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Extensive combined experimental and theoretical investigations of the linear evolution of unsteady (in general) Cross-Flow (CF) and three-dimensional (3D) Tollmien-Schlichting (TS) instability modes of 3D boundary layers developing on a swept airfoil section have been carried out. CF-instability characteristics are investigated in detail at an angle of attack of −5° when this kind of instability dominates in the laminar-turbulent transition process, while the 3D TS-instability characteristics are studied at an angle of attack of +1.5° when this kind of instability is predominant in the transition process. All experimental results are deeply processed and compared with results of calculations based on several theoretical approaches. For the first time, very good quantitative agreement of all measured and calculated stability characteristics of swept-wing boundary layers is achieved both for unsteady CF- and 3D TS-instability modes for the case of a boundary layer developing on a real swept airfoil. The first part of the present study (this paper) is devoted to the description of the case of CF-dominated transition, while the TS-dominated case will be described in detail in a subsequent second part of this investigation.

International audience; The transition from laminar to turbulent boundary layer has been largely investigated in the literature on the experimental side as well as on the numerical side but the processes specifically involved in both transitional and unsteady flows (unsteady aerodynamic or aeroelasticity) have to be studied more in details. This paper describes an experimental investigation achieved through a wind tunnel test of an oscillating swept wing in low speed range. In order to force the apparition of Cross Flow instabilities, the model was installed with a 60° sweep angle under favorable pressure gradient ensuring the absence of Tollmien Schlichting instabilities. The model was equipped with hot-film sensors to estimate the laminar or turbulent state of the boundary layer through the measurement of wall shear stress. The synchronous acquisition of dynamic motion and hot-film signals allowed to analyse the unsteady effects on the Cross Flow induced transition. The wind tunnel test program was achieved following a parametric approach through the variation of significant parameters: the amplitude and frequency of the dynamic actuation, the flow speed velocity and the mean value of the airfoil angle of attack. The paper describes the main insights of the test campaign and the associated database and presents how the test setup provided a well-adapted mean to study dynamic motions of the transition location. In comparison to steady configuration, the transition was not located at a fixed chordwise position but covered an area with an intermediary state of the boundary layer between the laminar and turbulent states. Several tools and post processing methods were specifically tuned to calculate quantities of the transition position (time resolved signal, mean properties through a phase averaging process, intermittency ratio) in addition to classical time and frequency analyses. One of the main outcomes pointed out the mean location of the laminar-turbulent transition was weakly influenced by unsteady motion. The general trend indicated a IFASD-2019-120 2 more upstream location of few percent in chordwise in comparison to the steady case without any motion. The experimental results constitute a first step in the understanding of the interaction of Cross Flow instabilities and dynamic motion and in the assessment of ONERA modelling and numerical simulation capabilities.

Two fully local model variants of a correlation-based transport model that predicts transition due to crossflow instability as well as two-dimensional transition mechanisms are introduced. Whereas ...

The paper is devoted to some results of a EC project RECEPT (FP7). The experiments were carried out in a three-dimensional boundary layer developing on an experimental model of a long-laminar-run airfoil (sweep angle of 35°, chord of 0.8 m). The model was mounted in a test section of a low-turbulence wind tunnel MTL (KTH, Stockholm) at an angle of attack of -5 degrees. The cross-flow instability modes (CF) were the most amplified ones, while the Tollmien–Schlichting instability modes were suppressed by a favorable pressure gradient. The main measurements were performed by means of a single-wire hot-wire probe at conditions of excitation of fully controlled, unsteady surface and flow perturbations. These perturbations were excited by special sources: (i) a surface membrane and (ii) a vibrating wire. The two sources oscillated at frequencies fs and fv, respectively. The membrane had a diameter of 6 mm. It was mounted into the model surface and located at a chordwise coordinate of 120 mm downstream the leading edge. The vibrating wire was mounted at a tension upstream the leading edge and parallel to it and adjusted in a way to provide location of one of the amplitude maxima of the excited quasi-2D vortex street near the boundary layer edge. The incident flow velocity was of about 10 m/s and varied in the chordwise direction. A rather efficient excitation of packets of three-dimensional CF-modes was found at frequency fs (due to action of a mechanism of the boundary-layer receptivity to surface vibrations) as well as at two combinational frequencies fsv+ = fs + fv and fsv- = fs - fv (due to action of a receptivity mechanism associated with scattering of the controlled freestream vortices on the controlled surface vibrations). The amplification curves (amplitudes and phases) of CF-modes were obtained in a broad range of problem parameters (spanwise wavenumbers and frequencies fs and fv), as well as the amplitudes and phases of the corresponding receptivity coefficients of 'vibrational' and 'vortex-vibration' receptivity mechanisms. Comparison with calculations carried out by means of linear (locally-parallel) stability theory (LST) was performed. A very good, quantitative agreement between results of measurements and calculations was found at surface frequency fs. It is found that the CF-modes excited at the combinational frequencies fsv evolve downstream in a way that is different from the LST. A thorough analysis of disturbance spanwise profiles of amplitudes and phases, their spectra, and the disturbance amplification curves showed that the features of downstream evolution of the CF-modes observed at frequency fsv can be explained by action of a distributed receptivity mechanism. This mechanism is associated with scattering of the controlled freestream vortices with frequency fv on the CF-instability modes excited by surface vibrations at frequency fs. Nowadays such distributed receptivity mechanism is remained completely unexplored. Therefore, the obtained experimental data seems to be of great basic and practical importance.

Boundary-layer measurements were performed in a Mach 6, low-disturbance wind tunnel on a 7∘ cone at 5.6∘ angle of incidence such that the model was primarily subject to crossflow instability. Constant-temperature hot-wire anemometry was used to measure the streamwise mass flux at a series of planes normal to the cone axis. A dominant stationary wave is observed to achieve nonlinear saturation at approximately 23% with maximum rms fluctuations of approximately 8%. Additionally, traveling crossflow waves were observed in a broad frequency band centered at f = 40 kHz and secondary instabilities were observed in a broad band centered at f = 100 kHz. Measurements show excellent agreement with previously published computational results and are qualitatively similar to studies performed in subsonic flows. Transition to turbulence was not observed to occur under these conditions.

This thesis deals with the instabilities of the incompressible boundary-layer flow thatis induced by a disk rotating in otherwise still fluid. The results presented include bothwork in the linear and nonlinear regime and are derived from direct numerical sim-ulations (DNS). Comparisons are made both to theoretical and experimental resultsproviding new insights into the transition route to turbulence. The simulation codeNek5000 has been chosen for the DNS using a spectral-element method (SEM) witha high-order discretization, and the results were obtained through large-scale paral-lel simulations. The known similarity solution of the Navier–Stokes equations for therotating-disk flow, also called the von K ́arm ́an rotating-disk flow, is reproduced by theDNS. With the addition of modelled small simulated roughnesses on the disk surface,convective instabilities appear and data from the linear region in the DNS are anal-ysed and compared with experimental and theoretical data, all corresponding verywell. A theoretical analysis is also presented using a local linear-stability approach,where two stability solvers have been developed based on earlier work. Furthermore,the impulse response of the rotating-disk boundary layer is investigated using DNS.The local response is known to be absolutely unstable and the global response, onthe contrary, is stable if the edge of the disk is assumed to be at radius infinity. Herecomparisons with a finite domain using various boundary conditions give a globalbehaviour that can be both linearly stable and unstable, however always nonlinearlyunstable. The global frequency of the flow is found to be determined by the Rey-nolds number at the confinement of the domain, either by the edge (linear case) or bythe turbulence appearance (nonlinear case). Moreover, secondary instabilities on topof the convective instabilities induced by roughness elements were investigated andfound to be globally unstable. This behaviour agrees well with the experimental flowand acts at a smaller radial distance than the primary global instability. The sharpline corresponding to transition to turbulence seen in experiments of the rotating diskcan thus be explained by the secondary global instability. Finally, turbulence datawere compared with experiments and investigated thoroughly. QC 20170203

Studies on the stability of three-dimensional hypersonic boundary layers for 7 and 15 deg. half-angle cones at 0 and 6 deg. angles of attack are reported. Measurements were carried out in the Hypersonic Ludwieg tube Braunschweig at Mach number 6. Surface mounted pressure and heat flux sensors were used to determine the spatial extension, frequency contents, and wave structure of second-mode and cross-flow instabilities. Infrared thermography was used to capture location, direction and growth of stationary cross flow instabilities. The experimental data are analyzed and compared to the results of corresponding stability computations.

This thesis deals with the instabilities of the incompressible boundary-layer flow that is induced by a disk rotating in otherwise still fluid. The results presented are mostly limited to linear instabilities derived from direct numerical simulations (DNS) but with the objective that further work will focus on the nonlinear regime, providing greater insights into the transition route to turbulence. The numerical code Nek5000 has been chosen for the DNS using a spectral-element method in an effort to reduce spurious effects from low-order discretizations. Large-scale parallel simulations have been used to obtain the present results. The known similarity solution of the Navier–Stokes equation for the rotating-disk flow, also called the von Karman flow, is investigated and can be reproduced with good accuracy by the DNS. With the addition of small roughnesses on the disk surface, convective instabilities appear and data from the DNS are analysed and compared with experimental and theoretical data. A theoretical analysis is also presented using a local linear-stability approach, where two stability solvers have been developedbased on earlier work. A good correspondence between DNS and theory is found and the DNS results are found to explain well the behaviour of the experimental boundary layer within the range of Reynolds numbers for small amplitude (linear) disturbances. The comparison between the DNS and experimental results, presented for the first time here, shows that the DNS allows (for large azimuthal domains) a range of unstable azimuthal wavenumbers β to exist simultaneously with the dominantβ varying, which is not accounted for in local theory, where β is usually fixed for each Reynolds number at which the stability analysis is applied. Furthermore, the linear impulse response of the rotating-disk boundary layer is investigated using DNS. The local response is known to be absolutely unstable. The global response is found to be stable if the edge of the disk is assumed to be at infinity, and unstable if the domain is finite and the edge of the domain is placed such that there is a large enough pocket region for the absolute instability to develop. The global frequency of the flow is found to be determined by the edge Reynolds number. QC 20140708

Rotating-disk flow has been investigated not only as a simple model of cross flow instability to compare with swept-wing flow but also for industrial flow applications with rotating configurations. However the exact nature of laminar-turbulent transi- tion on the rotating-disk flow is still major problem and further research is required for it to be fully understood, in particular, the laminar-turbulent transition process with absolute instability. In addition the studies of the rotating-disk turbulent boundary- layer flow are inadequate to understand the physics of three-dimensional turbulent boundary-layer flow. In present thesis, a rotating-rotating disk boundary-layer flow has been inves- tigated experimentally using hot-wire anemometry. A glass disk with a flat surface has been prepared to archieve low disturbance rotating-disk environment. Azimuthal velocity measurements using a hot-wire probe have been taken for various conditions. To get a better insight into the laminar-turbulent transition region, a new way to describe the process is proposed using the probability density function (PDF) map of azimuthal fluctuation velocity. The effect of the edge of the disk on the laminar-turbulent transition process has been investigated. The disturbance growth of azimuthal fluctuation velocity as a function of Reynolds number has a similar trend irrespective of the various edge conditions. The behaviour of secondary instability and turbulent breakdown has been in- vestigated. It has been found that the kinked azimuthal velocity associated with secondary instability just before turbulent breakdown became less apparent at a cer- tain wall normal heights. Furthermore the turbulent breakdown of the stationary mode seems not to be triggered by its amplitude, however, depend on the appearance of the travelling secondary instability. Finally, the turbulent boundary layer on a rotating disk has been investigated. An azimuthal friction velocity has been directly measured from the azimuthal velocity profile in the viscous sub-layer. The turbulent statistics normalized by the inner and outer sclaes are presented. QC 20120529

Laminar-turbulent transition of quasi-three-dimensional boundary-layer flows is investigated by nonlinear nonlocal instability theory based on parabolized stability equations (PSE). A strong TS-CF interaction scenario is described, which leads to a rapid rise in skin-friction coefficient indicating imminent breakdown of the laminar flow. The CF-CF interaction studies reproduce amplitude saturation observed in experiment, but do not provide an explanation for the final breakdown in crossflow-dominated boundary layers yet.