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The process of a laminar boundary layer becoming turbulent is known as boundary layer transition. This process is an extraordinarily complicated process which at present is not fully understood. However, as the result of many decades of intensive research, certain features have become gradually clear, and it is known that the process proceeds through a series of stages.

Transition stages

The initial stage of the natural transition process is known as the receptivity phase and consists of the transformation of external disturbances in the outer freestream flow over the boundary layer (such as freestream turbulence, surface roughness, acoustic noise etc.) into internal instability oscillations within the boundary layer. Upon entering the boundary layer, a wide spectrum of disturbances are present. Many of these disturbances decay, however, and only a limited number become amplified with further downstream development.

The second stage of the process, is the exponential growth of the few unstable disturbances. Since this stage is linear, it can be well-described by linear stability theory by following the most unstable mode. It is generally accepted that for subsonic incompressible boundary layers, these initial instabilities which cause transition and ultimately lead to turbulent flow, take the form of Tollmien–Schlichting waves.

In the third stage, the amplitudes of the disturbances now become large enough to introduce non-linearity effects. In low Reynolds number flows, the initial amplitude of the disturbances are insufficient to cause immediate transition. These waves must first develop within the boundary layer over a finite distance to trigger non-linear effects characteristic of the transition process. Here the uniform spanwise mean flow begins to become modulated by the non-linear interaction of the disturbances. In this third phase, the mean boundary layer profile now begins to become distorted and the boundary layer thickness varies strongly in the streamwise direction.

Due to the distortion of the boundary layer, inflexional mean profiles develop and a fourth stage is reached, where the boundary layer becomes unstable to three-dimensional high-frequency disturbances. The frequencies observed in this phase are typically an order-of-magnitude greater than those observed in the initial stages, and this is generally referred to as secondary instability. Finally, an explosive growth of these high-frequency disturbances initiates the fifth and final phase, the breakdown into turbulence.

Numerous experiments in recent decades have revealed that the extent of the amplification region, and hence the location of the transition point on the body surface, is strongly dependent not only upon the amplitude and/or the spectrum of external disturbances but also on their physical nature. Some of the disturbances easily penetrate into the boundary layer and turn into Tollmien-Schlichting waves, whilst others do not. Consequently, the concept of boundary layer transition is a complex one and still lacks a complete theoretical exposition.

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