The present work experimentally investigates two forcing strategies toward controlling stationary crossflow instability (CFI) induced transition manifesting on a swept wing at subsonic conditions. The effectiveness of upstream flow deformation (UFD) and the base-flow modification strategies, realized through the application of spanwise-modulated and spanwise-uniform dielectric barrier discharge plasma actuation, respectively, is compared experimentally. Specialized, patterned actuators that generate spanwise-modulated plasma jets have been fabricated using a spray-on technique and positioned near the leading edge. An array of discrete roughness elements (DREs) is installed upstream of the plasma forcing to lock the origin and evolution of the critical stationary CFI vortices in the boundary layer. The impact of the phase relation between the spanwise-modulated plasma jets and the incoming CFI vortices is inspected. Infrared thermography is employed to detect and quantify the transition location. A delay in transition is observed with all tested forcing configurations. However, as the incoming CFI vortices are highly amplified due to the application of DREs, the acquired results suggest that with spanwise-modulated forcing the control mechanism responsible for the observed transition delay is not purely UFD; rather the beneficial effects observed leverage on a combination of direct attenuation of the CFI vortices and localized base-flow modification, depending on the aforementioned phase relation. For all forcing strategies and configurations, a simplified drag reduction efficiency estimation is performed using the experimentally measured transition location and the electrical power use of the actuators. A net gain is found for selected configurations.@en

Gas-turbine aircraft engines use a gas turbine to produce high-pressure hot gasses. The available internal energy of the hot gasses is then converted into jet kinetic energy through nozzles or extracted from a turbine to produce thrust by means of fans and/or propellers. Propulsive-efficiency requirements have resulted in the continuous increase of engine bypass/core ratios experienced by modern high-bypass-ratio turbofans, turboprops, and unducted fans or propfans [1]; whereas the improvement of the core-compression ratios allows to obtain thermodynamic efficiencies higher than 0.6.@en

Very-large-scale structures in pipe flows are characterized using an extended Proper Orthogonal Decomposition (POD)-based estimation. Synchronized non-time-resolved Particle Image Velocimetry (PIV) and time-resolved, multi-point hot-wire measurements are integrated for the estimation of turbulent structures in a pipe flow at friction Reynolds numbers of 9500 and 20000. This technique enhances the temporal resolution of PIV, thus providing a time-resolved description of the dynamics of the large-scale motions. The experiments are carried out in the CICLoPE facility. A novel criterion for the statistical characterization of the large-scale motions is introduced, based on the time-resolved dynamically-estimated POD time coefficients. It is shown that high-momentum events are less persistent than low-momentum events, and tend to occur closer to the wall. These differences are further enhanced with increasing Reynolds number.@en

The present work is an experimental investigation of stationary crossflow (CF) instability-induced transition of the boundary layer over a 45°swept wing, under varying free-stream turbulence, surface roughness, angle of attack and Reynolds number. Key topological features of the transition front, such as the mean transition location and the jaggedness of the front, are retrieved via IR thermography. Linear Stability Theory (LST) is used to extract the N-factor of the most amplified stationary crossflow mode at the transition location, identified experimentally. Results show clear causality between free-stream turbulence, surface roughness, Reynolds number, angle of attack and transition. Large losses of laminarity and a consistent decrease in the transition N-factor are observed with rising turbulence and roughness. Remarkably, N-factor sensitivity to free-stream turbulence is found to vary significantly and non-linearly with angle of attack for the modest levels of turbulence explored in this campaign, whereas the N-factors scale linearly with the log of the surface roughness level, which is consistent with a receptivity mechanism, which is independent of the angle of attack. @en

A combined experimental and numerical approach to the analysis of the secondary stability of realistic swept-wing boundary layers is presented. Global linear stability theory is applied to experimentally measured base flows. These base flows are three-dimensional laminar boundary layers subject to spanwise distortion due to the presence of primary stationary crossflow vortices. A full three-dimensional description of these flows is accessed through the use of tomographic particle image velocimetry (PIV). The stability analysis solves for the secondary high-frequency modes of type I and type II, ultimately responsible for turbulent breakdown. Several pertinent parameters arising from the application of the proposed methodology are investigated, including the mean flow ensemble size and the measurement domain extent. Extensive use is made of the decomposition of the eigensolutions into the terms of the Reynolds-Orr equation, allowing insight into the production and/or destruction of perturbations from various base flow features. Stability results demonstrate satisfactory convergence with respect to the mean flow ensemble size and are independent of the handling of the exterior of the measurement domain. The Reynolds-Orr analysis reveals a close relationship between the type I and type II instability modes with spanwise and wall-normal gradients of the base flow, respectively. The structural role of the in-plane velocity components in the perturbation growth, topology and sensitivity is identified. Using the developed framework, further insight is gained into the linear growth mechanisms and later stages of transition via the primary and secondary crossflow instabilities. Furthermore, the proposed methodology enables the extension and enhancement of the experimental measurement data to the pertinent instability eigenmodes. The present work is the first demonstration of the use of a measured base flow for stability analysis applied to the swept-wing boundary layer, directly avoiding the modelling of the primary vortices receptivity processes.

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In this study, experiments are performed towards the identification and measurement of unsteady modes occurring in a transitional swept wing boundary layer. These modes are generated by the interaction between the primary stationary and travelling cross-flow instabilities or by secondary instability mechanisms of the stationary cross-flow vortices and have a crucial role in the laminar-to-turbulent breakdown process. Detailed hot-wire measurements were performed at the location of stationary instability amplitude-saturation. In order to deterministically capture the spatio-temporal evolution of the unsteady modes, measurements were phase- and frequency-conditioned using concurrent forcing by means of a dielectric barrier discharge plasma actuator mounted upstream of the measurement domain. The actuator effect, when positioned sufficiently upstream the secondary modes onset, was tuned such to successfully condition the high-frequency type-I and the low-frequency type-III modes without modifying the transition evolution. Two primary stationary cross-flow vortices of different amplitude were measured, revealing the effect of base-flow variations on the growth of travelling instabilities. The response of these two stationary waves to the naturally occurring and forced fluctuations was captured at different chordwise positions. Additionally, the deterministic conditioning of the instability phase to the phase of the actuation allowed phase-averaged reconstruction of the spatio-temporal evolution of the unsteady structures providing valuable insight on their topology. Finally, the effect of locating the actuator at a more downstream position, closer to the type-I mode branch-I, resulted in laminar-to turbulent breakdown for the high-frequency actuation while the low-frequency forcing showed milder effects on the transition evolution.

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Control of laminar-to-turbulent transition on a swept-wing is achieved by base-flow modification in an experimental framework, up to a chord Reynolds number of 2.5 million. This technique is based on the control strategy used in the numerical simulation by Dörr & Kloker (J. Phys. D: Appl. Phys., vol. 48, 2015b, 285205). A spanwise uniform body force is introduced using dielectric barrier discharge plasma actuators, to either force against or along the local cross-flow component of the boundary layer. The effect of forcing on the stability of the boundary layer is analysed using a simplified model proposed by Serpieri et al. (J. Fluid Mech., vol. 833, 2017, pp. 164–205). A minimal thickness plasma actuator is fabricated using spray-on techniques and positioned near the leading edge of the swept-wing, while infrared thermography is used to detect and quantify transition location. Results from both the simplified model and experiment indicate that forcing along the local cross-flow component promotes transition while forcing against successfully delays transition. This is the first experimental demonstration of swept-wing transition delay via base-flow modification using plasma actuators.

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In the present work, laminar flow control, following the discrete roughness elements (DRE) strategy, also called upstream flow deformation (UFD) was applied on a 45^◦ swept-wing at a chord Reynold’s number of Re_c = 2.1 · 10⁶ undergoing cross-flow instability (CFI) induced transition. Dielectric barrier discharge (DBD) plasma actuation was employed at a high frequency (f_ac = 10kHz) for this purpose. Specialized, patterned actuators that generate spanwinse-modulated plasma jets were fabricated using spray-on techniques and positioned near the leading edge. An array of DREs was installed upstream of the plasma forcing to lock the origin and evolution of critical stationary CFI vortices in the boundary layer. Two forcing configurations were investigated-in the first configuration the plasma jets were directly aligned against the incoming CF vortices while in the second the CF vortices passed between adjacent plasma jets. Infrared thermography was used to inspect transition location, while quantitative measurements of the boundary layer were obtained using particle image velocimetry. The obtained results show that the plasma forcing reduces the amplitude of stationary CF modes, thus delaying laminar-to-turbulent transition. In contrast to previous efforts [1], the plasma forcing did not introduce unsteady fluctuations into the boundary layer. The mechanism responsible for the observed transition delay appears to leverage more on localised base-flow modification rather than the DRE/UFD control strategy.

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The research presented in this booklet focusses on the cross-flow instability. Applying traditional and advanced flow diagnostics, the boundary layer evolution is studied in detail. The topology and evolution of both primary and secondary instability mechanisms is revealed with unprecedented detail for experimental research paving the way for new advanced-diagnostics investigations. Important confirmations of the outcomes of past experimental, numerical and theoretical studies are achieved together with the description of a newly-reported flow phenomenon. The latter consists of a low frequency motion of the "stationary" primary vortices. While this phenomenon is considered not relevant for the transition evolution, it is deemed important for experimental investigations as it encompasses very high levels of turbulent kinetic energy. Advanced flow control experiments based on alternating current dielectric barrier discharge plasma actuators are also performed following different instability control approaches. The primary instability is conditioned by the external forcing either in the wavenumber spectrum (by inducing selected spanwise modes) or in intensity (by weakening or enhancing the cross-flow velocity). The secondary instability modes are conditioned in the frequency spectrum and phase. These efforts achieved the intended scopes. Although, when selected stationary modes were forced, the boundary layer fluctuations were enhanced. These fluctuations can directly cause the turbulent breakdown vanishing the beneficial effect of the performed instability control. The cross-flow forcing, making use of newer actuators reaching higher frequencies, resulted successful yielding transition promotion or delay depending on the forcing direction. @en

In the current study, selective forcing of cross-flow instability modes evolving on a swept wing at is achieved by means of spanwise-modulated plasma actuators, positioned near the leading edge. In the perspective of laminar flow control, the followed methodology holds on the discrete roughness elements/upstream flow deformation (DRE/UFD) approach, thoroughly investigated by e.g. Saric et al. (AIAA Paper 1998-781, 1998), Malik et al. (J. Fluid Mech., vol. 399, 1999, pp. 85-115) and Wassermann & Kloker (J. Fluid Mech., vol. 456, 2002, pp. 49-84). The possibility of using active devices for UFD provides several advantages over passive means, allowing for a wider range of operating numbers and pressure distributions. In the present work, customised alternating current dielectric barrier discharge plasma actuators have been designed, manufactured and characterised. The authority of the actuators in forcing monochromatic stationary cross-flow modes at different spanwise wavelengths is assessed by means of infrared thermography. Moreover, quantitative spatio-temporal measurements of the boundary layer velocity field are performed using time-resolved particle image velocimetry. The results reveal distinct steady and unsteady forcing contributions of the plasma actuator on the boundary layer. It is shown that the actuators introduce unsteady fluctuations in the boundary layer, amplifying at frequencies significantly lower than the actuation frequency. In line with the DRE/UFD strategy, forcing a sub-critical stationary mode, with a shorter wavelength compared to the naturally selected mode, results in less amplified primary vortices and related fluctuations, compared to the critical forcing case. The effect of the forcing on the flow stability is further inspected by combining the measured actuators body force with the numerical solution of the laminar boundary layer and linear stability theory. The simplified methodology yields fast and computationally cheap estimates on the effect of steady forcing (magnitude and direction) on the boundary layer stability.

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In the present work, laminar flow control, following the distributed roughness elements (DRE) - upstream flow deformation (UFD) strategy, was attempted on a 45° swept wing at Re = 2:17·10⁶undergoing cross-flow (CF) instability transition. Active roughness-like AC-DBD plasma actuators were used for this purpose. Specialized patterned actuators have been designed, manufactured and tested to this goal. The effectiveness in forcing monochromatic stationary modes was assessed by means of infrared thermography. Finally, quantitative measurements of the transitional boundary layer were performed with high-speed PIV. This advanced flow diagnostics tool allows the investigation of the effects on the boundary layer of the unsteady forcing, inherent to the effect of the used actuators. It is found that, despite the actuators were operated at a frequency considerably larger than the frequency band of the (primary travelling) CF instability, the latter resulted massively amplified by the actuation. The causes of this phenomen are here only conjectured and require future dedicated investigation. The eventual beneficial effects of the DRE/UFD flow control technique are likely overwhelmed by the nocent amplification of travelling modes. The technological possibility of very high frequency forcing therefore can limit the usage of AC-DBD actuators for DRE/UFD laminar flow control in high Reynolds numbers flows.

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Time-resolved PIV measurements of the secondary instability modes of cross-flow vortices are presented. Measurements are performed on a large scale 45o swept wing at chord Reynolds number of 2.17 million in a low turbulence wind-tunnel facility. Using acquisition frequencies of 20 kHz, the present study is the first experimental demonstration of spatio-temporally resolved measurements of these structures. Statistical and spectral analysis reveals a fluctuating velocity field, strongly conditioned in space by the primary stationary cross-flow vortex. The flow structures related to the type-I high-frequency instability and type-III are captured by Proper Orthogonal Decomposition of the instantaneous flow-fields. Their temporal evolution is analysed showing good agreement with previous studies thus confirming that POD is correctly representing the flow structures of the relevant instability modes. The low frequency meandering oscillation of the stationary vortices, first reported by Serpieri &Kotsonis (2016b), is observed and characterised.@en

An experimental investigation of primary and secondary crossflow instability developing in the boundary layer of a 45° swept wing at a chord Reynolds number of 2.17 × 10⁶ is presented. Linear stability theory is applied for preliminary estimation of the flow stability while surface flow visualisation using fluorescent oil is employed to inspect the topological features of the transition region. Hot-wire anemometry is extensively used for the investigation of the developing boundary layer and identification of the statistical and spectral characteristics of the instability modes. Primary stationary, as well as unsteady type-I (z-mode), type-II (y-mode) and type-III modes are detected and quantified. Finally, three-component, three-dimensional measurements of the transitional boundary layer are performed using tomographic particle image velocimetry. This research presents the first application of an optical experimental technique for this type of flow. Among the optical techniques, tomographic velocimetry represents, to date, the most advanced approach allowing the investigation of spatially correlated flow structures in three-dimensional fields. Proper orthogonal decomposition (POD) analysis of the captured flow fields is applied to this goal. The first POD mode features a newly reported structure related to low-frequency oscillatory motion of the stationary vortices along the spanwise direction. The cause of this phenomenon is only conjectured. Its effect on transition is considered negligible but, given the related high energy level, it needs to be accounted for in experimental investigations. Secondary instability mechanisms are captured as well. The type-III mode corresponds to low-frequency primary travelling crossflow waves interacting with the stationary ones. It appears in the inner upwelling region of the stationary crossflow vortices and is characterised by elongated structures approximately aligned with the axis of the stationary waves. The type-I secondary instability consists instead of significantly inclined structures located at the outer upwelling region of the stationary vortices. The much narrower wavelength and higher advection velocity of these structures correlate with the higher-frequency content of this mode. The results of the investigation of both primary and secondary instability from the exploited techniques agree with and complement each other and are in line with existing literature. Finally, they present the first experimental observation of the secondary instability structures under natural flow conditions.

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The boundary layer evolving on the pressure side of a 45° swept wing at Re = 2.17 · 106 is experimentally investigated. This flow is characterized by laminar-turbulent transition, dominated by crossflow instability. This mechanism manifests, in low freestream turbulence flows, as stationary waves aligned approximately with the flow direction. These waves grow along the chord and deeply modify the boundary layer causing the destabilization of secondary high frequency vortices. The boundary layer at the location where primary modes saturation occurs, has been investigated here with hotwire anemometry and tomographic PIV. The optical technique allows the simultaneous measurement of all the velocity components within a volume of fluid. The comparison with the hotwire scans shows a very good match. The possibility of applying reduced order analyses based on the flow spatial coherence, such as proper orthogonal decomposition, led to the first experimental description of the spatial arrangement of the secondary instability modes under natural flow conditions.@en

Stationary waves approximately aligned with the flow direction develop in the laminar boundary layer of swept wings in low freestream turbulence conditions. These, so-called Crossflow vortices, undergo spatial amplification and amplitude saturation and deeply modify the boundary layer causing the destabilization of secondary high frequency instabilities. The rigorous amplification of these secondary modes has been identified as the cause of turbulent breakdown by recent investigations encompassing experimental, numerical and theoretical studies. In this paper experiments are conducted towards the identification and measurement of secondary crossflow instabilities. Detailed hotwire experiments are performed on a swept wing model at the location of primary instability saturation. In order to deterministically capture the spatio-temporal evolution of the secondary structures, measurements were phase conditioned using concurrent forcing of relevant frequencies by means of a DBD plasma actuator mounted upstream of the measured domain. The actuator effect was to enhance the amplitude of the forced modes. Additionally, the deterministic locking of the instability phase to the phase of the actuation allowed phase averaged reconstruction of the spatio-temporal evolution of these structures.

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