New Insights into Turbulent Spots

Transitional-turbulent spots bridge the deterministic laminar state with the stochastic turbulent state and affect the transition zone length in engineering flows. Turbulent spot research over the past four decades has expanded from incompressible flat-plate boundary layer and pipe flow to hypersonic boundary layer flow, turbomachinery flow, channel flow, plane Couette flow, and a range of more complex flows. Progress has been made on the origination, composition, demarcation, growth, mutual interaction, reproduction, sustainability, and self-organization of turbulent spots. The hypothesis that transitional-turbulent spots are a basic module of the fully turbulent boundary layer has been proven through the discovery of locally generated turbulent-turbulent spots dominating the wall layer. Splitting of transitional-turbulent spots in pipe flow has been linked to a life cycle localized in the spot frontal section. This review discusses these advances and outlines future research directions.

Automated summary

Transitional-turbulent spots play a crucial role in engineering flows by bridging the gap between laminar and turbulent states. Over the years, research on turbulent spots has expanded to various types of flows, including hypersonic boundary layer flow, turbomachinery flow, and complex flows. Important progress has been made in understanding the origin, growth, interaction, and self-organization of turbulent spots. Additionally, the hypothesis that transitional-turbulent spots are a fundamental module of fully turbulent boundary layers has been supported by the discovery of locally generated turbulent-turbulent spots. This review discusses these advancements and highlights future research directions.