This study reports the first time-resolved particle image velocimetry characterization of a planar two-phase mixing layer flow, whose velocity field is measured simultaneously in gas and liquid streams. Two parallel air and water flows meet downstream of a splitter plate, giving rise to an initially spanwise invariant configuration. The aim is to elucidate further the mechanisms leading to the flow breakup in gas-assisted atomization. The complete experimental characterization of the velocity field represents a database that could be used in data-driven reduced-order models to investigate the global behaviour of the flow system. After the analysis of a selected reference case, a parametric study of the flow behaviour is performed by varying the liquid and gas Reynolds numbers, and as a consequence also the gas-to-liquid dynamic pressure ratio , shedding light on both time-averaged (mean) and unsteady velocity fields. In the reference case, it is shown that the mean flow exhibits a wake region just downstream of the splitter plate, followed by the development of a mixing layer. By increasing both and, the streamwise extent of the wake decreases and eventually vanishes, the flow resulting in a pure mixing layer regime. The spectral analysis of the normal-to-flow velocity fluctuations outlines different flow regimes by variation of the governing parameters, giving more insights into the global characteristics of the flow field. As a major result, it is found that at high and values, the velocity fluctuations are characterized by low-frequency temporal oscillations synchronized in several locations within the flow field, which suggest the presence of a global mode of instability. The proper orthogonal decomposition of velocity fluctuations, performed in both gas and liquid phases, reveals finally that the synchronized oscillations are associated with a low-frequency dominant flapping mode of the gas-liquid interface. Higher-order modes correspond to interfacial wave structures travelling with the so-called Dimotakis velocity. For lower gas Reynolds numbers, the leading modes describe higher frequency fingers shedding at the interface.

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Images of compressible flows can be post-processed with digital imaging techniques to obtain accurate quantitative information about variables characterizing the flow. For example, the local flow Mach number can be obtained from the angle of Mach lines visualized with the schlieren method. These techniques were recently applied to supersonic flows of dense organic vapors, with the objective of obtaining accurate data to validate theory and CFD codes. Non-ideal compressible fluid dynamics (NICFD) is concerned with these flows, for which therefore the thermodynamic properties of the fluid can be modeled only with equations that are more complex than the ideal gas relations. NICFD flows are relevant, e.g., for applications in the power and chemical industry. However, currently employed image post-processing techniques used to obtain the local Mach number or shock wave angle from schlieren images, like the Hough transform, suffer from few drawbacks, namely a long computational time to obtain the relevant quantities and improvable accuracy. The investigation reported here concerns the application of known digital image processing methods to schlieren images, in this case Gabor filters and Radon transforms, to obtain the local Mach number and the shockwave angle of flows in NICFD conditions. The selected test case is the supersonic expansion of the dense vapor of hexamethyldisiloxane flowing through the nozzle test section of the ORCHID facility in operation at the Propulsion and Power laboratory of Delft University of Technology. The investigated digital image processing techniques provide values of the local Mach number with comparable uncertainty (within 5%) as the Hough transform approach. Moreover, Mach line orientations are computed for the whole field of view, together with Mach line wavelength. It was also proven that these methods are suitable for discerning Mach line orientation even in the case of very complex flow fields, with coexisting Mach waves and shock waves.

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A novel test setup called the asymmetric shock tube for experiments on nonideal rarefaction waves (ASTER) has been commissioned at Delft University of Technology. The ASTER, which works according to the principle of Ludwieg tubes, is designed to generate and measure the speed of small and finite amplitude waves propagating in the dense vapors of fluids formed by complex organic molecules, therefore in the nonideal compressible fluid dynamics regime. The ultimate goal of the associated research is to prove the existence of nonclassical gasdynamics. The setup consists of a high-pressure charge tube and a vacuum tank separated by a glass disk equipped with a breaking mechanism for rarefaction waves experiments. When the glass disk is broken, an expansion wave propagates into the tube in the direction opposite to the fluid flow. The propagation speed of this wave is measured using a time-of-flight method with the help of four fast-response pressure sensors placed equidistantly in the middle of the tube. The charge tube can withstand pressures and temperatures of up to 15 bar and 400∘C. Preliminary rarefaction experiments were successfully conducted using dodecamethylcyclohexasiloxane, D₆, as the working fluid and at pressures and temperatures of up to 9.4 bar and 372∘C, respectively. The results of an experiment featuring the initial state for which a theoretical model predicts the nonclassical acceleration of rarefaction waves show that the propagation is qualitatively different from that put into evidence by experiments for which the propagation is classic. Upcoming setup improvements and experimental campaigns are planned with the objective of experimentally verifying the existence of nonclassical gasdynamics. Graphical abstract: (Figure presented.)

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Compressible flows of fluids whose thermophysical properties are related by complex equations are quantitatively and can be qualitatively different from high-speed flows of ideal gases. Nonideal compressible fluid dynamics (NICFD) is concerned with these fluid flows, which are relevant in many processes and power and propulsion systems. Typically, NICFD effects occur if the fluid is an organic compound and its vapor state is close to the vapor–liquid critical point, at high-reduced temperature and pressure (even supercritical). Current design and analysis of devices operating in the nonideal compressible regime demand for validated simulation software, characterized in terms of uncertainty. Moreover, experiments are needed to further validate related theory. Experimental data are limited as generating and measuring these flows is challenging given their high pressure or temperature or both. In addition, flows of organic compounds can be flammable, can thermally decompose, and sealing may demand for special materials. Recently, more research has been devoted to the measurement of these flows using both intrusive and less intrusive techniques relying on optical access and lasers. The transparency and refractive properties of these dense vapors pose additional problems. The ORCHID (organic Rankine cycle hybrid integrated device) at the Aerospace Propulsion and Power Laboratory of Delft University of Technology is a closed-loop facility, used to generate a continuous nonideal supersonic flow of siloxane MM with the vapor at 4bar and 220 °C at the inlet of the test section. Within this work, we have employed particle image velocimetry for the first time to obtain the velocity field in a de Laval nozzle in such flows. Measured velocity fields (expanded uncertainty within 1.1% of the maximum velocity) have been compared with those resulting from a CFD simulation. The comparison between experimental and simulated data is satisfactory, with deviation ranging from 0.1 to 10 % from the throat to the outlet, respectively. This discrepancy is attributed to hardware limitations, which will be overcome in the future experiments. The feasibility of PIV with uncontrolled but fixed seeding density to measure high-speed vapors of organic vapors has been demonstrated, and future experimental campaigns will target flows for which nonideal effects are more pronounced, other paradigmatic configurations, and improvements to the measurement techniques.

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A novel method for control of convective boundary layer instabilities using metamaterial concepts is investigated. Attenuation of Tollmien-Schlichting (TS) waves with surface-embedded one-dimensional phononic crystals (PCs) is theoretically and numerically modeled, capitalizing on the inherent frequency band stop of PCs. The PC is tuned to the targeted TS wave characteristics through the use of analytical models derived from transfer matrix and interface response theories, verified using a finite elements analysis. The interaction between TS waves and a single PC is investigated using coupled two-dimensional fluid structure interaction simulations in the frequency domain. It is shown that TS waves are either amplified or attenuated depending on whether the PC free-face surface displacement and unsteady perturbation pressure at the wall are in-phase or out-of-phase, respectively. The perturbation pressure acts solely as the driver for the mechanical oscillation of the PC. The emerging hydrodynamic coupling between TS waves and the PC is found to be governed by a combination of the Orr mechanism and wall-normal velocity linear superposition near the wall. Finally, a metasurface comprised of an array of streamwise-distributed PCs is evaluated, resulting in an amplitude growth delay of 11.3% of the TS wavelength along the metasurface extent.

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This paper describes the design, functioning, and preliminary experiments in the Asymmetric Shock Tube for Experiments on Rarefaction Waves (ASTER), a Ludwieg-type shock tube designed and realised to measure waves propagating in dense vapour flows of organic fluids. The setup is designed to operate at pressures and temperatures of up to 15 bar and 400^∘ C. The high and low-pressure sections of the tube are separated by a glass-disk barrier to ensure quasi-instantaneous opening. When the glass disk is broken, a rarefaction wave propagates into the tube. The wave speed is measured using a time-of-flight method with the help of four pressure transducers placed at known distances from each other. Leakage rates of 2.2 × 10^{- 4} mbar · l · s^{- 1} at vacuum and 5 × 10^{- 4} mbar · l · s^{- 1} at superatmospheric pressures were measured, which is considered sufficient for the conceived experiments. Preliminary rarefaction experiments in the dense vapours of dodecamethylcyclohexasiloxane, D₆, were successfully performed at various thermodynamic conditions. Also, a method for the estimation of sound speeds from the pressure sensor recordings is proposed. Results are found to be within 2.5% of the values predicted by the state-of-the-art thermodynamic model for D₆.

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This work presents the first experimental characterization of the flow field in the vicinity of periodically spaced discrete roughness elements (DRE) in a swept wing boundary layer. The time-averaged velocity fields are acquired in a volumetric domain by high-resolution dual-pulse tomographic particle tracking velocimetry. Investigation of the stationary flow topology indicates that the near-element flow region is dominated by high- and low-speed streaks. The boundary layer spectral content is inferred by spatial fast Fourier transform (FFT) analysis of the spanwise velocity signal, characterizing the chordwise behaviour of individual disturbance modes. The two signature features of transient growth, namely algebraic growth and exponential decay, are identified in the chordwise evolution of the disturbance energy associated with higher harmonics of the primary stationary mode. A transient decay process is instead identified in the near-wake region just aft of each DRE, similar to the wake relaxation effect previously observed in two-dimensional boundary layer flows. The transient decay regime is found to condition the onset and initial amplitude of modal crossflow instabilities. Within the critical DRE amplitude range (i.e. affecting boundary layer transition without causing flow tripping) the transient disturbances are strongly receptive to the spanwise spacing and diameter of the elements, which drive the modal energy distribution within the spatial spectra. In the super-critical amplitude forcing (i.e. causing flow tripping) the near-element stationary flow topology is dominated by the development of a high-speed and strongly fluctuating region closely aligned with the DRE wake. Therefore, elevated shears and unsteady disturbances affect the near-element flow development. Combined with the harmonic modes transient growth these instabilities initiate a laminar streak structure breakdown and a bypass transition process.

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The wake-mixing layer flow developing past a splitter plate separating two parallel gas and liquid co-flowing currents is experimentally investigated in this work. Time-resolved particle image velocimetry (TR-PIV) measurements of the two-phase velocity field are simultaneously performed in gas and liquid streams, shedding light on both mean (time-averaged) and unsteady features of the flow configuration. A selected reference case is first analyzed, revealing the presence of a wake region within the flow field, right behind the splitter plate. By progressively moving downstream along the streamwise G direction, a pure mixing layer region is retrieved. The effect of two governing flow parameters, namely the gas Reynolds number '46 and the gasliquid dynamic pressure ratio ", is then investigated, focusing first on the mean flow topology. It is found that the streamwise extension of the wake GF is a monotonic decreasing function of '46, and it vanishes for the highest '46 value considered, the two-phase flow resulting in a pure mixing layer regime. The flow unsteady development is then characterized by means of the spectral analysis of normal-to-flow (i.e. along H direction) velocity fluctuating quantities E0 ¹C°, performed in both gas and liquid flows. As major results, it is found that frequency spectra are characterized by a high frequency content in the low '46 configuration, the peak frequency depending on the streamwise location G. On the other hand, by progressively increasing '46 the peak frequency shifts to lower values, and it becomes independent on the specific spatial location by increasing ". It is found that, at high '46 and " values, velocity fluctuations are characterized by low frequency temporal oscillations synchronized over a large spatial extent of the flow field. The different regimes outlined by variation of the flow governing parameters are found to be consistent with convective/absolute instability behaviors highlighted by spatiotemporal linear stability analyses of the flow recently presented in literature.

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The influence of a wall-embedded Helmholtz resonator on the development and stability of Tollmien-Schlichting (TS) waves is investigated numerically and experimentally for a range of frequencies extending from below to above resonance. Interactions are found to be limited in the near-wall region and toward the trailing edge of the resonator orifice while at the same time being linear nature. The dynamic response of the flow-excited resonator is shown to have a fixed phase relation with respect to the TS-waves, indicating that only amplification of the latter can be achieved. The same resonant behavior is maintained regardless of whether the resonator is flow-excited or acoustically excited. Thus, it is suggested that pressure perturbations propagate perpendicularly and acoustically within the resonator throat and cavity. The amplification observed in the vicinity of the resonator displays features typical of TS-wave scattering; however, it is confirmed that this is not solely the result of mean flow distortion due to the geometry and recirculation region. Instead, the results indicate that the phenomenology is a consequence of the combination of scattering, localized non-modal growth, and wall-forcing in the wall-normal direction due to resonance.

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The Parabolized Stability Equations (PSE), Adaptive Harmonic Linearized Navier-Stokes (AHLNS) and Harmonic Navier-Stokes (HNS) solvers are used to analyze the linear and nonlinear stability of swept-wing boundary layers under the influence of smooth wall deformations of varying size and geometry. Special attention is given to the validity of the slowly varying flow assumption of PSE via a comparison with AHLNS and HNS results. The surface deformations analyzed in this work are found to affect the development of the primary stationary crossflow instability mode as well as higher harmonics. Analysis of the locally most amplified mode reveals successive modulation of the growth rate in the vicinity of the surface deformation. This process was found to be largely governed by linear terms and driven by the base flow modification due to the deformed wall. Similarly, the base flow modification causes higher harmonics to experience a significant destabilization. This is followed by stabilization as nonlinear interactions become dominant. The PSE methodology proved capable of predicting the stability response for small wall deformations with only minor amplitude discrepancies compared to HNS results. The main difference was found to occur in the wall-normal velocity profiles of the mean flow distortion mode. The deviations of the PSE results compared to harmonic stability methods increased as the protuberance was made steeper. Moreover, the PSE framework was not able to converge for all cases nonlinearly.@en

This paper describes an experiment conducted within the nozzle test section of the Organic Rankine Cycle Hybrid Integrated Device (ORCHID) aimed at providing accurate data for the validation of NICFD flow solvers [5]. A supersonic flow of the dense vapor siloxane MM established in the nozzle of the setup was characterized by means of the schlieren technique and by pressure taps along the nozzle profile. The nozzle inlet conditions corresponded to a stagnation temperature and pressure of T0=253∘C and P0=18.36bara. At these inlet conditions, the compressibility factor of the fluid is Z₀= 0.58. The nozzle backpressure was equal to Pb=2.2bara. The experimental data-set includes: 1) the average mid-plane local Mach number, which was derived from the schlieren images by estimating the angle of the Mach waves originating from the roughness of the upper and lower nozzle surfaces, 2) the angle of a shock wave generated by a 5^∘ wedge placed at the nozzle exit, also detectable in the schlieren images, and 3) the static pressure distribution along the flow expansion acquired with a Scanivalve DSA3218 pressure scanner device. The Mach number at the nozzle exit estimated based on the schlieren images is M= 1.95 ± 0.05, very close to the expected value of M= 2 according to the design conditions of the experiment. The static pressure measurements have a maximum absolute uncertainty amounting to ± 1.80 kPa in the initial stages of the expansion. This information was used to assess the capability of the open-source SU2 flow solver in evaluating the NICFD effects in a supersonic flow of MM when the fluid thermodynamic properties are modeled with a cubic equation of state. For this purpose, two-dimensional Euler simulations were carried out with SU2 for the operating conditions achieved in the experiment. The numerical results are in good agreement with the experimental data. The largest deviation between the simulation and experiment is observed in the nozzle uniform region, where two dips in the Mach number occur due to a slight local decrease in flow velocity owing to two weak shock waves. The shock wave generated by the wedge located at the nozzle outlet propagates with two different angles, namely, β_above= 37. 6^∘± 0.86, and β_below= 31. 6^∘± 0.64, due to the axial misalignment of the wedge with respect to the flow.

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The present work details the steady and unsteady flow topology in the vicinity of an array of periodically spaced super-critical (i.e. causing flow tripping) discrete roughness elements (DRE) applied in a swept wing boundary layer. The stationary flow field is acquired by means of high-magnification dual-pulse tomographic particle tracking velocimetry (3D-PTV), while the unsteady instabilities are investigated through high-resolution hot wire anemometry (HWA). The 3D-PTV time-averaged velocity fields, indicate that the near-element flow region is dominated by the alternation of high- and low-speed streaks. A high-speed region substitutes the wake development shortly downstream of the DRE location, due to the high-speed streaks merging. This initiates a region of strong unsteady fluctuations that expands in the spanwise and wall-normal directions, ultimately leading to the boundary layer transition to turbulence. The spectral content of the stationary flow structures is investigated through a spanwise spatial Fourier transform. The extracted spectra and instability amplitudes, indicate the presence of non-modal mechanisms in the near-element stationary wake region. Nonetheless, the temporal spectral analysis of the HWA velocity signal, identifies the presence of strongly tonal shedding mechanisms initiating and the unsteady instabilities the element vicinity. Their rapid downstream growth and evolution retains a fundamental role in the transitional process. @en

This work presents the first reported experimental characterization of the flow field in the direct vicinity of discrete roughness elements (DRE), in a swept wing boundary layer. High magnification tomographic Particle Tracking Velocimetry (3D-PTV) measurements are used to acquire time-averaged velocity and standard deviation fields in a 3D volume directly aft of the DRE elements. The collected data detail the near-element flow topology, providing information on the developing wake and emerging flow structures, their organization and amplitude evolution. A transient growth behaviour is identified in the element wake, while onset and growth of crossflow instabilities is observed further downstream. As such, the near element flow is confirmed to be a fundamental part of the receptivity process, contributing in setting the initial amplitudes for the crossflow instability evolution.

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The effect of streamwise plasma vortex generators on the convective heat transfer of a turbulent boundary layer is experimentally investigated. A Dielectric Barrier Discharge (DBD) plasma-actuator array is employed to promote pairs of counter-rotating, streamwise-aligned vortices embedded in a well-behaved turbulent boundary layer over a flat plate. The study aims at elucidating the mechanism of interaction between the plasma-induced vortical structures and the convective heat transfer process downstream of them. The full three-dimensional mean flow field is measured with planar and stereoscopic PIV. The convective heat transfer is assessed with infrared thermography over a heat-flux sensor located downstream of the actuators. The combination of the flow field and heat transfer measurements provides a complete picture of the fluid-dynamic interaction of plasma-induced flow with local turbulent transport effects. The results show that the streamwise vortices are stationary and confined across the spanwise direction due to the action of the plasma discharge. Flow-field measurements show that the opposing plasma discharge causes a mass- and momentum-flux deficit within the boundary layer, leading to a low-velocity region that grows in the streamwise direction and which is characterised by an increase in displacement and momentum thicknesses. This low-velocity ribbon travels downstream, promoting streak-alike patterns of reduction in the convective heat transfer distribution. Near the wall, the plasma-induced jets divert the main flow. This phenomenon is a consequence of the DBD-actuator momentum injection and, thus, the suction caused on the surrounding fluid by the emerging jets. The stationarity of the plasma-induced vortices makes them persistent far downstream, reducing the convective heat transfer.

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The presented work introduces a cancellation technique, based on the linear superposition of stationary crossflow instabilities (CFIs) through the application of a streamwise series of optimally positioned discrete roughness element (DRE) arrays on a swept wing surface. The DRE arrays are designed and arranged with suitable amplitude and phase shift to induce velocity disturbance systems that destructively interact, ultimately damping the developing CFIs. The robustness of this technique is investigated for a smooth wing surface as well as in the presence of enhanced distributed surface roughness. The resulting flow fields are measured with infrared thermography and particle tracking velocimetry, allowing for the extraction of the laminar-to-turbulent transition front location and for the characterization of the local boundary layer development. The acquired data show that the superposition of suitably arranged DRE arrays can successfully suppress monochromatic CFIs, reducing their amplitude and growth and delaying the boundary layer transition to turbulence when applied on a smooth wing surface. However, the presence of elevated background roughness significantly reduces the effectiveness of the proposed method.

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We study an array of streamwise-oriented Dielectric Barrier Discharge (DBD) plasma actuators as an active control technique in turbulent flows. The analysis aims at elucidating the mechanism of interaction between the structures induced by the DBD-plasma actuators and the convective heat transfer process in a fully developed turbulent boundary layer. The employed flush-mounted DBD-plasma actuator array generates pairs of counter-rotating, stationary, streamwise vortices. The full three-dimensional, velocity field is measured with stereoscopic PIV and convective heat transfer at the wall is assessed by infrared thermography. The plasma actuator forcing diverts the main flow, yielding a low-momentum region that grows in the streamwise direction. The suction effect promoted on top of the exposed electrodes confines the vortices in the spanwise direction. Eventually, the pair of streamwise vortices locally reduces the convective heat transfer with a persistence of several outer lengthscales downstream of the actuation.

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The variation of streamwise and spanwise characteristic wavelengths of a NACA 0018 laminar separation bubble under natural and periodic excitation conditions is investigated experimentally. Periodic forcing is applied with an AC-DBD plasma actuator, and the response of the bubble is characterised in two orthogonal planes by means of time-resolved particle image velocimetry. Periodic excitation results in substantial time-averaged size reduction of the bubble. Linear stability analysis is used to establish that the most notable flow deformation is achieved when excitation is applied at the most unstable frequency, which does not significantly vary (<4%) for the range of excitation parameters investigated. At excitation frequencies well below the unstable frequency band, the shear layer does not lock to the excitation and is, instead, modulated. Lock-in is achieved at higher forcing frequencies, which are within the unstable band. For the case of modulated shedding, spanwise deformations become more significant than in the natural case; whereas when shedding becomes locked to the excitation frequency, the coherence of the rollers along the span increases. Characteristic streamwise and spanwise wavelengths are statistically quantified by means of spatial wavelet analysis, demonstrating that spanwise deformations attain wider range of wavelengths than the respective streamwise rollers. Analysis of these results suggests that spanwise deformation is associated to both the incoming boundary layer and shear layer stability characteristics.

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This work investigates the three-dimensional, spatio-Temporal flow development in the aft portion of a laminar separation bubble. The bubble is forming on a flat plate geometry, subjected to an adverse pressure gradient, featuring maximum reverse flow of approximately 2Â % of the local free-stream velocity. Time-resolved velocity measurements are performed by means of planar and tomographic particle image velocimetry, in the vicinity of the reattachment region. The measurements are complemented with a numerical solution of the boundary layer equations in the upstream field. The combined numerical and measured boundary layer is used as a baseline flow for linear stability theory analysis. The results provide insight into the dynamics of dominant coherent structures that form in the separated shear layer and deform along the span. Stability analysis shows that the flow becomes unstable upstream of separation, where both normal and oblique modes undergo amplification. While the shear layer roll up is linked to the amplification of the fundamental normal mode, the oblique modes at angles lower than approximately are also amplified substantially at the fundamental frequency. A model based on the stability analysis and experimental measurements is employed to demonstrate that the spanwise deformations of rollers are produced due to a superposition of normal and oblique instability modes initiating upstream of separation. The degree of the initial spanwise deformations is shown to depend on the relative amplitude of the dominant normal and oblique waves. This is confirmed by forcing the normal mode through a controlled impulsive perturbation introduced by a spanwise invariant dielectric-barrier-discharge plasma actuator, resulting in the formation of spanwise coherent vortices. The findings elucidate the link between important features in the bubble shedding dynamics and stability characteristics and provide further clarification on the differences in the development of coherent structures seen in recent experiments. Moreover, the results present a handle on the development of effective control strategies that can be used to either promote or suppress shedding in separation bubbles, which is of interest for system performance improvement and control of aeroacoustic emissions in relevant applications.

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The research presented in this thesis focuses on boundary layer separation and, more specifically, on the related physical mechanisms, such as laminar to turbulent transition, with a vision towards effective separation control. The work is divided in three parts, each corresponding to a different separation scenario. The supporting data is experimental, obtained by means of particle image velocimetry. The first and major part regards the phenomenon of laminar separation bubbles. The spatial and temporal response characteristics of a flat plate laminar separation bubble to impulsive forcing are first investigated in order to shed light on the processes of flapping and bursting. The impulsive disturbance is introduced twodimensionally with a dielectric barrier discharge plasma actuator. The disturbance develops into a wave packet that causes rapid shrinkage of the bubble in both upstream and downstream directions. This is followed by bubble bursting, during which the bubble elongates significantly, while vortex shedding strength in the aft portion of the bubble is reduced. Duration of the recovery of the bubble to its unperturbed state is independent of the forcing amplitude. At the same time, linear stability analysis shows that the growth rate and the frequency of the most unstable mode decreases for increasing forcing amplitude. Throughout recovery, growth rates are directly proportional to the shape factor, indicating that bursting and flapping mechanisms are driven by altered stability characteristics due to variations in incoming disturbances. It is found that the stability of the flow changes only when disturbances interact with the shear layer breakdown and reattachment processes, supporting the notion of a closed feedback loop.@en

The spatial and temporal response characteristics of a laminar separation bubble to impulsive forcing are investigated by means of time-resolved particle image velocimetry and linear stability theory. A two-dimensional impulsive disturbance is introduced with an alternating current dielectric barrier discharge plasma actuator, exciting pertinent instability modes and ensuring flow development under environmental disturbances. Phase-averaged velocity measurements are employed to analyse the effect of imposed disturbances at different amplitudes on the laminar separation bubble. The impulsive disturbance develops into a wave packet that causes rapid shrinkage of the bubble in both upstream and downstream directions. This is followed by bubble bursting, during which the bubble elongates significantly, while vortex shedding in the aft part ceases. Duration of recovery of the bubble to its unforced state is independent of the forcing amplitude. Quasi-steady linear stability analysis is performed at each individual phase, demonstrating reduction of growth rate and frequency of the most unstable modes with increasing forcing amplitude. Throughout the recovery, amplification rates are directly proportional to the shape factor. This indicates that bursting and flapping mechanisms are driven by altered stability characteristics due to variations in incoming disturbances. The emerging wave packet is characterised in terms of frequency, convective speed and growth rate, with remarkable agreement between linear stability theory predictions and measurements. The wave packet assumes a frequency close to the natural shedding frequency, while its convective speed remains invariant for all forcing amplitudes. The stability of the flow changes only when disturbances interact with the shear layer breakdown and reattachment processes, supporting the notion of a closed feedback loop. The results of this study shed light on the response of laminar separation bubbles to impulsive forcing, providing insight into the attendant changes of flow dynamics and the underlying stability mechanisms.

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