Turbulence in intermittent transitional boundary layers and in turbulence spots

Abstract

Direct numerical simulation data of bypass transition in flat-plate boundary layers are analysed to examine the characteristics of turbulence in the transitional regime. When intermittency is 50 % or less, the flow features a juxtaposition of turbulence spots surrounded by streaky laminar regions. Conditionally averaged turbulence statistics are evaluated within the spots, and are compared to standard time averaging in both the transition region and in fully turbulent boundary layers. The turbulent-conditioned root-mean-square levels of the streamwise velocity perturbations are notably elevated in the early transitional boundary layer, while the wall-normal and spanwise components are closer to the levels typical for fully turbulent flow. The analysis is also extended to include ensemble averaging of the spots. When the patches of turbulence are sufficiently large, they develop a core region with similar statistics to fully turbulent boundary layers. Within the tip and the wings of the spots, however, the Reynolds stresses and terms in the turbulence kinetic energy budget are elevated. The enhanced turbulence production in the transition zone, which exceeds the levels from fully turbulent boundary layers, contributes to the higher skin-friction coefficient in that region. Qualitatively, the same observations hold for different spot sizes and levels of free-stream turbulence, except for young spots which do not yet have a core region of developed turbulence.

Statistical transition to turbulence in plane channel flow

“…In plane Couette flow [28,49], the upstream and downstream spatial decay rates are equal, by virtue of symmetry, while those of pipe flow show a strong dependence of the upstream decay rate on Reynolds number [50]. Asymmetry between upstream and downstream spatial decay rates is also seen in turbulent spots in boundary layer flow [51] and in Poiseuille-Couette flow [52].…”

Low-frequency unsteadiness mechanisms in shock wave/turbulent boundary layer interactions over a backward-facing step

“…The Reynolds stresses from the current LES are also in a good agreement with the reference data. Since the current LES data is for a compressible boundary layer that has a higher momentum thickness Reynolds number and friction Reynolds number , the velocity profile has a slightly larger plateau value and the streamwise Reynolds stress profile has a higher peak value in the buffer layer (Marxen & Zaki 2019). Note that the grid sensitivity has been checked using two coarser grids with and .…”

Bypass transition in boundary layers subject to strong pressure gradient and curvature effects

“…More recently, the concept of edge state, which originates from the dynamics of systems, has been applied in combination with the receptivity of boundary layers to explain the nucleation mechanism for turbulent spots (Khapko et al 2016;Kreilos et al 2016). Based on conditional statistics of the bypass transition in flat-plate boundary layers, Marxen & Zaki (2019) found that turbulent spots can develop a core region with similar statistics to fully turbulent boundary layers, while the edge shows elevated levels of Reynolds stresses.…”