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The stabilization of Tollmien-Schlichting (TS) waves in a spanwise modulated Blasius boundary layer has been experimentally verified in the MTL wind tunnel at KTH. The alternating high and low speed streaks were created by regularly spaced cylindrical roughness elements mounted on the flat plate. It is shown that the larger the streak amplitude the larger is the damping effect of the TS-waves, which is in agreement with recent theoretical work.

Through a short perforated wall strip, local boundary layer suction is applied to a turbulent boundary layer to generate a quasi-laminar boundary layer with highintensity active turbulence. The retransition begins with algebraic growth of streamwise streaks from immediately behind the suction. Even in the early stage of the growth of low-speed streaks their spanwise spacing is found to be almost the same as that in the original turbulent boundary layer. After the growth saturation the streak instability and the associated breakdown of low-speed streaks occur to generate near-wall burst-like motions with quasi-streamwise vortices.

This work is devoted to the experimental study of the transition to turbulence of a flow confined in a narrow gap between a rotating and a stationary disk. When the fluid layer thickness is of the same order of magnitude as the boundary layer depth, the azimuthal velocity axial gradient is nearly constant and this rotating disk flow is a torsional Couette flow. As in the plane Couette flow or the cylindrical Couette flow, transition to turbulence occurs via the appearance of turbulent domains inside a laminar background. Nevertheless, we show that in the rotating disk case, the nucleation of turbulent spirals is connected to the birth of structural defects in a periodic underlying spiral roll pattern.

The nonlinear evolution of streamwise modulated three-dimensional waves in boundary layers was investigated. Three nonlinear stages were identified. The first was related to oblique transition and transient growth. The second involved Klebanoff instability. The third was a subharmonic instability. Associated with the modulation, a mechanism of production of subharmonic waves was also identified. This provided the deterministic seeds for the subharmonic instability.

Instantaneous transitional flow structures of standard K-type transition in in-compressible plane channel flow predicted by large-eddy simulations (LES) arecompared to fully-resolved DNS data. For the LES different subgrid-scale (SGS) models are compared. It is investigated how well the SGS models on coarse grids are able to predict the physically relevant mechanisms at successive stages of transition: Λ-vortices, rollup of shear layers, hairpin vortices. Additionally, results for the exact subgrid-scale dissipation are computed from the DNS data. The results suggest that SGS models including a three-dimensional relaxation regularization show similar transitional structures as present in the DNS, whereas the dynamic Smagorinsky model does not show hairpin vortices for the chosen coarse resolution.

Direct simulation of an isolated turbulent spot in a compressible isothermal wall boundary-layer flow has been performed. A bypass transition scenario at Mach 2,4 and 6 is considered. The flow field associated with the transitional and turbulent spots is studied in detail, with results in broad agreement with previous experimental work. The evolved spots are found to have an arrowhead shaped front with a leading edge overhang, followed by a turbulent core and a calmed region at the rear interface. The lateral spreading of the spot is found to decrease substantially with the flow. Evidence for a supersonic (Mack) mode is found in the Mach 6 case: spanwise-coherent structures are observed under the spot overhang region.

The evolution of a train of vortex pairs ejected from a slot into a Blasius boundary layer was studied experimentally, with the aim of active boundary layer tripping. The excitation was directed upstream or downstream, at a shallow angle, or perpendicular to the surface. Vorticity, circulation, trajectories and convection speeds were calculated and used to describe the vortices’ evolution. Intermittency, spectra and mean velocity profiles of the forced boundary layer were measured. It was found that shallow downstream directed excitation is very effective for promoting transition.

A novel con.guration has been devised to study the flow physics of separated airfoil flows. The configuration allows us to prescribe the size and extent of the separation zone. A systematic variation of key separation parameters then allows us to precisely delineate the processes that govern resonant-mode interactions and transition in these flows. Numerical simulations of this configuration indicate the presence of three distinct characteristic time scales associated with the shear layer, the separation zone and the vortex shedding in the wake.

The linear stability analysis of a plane Couette flow of viscoelastic fluid have been studied with the emphasis on two dimensional disturbances with wave number , where is Reynolds number based on maximum velocity and channel width. We employ three models to represent the dilute polymer solution: the classical Oldroyd-B model, the Oldroyd-B model with artificial diffusivity and the non-homogeneous polymer model. The result of the linear stability analysis is found to be sensitive to the polymer model used. While the plane Couette flow is found to be stable to infinitesimal disturbances for the first two models, the last one exhibits a linear instability.

The paper is devoted to an experimental study of controlled disturbances evolution in supersonic boundary layer of swept wing. We obtained, that the evolution of disturbances at frequencies of 10, 20, and 30 kHz are similar to the development of traveling waves for subsonic velocities. We shown, that mechanism of secondary cross-flow instability play main role in laminarturbulent transition in 3-D supersonic boundary layer. On the basis of the obtained data the technique of laminar-turbulent transition control with the help of the distributed roughness was developed. Usage of this technique, has allowed as to bring transition upstream by 30% to delay downstream by 40%.