Optical properties of atmospheric turbulence play an important role in atmospheric sciences, astronomy, and applications of optical engineering, such as adaptive optics, imaging, remote sensing, and optical communication in free space. Real atmospheric turbulence presents great spatial and temporal complexity. Practical light propagation experiments in a real atmosphere usually cost much and encounter many difficulties. Although these kinds of experiments have been carried out, it is usually difficult to obtain favorable results because of the non-homogenous and uncontrollable atmospheric conditions.

Thus, some controllable and size-limited atmospheric turbulence simulators in the laboratory have been built for light propagation effect study in scientific research and engineering applications. A typical laboratory turbulence simulator is a tank or chamber of meter size filled with a turbulent medium under a heating mechanism. The turbulent medium is usually water or air. An unstable vertical temperature gradient is produced to simulate turbulence. Some instruments for status monitoring are used to measure the temperature, velocity, and turbulence strength. Adjusting the gradient can change the turbulence strength. These artificial turbulence simulators have been proven to be useful facilities.

Much work has been done on these turbulence simulators. For example, the relationship between the phase compensation efficiency of an adaptive optics system and the Fried parameter r₀ of simulated turbulence was obtained through experiments with such laboratory turbulence simulators. Some light propagation effects were investigated. However, there were few laws for light propagation effects based on the results of experiments on such simulators.

With the development of light propagation and imaging in marine media, new optical engineering in earth environments, and special optical beam propagation in the atmosphere, artificial turbulence simulators are employed more widely. The experiments carried out in the simulated turbulent media can qualitatively or semi-quantitatively present light propagation effects similar to those effects in real atmospheric turbulence.

In many applications of optical engineering, the light propagation distance is several kilometers or even longer. In order to simulate the light propagation effects, the turbulence strength in the laboratory simulators must be much stronger than the real atmospheric turbulence. This requirement for turbulence strength has been fulfilled in most laboratory simulators. However, less attention has been paid to the similarity of spatial and temporal properties of the simulated turbulence with real atmospheric turbulence.

A favorable laboratory turbulence simulator with excellent performance should provide turbulence with stable properties that can be adjusted quantitatively. The properties of the simulated turbulence should be similar to those of the real atmospheric turbulence. The inertial range of turbulence should cover the scale range from millimeter to meter, and the temporal spectrum should cover a range from 0.1 Hz to 100 Hz or several kHz.

It must be reminded that only a small portion in the inner of the flow media of the simulator can present locally isotropic homogeneous turbulent status, and thus it is not suitable to employ propagation theory for a homogenous turbulent path to analyze the experimental results of these simulators.

More and more investigations on the optical properties of atmospheric turbulence at different places and time reveal that real atmospheric turbulence is very complicated, and in many cases, the real turbulence cannot be simply treated as locally homogenous and isotropic and described by Kolmogorov theory. It is very difficult in the laboratory to reliably simulate turbulence with properties of real atmospheric turbulence.

If we want to study light propagation effects quantitatively by using a simulator, we should design a simulator providing optical similarity with practical propagation conditions and obtain simultaneously detailed information about the structure of optical properties in the simulated turbulent media with the light propagation experiment. As more and more laboratory turbulence simulators are constructed, it is necessary to emphasize the physical similarity requirements. The first physical similarity is the fluidity similarity which concerns geometry, dynamics, Reynolds's number, etc. The second physical similarity is the light propagation condition. Some key spatial scales must be considered, including the scale of the light source, light wavelength, and propagation distance, the Fried parameter r₀, and the inner and outer scales of turbulence. When these similarity requirements are fulfilled, the turbulence strength should be created high enough to achieve the most severe propagation condition characterized by the Rytov index.

In order to make more proper use of a laboratory turbulence simulator in the scientific study of atmospheric optics and the system design of optical engineering, the spatial and temporal properties of the laboratory turbulence simulator should be investigated in detail by both measurements and numerical simulation of the fluid field. On the basis of these investigations, better laboratory turbulence simulators with more suitable geometry can be designed and constructed.