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.
Fig. 1
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.
Fig. 2
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.