Masahito Asai

The instability and breakdown of a near-wall low-speed streak

The instability of the three-dimensional high-shear layer associated with a near-wall low-speed streak is investigated experimentally. A single low-speed streak, not unlike the near-wall low-speed streaks in transitional and turbulent flows, is produced in a laminar boundary layer by using a small piece of screen set normal to the wall. In order to excite symmetric and anti-symmetric modes separately, well-controlled external disturbances are introduced into the laminar low-speed streak through small holes drilled behind the screen. The growth of the excited symmetric varicose mode is essentially governed by the Kelvin–Helmholtz instability of the in ectional velocity profiles across the streak in the normal-to-wall direction and it can occur when the streak width is larger than the shear-layer thickness. The spatial growth rate of the symmetric mode is very sensitive to the streak width and is rapidly reduced as the velocity defect decreases flowing to the momentum transfer by viscous stresses. By contrast, the anti-symmetric sinuous mode that causes the streak meandering is dominated by the wake-type instability of spanwise velocity distributions across the streak. As far as the linear instability is concerned, the growth rate of the anti-symmetric mode is not so strongly affected by the decrease in the streak width, and its exponential growth may continue further downstream than that of the symmetric mode. As for the mode competition, it is important to note that when the streak width is narrow and comparable with the shear-layer thickness, the low-speed streak becomes more unstable to the anti-symmetric modes than to the symmetric modes. It is clearly demonstrated that the growth of the symmetric mode leads to the formation of hairpin vortices with a pair of counter-rotating streamwise vortices, while the anti-symmetric mode evolves into a train of quasi-streamwise vortices with vorticity of alternate sign.

Detailed Observations of Interactions of Wingtip Vortices in Close-Formation Flight

The interactions of wingtip vortices in close-formation flight of two wings were examined experimentally. Both wings consisting of a NACA 23012 airfoil section and a rectangular planform of an aspect ratio of 5 were set in echelon formation in a wind tunnel. The lift-to-drag ratio increased the most effectively when the two wings were overlapped with each other by 5% of the wingspan with zero vertical offset for the present wing configuration. Flow visualizations and particle image velocimetry measurements showed that the trailing vortex of the lead wing collided at a slightly inboard point of the trail wing and was divided into two streamwise vortices in the most effective overlap condition. It was also observed that the tip vortices of the lead and trail wings interacted closely with each other to form a pair of counter-rotating vortices for smaller overlap conditions less than 5 % with zero vertical offset. For such small overlap cases, circulation of the trailing vortex of the trail wing was reduced markedly by influences of the trailing vortex of the lead wing.

Growth and breakdown of low-speed streaks leading to wall turbulence

Two-dimensional local wall suction is applied to a fully developed turbulent boundary layer such that near-wall turbulence structures are completely sucked out, but most of the turbulent vortices in the original outer layer can survive the suction and cause the resulting laminar flow to undergo re-transition. This enables us to observe and clarify the whole process by which the suction-surviving strong vortical motions give rise to near-wall low-speed streaks and eventually generate wall turbulence. Hot-wire and particle image velocimetry (PIV) measurements show that low-frequency velocity fluctuations, which are markedly suppressed near the wall by the local wall suction, soon start to grow downstream of the suction. The growth of low-frequency fluctuations is algebraic. This characterizes the streak growth caused by the suction-surviving turbulent vortices. The low-speed streaks obtain almost the same spanwise spacing as that of the original turbulent boundary layer without the suction even in the initial stage of the streak development. This indicates that the suction-surviving turbulent vortices are efficient in exciting the necessary ingredients for the wall turbulence, namely, low-speed streaks of the correct scale. After attaining near-saturation, the low-speed streaks soon undergo sinuous instability to lead to re-transition. Flow visualization shows that the streak instability and its subsequent breakdown occur at random in space and time in spite of the spanwise arrangement of streaks being almost periodic. Even under the high-intensity turbulence conditions, the sinuous instability amplifies disturbances of almost the same wavelength as predicted from the linear stability theory, though the actual growth is in the form of a wave packet with not more than two waves. It should be emphasized that the mean velocity develops the log-law profile as the streak breakdown proceeds. The transient growth and eventual breakdown of low-speed streaks are also discussed in connection with the critical condition for the wall-turbulence generation.

Boundary-layer transition triggered by hairpin eddies at subcritical Reynolds numbers

Subcritical transition in a flat-plate boundary layer is examined experimentally through observing its nonlinear response to energetic hairpin eddies acoustically excited at the leading edge of the boundary-layer plate. When disturbed by the hairpin eddies convecting from the leading edge, the near-wall flow develops local three-dimensional wall shear layers with streamwise vortices. Such local wall shear layers also evolve into hairpin eddies in succession to lead to the subcritical transition beyond the x -Reynolds number R x = 3.9 × 10 4 , where the momentum thickness Reynolds number R θ is 127 for laminar Blasius flow without excitation, and is about 150 under the excitation of energetic hairpin eddies. It is found that in terms of u - and v -fluctuations, the intensity of the near-wall activity at this critical station is of almost the same order as or slightly less than that of the developed wall turbulence. The development of wall turbulence structure in this transition is also examined.

Origin of the peak-valley wave structure leading to wall turbulence

A generation process for the three-dimensional wave which dominates the transition preceded by a Tollmien-Schlichting (T-S) wave is studied both experimentally and numerically in plane Poiseuille flow at a subcritical Reynolds number of 5000. In order to identify the origin of the three-dimensional wave in Nishioka et al. s laboratory experiment, the corresponding spanwise mean-flow distortion and two-dimensional T-S wave modes are introduced into a parabolic flow as the initial disturbance conditions for a numerical simulation of temporally growing type. Through reproducing the actual wave development into the peak-valley structure, the simulation pinpoints the origin to be the slight spanwise mean-flow distortion in the experimental basic flow. Furthermore, the simulation clearly shows that the growth of the three-dimensional wave requires the vortex stretching effect due to the streamwise vortices, which appear under the experimental conditions only when the amplitude of the two-dimensional T-S wave is above the observed threshold.

On the transition between distributed and isolated surface roughness and its effect on the stability of channel flow

The question of whether a system of roughness elements has to be viewed either as a distributed roughness or a set of individual, hydrodynamically independent roughness elements has been considered. The answer has been given in the context of definition of hydraulic smoothness proposed by Floryan [Eur. J. Mech. B/Fluids 26, 305 (2007)] where a roughness system that cannot destabilize the flow is viewed as hydraulically inactive. Linear stability characteristics have been traced from the distributed to the isolated roughness limits. It has been shown that an increase of distance between roughness elements very quickly stabilizes disturbances in the form of streamwise vortices; however, roughness elements placed quite far apart are able to affect evolution of disturbances in the form of traveling waves. Transition from the distributed to the isolated roughness limit is achieved much faster in the case of roughness elements in the form of “trenches” forming depressions below the reference surface than in the case of roughness elements in the form of “ridges” protruding above the reference surface.

Vortex Shedding and Noise Radiation from a Slat Trailing Edge

Vortex shedding and the associated noise radiation from a trailing edge were experimentally investigated for a leading-edge slat of a multi-element airfoil at stowed chord Reynolds number Re 2.1 x 10 5 , however, acoustic feedback becomes pronounced between the trailing-edge noise and the boundary-layer instability waves on the suction surface, so that multiple spectral peaks appear both in the velocity fluctuations and sound pressure. At and around the Reynolds number for the first appearance of tonal noise, Re = 1.9 x 10 5 , both of the instability modes coexist. Beyond Re = 2.1 x 10 5 , the boundary-layer instability waves excited by the acoustic feedback evolve into high-intensity vortices before reaching the trailing edge and suppress the absolute instability of the wake through diminishing the reversed-flow region in the wake.

Suppression of Tonal Trailing-Edge Noise From an Airfoil Using a Plasma Actuator

Suppression control of tonal noise generation at an airfoil trailing edge was conducted using a plasma actuator for a NACA0012 airfoil at a 2-deg angle of attack and at a Reynolds number 2.2×105 where the acoustic feedback responsible for the trailing-edge noise generation occurs in the pressure-side boundary layer. To minimize the possible interference of electrode installation in the boundary-layer stability a specially designed actuator with flush-mounted electrode configuration was employed. It was found that when the plasma actuator was installed at 55–60% chord location on the pressure surface a surface flow induced by the actuator stabilizes the downstream boundary-layer most significantly to suppress the strong growth of instability waves, which is responsible for the occurrence of the acoustic feedback. Consequently, suppression of the tonal trailing-edge noise was successfully achieved.

Certain aspects of channel entrance flow

Two-dimensional, developing laminar flow in the downstream zone of the channel entry is investigated experimentally for Reynolds number 3000⩽Re⩽6000. Measured evolution of the flow towards the fully developed state agrees with the theoretical predictions [R. M. Sadri and J. M. Floryan, “Entry flow in a channel,” Comput. Fluids 31, 133 (2002)].

Three-Dimensional Wave-Disturbances in Plane Poiseuille Flow

To clarify the initial 3D wave development in ribbon-induced transition of plane Poiseuille flow, double Fourier analyses, with respect to frequenicy and spanwise wavenumber, of u-velocity fields are made. The results indicate the validity of the concept of secondary instability, i.e. a linear instability of the near-equilibrium periodic flow (with 2D T-S wave) with respect to phase-locked 3D wave-disturbances, which has been proposed by Orszag & Patera and Herbert recently.