1,720,977 research outputs found
Unsteady disturbances in a swept wing boundary layer due to plasma forcing
Full text linkThis work investigates the response of a transitional boundary layer to spanwise-invariant dielectric barrier discharge plasma actuator (PA) forcing on a 45 ° swept wing at a chord Reynolds number of 2.17 × 10 6. Two important parameters of the PA operation are scrutinized, namely, the forcing frequency and the streamwise location of forcing. An array of passive discrete roughness elements is installed near the leading edge to promote and condition a set of critical stationary crossflow (CF) instability modes. Numerical solutions of the boundary layer equations and linear stability theory are used in combination with the experimental pressure distribution to provide predictions of critical stationary and traveling CF instabilities. The laminar-turbulent transition front is visualized and quantified by means of infrared thermography. Measurements of velocity fields are performed using hotwire anemometry scans at specific chordwise locations. The results demonstrate the inherent introduction of unsteady velocity disturbances by the plasma forcing. It is shown that, depending on actuator frequency and location, these disturbances can evolve into typical CF instabilities. Positive traveling low-frequency type III modes are generally amplified by PA in all tested cases, while the occurrence of negative traveling high-frequency type I secondary modes is favored when PA is operating at high frequency and at relatively downstream locations, with respect to the leading edge. </p
Conditioning of unsteady cross-flow instability modes using dielectric barrier discharge plasma actuators
No full textIn 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
Time-resolved PIV investigation of the secondary instability of cross-flow vortices
No full textTime-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
Spatio-temporal characteristics of secondary instabilities in swept wing boundary layers
No full textStationary 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
Tomographic PIV investigation of crossflow instability of swept wing boundary layers
No full textThe 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
Three-dimensional organisation of primary and secondary crossflow instability
No full textAn 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
Design of a swept wing wind tunnel model for study of cross-flow instability
No full textThe need to investigate the cross-ow instability resides on the fact that this is the main cause of laminar to turbulent transition for swept wing ows in free ight. In this paper the procedure to design a wind tunnel model for cross-ow instability investigation is illustrated. The steps that are presented involve the airfoil and the wall liners design and the estimation of the boundary layer growing on the model for the experiment con- ditions. Furthermore linear stability theory is applied to numerically computed boundary layer profiles in order to evaluate the ow stability to standing cross-ow waves of several wavelengths and for two different angles of attack. A preliminary evaluation of the wave- length of the most amplified cross-ow standing mode and its angle with respect to the free stream direction are presented. Wind tunnel tests encompassing oil ow visualization, surface pressure and boundary layer hot-wire measurements were performed validating the design procedure. The effectiveness of the wall liners to constrain the ow to a spanwise invariant arrangement is showed to be limited for the presented case. The results of linear stability theory are compared to the experimental observations showing good agreement for both the estimated wavelengths and wave angles. The validity of oil ow visualization for qualitative investigation of cross-ow instability is confirmed by hot-wire measurements. Natural occurring transition and forced cross-ow instability ows are investigated giving results in good agreement with published literature
Crossflow instabilities under plasma actuation: Design, commissioning and preliminary results of a new experimental facility
No full textPlasma-based flow control poses a simple and robust technique for transition delay on swept wings. However, a clear understanding of how plasma actuators affect crossflow instabilities is necessary to develop and mature crossflow control based on plasma actuators. In this paper, the design of a new swept wing model optimised for the study of crossflow receptivity and stability under plasma actuator is described in detail. First, a 2D wing shape is designed, to match the nearing leading edge pressure distribution of a reference high-Reynolds number swept wing model (M3J) which has been used extensively in past investigations. The aerodynamic performance of this new shape is investigated using CFD simulations and the results show a good agreement for the pressure coefficient. In manufacturing design, the wing model features provisions to accept plasma actuators, such as non-conductive material as well as an appropriately designed recess for the actuator assembly. The new model in conjunction with a recently refurbished low turbulence windtunnel facility are characterized in a preliminary experiment. The uniformity and quality of the flow is identified using pressure measurements and the results confirm the new model achieved near-invariant spanwise conditions until 40% of the chord. Infrared thermography is used to capture the surface footprint of stationary primary crossflow vortices. Clear formations of stationary vortices created by discrete roughness are captured and no visible transition is observed. Finally, the effects of plasma actuation on crossflow instabilities are inspected by Infrared Thermography and PIV scanning. The results validate the prediction of Linear Stability Theory with respect to the most unstable stationary mode and traveling mode. The appearance of secondary crossflow instabilities is observed at relatively upstream chord locations even without transition detected. The outcome positively confirms the ability of this new model to reproduce receptivity and initial growth of crossflow instabilities of the reference model (M3J) under plasma actuation.Virtual/online event due to COVID-19AerodynamicsWind Energ
Plasma-based base flow modification on swept-wing boundary layers: dependence on flow parameters
Full text linkThis work examines the control of cross-flow instabilities (CFIs) and laminar–turbulent transition on a swept wing, through the plasma-based base flow modification (BFM) technique. The effect of experimentally derived plasma body forces on the steady boundary layer base flow is explored through numerical simulations. Linear stability theory is subsequently used to predict the net BFM effect on CFIs. Based on these preliminary predictions, experiments are conducted in a low-turbulence wind tunnel where a spanwise-invariant plasma actuator is installed near the wing leading edge and operated at constant input voltage and frequency. Various flow parameters governing the plasma-based BFM technique are investigated, namely the Reynolds number, angle of attack and wavelength of excited stationary CFI modes. Stationary and travelling CFIs are quantified by planar particle image velocimetry while the transition topology and location are recorded by infrared thermography. The results confirm the stabilising effect of BFM on the swept-wing boundary layer. However, the plasma-based BFM is found to render the boundary layer more susceptible to travelling CFIs. In the presence of both net BFM effect and intrinsic plasma unsteady perturbations, the plasma-based BFM technique achieves transition delay with specific combinations of Reynolds number, angle of attack and wavelength of excited stationary CFI modes. The present findings provide insights into the fundamental principles of operating plasma actuators within the context of BFM control
Sensitivity of crossflow transition to free-stream conditions and surface roughness
No full textThe 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 exper-imentally. 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
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