Boundary layer turbulence and freestream turbulence interface, turbulent spot and freestream turbulence interface, laminar boundary layer and freestream turbulence interface

Abstract

We study the boundary-layer turbulence and freestream turbulence interface (BTFTI), the turbulent spot and freestream turbulence interface (TSFTI), and the laminar boundary-layer and freestream turbulence interface (LBFTI) using direct simulation. Grid spacings in the freestream are less than 1 Kolmogorov length scale during transition. Probability density functions of temperature and its derivatives are used to select the interface identification threshold, corroborated by a vorticity-based method. The interfaces so detected are confirmed to be physical a posteriori by the distinctive quasi-step-jump behavior in the swirling strength and temperature statistics along traverses normal to the BTFTI and TSFTI. No interface-normal inflection is detected across the LBFTI for either swirling strength, temperature, vorticity magnitude, Reynolds shear stress, streamwise velocity, normal velocity, or turbulence kinetic energy. The present direct numerical simulation data thus cast serious doubts on the shear-sheltering hypothesis/theory, which asserts that a subset of freestream fluctuations is blocked by the LBFTI. In the early stage of transition, quasi-spanwise structures exist on the LBFTI. The TSFTI shape is dominated by head prints of concentrated hairpin vortices. Further downstream, the BTFTI geometry is strongly modulated by groves of hairpin vortices (the boundary layer large-scale motions) with a distinct streamwise preferential orientation. Streamwise velocity and turbulence kinetic energy only exhibit minor plateaus (rather than quasi-step-jump) across the BTFTI and the TSFTI. We emphasize that it is more meaningful and important to acquire reproducible and reliable interface-normal statistics prior to considering any plausible substructures and elusive transient dynamics of the BTFTI, TSFTI, and LBFTI.

Uniform-momentum zones in a turbulent boundary layer subjected to freestream turbulence

“…Kozul et al (2020) explicitly compared their instantaneous structure to that of Hancock & Bradshaw (1989) and found qualitatively good agreement, however, the main focus of their study was the statistics of the temporal boundary layer subject to a sudden injection of FST. The DNS studies of You & Zaki (2019) and Wu et al (2019) roughly echo these results, however, they investigate only a single FST case at limited Reynolds numbers. This is understandable as it is computationally expensive to compare multiple cases and achieve higher Reynolds numbers.…”

“…Moreover, the TBL experiments of Laskari et al (2018) detected UMZs beneath a freestream of approximately 3 % turbulence. Finally, the recent study of Wu et al (2019) also identified an instantaneous separation between the freestream and the wall-bounded flow in DNS of a zero-pressure-gradient TBL. Thus, UMZs exist in flows with turbulence in the freestream, and the flow may be separated into the flow directly influenced by the wall and the turbulent flow in the freestream or core.…”

“…Given the aforementioned similarities between a TBL subjected to FST and a canonical TBL, one might expect to see similar instantaneous structures. To date, the only instantaneous images available for these flows are the flow visualisations of Hancock & Bradshaw (1989) and instantaneous fields extracted from the direct numerical simulations (DNS) of You & Zaki (2019), Wu, Wallace & Hickey (2019) and Kozul et al (2020); while Dogan et al (2019) had instantaneous flow fields, their work focussed on the spatial statistics. The visualisations of Hancock & Bradshaw (1989) appear to show a more jagged interface with the freestream for the turbulent case compared to the approximately laminar freestream.…”

“…This is understandable as it is computationally expensive to compare multiple cases and achieve higher Reynolds numbers. Moreover, there was no particular focus on UMZs, although Wu et al (2019) did identify an instantaneous 'boundary layer turbulence and freestream turbulence interface' separating the two flow regions for their one case. There is thus a need to perform detailed quantitative analysis on the instantaneous structure within these TBLs subjected to different levels of FST.…”

Negative skin friction during transition in a zero-pressure-gradient flat-plate boundary layer and in pipe flows with slip and no-slip boundary conditions

“…Furthermore, our DNS profiles for the rate of viscous dissipation of turbulent kinetic energy ε + = ε/(u 4 τ /ν) as a function of y + are in excellent agreement, not only in slope but also in intercept, over the logarithmic region, with the asymptotic theoretical relation ε + = 1/(κy + ). Further details about the ZPGSFPBL simulation can be found in Wu, Wallace & Hickey (2019).…”