The transition of fluid flows from a laminar behaviour to turbulence–referred to as laminar-to-turbulent transition–is one of the chief difficulties in classical physics. While laminar flows are orderly, turbulence involves irregular velocity fluctuations over a wide range of spatial and temporal scales. The fluctuations enhance the transfer of momentum and energy so that the drag and heat transfer of a body immersed in the flow may potentially be increased.
Separated shear layers occur when a flow separates from a solid surface–such as a stalled wing–or when two initially-separated, co-flowing streams are brought together. The primary instability mode in separated flows is the inviscid Kelvin-Helmholtz instability. Disturbances amplified by the Kelvin-Helmholtz instability mode produce a streamwise accumulation of spanwise vorticity in the shear layer and eventual roll-up of the shear layer. This is shown in Fig. 1, which is taken from a direct numerical simulation of the shear layer that separates from a low-pressure turbine blade from a gas-turbine engine.
Following the roll-up of the shear layer, packets of three-dimensional vortical structures are created, shown in Fig. 2. The growth of “hairpin vortices” within the packets leads to the complete transition to turbulence.
By performing detailed simulations of transition in separation bubbles, modifications to existing semi-empirical transition models can be proposed. This has huge potential for increased efficiency, safety, and decreased costs in aircraft gas-turbine engines, turbomachinery for liquefied natural gas (LNG) production and utilization, and wind-turbine aerodynamics.
For more information on this subject, refer to this paper.