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A new experimental method for panoramic contactless determination of the laminar-turbulent transition position in a three-dimensional boundary layer is described. It is demonstrated that this method allows accurate determination of the transition region boundaries at both low and enhanced levels of free-stream turbulence.

This paper is devoted to an experimental investigation of the distributed receptivity of a laminar swept-wing boundary layer to unsteady freestream vortices with streamwise orientation of the vorticity vector. The experiments were performed on a model of a swept wing with a sweep angle of at fully controlled disturbance conditions with freestream vortices generated by a special disturbance source. It is found that the unsteady streamwise vortices are able to provide very efficient excitation of cross-flow instability modes without requiring the presence of any surface non-uniformities. The developed experimental approach is shown to allow us to perform a detailed quantitative investigation of the mechanism of distributed excitation of unsteady boundary-layer disturbances due to scattering of freestream vortices on natural base-flow non-uniformity. This mechanism has been studied experimentally in detail. Part 1 of the present investigation (Borodulin et al., J. Fluid Mech., vol. 908, 2021, A14) was devoted to the description of the experimental approach and the base-flow structure, the method of excitation of fully controlled streamwise-elongated freestream vortices, the results of measurements of structure of these vortices and the experimental evidence of high efficiency of the distributed vortex receptivity mechanism under study. Meanwhile, the present paper (Part 2) is devoted to: (a) theoretical background and definition of the distributed receptivity coefficients and (b) obtaining experimental quantitative characteristics of the distributed vortex receptivity including values of the corresponding receptivity coefficients for their three different definitions as functions of the disturbance frequency, spanwise wavenumber and wave propagation angle.

The paper is devoted to experimental investigation of an efficient mechanism of distributed excitation of nonstationary Gortler vortices in a boundary layer on concave wall due to scattering of three-dimensional, streamwise oriented freestream vortices on natural two-dimensional base-flow nonuniformity associated with the boundary layer growth. The investigations are performed in a broad range of disturbance frequencies and spanwise wavenumbers. The measurements have shown that the distributed receptivity mechanism is able to modify significantly the disturbance growth rates. This mechanism is found to be able to compete successfully with the linear instability mechanism. It is found that at relatively low frequencies and not too large freestream disturbance amplitudes, the Gortler modes’ development is dominated basically by the corresponding linear instability mechanism. Meanwhile, this mechanism becomes weaker with growth of frequency and, simultaneously, the distributed receptivity mechanism gets stronger. This leads to enhancement of role of the latter in amplification of Gortler instability modes, and at high frequencies the role of the distributed receptivity mechanism becomes very important and often even decisive one. Based on deep processing of the experimental data, quantitative values of the distributed linear receptivity coefficients are obtained for the case of excitation of unsteady Gortler modes and estimated for the case of steady Gortler vortices by means of extrapolation to the zero frequency. A complementary experiment devoted to investigation of the problem of linear unsteady Gortler instability is performed for the particular conditions of the distributed receptivity experiments. All main stability characteristics are obtained and compared with calculated ones. In general, the experimental information presented in this paper can be used for verification of various instability and vortex receptivitytheories.

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.

The paper is based on results obtained within an international project ‘RECEPT’ of the European Framework Program FP7. The experiments were carried out in a three-dimensional boundary layer developing on an experimental model of a long-laminar-run swept airfoil (sweep angle of 35°). The model was mounted in a test section of the lowturbulence wind-tunnel MTL (KTH, Stockholm) at an angle of attack of −5° and equipped with sidewalls provided satisfaction of infinite-span conditions. The cross-flow (CF) instability modes were predominant in this case, while the Tollmien–Schlichting (TS) waves were suppressed by a favorable pressure gradient. The main measurements were carried out by means of hot-wire anemometry at conditions of excitation of fully controlled, unsteady surface and flow perturbations. These perturbations were excited by special sources: (1) a row of oscillating membranes and (2) a vibrating wire, at frequencies of fs and fv, respectively. A very good, quantitative agreement between the measured and calculated (by linear stability theory based on PSE approach) amplification curves was found at surface frequency fs. However, the evolution of the CF-modes excited at difference combination frequency fsv– = fs – fv turned out to be very much different from the theoretical one. Thorough analysis of the obtained results has shown that the only explanation of these discrepancies can be associated with presence of a distributed receptivity mechanism due to scattering of freestream vortices on the CF-instability waves excited by surface vibrations. Another unusual and unexpected phenomenon found in the present experiments is associated with anomalous amplification of difference combination modes with the zero spanwise wavenumbers E′. This phenomenon was observed in the flow, which is stable with respect to both CF- and TSwaves having E′ = 0 for all frequencies. There is no explanation of this finding at present.

There are several kinds of velocity disturbances, which may affect the transition to turbulence in a swept wing boundary layer. Tollmien-Schlichting (TS) waves are among most important of them. The properties of TS waves and their potential competition with cross-flow waves on a swept wing are poorly studied in theoretical works and were not studied experimentally at all. This paper presents the method of excitation of fully controlled 3D TS waves via interaction of free-stream vortices and surface vibrations. The experimental approach developed here will be used for investigation of the corresponding receptivity problem.

This paper is devoted to an experimental investigation of receptivity of a laminar swept-wing boundary layer due to scattering of free-stream vortices on localized (in the streamwise direction) surface vibrations. The experiments were conducted under completely controlled disturbance conditions by means of a hot-wire anemometer on a model of a swept wing with a sweep angle of 25°. Both the free-stream vortices and the surface vibrations were generated by disturbance sources; their frequency–wavenumber spectra were measured thoroughly. The free-stream vorticity vectors were directed perpendicular to the incident-flow velocity vector and parallel to the swept-wing-model surface. The linearity of the receptivity mechanism under investigation (in a sense that the corresponding receptivity coefficients are independent of the disturbances amplitudes) has been checked carefully. The main goal of this experiment was to estimate the vibration-vortex receptivity coefficients as functions of the disturbance frequency, spanwise wavenumber and vortex offset parameter. This goal has been attained. Being defined in Fourier space, the obtained receptivity coefficients are independent of the specific surface vibration shape and can be used for verification of various receptivity theories.

Extensive combined experimental and theoretical investigations of the linear evolution of three-dimensional (3D) Tollmien-Schlichting (TS) instability modes of 3D boundary layers developing on a swept airfoil section have been carried out. The flow under consideration is the boundary layer over an airfoil at 35 degrees sweep and an angle of attack of +1.5 degree. At these conditions, TS instability is found to be the predominant one. Perturbations with different frequencies and spanwise wavenumbers are generated in a controlled way using a row of elastic membranes. All experimental results are deeply processed and compared with results of calculations based on theoretical approaches. Very good quantitative agreement of all measured and calculated stability characteristics of swept-wing boundary layers is achieved.

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.

The present study is devoted to the investigation of several, presumably most efficient, mechanisms of the production of non-stationary Gortler vortices in a laminar boundary layer on a concave wall due to scattering of 2D and 3D free-stream vortices by streamwise localized 3D and 2D surface and flow non-uniformities. The experiments were carried out by means of the method of controllable non-stationary disturbances. The interaction of downstream-propagating 3D free-stream vortices with the growing boundary layer, presenting natural 2D bas-flow non-uniformity, was found to lead to a rather efficient excitation of unsteady Gortler modes. This mechanism of distributed receptivity is able to modify considerably the growth rates of the excited Gortler vortices in comparison with the linear stability laws. In the present paper, definitions of the coefficients of distributed vortical receptivity are given and some estimates of values of these coefficients are reported. In spite of a high measurement accuracy and a rather broad range of examined parameters, no excitation of Gortler vortices due to other examined mechanisms was identified.

The paper is devoted to the first experimental study of distributed excitation of Görtler instability modes due the distributed mechanism of receptivity of a concave-wall boundary layer to streamwise freestream vortices. Experiments are carried out in the following range of problem parameters: Görtler numbers G* = 7,4÷21,3, frequencies f = 15, 20, and 26 Hz (nondimensional frequency parameters are F = 17,04; 22,72 and 29,54), and a broad range of spanwise scales of disturbances z = 8÷24 мм (nondimensional scales are Λ = 149÷774). It is found that this receptivity mechanism is quite efficient and can lead to amplification of unsteady Görtler vortices even in regimes where the boundary layer is linearly stable to these boundary-layer disturbances. Estimations of quantitative characteristics of the investigated physical phenomenon: the complex values of the distributed vortex receptivity coefficients are obtained in the present study for the first time. It is found that examined receptivity mechanism is especially effective for vortices with spanwise wavelengths close to the most dangerous in terms of the linear stability theory. The amplitudes of the receptivity coefficients are found to decrease with the streamwise coordinate

The present experimental study is devoted to examination of the vortex receptivity mechanism associated with excitation of unsteady cross-flow (CF) waves due to scattering of unsteady free-stream vortices on localized steady surface non-uniformities (roughness). The measurements are carried out in a low-turbulence wind tunnel by means of a hot-wire anemometer in a boundary layer developing over a $25\textdegree $ swept-wing model. The harmonic-in-time free-stream vortices were excited by a thin vibrating wire located upstream of the experimental-model leading edge and represented a kind of small-amplitude von Kármán vortex street with spanwise orientation of the generated instantaneous vorticity vectors. The controlled roughness elements (the so-called ‘phased roughness’) were placed on the model surface. This roughness had a special shape, which provided excitation of CF-waves having basically some predetermined (required) spanwise wavenumbers. The linearity of the stability and receptivity mechanisms under study was checked accurately by means of variation of both the free-stream-vortex amplitude and the surface roughness height. These experiments were directed to obtaining the amplitudes and phases of the vortex-roughness receptivity coefficients for a number of vortex disturbance frequencies. The vortex street position with respect to the model surface (the vortex offset parameter) was also varied. The receptivity characteristics obtained experimentally in Fourier space are independent of the particular roughness shape, and can be used for validation of receptivity theories.

The mechanism of production of nonstationary Gortler vortices in a boundary layer on concave wall by surface nonuniformities (vibrations and roughness) has been experimentally examined. The nonuniformities were produced by a specially developed disturbance source. They were controlled, localized along the streamwise coordinate, and periodic over the span of the experimental model. Tests in a low-turbulence wind tunnel have proved that the disturbance source is an efficient means of experimental study of the receptivity and stability problem for boundary layers dominated by Gortler instability. The operation of the disturbance source leads to the production of small-amplitude nonstationary Gortler vortices (tenth or hundredth fractions of a per cent of the free-stream velocity) with predefined characteristics (frequency and spanwise wavelength). In our experiments, we quantitatively examined the problem of linear receptivity of boundary layer to surface nonuniformities in a broad range of frequencies for the most dangerous spanwise scales of Gortler vortices. The values of the amplitudes and phases of the receptivity coefficients were determined. The amplitudes proved to be much smaller in magnitude in comparison with the excitation of modes of hydrodynamic instabilities of other types (Tollmien-Schlichting waves and cross-flow-instability modes). It was found that, with increasing the frequency, the amplitudes of the receptivity coefficients showed a distinct growth while for high frequencies those amplitudes also exhibited a growth with the spanwise scale of perturbations, although for stationary surface roughness no effect due to this scale was observed. It was found that the dependences on frequency of the efficiency of the mechanisms of stability and receptivity showed opposing behaviors, were in competition, and could partially compensate each other, promoting, thus, the production of boundary-layer Gortler vortices in a broad range of frequencies.

The paper describes the results of experiments on robustness of laminar-turbulent transition control on a swept-wing using distributed micro-sized roughness (DMSR) elements. These elements introduce controlled stationary vortices which are able to significantly modify the base flow and its stability characteristics. We have performed parametric study first varying height and period of the DMSR elements in order to find the most stabilizing effect on boundary later flow in compare to uncontrolled reference case without DMSR. Significant downstream shift of laminar-turbulent transition position due to application of DMSR is found and well documented with help of thermography. The robustness of this flow control method was studied by variation of the wind-tunnel flow quality introducing significant sound background or introducing enhanced turbulence level (applying turbulizing grids). The wind-tunnel tests performed with turbulence-generating grids (at enhanced turbulence levels) have shown that laminar-turbulent transition moves upstream in this case, while DMSR-elements loose their effectiveness for transition control (no matter in quiet sound conditions or at elevated sound background). The experiments on acoustic influence have shown that without DMSR acoustic does not effect transition location. However, in case then laminar-turbulent transition is delayed by presence of DMSR, an additional transition delay was observed when harmonic acoustic waves of certain frequency were excited.The paper describes the results of experiments on robustness of laminar-turbulent transition control on a swept-wing using distributed micro-sized roughness (DMSR) elements. These elements introduce controlled stationary vortices which are able to significantly modify the base flow and its stability characteristics. We have performed parametric study first varying height and period of the DMSR elements in order to find the most stabilizing effect on boundary later flow in compare to uncontrolled reference case without DMSR. Significant downstream shift of laminar-turbulent transition position due to application of DMSR is found and well documented with help of thermography. The robustness of this flow control method was studied by variation of the wind-tunnel flow quality introducing significant sound background or introducing enhanced turbulence level (applying turbulizing grids). The wind-tunnel tests performed with turbulence-generating grids (at enhanced turbulence levels) have shown that laminar-turb...

An extensive experimental investigation of linear evolution of Cross-Flow (CF) and Tollmien-Schlichting (TS) modes of 3D boundary layer oscillations on a swept wing has been carried out. TS-instability characteristics have been studied experimentally for the first time. The characteristics of development of the two kinds of instability modes are compared with calculations and display a very good agreement. The whole dataset may be used for promotion of theoretical methods of investigation of laminar-turbulent transition in swept wing boundary layers.

The subject of the present combined experimental and theoretical investigation is the steady and unsteady linear Gortler instability. The majority of previous experiments were devoted to the steady Gortler vortices, despite the unsteady ones are also observed in real transitional flows. Moreover, even for the steady Gortler vortices no quantitative agreement between the experimental and theoretical linear-stability characteristics was obtained, especially for disturbance amplification rates. The experimental difficulties were connected, in particular, with a rather poor accuracy of measurements at zero disturbance frequency, a possible influence of nonlinearity, and an admixture of non-modal (transient) growth mechanism. All these difficulties have been overcome in the experimental part of the present study by means of: (i) tuning-out of the exact zero frequency of Gortler vortices and working, instead, with quasi-steady perturbations of very low frequencies, (ii) performing measurements at low disturbance amplitudes, and (iii) minimization and careful estimation of the disturbance-source near-field by means of utilizing a special controlled disturbance source and performing special numerical computations for exact experimental conditions. A detailed study of all linear-stability characteristics for essentially unsteady Gortler vortices was performed in this paper as well. The results are obtained in a range of Gortler numbers 13 ≲ Go ≲ 17.3, frequency parameters F = 0.56–22.70, and spanwise wavelength parameters Λ = 149–775 (close to the most amplified Gortler modes). Appropriate calculations based on locally-parallel and non-local non-parallel linear-stability theories were performed and compared quantitatively with experimentally obtained linear-stability characteristics. For the first time all stability characteristics measured for steady Gortler vortices (in quasi-steady regimes) are found to agree very well with those calculated for the most amplified first discrete-spectrum mode of the linear Gortler-instability problem. Similar good agreement is obtained for essentially unsteady Gortler vortices. The roles of effects of the base-flow non-parallelism and the disturbance-source near-field are examined.

The distributed (over the longitudinal coordinate) excitation of two-dimensional (2D) Tollmien — Schlichting (TS) waves by weak non-stationary free-stream vortices propagating along the edge of the laminar boundary layer developing over a surface with small-amplitude 2D roughness is examined. The vorticity vector of the free-stream vortices was oriented over the span of the model, i. e., did not depend on the transverse coordinate. The theoretical analysis of the excitation mechanism reported in [1] was refined to develop, around it, a procedure making it possible to experimentally determine the coefficients of distributed vortical receptivity of the flow and the coefficients of “vortex-roughness” receptivity by fitting the experimental distributions with analytical solutions. Under conditions with controllable excitation of disturbances, a detailed hot-wire study of free-stream disturbances and boundary-layer disturbances at several vortex frequencies and at several surface-roughness periods was performed, and the shape of the controllable surface roughness was measured. The point-source method was employed to experimentally examine the characteristics of linear three-dimensional (3D) stability of the flow to TS waves, necessary for determination of the coefficients of distributed receptivity. It was found that the free-stream vortices with transverse orientation of the vorticity vector excited boundary-layer TS waves via two receptivity mechanisms: (a) on the smooth surface (due to natural non-uniformity of the flow) and (b) during interaction of the vortices with the surface roughness. The developed approach was used to experimentally estimate the amplitudes and phases of the coefficients of both types of distributed vortical receptivity as dependent on problem parameters. The absolute values of both types of receptivity coefficients were found to rapidly grow in value with increasing vortex frequency. It is shown that the most efficient excitation of TS waves is observed in the situation with satisfied resonance conditions for streamwise vortex, surface-roughness, and TS-wave streamwise wavenumbers, resulting in strong deviation of the increments of the TS waves from the linear-stability increments. Under no-resonance conditions, only amplitude beats of boundary-layer disturbances were observed.

The linear receptivity of a swept-wing three-dimensional boundary layer is studied experimentally and theoretically. Cross-flow instability normal modes are excited by means of surface vibration or roughness perturbations. The resulting disturbances are investigated, and the normal modes are linked to the source perturbations. Experiments are performed under controlled disturbance conditions with a time-harmonic source that is localized in the spanwise direction. A localized surface vibration is used to excite wave trains consisting of cross-flow instability waves. Normal oblique modes (harmonic in time and space) are obtained by Fourier decomposition of the wave trains. This provides the spatial variation of the normal modes and, in particular, the initial amplitudes and phases of the modes at the source location. The shape of the surface vibrator is measured and used to determine the complex receptivity coefficients for the normal modes (i.e. for various spanwise wavenumbers, wave propagation angles, and disturbance frequencies – including zero frequency). The experimental receptivity coefficients are independent of the specific shape of the surface non-uniformities and can be directly compared with calculations. The theoretical work is based on a linear approximation to the disturbance source – valid for small forcing amplitudes. Like earlier studies on roughness-induced receptivity, the basic flow is locally assumed to satisfy the parallel-flow approximation. The modal response for the cross-flow instability is determined from the residue associated with the least-stable eigenmode.A detailed quantitative comparison between the experimental and theoretical receptivity characteristics is carried out. Good agreement is found for the roughness–vibrational receptivity coefficients of the swept-wing boundary layer (especially for the most-unstable cross-flow modes) over a range of disturbance frequencies and spanwise wavenumbers. The theory correctly predicts the initial spectra for the travelling and stationary cross-flow instabilities excited by the surface vibrations and surface roughness, respectively. The good agreement between theory and experiment suggests that the linear receptivity theory can be used effectively in engineering methods for transition prediction. The experimental data can also be used for validation of other theoretical approaches to the problem.

The 3D receptivity of 2D laminar boundary layers to localized surface vibrations has been investigated both experimentally and theoretically for two types of basic flow: (i) the Blasius boundary layer and (ii) a boundary layer with a negative streamwise pressure gradient (Hartree parameter βH=0.10). For the boundary-layer excitation, a specially designed surface vibrator was used. The development of the excited wave-trains was measured by means of hot-wire anemometry and decomposed into oblique normal Tollmien–Schlichting-modes. The initial spectra of the excited perturbations at the position of the vibrator was obtained by two different techniques. The first used an additional source which was mounted upstream and provided the amplification curves for the instability modes in the vicinity of the vibrator, the second was based on linear stability calculations. The receptivity coefficients were defined as the ratio of the initial wavenumber spectrum of the excited TS-waves and the corresponding resonant spectrum of the surface vibrations. They were determined for each fixed frequency as a function of the spanwise wavenumber. The boundary value problem for the disturbance produced by the vibrating membrane was solved theoretically for the same conditions as in the experiments in the framework of the classical hydrodynamic stability theory. The Navier–Stokes equations were linearized around a incompressible basic flow described by a solution of the Falkner–Skan equation. Comparisons of the theoretical and experimental results on the 3D receptivity show good quantitative agreement. It is concluded that the favorable pressure gradient increases the boundary-layer receptivity to surface vibrations.

This experimental work is devoted to a parametric study of characteristics of the laminar-turbulent transition occurred in a model of a crossflow-dominated swept-wing boundary layer. The experiments are performed in 39 different regimes with low and elevated freestream turbulence levels both in the absence and in the presence of steady freestream vortices of various spanwise scales. The measurements are performed at several values of freestream velocity for two types of distributed surface roughness. Transition scenarios and criteria of turbulence onset are investigated and the problem of transition prediction is examined. It is shown that in all studied cases, the turbulence onset starts with the appearance of local high-frequency secondary instability of the base flow perturbed by primary instability modes. It is found that the 30% level of the threshold zero-to-peak amplitude of combined boundary-layer disturbances (i.e., the steady ones plus unsteady ones) can be used in all studied cases as a simplif...

The present work is a continuation of previous experiments (carried out in the Blasius boundary layer) and devoted to quantitative investigation of influence of an adverse pressure gradient on two efficient mechanisms of excitation of 3D TS instability waves due to a distributed boundary layer receptivity to free-stream vortices. These mechanisms are associated with distributed scattering of 3D amplified free-stream vortices both on the natural boundary-layer nonuniformity (on smooth surface) and on 2D surface nonuniformities (waviness). The corresponding detailed hotwire measurements were carried out in a self-similar boundary layer with Hartree parameter βH = –0.115 in a wide range of the problem parameters. Complex values of quantitative characteristics of the physical phenomenon under study (the distributed receptivity coefficients) are evaluated by based on the obtained experimental data. It is found that the adverse pressure gradient leads to reduction of efficiency of the investigated vortexroughness receptivity mechanism.

In the present paper, the linear 3D acoustic-roughness receptivity of the 2D boundary layer of an airfoil is investigated systematically with respect to main influence parameters by means of wind tunnel experiments and Direct Numerical Simulations (DNS). A plane acoustic wave with frequency f ac scatters at a specially designed localized roughness element, that is capable to vibrate with frequency f v . The resulting Tollmien-Schlichting (TS) waves appear at combination frequencies (f 1,2 = f ac ∓ f v ) and develop as a wave train downstream of the surface roughness. The complex receptivity function is evaluated by normalization of the initial TS-spectra with the related amplitudes and phases of the surface vibrator and the acoustics. The main influence parameters investigated are the frequency (of the acoustic and the TS wave), the pressure gradient (given by the streamwise position of the surface roughness) and the influence of surface vibrations on the acoustic receptivity. Additionaly pure vibrational receptivity is studied. The main goal of this investigation is to study and compare systematically different types of receptivity in the same base flow developing on an airfoil at a Reynolds number typical for glider applications.

The paper is devoted to experimental study of the linear receptivity of a swept-wing boundary layer to localized surface vibrations. Namely, the generation and downstream development of wave trains produced by a localized surface vibrator are investigated. A decomposition of these wave trains (consisted of the cross-flow instability modes) into oblique harmonic (in time and space) modes is carried out. The amplitudes and phases of these normal modes are extrapolated to the position of the disturbance source to determine their initial values. The shape of the surface vibrations is measured with high accuracy by means of two different experimental techniques. Then the vibrational amplitudes and phases are determined for those modes of the frequency-wavenumber spectrum which correspond to the instability waves detected within the boundary layer downstream the source. And, finally, the complex receptivity coefficients of the swept-wing boundary layer to the surface vibrations are determined as functions of the spanwise wavenumber.

The paper is devoted to experimental study of the linear stability a 3D boundary layer on a model of a swept wing. Namely, the development of spatial wave packets (wave trains) generated by localized disturbance source is investigated. Decomposition of these wave trains to oblique harmonic (in time and space) modes is carried out. These wave packets are shown to consist mainly of the cross-flow instability modes. A complete set of stability and dispersion characteristics of the flow with respect to traveling waves inclined at various angles to the flow direction is obtained.

Noise emitted from 2D subsonic airfoil sections depends strongly on the specific properties of the turbulent boundary-layer passing the trailing edge. The intention of the present paper is three folds: (1) wind tunnel measurement of the two-point turbulent velocity correlations to analyze the trailing-edge near wake flow structure in detail, (2) assessment of the measurement data and post-processing of the turbulence properties in a way so that it can be comparable to numerical results, and (3) development of a relationship between isotropic theory and anisotropic measurement data to approximate quantities like isotropic longitudinal length scale Λf . Measurements of two-point turbulent velocity correlations were conducted in 2D turbulent boundary layer flows over two airfoils (NACA 0012 & NACA 643-418 ) at high Reynolds numbers (Re=1.5e6 and Re=2.5e6) in the Laminar Wind Tunnel (LWT) of the University of Stuttgart. Correlation tensor components were measured by two X-wire probes shortly downstream of the airfoil trailing edge with separation in vertical direction for different cases of boundary layer development (angles-of-attack, natural and fixed transition). Two different approaches to evaluate turbulence integral correlation length scales from the measured two-point correlation data are presented. The first methods provides anisotropic turbulence integral length scales estimated by fitting an exponential function to the measured two-point velocity correlation coefficient. The second method established a local relationship between the results from isotropic theory and anisotropic experimental data. This relationship permits efficient approximation of the isotropic longitudinal integral length scale (Λf), and enables the consideration of anisotropy effects in RANS predicted data. The outcome of both methods are been compared with RANS results and lead to significant improvement of the prediction of the turbulence noise sources. The benefit of these efforts will be further applied to a Turbulent Boundary-Layer Trailing-Edge (TBLTE) interaction noise prediction method (Rnoise) to analyze anisotropy effects of the airfoil trailing-edge near-wake flow.

We have examined, both experimentally (using fully controlled disturbances) and theoretically, the weakly-nonlinear development stages of unsteady (in general) Gortler instability of a boundary layer over a concave surface. Primary attention was given to early manifestations of nonlinearity in the development of unsteady Gortler vortices belonging to the first, most rapidly growing, mode in the discrete spectrum of the stability problem. We have investigated the manifestations of instability versus the frequency of the fundamental (primary) Gortler mode and the initial disturbance amplitude. The weakly-nonlinear stage of development of unsteady Gortler vortices was found to display the following characteristic features: (a) nonlinear interaction among the combination modes in the frequency-wavenumber spectrum, (b) distortion of the wall-normal profiles of disturbance amplitudes and phases, (c) reduction of the growth rate of the fundamental Gortler mode and the majority of combination modes, and (d) a decrease in the phase velocities of unsteady disturbances. It was found that the disturbances enter the region of weakly-nonlinear development after the amplitude of the fundamental frequency-wavenumber mode reaches a threshold of 4–6 %, this value being much greater than that for Tollmien — Schlichting waves (1–2 %) but significantly lower than that for the cross-flow instability modes in three-dimensional boundary layer (more than 10 %).