Views Icon Views Article contents Figures & tables Video Audio Supplementary Data Peer Review Share Icon Share Twitter Facebook Reddit LinkedIn Tools Icon Tools Reprints and Permissions Cite Icon Cite Search Site Citation S. V. Kirilovskiy, A. V. Boiko, V. I. Borodulin, K. V. Demyanko, A. V. Ivanov, D. A. Mischenko, Y. M. Nechepurenko, T. V. Poplavskaya; On predicting the onset of the transition to turbulence in the three-dimensional boundary layer on a swept wing. AIP Conference Proceedings 16 February 2023; 2504 (1): 030088. https://doi.org/10.1063/5.0133391 Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentAIP Publishing PortfolioAIP Conference Proceedings Search Advanced Search |Citation Search

An extensive experimental investigation of two receptivity mechanisms of localized excitation of unsteady (in general) Cross-Flow (CF) instability modes in a three-dimensional (3D) swept-airfoil boundary layer has been carried out within one extended set of measurements. Two mechanisms of the boundary-layer receptivity have been investigated: (i) to surface nonuniformities (vibrations, in general) and (ii) to scattering of freestream vortices on surface nonuniformities. All obtained results are deeply processed and analyzed. Part 1 of the present study (Borodulin et al., 2023), was devoted to description of the results obtained for problem (i). Meanwhile, results of detailed investigation of problem (ii) are presented in Part 2 of this study (the present paper). & COPY; 2023 Elsevier Masson SAS. All rights reserved.

An extensive experimental investigation of two receptivity mechanisms of localized excitation of unsteady (in general) Cross-Flow (CF) instability modes in a three-dimensional (3D) swept-airfoil boundary layer has been carried out within one extended set of measurements. Two mechanisms of the boundary-layer receptivity have been investigated: (i) to surface nonuniformities (vibrations, in general) and (ii) to scattering of freestream vortices on surface nonuniformities. All obtained results are deeply processed and analyzed. Part 1 of the present study (this paper) is devoted to description of the results obtained for problem (i). Meanwhile, results of detailed investigation of problem (ii) are presented in Part 2 of this study (the next paper Borodulin et al. (2023)). & COPY; 2023 Elsevier Masson SAS. All rights reserved.

The problem of accurate experimental detection of laminar-turbulent transition in three-dimensional boundary layers is discussed. A high-resolving panoramic experimental technique to estimate the on -set and length of the transition for aerodynamic applications is proposed and described in detail. The technique includes multistage processing of recorded infrared image sequences to provide a robust de-tection of the transition in a broad range of base flow parameters. The data processing is fast and virtually hands-free that allows extensive parametric investigation of the transition in experiments. Moreover, an application of statistics procedures to obtain various spanwise-averaged values quantifying different as-pects of the transition important for calibrating and validating engineering transition prediction methods is possible. (c) 2021 Elsevier Ltd. All rights reserved.

The paper presents the first results of an experimental study of a new effective control mechanism of evolution of fast-growing unsteady Görtler modes by modifying the base flow (the boundary layer on a concave surface) by means of quasi-steady (physically steady) Görtler vortices of finite amplitude. It is found that this mechanism is able to strongly suppress growth rates of the disturbances under control. The suppression efficiency depends on the phase relations between the control and controlled perturbations, but does not depend on amplitudes of the latter.

The problem of calibration of semi-empirical eN-method for prediction of laminar-turbulent transition in three-dimensional boundary layers is discussed. A high-resolving panoramic experimental technique to estimate the onset and length of the transition for aerodynamic applications is proposed and described. The position of laminar-turbulent transition is estimated also using the developed techniques for computing three-dimensional laminar-turbulent flow at a swept wing using an original transition module developed on the basis of the author’s software bundle LOTRAN 3, the module being integrated into ANSYS Fluent.

The laminar-turbulent transition in the boundary layer of a 45° swept wing model installed at zero attack angle in the test section of a subsonic wind-tunnel was detected with the help of an infrared camera. The camera recorded sequences of frames, the evolution of the preheated model surface temperature acquired and used for differentiating between the laminar and turbulent regions. The transition onset was evaluated at both sides of the model. Corresponding main flow computations in the virtual wind tunnel test section were performed at the same flow conditions with ANSYS Fluent. The computed main-flow velocity profiles along inviscid streamlines were used for analysis of hydrodynamic stability of the boundary layer with respect to Tollmien-Schlichting waves and stationary cross-flow vortices to obtain N-factor distributions along the model chord. A comparison of the experimental and the computed transition onsets was performed.

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.

The paper is devoted to the problem of creation of experimental database obtained due to systematic extended series of experiments aimed at registering the laminar-turbulent transition in boundary layers on a swept wing model. The experiments were carried out in a low-turbulence wind tunnel in boundary layers developed on ‘suction’ and ‘pressure’ sides of experimental model of an airfoil in a broad range of problem parameters (angle of attack, free-stream velocity, free-stream turbulence etc.). The main feature of performed experiments is using of a new panoramic experimental technique based on application of sensitive infrared camera. Digital postprocessing of obtained thermograms that gave a possibility to detect the position of transition occurred due to amplification of cross-flow instability modes, TS-waves or both of them. The article includes description of the approaches used and the first preliminary results.

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 devoted to the first results of an experimental quantitative study of the receptivity mechanism of a swept-wing laminar boundary layer related to scattering of 2D freestream vortices (with frequency f(v)) at 3D local surface vibrations (with frequency fs) resulting in an excitation of Tollmien-Schlichting (TS) waves (having combination frequencies f(+) = f(s) + f(v) and f(-) = f(s) - f(v)). The experiments were carried out in a low-turbulence level wind tunnel on a high-precision experimental model of long-laminar-run swept airfoil (sweep angle of 35 degrees) at a freestream speed of about 10 m/s. Controlled localized 3D surface vibrations and 2D freestream vortices were generated by special disturbance sources. Quantitative characteristics of the studied receptivity mechanism (receptivity coefficients) were estimated.

A new experimental technique of panoramic determination of the position of the laminar-turbulent transition in three-dimensional boundary-layer flows with a thermal imaging camera is described. It is shown that it is suitable for parametric measurements of the transition providing a sharp location of the transition at both low and high degrees of free-stream turbulence.

In recent works [1, 2] an effective influence of oblique streaky surface relief to stationary cross-flow vortices amplification was established theoretically and experimentally. In the experimental work [2] the wave trains of stationary cross-flow vortices exited due to artificial 3D localized surface roughness were developed downstream both over the smooth surfaces (as baseline case) and in cases when the model surface was modified with surface relief (comprising elongated strips oriented oblique to the local flow direction). The obliqueness of relief strips with respect to the flow vector was varied and clear influence of the relief obliqueness on cross-flow vortex amplification was found. In particular in case when the surface relief strips were oriented parallel to the leading edge a remarkable cross-flow vortex amplitudes reduction was detected. Even stronger stabilizing effect was observed when the relief strips were oriented in between the leading edge and the local flow direction. Based on the obtained results a new concept of passive swept-wing flow laminarization was suggested. In contrast to experiments [2] performed with hot-wire on 35-degree swept-plate model the present experimental study is performed on 45-degree 3D swept-wing section [RODTRAC]. The distributed leading edge roughness of the swept-wing model provides excitation of unstable cross-flow vortexes the amplification of those leads to "saw-tooth-like" laminar-turbulent transition of 3D boundary layer flow. The surface of wing model was modified by applying of surface relief comprising strips oriented oblique to flow direction. Integral influence of surface relief on laminar-turbulent transition position was visualized with help of infrared thermography. It is proved that application of surface relief with proper orientation (according to findings in [1, 2]) is able to delay cross-flow dominated transition to turbulence on a swept-wing.

A new method of excitation of fully controlled unsteady (including quasi-steady) perturbations in boundary layer is developed in the present study. The method is based on significant improvement of an experimental technique, the key element of which is the universal disturbance source VS-II. Due to implementation of modern technologies a new mechanical part of the source was created. The design of the new mechanical part provides significant improvement of the source's characteristics and expands its application area. Special tests carried out in a low-turbulence wind tunnel have shown, in particular, that the new source is able to excite fully reproducible three-dimensional Tollmien-Schlichting waves with very large initial amplitudes. In the present study the new source was used for the first time for investigation of a new (unstudied previously) mechanism of control of evolution of unsteady Gortler modes by steady Gortler vortices. Preliminary calculations (carried out by means of a computer codes of A.V. Boiko), as well as experiments in a low-turbularce wind tunnel, have shown that this mechanism is able to reduce significantly growth rates of unsteady Gortler modes.

Efficiency of a new concept of passive laminar-turbulent transition control on a swept-wing with help of oblique non-uniformities on the wing surface has been verified and proved in wind-tunnel experiments performed on a model of 35-degree swept-wing. The performed measurements have revealed that oblique 2D non-uniformities on a swept-wing surface are able to influence (suppress or intensify) cross-flow vortex amplification depending on the angle between non-uniformities fronts and local flow velocity vector. In particular, it is found that surface non-uniformities with their fronts parallel to swept-wing leading edge provide stabilizing effect to cross-flow vortices. The performed experiments have shown that stabilizing/destabilizing effect of controlling surface non-uniformities may be increased with growth of non-uniformity number applied, while impact of single oblique surface non-uniformity is weak. The performed direct measurements of laminar-turbulent transition position on the swept-wing model also proved elongation of laminar flow region when stabilizing pattern of surface non-uniformities is applied. In general, during the performed experiments the proposed concept of passive flow control has shown promising effectiveness, rather high technological tolerance and robustness.

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 degrees). The model was mounted in a test section of the low-turbulence wind-tunnel MTL (KTH, Stockholm) at an angle of attack of -5 degrees 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 f(s) and f(v), 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 f(s). However, the evolution of the CF-modes excited at difference combination frequency f(sv-) =f(s) -f(v) 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 beta'. This phenomenon was observed in the flow, which is stable with respect to both CF- and TS-waves having beta' = 0 for all frequencies. There is no explanation of this finding at present.

Results of the EU project RECEPT are presented. The experiments in a three-dimensional swept-wing boundary layer were carried out in the low-turbulence wind tunnel MTL at KTH Stockholm. 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. Comparisons with calculations carried out by means of linear (locally-parallel) stability theory (LST) were performed. A thorough analysis of the disturbance development showed that the features of the downstream evolution of the CF-modes observed at the combinational frequency can be explained by the action of a distributed receptivity mechanism.

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 freestream vortices and surface vibrations. The experimental approach developed here will be used for investigation of the corresponding receptivity problem.

An experimental study of two efficient receptivity mechanisms of excitation of cross-flow (CF) instability modes is carried out in a boundary layer of a real airfoil section of a swept wing due to: (i) action of localized surface vibrations, and (ii) scattering of 2D freestream vortices on them. It is found that the two mechanisms lead to rather efficient excitation of CF-modes both at surface vibration frequency and at combination 'vortexvibration' frequencies. First estimations of the corresponding localized receptivity coefficients are obtained. Direct comparison of the experimental amplification curves of the excited CF-modes with those calculated based on the linear stability theory (LST) has shown that the experimental data obtained at vibration frequency are in excellent agreement with the LST. At the same time, growth rates of the CF-modes excited at combination frequencies are found to be completely inconsistent with the LST. A possible explanation of this phenomenon via action of a new efficient distributed receptivity mechanism is suggested. This mechanism is associated with scattering of freestream vortices on rather high-amplitude CF-modes excited by surface vibrations.

The paper is devoted to results of quantitative experimental study of a mechanism of excitation of three-dimensional Tollmien-Schlichting (TS) waves in a boundary layer of a 35-degree swept wing due to scattering of unsteady free-stream vortices on streamwise localized surface vibrations. The experiments are performed in the Royal Institute of Technology in Stockholm (Sweden) in the framework of studies ‘B’ of Working Package 1 of project ‘RECEPT’ of the Seventh European Framework Program. The measurements are carried out on a model of a real swept wing of an infinite span at conditions of controlled surface perturbations. Calculations and measurements have shown that at the selected angle of attack of +1.5°, the TS instability dominates in the boundary layer, while the cross-flow instability is absent due to very low cross-flow intensity at conditions of very weak streamwise pressure gradient. The base-flow parameters, as well as the parameters of the free-stream and surface perturbations (such as frequencies and range of spanwise wavenumbers) were selected based on special calculations, performed for the particular experimental conditions, as well as on preliminary experimental studies of the TS instability. The calculations have shown a very good agreement of the TS-stability characteristics with the measurements obtained in a broad range of parameters. Deep experimental data processing enables obtaining estimates of values of the coefficients of linear flow receptivity to freestream vortices (with vorticity vectors parallel to the wing leading edge) at their scattering on streamwise localized surface vibrations. This mechanism responsible for excitation of 3D TS waves has been investigated for various relations of the vortex and surface frequencies in a great range of spanwise wavenumbers and propagation angles of the excited TS instability modes