This investigation explores the utility of Alternating Current, Dielectric Barrier Discharge (AC-DBD) plasma actuators for producing three-dimensional disturbances of a desired spanwise wavelength via superposition. The technique utilizes two pairs of exposed and covered electrodes on a single dielectric layer arranged in streamwise succession. Two-dimensional forcing is achieved through operation of the upstream, spanwise uniform electrode pair, while three-dimensional forcing at a prescribed spanwise wavelength is attained by operating both electrode pairs simultaneously, with the downstream actuator spanwise modulating the upstream, two-dimensional output. The ability to produce disturbances of different spanwise wavelengths but with equal streamwise wavelength, frequency and total momentum is established through a combined characterization effort that considers quiescent and in-flow conditions. A demonstration of the technique in an exemplary wall-bounded shear flow, a laminar separation bubble, is provided, revealing spanwise wavelength dependent disturbance growth in the flow that could be exploited for performance gains in future flow control endeavours. Graphical abstract: [Figure not available: see fulltext.]

Three-dimensional flow development is experimentally assessed in a convectively unstable laminar separation bubble formed over a NACA 0018 airfoil at a chord Reynolds number of 125,000, an angle of attack of 4 deg, and a freestream turbulence intensity of 0.07%. The flow is weakly excited in a spanwise uniform manner at a frequency matching that of the most unstable disturbances in the natural separation bubble, leaving the base flow unmodified while enabling three-dimensional reconstructions of the dominant coherent structures using phase-locked planar particle image velocimetry measurements. Time-averaged flowfield reconstructions show a strongly two-dimensional topology of the separation bubble for both the natural and weakly excited cases. Analysis of the flow development demonstrates that, for both the natural and excited flows, spanwise-oriented and strongly two-dimensional shearlayer vortices form in the separation bubble upstream of the mean maximum height location. Spanwise undulations develop in the vortex filaments that continually intensify with downstream convection as a result of the streamwise forward sections of the filaments lifting away from the surface. This motion reorients the vorticity of the primary structures from the spanwise direction into the streamwise and wall-normal directions, forming hairpinlike structures above the vortex core region. These findings offer new quantitative insight into the vortex dynamics and breakdown process of the shear-layer vortices in two-dimensional, convectively unstable laminar separation bubbles subject to low freestream turbulence levels.

Vortex merging in a laminar separation bubble (LSB) is studied experimentally. The bubble is formed on the suction side of a NACA 0018 airfoil at an angle of attack of 4° and a Reynolds number of 125 000. The merging process in the bubble is manipulated through acoustic forcing applied as a tone at either the fundamental vortex shedding frequency or at the first subharmonic of this frequency. The flow field is assessed using time-resolved, two-component particle image velocimetry. A method for detecting merged structures using wavelet analysis is introduced, allowing for reliable quantification of merging events. The results show that vortex merging occurs naturally in the separation bubble, while forcing at the subharmonic and fundamental frequencies promotes and inhibits merging, respectively. While these trends are similar to those observed for free shear layers, the subharmonic forcing of an LSB is found to directly promote disturbance development at the subharmonic frequency. For all cases, the majority of merging events take place in the aft portion of the bubble, i.e., downstream from the maximum bubble height location and upstream of mean reattachment, with subharmonic forcing causing merging to shift upstream. The merged structures are found to be the most energetic flow features; however, promoting vortex merging through subharmonic forcing does not lead to significant changes in the mean bubble topology. The spanwise behavior of the vortex merging process is studied, revealing that structures merge in a spanwise nonuniform manner, with localized merging occurring away from where forward or rearward streamwise bugles develop in the vortex filaments. Statistical characterization reveals that merging tends to occur more often over some specific spanwise segments, with the number of primary structures that merge varying by as much as 50% between spanwise locations. These findings offer insight into vortex merging in laminar separation bubbles and the attendant influence of forcing, while also highlighting the need to consider spanwise aspects of flow development.

This work examines flow development in a laminar separation bubble (LSB) undergoing natural transition and transition controlled with two-dimensional and spanwise modulated disturbances. The investigation is carried out in a series of wind tunnel tests, with the separation bubble formed over a flat plate subjected to an adverse pressure gradient. Velocity field measurements are performed using time-resolved, two-component Particle Image Velocimetry (PIV). Disturbances are produced using surface-mounted plasma actuators in a novel configuration that allows for the introduction of controlled disturbances that are two-dimensional or of a prescribed spanwise wavelength. The natural transition process is dominated by shear layer vortex shedding which is characterized by significant spanwise deformations in the aft portion of the bubble. When the flow is subjected to either two or three-dimensional forcing, vortex formation within the separation bubble is rendered two-dimensional. However, while the two-dimensionally forced perturbations remain largely two-dimensional until breakdown, a clear spanwise wavelength that matches the input wavelength of the forcing develops when the flow is subjected to the spanwise modulated forcing. The reported findings point to the presence of a secondary instability in the separation bubble, which leads to the amplification of the initially weak spanwise component of input disturbances, causing the shear layer vortices to develop significant spanwise undulations.

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.

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.

Transition and flow development in a separation bubble formed on an airfoil are studied experimentally. The effects of tonal and broadband acoustic excitation are considered since such acoustic emissions commonly result from airfoil self-noise and can influence flow development via a feedback loop. This interdependence is decoupled, and the problem is studied in a controlled manner through the use of an external acoustic source. The flow field is assessed using time-resolved, two-component particle image velocimetry, the results of which show that, for equivalent energy input levels, tonal and broadband excitation can produce equivalent changes in the mean separation bubble topology. These changes in topology result from the influence of excitation on transition and the subsequent development of coherent structures in the bubble. Both tonal and broadband excitation lead to earlier shear layer roll-up and thus reduce the bubble size and advance mean reattachment upstream, while the shed vortices tend to persist farther downstream of mean reattachment in the case of tonal excitation. Through a cross-examination of linear stability theory (LST) predictions and measured disturbance characteristics, nonlinear disturbance interactions are shown to play a crucial role in the transition process, leading to significantly different disturbance development for the tonal and broadband excited flows. Specifically, tonal excitation results in transition being dominated by the excited mode, which grows in strong accordance with linear theory and damps the growth of all other disturbances. On the other hand, disturbance amplitudes are more moderate for the natural and broadband excited flows, and so all unstable disturbances initially grow in accordance with LST. For all cases, a rapid redistribution of perturbation energy to a broad range of frequencies follows, with the phenomenon occurring earliest for the broadband excitation case. By taking nonlinear effects into consideration, important ramifications are made clear in regards to comparing LST predictions and experimental or numerical results, thus explaining the trends reported in recent investigations. These findings offer new insights into the influence of tonal and broadband noise emissions, resulting from airfoil self-noise or otherwise, on transition and flow development within a separation bubble.

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.

The steady and transient response of a laminar separation bubble to flow disturbances is examined experimentally. Wind tunnel experiments are performed on a NACA 0012 aerofoil at a chord Reynolds number of 130 000 and angle of attack of . Under the investigated conditions, a laminar separation bubble forms on the suction side of the aerofoil in the unperturbed flow. Periodic disturbances are introduced into the boundary layer just upstream of separation by means of a surface-mounted dielectric barrier discharge plasma actuator. Two-component, time-resolved particle image velocimetry measurements are performed to characterise both quasi-steady and transient response of the flow to periodic disturbances. The results show that the dynamics of the laminar separation bubble is dominated by the periodic shedding of shear layer vortices, forming upstream of the mean reattachment location due to the amplification of unstable flow disturbances. Introducing the controlled perturbations leads to significant changes in separation bubble topology and the characteristics of the dominant coherent structures, with the effect dependent on both amplitude and frequency of disturbances. Linear stability analysis demonstrates that the induced changes to the mean bubble topology affect the stability characteristics, reducing the maximum growth rate and the frequency of the most amplified disturbances by 35 % and 20 %, respectively, when the bubble length is reduced by up to 40 %. The observed changes in stability characteristics are shown to correlate with the attendant variations in the shape factor. The transient response of the bubble is associated with significant changes in the separation bubble dynamics, with significant differences observed between the relative duration of the transients flow response associated with the introduction and removal of the controlled disturbances. A detailed analysis of the results offers new insight into the response of laminar separation bubbles to changes in the disturbance environment.

This work examines the effect of local active flow control on stability and transition in a laminar separation bubble. Experiments are performed in a wind tunnel facility on a NACA 0012 airfoil at a chord Reynolds number of 130 000 and an angle of attack of 2 degrees. Controlled disturbances are introduced upstream of a laminar separation bubble forming on the suction side of the airfoil using a surface-mounted Dielectric Barrier Discharge plasma actuator. Time-resolved two-component Particle Image Velocimetry is used to characterise the flow field. The effect of frequency and amplitude of plasma excitation on flow development is examined. The introduction of artificial harmonic disturbances leads to significant changes in separation bubble topology and the characteristics of coherent structures formed in the aft portion of the bubble. The development of the bubble demonstrates strong dependence on the actuation frequency and amplitude, revealing the dominant role of incoming disturbances in the transition scenario. Statistical, topological and linear stability theory analysis demonstrate that significant mean flow deformation produced by controlled disturbances leads to notable changes in stability characteristics compared to those in the unforced baseline case. The findings provide a new outlook on the role of controlled disturbances in separated shear layer transition and instruct the development of effective flow control strategies.