "eddies" of different sizes. The sizes define a characteristic length scale for the eddies, which are also characterized by flow velocity scales and time scales (turnover time) dependent on the length scale. The large eddies are unstable and eventually break up originating smaller eddies, and the kinetic energy of the initial large eddy is divided into the smaller eddies that stemmed from it. These smaller eddies undergo the same process, giving rise to even smaller eddies which inherit the energy of their predecessor eddy, and so on. In this way, the energy is passed down from the large scales of the motion to smaller scales until reaching a sufficiently small length scale such that the viscosity of the fluid can effectively dissipate the kinetic energy into internal energy.

Richardson's notion of turbulence was that a turbulent flow is composed by "eddies" of different sizes. The sizes define a characteristic length scale for the eddies, which are also characterized by flow velocity scales and time scales (turnover time) dependent on the length scale. The large eddies are unstable and eventually break up originating smaller eddies, and the kinetic energy of the initial large eddy is divided into the smaller eddies that stemmed from it. These smaller eddies undergo the same process, giving rise to even smaller eddies which inherit the energy of their predecessor eddy, and so on. In this way, the energy is passed down from the large scales of the motion to smaller scales until reaching a sufficiently small length scale such that the viscosity of the fluid can effectively dissipate the kinetic energy into internal energy.

In his original theory of 1941, Kolmogorov postulated that for very high Reynolds numbers, the small-scale turbulent motions are statistically isotropic (i.e. no preferential spatial direction could be discerned). In general, the large scales of a flow are not isotropic, since they are determined by the particular geometrical features of the boundaries (the size characterizing the large scales will be denoted as L). Kolmogorov's idea was that in the Richardson's energy cascade this geometrical and directional information is lost, while the scale is reduced, so that the statistics of the small scales has a universal character: they are the same for all turbulent flows when the Reynolds number is sufficiently high.

In his original theory of 1941, Kolmogorov postulated that for very high Reynolds numbers, the small-scale turbulent motions are statistically isotropic (i.e. no preferential spatial direction could be discerned). In general, the large scales of a flow are not isotropic, since they are determined by the particular geometrical features of the boundaries (the size characterizing the large scales will be denoted as ). Kolmogorov's idea was that in the Richardson's energy cascade this geometrical and directional information is lost, while the scale is reduced, so that the statistics of the small scales has a universal character: they are the same for all turbulent flows when the Reynolds number is sufficiently high.

Thus, Kolmogorov introduced a second hypothesis: for very high Reynolds numbers the statistics of small scales are universally and uniquely determined by the kinematic viscosity ν and the rate of energy dissipation ε. With only these two parameters, the unique length that can be formed by dimensional analysis is

Thus, Kolmogorov introduced a second hypothesis: for very high Reynolds numbers the statistics of small scales are universally and uniquely determined by the kinematic viscosity and the rate of energy dissipation . With only these two parameters, the unique length that can be formed by dimensional analysis is

$\displaystyle{ \eta = \left(\frac{\nu^3}{\varepsilon}\right)^{1/4} \,. }$

$\displaystyle{ \eta = \left(\frac{\nu^3}{\varepsilon}\right)^{1/4} \,. }$

\eta = \left(\frac{\nu^3}{\varepsilon}\right)^{1/4} \,

This is today known as the Kolmogorov length scale (see Kolmogorov microscales).

This is today known as the Kolmogorov length scale (see Kolmogorov microscales).

A turbulent flow is characterized by a hierarchy of scales through which the energy cascade takes place. Dissipation of kinetic energy takes place at scales of the order of Kolmogorov length η, while the input of energy into the cascade comes from the decay of the large scales, of order L. These two scales at the extremes of the cascade can differ by several orders of magnitude at high Reynolds numbers. In between there is a range of scales (each one with its own characteristic length r) that has formed at the expense of the energy of the large ones. These scales are very large compared with the Kolmogorov length, but still very small compared with the large scale of the flow (i.e. ηrL). Since eddies in this range are much larger than the dissipative eddies that exist at Kolmogorov scales, kinetic energy is essentially not dissipated in this range, and it is merely transferred to smaller scales until viscous effects become important as the order of the Kolmogorov scale is approached. Within this range inertial effects are still much larger than viscous effects, and it is possible to assume that viscosity does not play a role in their internal dynamics (for this reason this range is called "inertial range").

A turbulent flow is characterized by a hierarchy of scales through which the energy cascade takes place. Dissipation of kinetic energy takes place at scales of the order of Kolmogorov length , while the input of energy into the cascade comes from the decay of the large scales, of order . These two scales at the extremes of the cascade can differ by several orders of magnitude at high Reynolds numbers. In between there is a range of scales (each one with its own characteristic length ) that has formed at the expense of the energy of the large ones. These scales are very large compared with the Kolmogorov length, but still very small compared with the large scale of the flow (i.e. ). Since eddies in this range are much larger than the dissipative eddies that exist at Kolmogorov scales, kinetic energy is essentially not dissipated in this range, and it is merely transferred to smaller scales until viscous effects become important as the order of the Kolmogorov scale is approached. Within this range inertial effects are still much larger than viscous effects, and it is possible to assume that viscosity does not play a role in their internal dynamics (for this reason this range is called "inertial range").

Hence, a third hypothesis of Kolmogorov was that at very high Reynolds number the statistics of scales in the range ηrL are universally and uniquely determined by the scale r and the rate of energy dissipation ε.

Hence, a third hypothesis of Kolmogorov was that at very high Reynolds number the statistics of scales in the range are universally and uniquely determined by the scale and the rate of energy dissipation .

The way in which the kinetic energy is distributed over the multiplicity of scales is a fundamental characterization of a turbulent flow. For homogeneous turbulence (i.e., statistically invariant under translations of the reference frame) this is usually done by means of the energy spectrum function E(k), where k is the modulus of the wavevector corresponding to some harmonics in a Fourier representation of the flow velocity field u(x):

The way in which the kinetic energy is distributed over the multiplicity of scales is a fundamental characterization of a turbulent flow. For homogeneous turbulence (i.e., statistically invariant under translations of the reference frame) this is usually done by means of the energy spectrum function , where is the modulus of the wavevector corresponding to some harmonics in a Fourier representation of the flow velocity field :

$\displaystyle{ \mathbf{u}(\mathbf{x}) = \iiint_{\mathbb{R}^3} \hat{\mathbf{u}}(\mathbf{k})e^{i \mathbf{k \cdot x}} \, \mathrm{d}^3\mathbf{k} \,, }$

$\displaystyle{ \mathbf{u}(\mathbf{x}) = \iiint_{\mathbb{R}^3} \hat{\mathbf{u}}(\mathbf{k})e^{i \mathbf{k \cdot x}} \, \mathrm{d}^3\mathbf{k} \,, }$

{u}(\mathbf{x}) = \iiint_{\mathbb{R}^3} \hat{\mathbf{u}}(\mathbf{k})e^{i \mathbf{k \cdot x}} \,

where û(k) is the Fourier transform of the flow velocity field. Thus, E(k) dk represents the contribution to the kinetic energy from all the Fourier modes with k < 模板:Abs < k + dk, and therefore,

where is the Fourier transform of the flow velocity field. Thus, represents the contribution to the kinetic energy from all the Fourier modes with < k + dk}}, and therefore,

$\displaystyle{ \tfrac12\left\langle u_i u_i \right\rangle = \int_0^\infty E(k) \, \mathrm{d}k \,, }$

$\displaystyle{ \tfrac12\left\langle u_i u_i \right\rangle = \int_0^\infty E(k) \, \mathrm{d}k \,, }$

\tfrac12\left\langle u_i u_i \right\rangle = \int_0^\infty E(k) \,

where 模板:Sfracuiui is the mean turbulent kinetic energy of the flow. The wavenumber k corresponding to length scale r is k = 模板:Sfrac. Therefore, by dimensional analysis, the only possible form for the energy spectrum function according with the third Kolmogorov's hypothesis is

where ⟨uiui⟩}} is the mean turbulent kinetic energy of the flow. The wavenumber corresponding to length scale is r is k = 模板:Sfrac. Therefore, by dimensional analysis, the only possible form for the energy spectrum function according with the third Kolmogorov's hypothesis is

$\displaystyle{ E(k) \propto k^{-p} \,, }$

$\displaystyle{ E(k) \propto k^{-p} \,, }$

E(k) \propto k^{-p}

with 1 < p < 3, the second order structure function has also a power law, with the form

with , the second order structure function has also a power law, with the form

$\displaystyle{ \Big\langle \big (\delta \mathbf{u}(r)\big )^2 \Big\rangle \propto r^{p-1} \,, }$

$\displaystyle{ \Big\langle \big (\delta \mathbf{u}(r)\big )^2 \Big\rangle \propto r^{p-1} \,, }$

mathbf{u}(r)\big )^2 \Big\rangle \propto r^{p-1}

Since the experimental values obtained for the second order structure function only deviate slightly from the 模板:Sfrac value predicted by Kolmogorov theory, the value for p is very near to 模板:Sfrac (differences are about 2%). Thus the "Kolmogorov −模板:Sfrac spectrum" is generally observed in turbulence. However, for high order structure functions the difference with the Kolmogorov scaling is significant, and the breakdown of the statistical self-similarity is clear. This behavior, and the lack of universality of the Cn constants, are related with the phenomenon of intermittency in turbulence. This is an important area of research in this field, and a major goal of the modern theory of turbulence is to understand what is really universal in the inertial range.

Since the experimental values obtained for the second order structure function only deviate slightly from the value predicted by Kolmogorov theory, the value for is very near to (differences are about 2%). Thus the "Kolmogorov − spectrum" is generally observed in turbulence. However, for high order structure functions the difference with the Kolmogorov scaling is significant, and the breakdown of the statistical self-similarity is clear. This behavior, and the lack of universality of the constants, are related with the phenomenon of intermittency in turbulence. This is an important area of research in this field, and a major goal of the modern theory of turbulence is to understand what is really universal in the inertial range.

## References and notes

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### General

• Davidson, P. A. (2004). Turbulence – An Introduction for Scientists and Engineers. Oxford University Press. ISBN 978-0198529491.

• Cardy, J.; Falkovich, G.; Gawedzki, K. (2008). Non-equilibrium Statistical Mechanics and Turbulence. Cambridge University Press. ISBN 9780521715140.

• Durbin, P. A.; Pettersson Reif, B. A. (2001). Statistical Theory and Modeling for Turbulent Flows. John Wiley & Sons. ISBN 978-0-470-68931-8.

• Bohr, T.; Jensen, M. H.; Paladin, G.; Vulpiani, A. (1998). Dynamical Systems Approach to Turbulence. Cambridge University Press. ISBN 9780521475143.

• Nieuwstadt, F. T. M.; Boersma, B. J.; Westerweel, J. (2016). Turbulence – Introduction to Theory and Applications of Turbulent Flows (Online ed.). Springer. ISBN 978-3-319-31599-7.

### Original scientific research papers and classic monographs

• Batchelor, G. K. (1953). The Theory of Homogeneous Turbulence. Cambridge University Press.