Abstract
This paper is a short review of our recent DNS work on physics of late boundary layer transition and turbulence. Based on our DNS observation, we propose a new theory on boundary layer transition, which has five steps, that is, receptivity, linear instability, large vortex structure formation, small length scale generation, loss of symmetry and randomization to turbulence. For turbulence generation and sustenance, the classical theory, described with Richardson's energy cascade and Kolmogorov length scale, is not observed by our DNS. We proposed a new theory on turbulence generation that all small length scales are generated by “shear layer instability” through multiple level ejections and sweeps and consequent multiple level positive and negative spikes, but not by “vortex breakdown.” We believe “shear layer instability” is the “mother of turbulence.” The energy transferring from large vortices to small vortices is carried out by multiple level sweeps, but does not follow Kolmogorov's theory that large vortices pass energy to small ones through vortex stretch and breakdown. The loss of symmetry starts from the second level ring cycle in the middle of the flow field and spreads to the bottom of the boundary layer and then the whole flow field.
Highlights
These comments clearly show that the mechanism of turbulence formation and sustenance is still a mystery for research
Dissipation is inevitable during the energy transfer; (9) all smaller vortices are newly generated by shear layer instability, but not by original vortex “breakdown;” (10) smallest length scales are all generated near the wall surface which must be related to the surface configuration and cannot be “isotropic;” (11) surface drag is determined by small length scale generation, which must be closely related to turbulence modeling; (12) “shear layer” control is the key issue for flow transition control, turbulent flow control, and drag reduction
In our recent direct numerical simulation (DNS) work [20,21,22,23,24,25,26,27,28,29,30,31,32, 36, 37] we have made a number of new findings on late flow transition in a boundary layer including the mechanism of large vortex structure formation, small-length scale generation and flow randomization
Summary
These comments clearly show that the mechanism of turbulence formation and sustenance is still a mystery for research. The classical theory, which considers “vortex breakdown” as the last stage of boundary layer transition on a flat plate, is challenged and the phenomenon of “hairpin vortex breakdown to smaller structures” is not observed by our new DNS [24, 27, 28]. The new theory of boundary layer transition can be described by five stages: (1) boundary layer receptivity; (2) linear instability; (3) large vortex structure formation; (4) small vortices generation; (5) symmetry loss and turbulence formation. “Shear layer instability” is considered as the “mother of turbulence.” This new theory may give a universal mechanism for turbulence generation and sustenance—the energy is brought by large vortex structure through multiple level sweeps in a boundary layer “Shear layer instability” is considered as the “mother of turbulence.” This new theory may give a universal mechanism for turbulence generation and sustenance—the energy is brought by large vortex structure through multiple level sweeps in a boundary layer
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