Stages Of Boundary-layer Transition Research Articles

The Interaction of Turbulent Spots With Low-Speed Streaks

Abstract Turbulent spots are regions of turbulence surrounded by laminar flow that appear during the late stages of boundary layer transition. While turbulent spots are often studied in isolation, they usually occur near low-speed streaks and other disturbances during transition. This paper investigates the interaction between a turbulent spot and a subcritical low-speed streak using direct numerical simulations. The results, analyzed from streak instability and vorticity points of view, reveal mechanisms of the destabilization of the streak by the spot and provide insights into spot evolution in a realistic environment. Additional simulations involving intentional local control of portions of the streak provide further insight into the interaction mechanisms and potential transition mitigation strategies.

Tollmien–Schlichting waves in the subsonic regime

AbstractThe Tollmien–Schlichting (T‐S) waves play a key role in the early stages of boundary layer transition. In a breakthrough work, Grenier, Guo, and Nguyen gave the first rigorous construction of the T‐S waves of temporal mode for the incompressible fluid. Yang and Zhang recently made an important contribution by constructing the compressible T‐S waves of temporal mode for certain boundary layer profiles with Mach number . In this paper, we construct the T‐S waves of both temporal mode and spatial mode to the linearized compressible Navier–Stokes system around the boundary layer flow in the whole subsonic regime , including the Blasius profile. Our approach is based on a novel iteration scheme between the quasi‐incompressible and quasi‐compressible systems, with a key ingredient being the solution of an Orr–Sommerfeld‐type equation using a new Airy–Airy–Rayleigh iteration instead of Rayleigh–Airy iteration introduced by Grenier, Guo, and Nguyen. We believe that the method developed in this work can be applied in solving other related problems for subsonic flows.

Characteristics of boundary-layer transition driven by diverse streamwise vortices

The present direct numerical simulations investigate the dynamics of diverse streamwise vortices in a zero-pressure gradient incompressible laminar boundary layer and the onset of turbulence. Due to the critical role of streamwise vortices in bypass transitions, we compare the transition mechanisms induced by a single vortex and vortex pairs. We initially examine the evolution of a single vortex by employing streamwise vortex profiles at two wall-normal locations. The single streamwise vortex will prompt a more rapid eruption from the bottom part of the boundary layer if moved nearer to the wall, as observed in the experimental study by Manu et al. [“Evolution of isolated streamwise vortices in the late stages of boundary-layer transition,” Exp. Fluids 48, 431–440 (2010)]. In the late stages of boundary-layer transition, the vortex–wall interaction emerges to be particularly pronounced. The second set of simulations triggers flow transition by imposing counter-rotating vortex pairs at the inlet of the computational domain. Streamwise vortex pairs with net upward flows cause intense, sporadic ejections of near-wall fluids into the boundary-layer edge, providing the first signs of inflectional instability in all considered cases. Instead of vortex–wall interactions, flow structures created by vortex pairs penetrate deeply into the inviscid region, resulting in substantial unsteady viscous-inviscid interactions. When counter-rotating vortices accompany to form a net downward flow, the initial formation of each vortex is analogous to that of a single vortex. The instability frequency and wavelength of the transitional flow produced by a vortex introduced in the middle of the boundary layer are lower than those imposed near the wall. The transitional flow generated by the vortex pair exhibits longer-wavelength instability than the single vortex cases.

Evolution of isolated streamwise vortices in the late stages of boundary layer transition

Experiments were conducted with two, smooth hills, lying well within the boundary layer over a flat plate mounted in a wind tunnel. One hill was shallow, with peak height 1.5 mm and width 50 mm; the other, steep, 3 mm high and 30 mm wide. Since the hills occupied one-half of the tunnel span, streamwise vorticity formed near the hills' edge. At a freestream speed of 3.5 m/s, streaks formed with inflectional wall-normal and spanwise velocity profiles but without effecting transition. Transition, observed at 7.5 m/s, took different routes with the two hills. With the steep hill, streamwise velocity signals exhibited the passage of a wave packet which intensified before breakdown to turbulence. With the shallow hill there was a broad range of frequencies present immediately downstream of the hill. These fluctuations grew continuously and transition occurred within a shorter distance. Since the size of the streamwise vorticity generated at the hill edge is of the order of the hill height, the shallow hill generates vorticity closer to the wall and supports an earlier transition, whereas the steep hill creates a thicker vortex and associated streaks which exhibit oscillations due to their own instability as an additional precursor stage before transition.

Recent progress in dynamics of boundary layer transition

The dynamics in transitional boundary layers involve interactions among various flow structures. It is not easy to identify unambiguously the dominant structures which give the major turbulent production. In this paper, we study the flat plate boundary layer was studied by means of using two-dimensional hot film velocity measurements and flow visualization. Our results present direct evidences of the existence of soliton-like coherent structures that are dominant flow structures in turbulent production processes for both the early and late stages of boundary layer transition. Furthermore, we observe it is concluded that in boundary layer flows these soliton-like coherent structures are the building blocks to produce other coherent structures.

An Attempt to Induce the Three-Dimensionalization Process of T-S Waves in Boundary-Layer Transition.

The of a three-dimensional roughness element on a flat-plate boundary layer to outer acoustic disturbances is investigated by wind-tunnel experiments. The term receptivity describes the process through which external disturbances are transformed into instability waves in a boundary layer. It is known that several types of oblique waves can be found in a three-dimensionalization process in the later stages of boundary-layer transition. In the present experiment, we investigate the behavior of oblique waves and try to control boundary-layer transition by utilizing receptivity. It is shown that oblique waves can be generated on a flat-plate boundary layer with an inclined roughness element under acoustic forcing. An attempt to induce three-dimensionalization modes which are shown in the later stages of boundary-layer transition is carried out. However, flow structure generated by our method did not match the natural three-dimensional modes.

Artificial Control of the Three-Dimensionalization Process of T-S Waves in Boundary-Layer Transition.

An attempt to control the three-dimensionalization process of T-S waves by utilizing the receptivity of a three-dimensional roughness element on a flat-plate boundary layer to outer acoustic disturbances is carried out by wind-tunnel experiments. The term receptivity(1)describes the process through which external distur-bances are transformed into instability waves in a boundary layer. It is known that several types of oblique waves can be found in the three-dimensionalization process in the later stages of the boundary-layer transition. In the present experiment, we artificially introduce oblique waves in the boundary layer in order to control the boundary-layer transition. It is shown that an oblique wave can be generated in a flat-plate boundary layer with a roughness element attached to the plate inclined and an acoustic forcing. An attempt to induce three-dimensionalization modes that are observed in the later stages of boundary-layer transition is also carried out. However, the natural three-dimensional mode flow structures could not be generated by our method.

A dynamical systems approach to the early stages of boundary-layer transition

A series of experiments has been performed on a laminar flat-plate boundary layer undergoing transition to turbulence. Reproducible disturbances were introduced via a loudspeaker embedded at some upstream location. Time series of the velocity fluctuations were obtained at a sequence of downstream locations using hot-wire anemometry and the phase portraits were reconstructed at each position. A new technique has been used to estimate the number of independent modes. Nonlinear maps were then fitted that transform the portrait at one streamwise location onto the portrait at the neighbouring downstream position. In this way the spatial evolution of disturbances is modelled explicitly

Numerical simulation of boundary-layer transition at Mach two

The linear and early nonlinear stages of boundary-layer transition at free-stream Mach number M ∞ =2.0 are investigated by direct numerical simulation of the compressible NavierStokes equations. Results from simulations with a large computational box and small-amplitude random initial conditions are compared with linear stability theory. The growth rates of oblique waves are reproduced correctly. Two-dimensional waves show a growth that is modulated in time, indicating the presence of an extra unstable mode which moves supersonically relative to the free stream. Further simulations are conducted to investigate the nonlinear development of two- and three-dimensional disturbances The transition due to oblique disturbance waves is the most likely cause of transition at this Mach number, and is found to lead to the development of strong streamwise vortices.