What is Turbulence?

Turbulence is a state of fluid flow. When the flow velocity is very small, the fluid flows in layers without mixing with each other, which is called laminar flow, also known as steady flow or sheet flow. When the flow rate is gradually increased, the fluid's streamline begins to wavy, and the frequency and amplitude of the swing follow the flow velocity. This flow condition is called transitional flow; when the flow velocity increases to a large value, the flow lines are no longer clearly identifiable. There are many small vortices in the flow field, and laminar flow is destroyed. Swipe, and blend. At this time, the fluid makes irregular movement, and a partial velocity perpendicular to the axis of the flow tube is generated. This movement is called turbulence, and it is also called turbulence, turbulence or turbulence.

Turbulence is a state of fluid flow. When the flow velocity is very small, the fluid flows in layers without mixing with each other, which is called laminar flow, also known as steady flow or sheet flow. When the flow rate is gradually increased, the fluid's streamline begins to wavy, and the frequency and amplitude of the swing follow the flow velocity. This flow condition is called transitional flow; when the flow velocity increases to a large value, the flow lines are no longer clearly identifiable. There are many small vortices in the flow field, and laminar flow is destroyed. Swipe, and blend. At this time, the fluid makes irregular movement, and a partial velocity perpendicular to the axis of the flow tube is generated. This movement is called turbulence, and it is also called turbulence, turbulence or turbulence.

Chinese name

Turbulent

Foreign name

turbulent flow

Basic introduction to turbulence

Turbulent

Turbulence is a state of fluid flow. When the flow velocity is very small, the fluid flows in layers without mixing with each other, which is called laminar flow, also known as steady flow or sheet flow. When the flow rate is gradually increased, the fluid's streamline begins to wavy, and the frequency and amplitude of the swing follow the flow velocity. This flow condition is called transitional flow; when the flow velocity increases to a large value, the flow lines are no longer clearly identifiable. There are many small vortices in the flow field, and laminar flow is destroyed. Sliding, as well as mixing, form turbulence, also known as turbulence, turbulence or turbulence.

In nature, we often encounter fluids as turbulence, such as river rapids, air flow, and chimney exhaust.

Turbulence occurs under a large Reynolds number. When the Reynolds number is small, the effect of viscous force on the flow field is greater than the inertial force. The disturbance of the flow velocity in the flow field will be attenuated by the viscous force, and the fluid flow will be stable. If the Reynolds number is large, the effect of inertial force on the flow field is greater than the viscous force, the fluid flow is more unstable, and small changes in flow velocity are easy to develop and enhance, forming a turbulent and irregular turbulent flow field.

The Reynolds value at the time of flow transition is called the critical Reynolds number. Generally, the Reynolds number of the pipeline Re = 4000 is the turbulent state, and Re = 2320 4000 is the transition state.

The basic characteristic of turbulence is the randomness of the movement of fluid micelles. Turbulent micelles have not only lateral pulsations, but also

Turbulent flow after uniform smoke passing through thick plates

Relative to the total motion of the fluid, the trajectory of the fluid micelles is extremely disordered and changes rapidly with time (see Figure 2). The most important phenomenon in turbulence is the transfer of momentum, heat and mass caused by this random movement, whose transfer rate is several orders of magnitude higher than laminar flow.

Turbulence has both advantages and disadvantages. On the one hand, it strengthens the transmission and reaction processes; on the other hand, it greatly increases frictional resistance and energy loss. Whereas turbulence is a common state of fluid movement in nature and various technical processes (for example, wind and river currents, circulation around aircraft and ship surfaces, fluid movement in fluid machinery, combustion chambers, reactors and

The movement of working medium in the heater, the diffusion of pollutants in the atmosphere and water, etc.), research, prediction and control of turbulence are one of the important topics in understanding natural phenomena and developing modern technology. There are two basic types of turbulence research: elucidating how turbulence occurs; and understanding turbulence characteristics. Due to the random nature of turbulent motion, statistical mechanics or statistical averaging must be used to study turbulence. The methods of studying turbulence include theoretical analysis, numerical calculation and experiment. The latter two have important engineering practical significance.

Turbulence theory

The central problem is to find the statistical solution of the Navier-Stokes equation of the basic turbulence equation. Due to the nonlinearity of this equation and the irregularity of the turbulence solution, turbulence theory has become the most difficult and fascinating field in fluid mechanics. Although turbulence has been studied for more than 100 years, there is no mature precise theory so far, and many basic technical problems have not been explained theoretically.

In 1895, O. Xue Nuo firstly averaged the turbulent instantaneous velocity and instantaneous pressure, and derived the basic equation of the turbulent average flow field from the Navier-Stokes equation, the Reynolds equation, which established the theoretical basis of turbulence. Later, the semi-empirical theory (centered on the mixed length hypothesis) and various turbulence models were developed, which provided a certain effective theoretical basis for solving various urgent technical problems. Since the 1930s, turbulence statistical theory, especially the ideal uniform isotropic turbulence theory, has made great progress, but it is still far from solving practical problems. Since the 1960s, applied mathematicians have used mathematical tools such as functionals, topology, and group theory to explore new approaches to turbulence theory from different perspectives, such as statistical mechanics and quantum field theory. Since the 1970s, due to the establishment of the concept of turbulent coherent structure (also known as pseudo-sequence structure), experts have tried to establish a deterministic turbulence theory. There have been important advances in non-linear theories about how turbulence evolved from laminar flow, such as bifurcation theory, chaos theory, and strange attractors.

Turbulent numerical calculation

It is essentially a numerical solution to the basic equation of turbulence. On the one hand, turbulence theory is very difficult. On the other hand, the solvability of turbulence problems increases with the improvement of computer performance. Therefore, the role of numerical calculation of turbulence is more and more important. Previous numerical calculations of turbulence were mainly based on semi-empirical theory. Before the 1960s, the integration method and ordinary differential equation method became routine algorithms in the engineering department. Since the mid-1960s, various complex turbulence models and calculation methods have been proposed due to the application of high-speed electronic computers. The method of partial differential equations has developed rapidly. In particular, since the 1970s, due to the use of fourth-generation giant high-speed computers, turbulent numerical calculations have developed to a higher stage of large-scale numerical simulation. It can be expected that with the advancement of computers, the numerical calculation of turbulence will have greater development.

Turbulence experiment

Under controlled experimental conditions, various test instruments and data processing systems are used to measure the characteristic parameters of turbulence or display the flow field. Turbulence experiments can not only directly obtain useful technical data, but also a means to understand the structure of turbulence and develop new concepts and models of turbulence. The invention of the hot wire anemometer in the 1930s allowed people to measure the pulsating speed of turbulence, test and develop theories and semi-empirical theories. With the improvement of electronic instruments in the 1950s, the experiments focused on studying the spectral distribution of turbulent energy, especially the fine structure of turbulence. After the mid-1960s, because of improved flow field display technology and the use of conditional sampling methods, it was found that large-scale coherent structures with a certain order exist in the irregular turbulence. Since then, turbulent coherent structures have become a new subject in turbulent experiments. (See turbulence theory, numerical calculation of turbulence, turbulence experiments)

Turbulence references

J. 0. Hinze, Turbulence , 2nd ed., McGraw-Hill, New York, 1975.