Cunbiao Lee

Transition in Wall-Bounded Flows

In this paper, we present direct comparisons of experimental results on transition in wall-bounded flows obtained by flow visualizations, hot-film measurement, and particle-image velocimetry, along with a brief mention of relevant theoretical progresses, based on a critical review of about 120 selected publications. Despite somewhat different initial disturbance conditions used in experiments, the flow structures were found to be practically the same. The following observed flow structures are considered to be of fundamental importance in understanding transitional wall-bounded flows: the three-dimensional nonlinear wave packets called solitonlike coherent structures (SCSs) in boundary layer and pipe flows, the Λ-vortex, the secondary vortex loops, and the chain of ring vortices. The dynamic processes of the formation of these structures and transition as newly discovered by recent experiments include the following: (1) The sequential interaction processes between the Λ-vortex and the secondary vortex loops, which control the manner by which the chain of ring vortices is periodically introduced from the wall region into the outer region of the boundary layer. (2) The generation of high-frequency vortices, which is one of the key issues for understanding both transitional and developed turbulent boundary layers (as well as other flows), of which several explanations have been proposed but a particularly clear interpretation can be provided by the experimental discovery of secondary vortex loops. The ignorance of secondary vortex loops would make the dynamic processes and flow structures in a transitional boundary layer inconsistent with previous discoveries. (3) The dominant role of SCSs in all turbulent bursting, which is considered as the key mechanism of turbulent production in a low Reynolds-number turbulent boundary layer. Of direct relevance to bursting is the low-speed streaks, whose formation mechanism and link to the flow structures in wall-bounded flows can be answered more clearly than before in terms of the SCS dynamics. The observed SCSs and secondary vortex loops not only enable revisiting the classic story of wall-bounded flow transition, but also open a new avenue to reconstruct the possible universal scenario for wall-bounded flow transition.

Experimental study of freely falling thin disks: Transition from planar zigzag to spiral

Using a stereoscopic vision method, we have experimentally investigated the time evolution of a free thin disk motion with six degrees of freedom for the first time. It is found that, as the dimensionless moment of inertia I∗ decreases, the trajectory of the disk transits from planar to nonplanar. New types of free falling motions were identified for small I∗ values, including the spiral state and the transitional state. An extended Re−I∗ phase diagram corresponding to different flow regimes was given. The underlying physics associated with the transition is found to be connected to the interactions between the moving object and induced vortices.Using a stereoscopic vision method, we have experimentally investigated the time evolution of a free thin disk motion with six degrees of freedom for the first time. It is found that, as the dimensionless moment of inertia I∗ decreases, the trajectory of the disk transits from planar to nonplanar. New types of free falling motions were identified for small I∗ values, including the spiral state and the transitional state. An extended Re−I∗ phase diagram corresponding to different flow regimes was given. The underlying physics associated with the transition is found to be connected to the interactions between the moving object and induced vortices.

Dominant structure for turbulent production in a transitional boundary layer

The nonlinear evolution of the three-dimensional soliton-like coherent structure (SCS) in a transitional boundary layer is described in this paper. Hydrogen bubble visualizations and two-dimensional hot film measurements were used to analyze the dynamic relationships between the SCS and the Λ -vortex, the secondary closed vortices and the low-speed streak. The results show that the three-dimensional SCS is the dominant flow structure in almost all dynamic processes in both the early and later stages of boundary-layer transitions as well as in a turbulent boundary layer. Further, the SCS is shown to be the building block that produces other coherent structures in wall-bounded flows.

New features of CS solitons and the formation of vortices

Abstract This work investigates, mainly by means of flow visualization, experimental aspects of nonlinear stages, aiming at the K-regime in boundary layer transition. Soliton-like coherent structures (CS solitons) and a new physical mechanism for the formation of vortices along the border of the CS soliton are found.

Development of second-mode instability in a Mach 6 flat plate boundary layer with two-dimensional roughness

Particle image velocimetry, PCB pressure sensors, and planar Rayleigh scattering are combined to study the development of second-mode instability in a Mach 6 flow over a flat plate with two-dimensional roughness. To the best of the authors’ knowledge, this is the first time that the instantaneous velocity fields and flow structures of the second-mode instability waves passing through the roughness are shown experimentally. A two-dimensional transverse wall blowing is used to generate second-mode instability in the boundary layer and seeding tracer particles. The two-dimensional roughness is located upstream of the synchronization point between mode S and mode F. The experimental results showed that the amplitude of the second-mode instability will be greatly increased upstream of the roughness. Then it damps and recovers quickly in the vicinity downstream of the roughness. Further downstream, it acts as no-roughness case, which confirms Fong’s numerical results [K. D. Fong, X. W. Wang, and X. L. Zhong, “N...

Experimental investigations of the initial growth of flow asymmetries over a slender body of revolution at high angles of attack

This paper describes an experimental investigation of the initial growth of flow asymmetries over a slender body of revolution at high angles of attack with natural and disturbed noses. Time-resolved particle image velocimetry was used to investigate the flow field around the body. The experimental results show that initially different amplitudes of unsteady disturbances near the tip are established owing to the tip imperfections. These unsteady disturbances experience a super-exponential growth near the tip and continue to grow exponentially due to linear instabilities. Attachment of a piece to the tip brings a larger initial difference and extends the super-exponential growth region. Thus, the disturbance amplitudes and their differences are larger for the disturbed case than for the natural case before reaching the neutral point of linear instability. The amplified disturbances lead to different instability vortex strengths in the separated shear layers, which feed continuously into the two primary concentrated vortices. As a result, the primary vortex strengths differ, which result in the initial vortex asymmetry. The experiment results demonstrate that the initial flow asymmetry arises from an asymmetric development of the boundary layer instability.

Aerodynamic heating in transitional hypersonic boundary layers: Role of second-mode instability

The evolution of second-mode instabilities in hypersonic boundary layers and its effects on aerodynamic heating are investigated. Experiments are conducted in a Mach 6 wind tunnel using fast-response pressure sensors, fluorescent temperature-sensitive paint, and particle image velocimetry. Calculations based on parabolic stability equations and direct numerical simulations are also performed. It is found that second-mode waves, accompanied by high-frequency alternating fluid compression and expansion, produce intense aerodynamic heating in a small region that rapidly heats the fluid passing through it. As the second-mode waves decay downstream, the dilatation-induced aerodynamic heating decreases while its shear-induced counterpart keeps growing. The latter brings about a second growth of the surface temperature when transition is completed.

PATHS OF FREELY FALLING DISKS

The free falling motion of round thin disks in quiescent water was investigated experimentally. The mean density of the disks were about 1.05 g/cm3, slightly larger than water, and thickness to diameter ratio is 0.1. In this letter, We reported two cases with different mass distributions, in one case the disks center of mass coincided with the geometry center and in another case it was deviated. Two CCD cameras were used to photograph the falling disk simultaneously, the disks six degrees of freedom were obtained via a stereoscopic vision method. In the first case fluttering motion was observed, the disks oscillatory motion confined in a vertical plane, and the pitching motion occurred about an axis normal to the plane. In the second case, steady helical falling was observed, the disk moved in a helical path at constant speed, the attacking angle is constant.

PERIODIC TRIPLING AND JET ERUPTION OF FORCED STEEP GRAVITY WAVES

Period tripling of standing waves in a circular tank generated by the side-wall excitation is investigated for the first time. With the increasing of forcing acceleration after the appearance of axisymmetric standing wave, period-tripled non-breaking standing waves and violent jet eruption between the modes are observed. However, the physical mechanisms for the generation of the interesting period-tripled standing waves are still unknown and need further research.

Rotor boundary layer development with inlet guide vane (IGV) wake impingement

This paper examines the transition process in a boundary layer on a rotor blade under the impingement of an inlet guide vane wake. The effects of wake strengths and the reduced frequency on the unsteady boundary layer development on a low-speed axial compressor were investigated using particle image velocimetry. The measurements were carried out at two reduced frequencies (fr = fIGVS0/U2i, fr = 1.35, and fr = 0.675) with the Reynolds number, based on the blade chord and the isentropic inlet velocity, being 97 500. At fr = 1.35, the flow separated at the trailing edge when the wake strength was weak. However, the separation was almost totally suppressed as the wake strength increased. For the stronger wake, both the wake’s high turbulence and the negative jet behavior of the wake dominated the interaction between the unsteady wake and the separated boundary layer on the suction surface of the airfoil. The boundary layer displacement thickened first due to the negative jet effect. Then, as the disturbances ...