What is strip flow? It is about fluid flow! Accurately detailed description!

It should be turbulent flow. Turbulent flow Turbulent flow is a term in fluid mechanics, which refers to a disordered state in the process of fluid changing from one stable state to another. Specifically, it means that when the fluid flows, the inertial force between the particles plays a dominant role, and the particles of the fluid flow irregularly.

Turbulent flow is generally relative to "laminar flow". Generally determined by Reynolds number. The small Reynolds number means that the viscous force between the particles plays a dominant role when the fluid flows. The particles of the fluid flow regularly parallel to the inner wall of the pipe, showing a laminar flow state. A large Reynolds number means that inertial force dominates and the fluid is in a turbulent flow state. Generally, a pipeline Reynolds number Re<2000 is a laminar flow state, Re>4000 is a turbulent flow state, and Re=2000~4000 is a transitional state. The movement rules of fluid under different flow conditions. The distribution of flow velocity is different, so the ratio of the average flow velocity and the maximum flow velocity of the fluid in the pipeline is also different. Therefore, the Reynolds number determines the flow characteristics of viscous fluids.

The fluid movement in which physical quantities such as speed and pressure pulsate in time and space is also called turbulence. The main characteristics of turbulent flow

are: ①The movement of fluid particles is extremely irregular, and the values ??of various flow parameters in the flow field have pulsating phenomena. ②Due to the rapid mixing of pulsation, the diffusion rate of fluid momentum, energy, temperature and content concentration is greater than that of laminar flow. ③ Turbulent flow has vortex movement and has three-dimensional characteristics.

In 1883, O. Reynold published his article on observing laminar and turbulent flow regimes, and in 1894 he derived the basic equation of the time-averaged flow

of cable flow— —Formula Renault. Since the 1920s, various semi-empirical theories and various turbulence models have been developed to quantitatively analyze turbulence problems. Since the 1930s, the statistical theory of turbulence, especially the uniform isotropic turbulence theory of G.I. Taylor, has been developed; in the 1940s, A.H. Kolmogorov of the Soviet Union proposed local isotropy. Turbulent flow

Theory. In the 1950s, Zhou Peiyuan of China proposed a vortex structure theory for uniform isotropic turbulent flow; at the same time, experimental research on turbulent flow

enabled people to further understand the nature of turbulent flow. After the 1960s, the use of measurement technologies such as hydrogen bubble method and high-speed photography

further revealed the mechanism of turbulent flow; the application of electronic computers also simplified the processing of measurement data, thereby understanding turbulence

We have a deep understanding of the origin of p>

and the internal structure of turbulent flow. A picture of the burst phenomenon is proposed for the origin of wall turbulence. However, from a practical point of view, there is not yet a relatively mature turbulence theory, and many basic technical problems cannot be completely solved by turbulence theory. Semi-empirical formula.

Turbulence can be divided according to its flow characteristics: ① Isotropic uniform turbulence is a hypothetical turbulence model, and its turbulence characteristics

(such as turbulence intensity ) are the same at all points in space (uniformity) and the same in all directions (isotropy). In this kind of turbulent flow, there is no velocity gradient and therefore no shear stress. Local isotropic turbulence is a turbulence model that only considers small-scale vortices to be isotropic. ② Shear turbulence refers to the turbulent flow with time-averaged flow velocity gradient and thus shear stress. It can be divided into free turbulence (the development of turbulence is not restricted by the solid wall) and wall turbulence. Flow (velocity gradient is caused by solid walls).

Studying turbulence can be carried out from both theoretical and experimental aspects.

Turbulence theory: The problem of laminar flow stability and fully developed turbulence characteristics are important contents in turbulence theory.

Laminar flow stability problem Laminar flow has a certain ability to suppress various external disturbances. This ability is called flow stability

Stability. The inertia of the fluid expands the disturbance, but the viscosity of the fluid suppresses the disturbance, so the stability of the flow weakens as the Reynolds number increases. The Reynolds number at which laminar flow begins to transform into turbulent flow is called the critical Reynolds number. The small perturbation method is an important theory for analyzing flow stability

. In most cases, the disturbance in the wall shear flow gradually increases, destabilizing the flow and forming turbulent flow spots, and finally forming turbulent flow.

Basic equations of turbulent flow For the study of fully developed turbulent flow characteristics, most scholars still start from the Navier-Stokes equations

and express each quantity in the formula Becomes the sum of the time-averaged quantity and the pulsating quantity (see Reynolds equation). After taking the time

average of this equation, we can get

(1)

This equation The difference from the Navier-Stokes equation is that there are additional terms of Reynolds stress U′U′ in the equation; this is an "apparent stress" formed by turbulent exchange. is an unknown quantity, so the set of equations composed of Reynolds equation and continuity equation cannot be closed. Therefore, a central issue in turbulence theory is to find a way to close the set of equations.

At present, one is to use semi-empirical theory to establish the relationship between Reynolds stress and time-averaged flow velocity without increasing the number of basic equations; the other is to establish a new turbulence theory The flow model increases the number of equations and makes the system of equations closed.

Semi-empirical theory of turbulence The earliest semi-empirical theory is the turbulence viscosity coefficient concept and eddy viscosity model theory proposed by J.V. Buseniesk in 1877. In 1925, L. Prandtl proposed the mixing length theory. He believed that the turbulent mass had to travel a certain distance before it mixed with the surrounding fluid and lost its original characteristics. Within this distance, the mass maintained its original characteristics.

He called this distance the mixing length l. Assumptions:

(2)

In the formula, U' is the pulsating flow velocity; u is the time-averaged flow velocity; the subscripts i and j represent two directions that are perpendicular to each other, so

p>

(3)

Assume that in free turbulent flow, l is a constant on the cross-section and is proportional to the mixing length of the section in question. In wall turbulent flow, l=kxj, where xj is the normal distance from the wall, and k is called Karman’s constant. When k≈0.4, the theoretical results are consistent with the actual results

The measured data agree well. In 1915, G.I. Taylor proposed the vortex transfer theory. The main point is to treat vortex as a transmittable star. Under the action of pulsating flow rate, the fluid mass with vortex must travel a certain distance. After that, its vorticity changes,

And within this distance lw, the vorticity is constant; the Reynolds stress expression he obtained is

(4)

Generally, lw= KnXj, and actual measurements show that k≈0.2 is acceptable.

In 1930, T. von Karman proposed the local similarity hypothesis of turbulence. He assumed that: except for the area close to the wall, the mechanism of turbulence has nothing to do with the viscosity of the fluid. In a statistical sense, the local ranges near each point in the pulsating flow velocity field are similar to each other, and they only differ in length and time scales. Starting from these two points, he concluded that the statistical theory of mixed long turbulence can not only be used to study turbulence based on the Navier-Stokes equation, but can also be used to deal with random phenomena

< p>Xiang's statistical method to study turbulence. G.I. Taylor was the first to apply this method. In 1921, he proposed the related concept of pulsating flow velocity at different times at the same space point, and called it Lagrangian correlation or autocorrelation. In 1935, he proposed the concept of correlation between pulsating flow velocities at different spatial points at the same time, also called Euler correlation or cross-correlation. The two correlation coefficients are expressed as follows: From

Correlation coefficient

In the formula, i and j can be two different directions at the same point, or they can be two at different points. Different directions or the same direction.

In addition to the above-mentioned second-order correlation between pulsating flow velocity, there is also the correlation between pulsating flow velocity and pulsating pressure and the

third-order correlation of pulsating flow velocity, etc. Carry out correlation analysis between turbulent pulsation quantities, establish a motion differential equation represented by a correlation tensor and then solve it.

Currently, this approach is limited to the study of uniform isotropic turbulence, and has achieved certain results. Results.

The probability distribution of pulsation amount is also a characteristic of turbulent motion. In uniform turbulent flow, the probability distribution of pulsating flow velocity is close to the normal distribution; but in shear turbulent flow, the probability distribution is often not normal distribution. The closer to the inlet wall or the closer to the free

The edge of turbulent flow deviates from the normal distribution. In order to more accurately represent the probability distribution characteristics of pulsation volume, sometimes it is necessary to study the third-order moment (skewness) and fourth-order moment (kurtosis parameter) of pulse

momentum. In statistical theory, another important component is energy spectrum analysis. Since the 1960s, due to the advancement of flow display and measurement technology, people have discovered that turbulent flow can be regarded as a flow composed of many vortices of different sizes. Large eddies obtain energy from the time-averaged flow, transfer it to small eddies step by step, and are finally dissipated through viscous effects. Vortices of different sizes cause pulsations of different frequencies (domain wave numbers). Therefore, the pulsation energy in turbulent flow can be decomposed according to frequency (or wave number) to obtain various frequencies (or wave numbers). ) The distribution of pulsating energy of the vortex is called spectrum (or wave spectrum) or turbulent energy spectrum. The -dimensional energy spectral density Ei(n) of the pulsating flow rate (t) can be expressed as

(7)

In the formula, n is the number of pulsations per unit time, which is called frequency ; RE(t) is the autocorrelation coefficient. One-dimensional energy spectrum is prone to confusion

so sometimes three-dimensional energy spectrum is used. The energy spectrum curve with wave number k as a variable is shown in the figure.

Schematic diagram of energy spectrum curve

Numerical calculation of turbulent flow In order to seek closure of Reynolds equation and continuity equation, more and more factors are considered in various turbulent flows

Models appear one after another. The development of high-speed and large-capacity electronic computers has made great progress in the numerical calculation of the basic equations of turbulence.

The main content of experimental research on turbulence is to observe turbulence phenomena and measure various turbulence parameters.

Methods commonly used to observe phenomena include schlieren method, interference method, staining method, hydrogen bubble method, etc. In recent years, laser interference method and holography technology have also been widely used. As for data processing, real-time spectrum analyzers, x-y coordinate meters, etc. can now be used to provide spectrum, correlation function, probability density and other data related to turbulent flow while measuring.

That’s detailed enough!