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Suction through porous aircraft wings is a promising concept to reduce its drag, but the modified boundary layer can be more susceptible to external forcing, like surface roughness and inhomogenities in the suction distribution. The disturbance amplitudes triggered by this forcing are quantified by the receptivity coefficients. This paper analyzes the effects of suction on receptivity in the boundary layer and shows its detrimental effects on steady crossflow instabilities, which can be the dominant instabilities in swept-wing boundary layers. Employing a compressible adjoint approach, the receptivity coefficients for a Falkner-Skan-Cooke similarity solution and an A320 vertical tail plane with and without suction are given.

Laminar-turbulent transition caused by modal disturbance growth in the wake flow of isolated roughness elements on blunt re-entry capsules is studied numerically at typical cold hypersonic wind-tunnel conditions. Two fundamentally different heat shield shapes are considered. On the sphere-cone forebody the wake flow of the roughness is exposed to an adverse pressure gradient, whereas the spherical heat shield exhibits a strongly favorable pressure gradient. The pressure gradient effects on the development of the stationary wake flow and its modal instability characteristics are discussed for various heights and diameters of the cylindrical roughness element. Regions of increased shear develop in its wake, which persist longer in the adverse pressure gradient case. Consequently, the results of spatial two-dimensional eigenvalue analyses reveal that the unstable wake-flow region extends much further downstream and the wake-mode instabilities are considerably more amplified. The disturbance kinetic energy production terms are used to assess the contributions of the different shear-layer regions to the mode growth and its dependence on the pressure gradient.

In the presence of surface imperfections, the extent of the laminar flow regions on natural laminar flow (NLF) aircraft components can be significantly reduced, as compared to design. The present work focuses on surface bumps, which can occur, e.g., at or between structural joints, ribs and stringers. Bumps can induce a marked amplification of boundary-layer instabilities and thus lead to premature transition to turbulence. The influence of bumps on boundary-layer transition was systematically studied in this work in combination with the effect of variations in the following parameters: streamwise pressure gradient, freestream Mach number and chord Reynolds number. The experiments were conducted in a quasi-two-dimensional flow in the Cryogenic Ludwieg-Tube Göttingen, where chord Reynolds numbers up to 10 million, freestream Mach numbers up to 0.77 and various streamwise pressure gradients were examined. Essentially two-dimensional bumps, with a sinusoidal shape in the streamwise direction (positive half of a sine), fixed length and three different heights, were mounted on a two-dimensional flat-plate configuration. The wind-tunnel model was instrumented with a temperature-sensitive paint for non-intrusive, global transition detection, and also with pressure taps for the measurement of the surface pressure distributions. Boundary-layer transition was found to occur at a more upstream location with increasing bump height-to-length ratio. Transition induced by the small bump was shown to be dependent on streamwise pressure gradient, freestream Mach number and chord Reynolds number. In the case of bumps with larger height-to-length ratios, however, the influence of the aforementioned parameters on bump-induced transition was significantly reduced. The measured surface pressure distributions served also as an input for boundary-layer computations, and the calculated boundary-layer velocity profiles were analyzed according to linear, local stability theory. The results of the stability analysis were then correlated with the transition locations measured in the experiments. Detailed results will be presented in the final version of this contribution, where the importance of considering the effect on transition of pressure gradient, Mach number and Reynolds number in criteria for allowable size of bumps on NLF surfaces will be also discussed.

A realistic wing is not expected to be an ideal smooth surface: the influence of junctions or wing-panels must be taken into account in the laminar wing design. The presence of these spanwise invariant two-dimensional surface perturbations amplify the streamwise instabilities existing on a smooth surface, potentially causing an early transition to turbulence. In the present work, the effect of surface irregularities such as steps, gaps and humps on the development of Tollmien-Schlichting (TS) waves in an incompressible boundary layer is investigated using direct numerical simulations (DNS). The calculations are performed with the open source spectral element code NEK5000 and consist of a DNS of the steady laminar two-dimensional base flows and succeeding DNS for the disturbance propagation. A Blasius profile is specified at the inflow boundary and when the base flow reaches its steady state the TS waves are artificially excited. This is done either by superimposing the eigensolutions of the Orr-Sommerfeld equations onto the Blasius profiles, or by periodic blowing and suction through a narrow strip at the wall. Arbitrary two- or three-dimensional (oblique) disturbance waves can be generated. For the oblique waves, the two-dimensional base flow is extruded in the spanwise direction over a spanwise width that is fixed to the spanwise wavelength of the introduced disturbance. After a transient phase where the perturbations propagate through the entire domain, a time-periodic flow field in the unsteady DNS is reached. This enables a Fourier analysis in time of the streamwise component of the disturbance velocity to obtain the perturbation amplitudes at different streamwise locations. The N-factor is then used to assess the growth of the disturbances. The amplitudes of the introduced disturbances are chosen to be small enough to grant a purely linear development throughout the domain. The geometric irregularities are directly meshed or defined by analytical mapping of the Gauss-Lobatto-Legendre (GLL) points, used to build the velocity Lagrange polynomial interpolants in each spectral element. The latter procedure requires the irregularities to be localised and smooth. Depending on their size, the surface imperfections give rise to a local separation bubble which interacts with the oncoming TS waves. Results show, as expected, that these surface perturbations have a destabilizing effect when compared to the case of the smooth flat plate. Detailed results on the amplification of two-dimensional and oblique TS waves encountering steps, gaps, humps and indentations of different heights and widths will be presented.

Laminar-turbulent transition prediction is of practical interest in aircraft design since transition affects important aerodynamic quantities such as drag and heat transfer. Extended laminar flow on aerodynamic surfaces is an effective way of reducing aircraft drag. One of the major challenges for the implementation of laminar-flow surfaces is the potential for any irregularity to move transition upstream. Under low-disturbance environment, boundary-layer transition results from the growth and breakdown of different flow instabilities. In 2-D flows the scenario is dominated by Tollmien-Schlichting (TS) instabilities. Common wing-surface irregularities, such as two-dimensional steps, gaps or waviness can alter the growth characteristics of TS waves and therefore must be taken into account at the design stage.

The interest in Hybrid Laminar Flow Control (HLFC) wings lies in the potential of suction for reducing the viscous drag on commercial aircraft wings by delaying laminar-turbulent transition. However, the joint between the leading edge suction panel and the wing box may introduce surface irregularities (e.g. backward/forward-facing steps (BFS/FFS), gaps, etc.). Such surface irregularities can promote transition by enhancing the spatial development of flow instabilities (e.g. Tollmien-Schlichting (TS) waves or crossflow (CF) vortices) and, therefore, may reduce the potential benefit of the suction system. The presence of surface irregularities can cause regions of localized strong streamwise gradients in the base flow quantities. Standard methodologies for transition prediction like Parabolized Stability Equations (PSE) can still be applied in regions far from the surface irregularities, where the streamwise variations are small. However, the PSE formulation is not suited for handling the presence of such large streamwise gradients. The Adaptive Harmonic Linearized Navier-Stokes (AHLNS) equations on the other hand can handle these large streamwise gradients properly. Because AHLNS still takes advantage of the wave-like character of the boundary-layer instabilities by using the same wave approach as in PSE, it is able to reproduce the results from linear Direct Numerical Simulations (DNS) at orders of magnitude lower computational costs. The AHLNS approach has already been applied to study the effect of surface irregularities on laminar nacelles at flight Reynolds numbers relevant for transonic transport aircraft. Within the European project HLFC-WIN, part of the Clean Sky 2 Joint Undertaken, AHLNS is used to assess the detrimental effects of different surface irregularities immediately downstream of the suction panel of the transonic HLFC-WIN swept-wing configuration, thus contributing to the selection of a suitable joint design and the specification of the corresponding manufacturing tolerances. As an initial result, gap geometries considered do not significantly affect the expected laminar-turbulent transition location on the HLFC system. An overview of these on-going numerical studies will be given.

The Business Unit GUK of DNW proposed a research and development support in cooperation with the Institute of Aerodynamics and Flow Technology (DLR--AS) entitled "3D-Adaptation Procedure for the 2D-Adaptive Walls in the DNW-KRG Test Section''. The research objective was to verify an existing 3D-adaptation procedure at the Cryogenic Ludwieg Tube Göttingen (DNW-KRG) which allows an optimal adjustment of the test sections 2D-adaptive walls to achieve a minimum of wall interference during measurements with a built-in 3D-object.

Abstract Shock-wave/boundary-layer interactions (SW–BLIs) play an important role in a wide range of transonic, supersonic and hypersonic applications. Fundamental studies on stationary interactions have been conducted extensively during the last 60 years. However, unsteady SWBLIs with traveling shock fronts have been little studied on canonical geometries. In the present experimental investigation, the influence of a uniformly moving impinging shock on the separated SWBLI flow is analyzed, with a freestream Mach number of 3 and a traveling Mach number in upstream direction of 0.5. To evaluate this effect, stationary reference SWBLIs have been investigated in a wide ranging study. A scaling method from the literature has been enhanced to drastically reduce the data scattering using a new approach accounting for the Reynolds number influence. The results gathered from the traveling interactions were within the spread of reference data, considering the true shock-wave Mach number of 3.5. The validity of the modified scaling approach to describe the interaction length in cases with steady and traveling shock-wave/turbulent-boundary-layer interactions is discussed. Graphic abstract

Different concepts for controlling a non-slender delta wing are considered and compared with each other. The concepts are based on geometrical modifications of the leading edges. The strategy aims at producing an asymmetric distribution of lift due to vortices on the left and right wing half and thus, for example, rolling moments to be used in controlling the model. The effectiveness of different approaches is determined in lowspeed wind tunnel tests both by measuring forces and moments acting on the model and by investigating the flow field around the model by particle image velocimetry and oilflow visualization of skin friction lines. Two approaches are pursued. One is based on static and dynamic actuators acting at the left leading edge. The other uses different combinations of leading edge shapes on the two halves of the model. Finally, the effect of asymmetric boundary layer tripping is briefly considered.

Abstract Wall shear stress and wall streamline direction measurements were conducted on a generic delta wing model to demonstrate the potential of the white-light oil-film interferometry in three-dimensional flows. With the proposed algorithm, an automatic oil-film thickness calculation is possible and yields reproducible and plausible wall shear stress distributions on the leeward side of the delta wing for various angles of attack. Graphic abstract

The unsteady boundary layer behavior of a supercritical laminar airfoil model, which undergoes limit cycle oscillations in pitch, is investigated by the application of hot-film anemometry. The data basis is a 2D flutter experiment under transonic flow conditions. The laminar airfoil model was elastically mounted with a single degree of freedom in pitch and performed self-excited limit cycle oscillations at a Mach number of$${\text {Ma}}=0.73$$Ma=0.73and a Reynolds number of$${\text {Re}}\approx 2\times 10^6$$Re≈2×106. An analysis of the hot-film signals on the basis of the quasi-wall shear stress is carried out, with which the boundary layer state for steady and unsteady flow is resolved. An algorithm is presented which allows an automated detection of the transition position, so that a correlation between airfoil motion and transition location movement can be quantified. A sudden movement of the boundary layer transition is observed at the upper and lower reversal points of the limit cycle oscillation, while in parts of the up- and downstroke of the laminar airfoil a shock-induced transition occurs. During the limit cycle oscillation, a delayed response of the boundary layer occurs, resulting in a significant phase lag between the movement of the boundary layer transition and the motion of the laminar airfoil.

The integration of advanced propulsion systems into the aircraft, and the assessment of their benefits related to e.g. fuel burn, noise emission and potential further measures, is one of the major activities of the Large Passenger Aircraft (LPA) Platform 1 of the Clean Sky 2 initiative. In order to optimize and assess the potential of advanced propulsion integration, it is necessary to establish suitable design, evaluation, and measurement tools. The so-called Cross-Demonstrator-Capabilities (XDC-) activities have been set up to develop and demonstrate powerful numerical and experimental methods for aerodynamic, aeroacoustics and aeroelastic simulation and measurement tasks, supporting the dedicated demonstrators of LPA. The paper will give an overview of the XDC activities and some examples of the developments related to this demonstrator.

In the present thesis a Machine Learning approach in Fluid Mechanics field was investigated. In particular Artificial Neural Networks were used to predict lift and drag coefficient of NACA 4 digit airfoils. In the last years the application of Artificial Intelligence and in particular of Machine Learning to scientific disciplines increased substantially. Machine Learning offers techniques to extract information and knowledge from data and it provides the possibility to handle with massive quantitative of data. The ojective of the related work was to investigate how Machine Learning is working, in particular Neural Networks, and how it has to be applied in order to make a prediction. The preliminary phase of the work was to create the data-set necessary for the secondary phase, the Neural Network analysis. The generation of the dataset involved CFD simulations. Those were performed with DLR-TAU code, a finite volume method for RANS equations. A tool as a code for RANS equations were used because of its ability to capture the aerodynamic coefficients of interest in the related work. The preliminary phase includes also all the steps that a CFD simulation concerns: CAD generation performed with Geocreate, mesh generation with Centaur and numerical simulation with DLR-TAU code. The related theoretical background is given in chapter 1, instead in chapter 3 numerical simulations are presented. In the context of neural network approach the software package Google Tensorflow 2 via Python3 interface was used. Therefore, in the context of this thesis, artificial neural networks were used to manage lift and drag coefficient generated from CFD simulations. This work presents in chapter 2 an overview of what is Machine Learning and a detailed introduction to artificial neural networks. Final results and considerations are shown in chapter 4.

The presence of large surface irregularities such as humps, where the height is similar to the local boundary-layer (BL) displacement thickness, introduces regions of localized strong streamwise gradients in the base flow quantities. These large gradients can significantly modify the spatial development of incoming disturbances that lead to laminar–turbulent transition in wall-bounded flows [e.g., Tollmien–Schlichting (TS) waves]. Techniques such as Parabolized Stability Equations (PSE) are not suited for BL instability analysis in such regions: their formulation assumes that streamwise variations of base flow and disturbance quantities are small, allowing a marching procedure for their resolution. On the other hand, the Adaptive Harmonic Linearized Navier–Stokes (AHLNS) equations can handle these large streamwise gradients by using a fully elliptic system of equations, similar to Linearized Navier–Stokes (LNS), Harmonic LNS (HLNS), or Direct Numerical Simulation (DNS). Moreover, in AHLNS (as in PSE), a wave-like character of the instabilities is assumed, leading to a significant reduction in the number of streamwise grid points required compared with LNS, HLNS, or DNS computations. In the present study, an efficient combination of PSE and AHLNS is used to investigate the effect of height, length, and shape of a single hump placed on a flat plate in a two-dimensional flow field at Ma=0.5 without pressure gradient. The effect of this hump on the spatial evolution of TS waves, in terms of N-factors, is presented. An expected laminar–turbulent transition onset, via the eN methodology, is also described. It is shown that the shape of the surface irregularity, together with the height and length, plays an important role for the location of laminar–turbulent transition onset in convectively unstable flows.

Strömungsvorgänge in Fluiden spielen in Naturwissenschaft und Technik eine bedeutende Rolle. Um sie besser verstehen zu können, werden verschiedene Messverfahren eingesetzt und numerische Simulationen durchgeführt. Mit Hilfe der gewonnenen Daten können eine Vielzahl von Analysen betrieben werden. In der vorliegenden Arbeit geht es um die Implementierung zwei dieser Auswertemethoden, der Empirical Mode Decomposition (EMD) und der Clauser Chart Methode. Insbesondere die EMD stellt dabei in der Strömungsanalyse ein relativ neues Verfahren dar. Mit der Multidimensional Ensemble EMD (MEEMD) und der Fast and Adaptive Multivariate and Multidimensional EMD (FA-MVEMD) werden zwei Variationen der EMD betrachtet und hinsichtlich ihrer Anwendbarkeit in der Strömungsanalyse untersucht. Das zweite implementierte Verfahren, die Clauser Chart Methode, kann hingegen als Standardverfahren betrachtet werden und dazu genutzt werden, Messergebnisse sowie die Ergebnisse der EMD zu verifizieren. Sowohl EMD, als auch die Clauser Chart Methode werden erfolgreich in eine Toolbox eingebunden und getestet. Im Hinblick auf die EMD zeigen sowohl die MEEMD, als auch die FA-MVEMD keine voll zufriedenstellende Lösung. Für einen der größten Nachteile der MEEMD, die hohe Berechnungszeit, wird daher eine Möglichkeit vorgestellt, den Rechenaufwand zu reduzieren.

A measuring method to detect wall flow topologies by active infrared thermography was developed. Quadcopter rotor blades were studied in typical hover and forward flight conditions, while using a high-power infrared laser to heat the blade surface. By this, the distance between heat source and measuring object could be significantly increased compared to conventional heat sources. The resulting images show little blurring at high signal-to-noise ratios. In steady-state flow conditions, wall flow topologies could be detected reliably and show good agreement with performed oil-film interferometry measurements. Wind tunnel experiments at forward flight conditions were conducted and indicate unsteady shifts of the transition location.

The recent introduction of the Multi-Pulse Shake-The-Box technique (MP-STB [3]) extended the capabilities of the Shake-The-Box (STB [4]) 3D Lagrangian particle tracking (LPT) algorithm in terms of applicable flow velocity. In particular, the MP-STB allows for 3D LPT investigations at velocities relevant for industrial and aerodynamic applications in relatively large volumes where the adoption of a time-resolved recording strategy would result in significant compromises in terms of spatial range or resolution (in order to limit the particle displacement between subsequent recordings). Multi-pulse sequences are obtained by synchronizing multiple illumination systems to generate bursts of laser pulses (typically four) where the time separation can be freely adjusted down to less than a microsecond (e.g. for supersonic applications). A 3D imaging system consisting of dual-frame cameras is adopted, where each frame is double-exposed to record the four pulses [2]; the MP-STB technique makes use of an iterative approach to overcome the limitations posed by the short observation time offered by the multi-pulse recording sequence, and allows for the reconstruction of individual four-pulse particle tracks in 3D [3]. The acquisition of long sequences of four-pulse recordings at low frequency (10-15Hz) enables the evaluation of flow statistics. In the present study, the MP-STB technique is employed for the investigation of turbulent boundary layer (TBL) flows at relatively high Reynolds numbers. A TBL at 15 m/s (Reθ ≈ 10; 000) is investigated in the SWG facility at DLR Göttingen; approximately 95; 000 individual tracks are reconstructed within the 80×120×10 mm^3 domain for each four-pulse recording. Spatial gradients, as well as the instantaneous 3D pressure field, are evaluated by means of the FlowFit data assimilation algorithm [1] (fig.1-left). A TBL with adverse pressure gradient (APG) at 29 m/s (Reθ ≈ 30; 000) is investigated in the atmospheric wind tunnel of the University of Armed Forces in Munich; both planar PIV and MP-STB in a 80 × 90 × 7 mm^3 domain are employed; the direct comparison of the results in terms of turbulent fluctuations (fig.1-right) highlights the higher spatial resolution of the LPT approach which, unlike PIV, does not suffer from the signal modulation caused by the finite size of the cross-correlation window. In the final presentation the MP-STB processing technique will be introduced together with the main parameters affecting its performances. Moreover, results regarding the TBL structure and flow statistics (e.g. TBL profiles, two-point correlations) will be presented.

Heat flux measurements of transitional shock-wave/boundary-layer interactions (SWBLI) were conducted by quantitative infrared thermography. The shock intensity and mutual position of the impingement and transition locations were varied independently. The shock induced Stanton number peaks are maximal for transitional SWBLI and stronger shocks cause higher heat transfer amplification. The highest amplification rates for each shock intensity depend on the transitional state at the impingement location. Weaker shocks require a further developed transitional state at the impingement location for the highest amplification due to less shock induced turbulence. Numerical simulations with the DLR TAU-Code were conducted for the limiting boundary layer cases (laminar & turbulent) and cover the entire experimental parameter study.

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 recent introduction of the Multi-Pulse Shake-The-Box (MP-STB) method opened the possibility of extending 3D Lagrangian particle tracking (LPT) to the investigation of high-speed flows, where long time-resolved sequences of recordings are currently not available due to the limited acquisition frequency of high-speed systems. The MP-STB technique makes use of an iterative approach to overcome the limitations posed by the short observation time offered by a multi-pulse recording sequence. Multi-pulse sequences are typically obtained by synchronizing multiple illumination systems to generate bursts of laser pulses where the time separation can be freely adjusted down to less than a microsecond. Several strategies can be adopted for the recording of multi-pulse sequences; a dual camera system can be adopted to separate the single pulses onto the camera frames (either by means of polarization or timing), while the use of multi-exposed frames allows for the employment of a single imaging system, largely reducing the complexity and cost of the experimental setup. The main strategies to generate multi-pulse recording sequences are presented here; the application and performances of the MP-STB method are discussed based on the analysis of experimental data from the investigation of three turbulent boundary layer flows at velocities ranging from 10 to approximately 30 m/s. Results show the capability of the MP-STB technique in reconstructing accurate track fields which can be exploited both to describe instantaneous flow structures and to produce highly spatially resolved statistics by means of ensemble average in small bins. The iterative reconstruction and tracking strategy for MP-STB can be successfully adapted to the case of multi-exposed frames. Results suggest that, despite the increase in particle image density resulting from the double-exposed particle images, the adoption of multi-exposed recordings has the potential to become the technique of choice for the recording of multi-pulse sequences suitable for Lagrangian particle tracking in high-speed flows.

Masterarbeit Entwicklung und Anwendung eines Computerprogramms zur automatisierten Auswertung der Ölfilminterferometrie

Nichtnewtonsche Fluide, zu denen die verallgemeinerten newtonschen Fluide zählen, zeichnen sich dadurch aus, dass ihre Viskosität nicht nur von Druck und Temperatur abhängt. Vielmehr wirkt sich die Strömung selbst auf die Viskosität der Fluide aus. Das macht die Physik und Strömungsmechanik dieser Fluide außerordentlich komplex. Das Buch führt daher umfassend in die Theorie und Anwendung der verallgemeinerten newtonschen Fluide ein. Neben ausführlichen Herleitungen der Gesetzmäßigkeiten werden Lesern die strömungsphysikalischen Phänomene, die bei diesen Fluiden auftreten, anhand analytischer Beispiele und praxisnaher Strömungssimulationen veranschaulicht. Die gängigsten Fluidmodelle werden vorgestellt und ihre jeweiligen Modellierungsgrenzen aufgezeigt. Viskoelastische Fluide werden im Gegensatz zur gängigen Fachliteratur bewusst nur am Rande behandelt, vielmehr wird der Temperatureinfluss auf die rheologischen Fluideigenschaften betrachtet. Für Studierende der Ingenieurwissenschaften sowie für Praktiker sind die strömungsmechanischen Aspekte der Rheologie von besonderer Bedeutung, denn diese Fluide sind für zahlreiche Industriesparten relevant. Ohne Kenntnisse ihrer Eigenheiten können viele Problemstellungen in der Chemie-, Prozess- und Verfahrenstechnik nicht bearbeitet werden. Bei der Produktion von Farben, Lacken, Dispersionen und Emulsionen beispielsweise, aber auch bei Kunststoffschmelzen sind die Effekte strömungsbedingt veränderlicher Viskosität entscheidend für die Qualität des Endprodukts. Ähnliches gilt für die Prozesstechnik, Hydraulik oder die Getriebe- und Motorenentwicklung.

Das Projekt BELARWEA "Blattspitzen für Effiziente und LärmArme Rotoren von WindenergieAnlagen" widmet sich der Weiterentwicklung und Validierung von numerischen Schallvorhersagenmethoden für Windenergierotoren, um zukünftig den rechnergestützten Entwurf von gleichzeitig effizienten und geräuscharmen Rotorblättern zu ermöglichen. Der Projektfokus lag auf der Blattspitzenregion (äußere 20% des Rotorradius), die aufgrund der dort höchsten Relativgeschwindigkeiten am Rotor für die Schallerzeugung entscheidend ist.

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.

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 aim of this work is to capture the spatiotemporal dynamics of large-scale flow structures in a turbulent boundary layer flow in a volumetric and time-resolved way. The applied means are dense Lagrangian Particle Tracking of Helium-Filled Soap Bubbles (HFSBs) using the Shake-The-Box (STB) algorithm and a subsequent interpolation to a regular grid using the physically regularized FlowFit method. The presented experiment was performed as a pre-test to a full campaign to be held in August 2018 at the Atmospheric wind tunnel at the University of the armed forces in Munich, Germany. This test demonstrates the principle applicability of the measurement techniques to a low-speed wind tunnel environment and shows that high spatial accuracy and resolution can be reached despite of a large measurement volume. Problems were encountered achieving a homogeneous bubble distribution and in deploying large amounts of bubbles, while not disturbing the flow.

Non-Newtonian fluid properties can change the flow behaviour significantly, its prediction is more difficult when thermal effects come into play. Hence, the focal point of this work is the wake flow behind a heated circular cylinder in the laminar vortex shedding regime for thermo-viscous shear thinning fluids. In the case of isothermal flows of Newtonian fluids the vortex shedding regime is characterised by a distinct Reynolds number and an associated Strouhal number. In the case of thermo-viscous shear thinning fluids the flow regime can significantly change in dependence of the temperature of the viscous wall of the cylinder. The Reynolds number alters locally and, consequentially, the Strouhal number globally. In the present CFD study the temperature dependence of the Reynolds and Strouhal number is investigated for the flow of a Carreau fluid around a heated cylinder. The temperature dependence of the fluid viscosity has been modelled by applying the standard Williams-Landel-Ferry (WLF) equation. In the present simulation campaign thermal boundary conditions have been varied over a wide range in order to derive a relation between dimensionless heat transfer, Reynolds and Strouhal number. Together with the shear thinning due to the high shear rates close to the cylinder wall this leads to a significant decrease of viscosity of three orders of magnitude in the nearfield of the cylinder and a reduction of two orders of magnitude in the wake field. Yet the shear thinning effect is able to change the flow topology: a complex K´arm´an vortex street occurs, also revealing distinct characteristic frequencies associated with the dominant and sub-dominant vortices. Heating up the cylinder wall leads to a delayed flow separation and narrower wake flow, giving lesser space for the sequence of counter-rotating vortices. This spatial limitation does not only reduce the amplitude of the oscillating wake flow it also shifts the dominant frequency to higher frequencies, furthermore it damps higher harmonics. Eventually the locally heated wake flow smears out. Eventually, the CFD simulation results of the systematically varied thermal flow parameter study have been used to describe a relation for the main characteristic order parameters., {"references":["Andrade, E. N. D.: A theory of the viscosity of liquids. part 1.\nPhilosophical Magazine, Vol. 17, pp. 497-511, 1934.","CentaurSoftware, http://www.centaursoft.com.","Bird, R. B.: Non-Newtonian behavior of polymeric liquids. Physica A:\nStatistical and Theoretical Physics, Vol. 118, No. 1-3, pp. 3-16, 1983.","Bird, R. B., Curtiss, C. F.: Nonisothermal polymeric fluids. Rheol. Acta,\n35:103109, 1983.","Bird, R. B., Hassager, O.: Dynamics of Polymeric Liquids: Vol.1: Fluid\nMechanics. Series: Dynamics of Polymeric Liquids, London, New York,\nJohn Wiley & Sons, 1987.","Bogn´ar, G., Kov´acs, J.: Non-isothermal steady flow of power-law fluids\nbetween parallel plates. International Journal of Mathematical Models and\nMethods in Applied Science, 6(1), 2012.","Carreau, P. J.: Rheological equations from molecular network theories. J.\nRheol., Vol. 16, No. 1, pp. 99-127, 1972.","Chabral, B.; Leedom, L. C.: Imaging Vector Fields Using Line Integral\nConvolution. In: Proceedings of SIGGRAPH 93. pp. 263-270, New York,\n1993.","Coelho, P. M.; Pinho, F. T. Vortex shedding in cylinder flow of\nshear-thinning fluids. II. Flow characteristics. J. Non-Newtonian Fluid\nMech. Vol. 110, pp. 177-193, 2003.\n[10] Eisenlohr, H., Eckelmann, H.: Vortex splitting and its consequences in\nthe vortex street wake of cylinders at low Reynolds number. Phys. Fluids\nA 1 (2), pp. 189192, 1989.\n[11] Ferry, J. D.: Viscoleastic Properties of Polymers. Third edition, New\nYork; John Wiley & Sons, 1980.\n[12] Fey, U., K¨onig, M., Eckelmann, H.: A new Strouhal Reynolds number\nrelationship for the circular cylinder in the range 47 ¡ Re ¡ 2 105. Phys.\nFluids 10, pp. 15471549, 1998.\n[13] von Ka´rma´n, Th.: U¨ ber den Mechanismus des Widerstandes, den\nein bewegter K¨orper in einer Fl¨ussigkeit erf¨ahrt. Nachrichten\nder K. Gesellschaft der Wissenschaften zu G¨ottingen,\nMathematisch-physikalische Klasse, 1911.\n[14] von Ka´rma´n, Th., Rubach. H.: U¨ ber den Mechanismus des Flu¨ssigkeitsund\nLuftwiderstandes. Physikalische Zeitschrift, 13, pp. 4959, 1911.\n[15] Knopp, T., Zhang, X., Kessler, R. and Lube, G.: Enhancement\nof an industrial finite-volume code for large-eddy-type simulation of\nincompressible high Reynolds number flow using near-wall modelling.\nJournal of Computer Methods in Applied Mechanics and Engineering,\nVol. 199, pp. 890-902, 2010.\n[16] Monkewitz P. A., Williamson, C. H. K., Miller, G. D.: Phase dynamics\nof K´arm´an vortices in cylinder wakes, Phys. Fluids 8, pp. 9196, 1996.\n[17] Ostwald, W.: Ueber die rechnerische Darstellung des Strukturgebietes\nder Viskosit¨at, Kolloid Zeitschrift 47 (2), pp. 176-187, 1929.\n[18] Owens, R. G.,Phillips, T. N.: Computational Rheology. Computational\nRheology. Imperial College Press, ISBN 9781860941863, 2002.\n[19] Skelland, A. H. P.: Non-Newtonian flow and heat transfer. John Wiley\n& Sons, New York, 1967. [20] Soares, A. A., Ferreira, J. M., Chhabra, R. P.: Flow and Forced\nConvection Heat Transfer in Crossflow of Non-Newtonian Fluids over\na Circular Cylinder. Ind. Eng. Chem. Res. Vol. 44, pp. 5815-5827, 2003.\n[21] V´ıt, T., Ren, M., Tr´avn´ıˇcek, Z.,: Marˇs´ık, F.: The influence of temperature\ngradient on the StrouhalReynolds number relationship for water and air.\nExperimental Thermal and Fluid Science, Vol. 31, pp. 751-760, 2007.\n[22] Wang, A.-B. , Tr´avn´ıˇcek, Z., Chia, K.-C.: On the relationship of effective\nReynolds number and Strouhal number for the laminar vortex shedding\nof a heated circular cylinder. Phys. Fluids 12 (6), pp. 1401-1410, 2000.\n[23] Wendt, J. F. (Ed.): Computational Fluid Dynamics - An Introduction.\nThird edition, Berlin, Heidelberg; Springer, 2009.\n[24] Williams, M. L., Landel, R. F., Ferry, J. D: The Temperature\nDependence of Relaxation Mechanisms in Amorphous Polymers and\nOther Glass-forming Liquids. Journal of the American Chemical Society,\nVol. 77, pp. 3701-3707, 1955.\n[25] Williamson, C. H. K.: Vortex dynamics in the cylinder wake, Ann. Rev.\nFluid. Mech. 28, pp. 477539, 1996."]}

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.

Knowledge about laminar–turbulent transition on operating multi megawatt wind turbine (WT) blades needs sophisticated equipment like hot films or microphone arrays. Contrarily, thermographic pictures can easily be taken from the ground, and temperature differences indicate different states of the boundary layer. Accuracy, however, is still an open question, so that an aerodynamic glove, known from experimental research on airplanes, was used to classify the boundary-layer state of a 2 megawatt WT blade operating in the northern part of Schleswig-Holstein, Germany. State-of-the-art equipment for measuring static surface pressure was used for monitoring lift distribution. To distinguish the laminar and turbulent parts of the boundary layer (suction side only), 48 microphones were applied together with ground-based thermographic cameras from two teams. Additionally, an optical camera mounted on the hub was used to survey vibrations. During start-up (SU) (from 0 to 9 rpm), extended but irregularly shaped regions of a laminar-boundary layer were observed that had the same extension measured both with microphones and thermography. When an approximately constant rotor rotation (9 rpm corresponding to approximately 6 m/s wind speed) was achieved, flow transition was visible at the expected position of 40% chord length on the rotor blade, which was fouled with dense turbulent wedges, and an almost complete turbulent state on the glove was detected. In all observations, quantitative determination of flow-transition positions from thermography and microphones agreed well within their accuracy of less than 1%.

Laminar-turbulent transition caused by modal disturbance growth in the wake flow of isolated roughness elements on blunt re-entry capsules is studied numerically at typical cold hypersonic wind tunnel conditions. Two fundamentally different heat shield shapes are considered. On the sphere-cone forebody the wake flow of the roughness is exposed to an adverse pressure gradient, whereas the spherical heat shield exhibits a strongly favourable pressure gradient. The pressure gradient effects on the development of the stationary wake flow and its modal instability characteristics are discussed for various heights and diameters of the cylindrical roughness element. Regions of increased shear develop in its wake which persist longer in the adverse pressure gradient case. Consequently, the results of spatial two-dimensional eigenvalue analyses reveal that the unstable wake flow region extends much further downstream and the wake mode instabilities are considerably more amplified. The disturbance kinetic energy production terms are used to assess the contributions of the different shear-layer regions to the mode growth and its dependence on the pressure gradient.

Numerical results on modal disturbance growth in the wake flow downstream of a roughness element submerged in the boundary layer of a typical reentry capsule at an angle of attack are presented. Laminar base flow computations were conducted for different roughness heights and planform shapes. The modal instability characteristics of the wake flow were studied by spatial two-dimensional (2D) eigenvalue analysis. For all cases considered, the varicose wake modes are most amplified in terms of maximum N-factors, with the cylindrical roughness element being the most effective shape.

The present paper describes an experiment performed in a transonic wind tunnel facility where a new test section has been developed especially for the investigation of the unsteady flow above oscillating airfoils under dynamic stall conditions. Dynamic stall is characterized by the development, movement and shedding of one or more concentrated vortices on the airfoils upper surface. The hysteresis loops of lift-, drag- and pitching moment are highly influenced by these vortices. To understand the very complicated unsteady flow involved, a detailed knowledge of the instantaneous flow fields is of crucial importance. With the application of the described measuring techniques it is expected to gain more insight into the problem. Results from these tests are ready for comparison with numerical data.

Hybrid laminar flow control (HLFC) wings have captured the interest of several research projects, in particular the HLFC-WIN project [1], because of the potential of suction for reducing drag on commercial aircraft [2]. This reduction in drag is mainly due to the impact on laminarturbulent transition, extending the region along the wings where the flow remains laminar. Typically, on HLFC wings, suction is applied only in the leading edge region (before the front spar). The flow suction modifies the boundary-layer (BL) profiles in such a way that the growth of flow instabilities that might trigger the above mentioned laminar-turbulent transition location, e.g. Tollmien-Schlichting (TS) waves or crossflow (CF) vortices, is delayed. However, the joint between the leading edge suction panel and the wing box may introduce surface irregularities (e.g. backward/forward-facing steps (BFS/FFS), gaps, etc.) that might enhance the spatial development of those flow instabilities and, therefore, reduce the potential benefit of the suction system [3]. For this reason, it is crucial during the design process to quantify the influence of such surface irregularities in terms of BL instability.