Parametric study of separation and transition characteristics over an airfoil at low Reynolds numbers

Time-resolved surface pressure measurements are used to experimentally investigate characteristics of separation and transition over a NACA 0018 airfoil for the relatively wide range of chord Reynolds numbers from 50,000 to 250,000 and angles of attack from 0° to 21°. The results provide a comprehensive data set of characteristic parameters for separated shear layer development and reveal important dependencies of these quantities on flow conditions. Mean surface pressure measurements are used to explore the variation in separation bubble position, edge velocity in the separated shear layer, and lift coefficients with angle of attack and Reynolds number. Consistent with previous studies, the separation bubble is found to move upstream and decrease in length as the Reynolds number and angle of attack increase. Above a certain angle of attack, the proximity of the separation bubble to the location of the suction peak results in a reduced lift slope compared to that observed at lower angles. Simultaneous measurements of the time-varying component of surface pressure at various spatial locations on the model are used to estimate the frequency of shear layer instability, maximum root-mean-square (RMS) surface pressure, spatial amplification rates of RMS surface pressure, and convection speeds of the pressure fluctuations in the separation bubble. A power-law correlation between the shear layer instability frequency and Reynolds number is shown to provide an order of magnitude estimate of the central frequency of disturbance amplification for various airfoil geometries at low Reynolds numbers. Maximum RMS surface pressures are found to agree with values measured in separation bubbles over geometries other than airfoils, when normalized by the dynamic pressure based on edge velocity. Spatial amplification rates in the separation bubble increase with both Reynolds number and angle of attack, causing the accompanying decrease in separation bubble length. Values of the convection speed of pressure fluctuations in the separated shear layer are measured to be between 35 and 50% of the edge velocity, consistent with predictions of linear stability theory for separated shear layers.     

Experimental analysis of the flow field over a novel owl based airfoil

The aerodynamics of a newly constructed wing model the geometry of which is related to the wing of a barn owl is experimentally investigated. Several barn owl wings are scanned to obtain three-dimensional surface models of natural wings. A rectangular wing model with the general geometry of the barn owl but without any owl-specific structure being the reference case for all subsequent measurements is investigated using pressure tabs, oil flow pattern technique, and particle-image velocimetry. The main flow feature of the clean wing is a transitional separation bubble on the suction side. The size of the bubble depends on the Reynolds number and the angle of attack, whereas the location is mainly influenced by the angle of attack. Next, a second model with a modified surface is considered and its influence on the flow field is analyzed. Applying a velvet onto the suction side drastically reduces the size of this separation at moderate angles of attack and higher Reynolds numbers.     

Application of a Synthetic Jet to Control Boundary Layer Separation under Ultra-High-Lift Turbine Pressure Distribution

The transition and separation processes of the boundary layer developing on a flat plate under a prescribed adverse pressure gradient typical of Ultra-High-Lift low-pressure turbine profiles have been investigated, with and without the application of a synthetic jet (zero net mass flow rate jet). A mechanical piston has been adopted to produce an intermittent flow with zero net mass flow rate. The capability of the device to suppress or reduce the large laminar separation bubble occurring under steady inflow condition at low Reynolds numbers has been experimentally investigated by means of hot-wire measurements. Wall static pressure measurements complement the hot-wire time-resolved velocity results. The paper reports the investigations performed for both steady and controlled conditions. The active device is able to control the laminar separation bubble induced at low Reynolds number conditions by the strong adverse pressure gradient. An overall view of the time-dependent evolution of the controlled boundary layer is provided by the phase-locked ensemble averaging technique, triggered at the synthetic jet frequency. The separated flow transition process, which is detected for the uncontrolled condition, is modified by the synthetic jet in different ways during the blowing and suction phases. Overall, the phase-locked velocity distributions show a reduced separated flow region for the whole jet cycle as compared to the uncontrolled condition. The phase-locked distributions of the random unsteadiness allow the identification of vortical structures growing along the shear layer mainly during the blowing phase.     

Time-resolved and volumetric PIV measurements of a transitional separation bubble on an SD7003 airfoil

To comprehensively understand the effects of Kelvin–Helmholtz instabilities on a transitional separation bubble on the suction side of an airfoil regarding as to flapping of the bubble and its impact on the airfoil performance, the temporal and spatial structure of the vortices occurring at the downstream end of the separation bubble is investigated. Since the bubble variation leads to a change of the pressure distribution, the investigation of the instantaneous velocity field is essential to understand the details of the overall airfoil performance. This vortex formation in the reattachment region on the upper surface of an SD7003 airfoil is analyzed in detail at different angles of attack. At a Reynolds number Re _c < 100,000 the laminar boundary layer separates at angles of attack >4°. Due to transition processes, turbulent reattachment of the separated shear layer occurs enclosing a locally confined recirculation region. To identify the location of the separation bubble and to describe the dynamics of the reattachment, a time-resolved PIV measurement in a single light-sheet is performed. To elucidate the spatial structure of the flow patterns in the reattachment region in time and space, a stereo scanning PIV set-up is applied. The flow field is recorded in at least ten successive light-sheet planes with two high-speed cameras enclosing a viewing angle of 65° to detect all three velocity components within a light-sheet leading to a time-resolved volumetric measurement due to a high scanning speed. The measurements evidence the development of quasi-periodic vortex structures. The temporal dynamics of the vortex roll-up, initialized by the Kelvin–Helmholtz (KH) instability, is shown as well as the spatial development of the vortex roll-up process. Based on these measurements a model for the evolving vortex structure consisting of the formation of c-shape vortices and their transformation into screwdriver vortices is introduced.     

Investigation of the vortex induced unsteadiness of a separation bubble via time-resolved and scanning PIV measurements

A transitional separation bubble on the suction side of an SD7003 airfoil is considered. The transition process that forces the separated shear layer to reattach seems to be governed by Kelvin–Helmholtz instabilities. Large scale vortices are formed due to this mechanism at the downstream end of the bubble. These vortices possess a three-dimensional structure and detach from the recirculation region, while other vortices are formed within the bubble. This separation of the vortex is a highly unsteady process, which leads to a bubble flapping. The structure of these vortices and the flapping of the separation bubble due to these vortices are temporally and spatially analyzed at angles of attack from 4° to 8° and chord-length based Reynolds numbers Re _c = 20,000–60,000 using time-resolved PIV measurements in a 2D and a 3D set-up, i.e., stereo-scanning PIV measurements are done in the latter case. These measurements complete former studies at a Reynolds number of Re _c = 20,000. The results of the time-resolved PIV measurements in a single light-sheet show the influence of the angle of attack and the Reynolds number. The characteristic parameters of the separation bubble are analyzed focusing on the unsteadiness of the separation bubble, e.g., the varying size of the main recirculation region, which characterizes the bubble flapping, and the corresponding Strouhal number are investigated. Furthermore, the impact of the freestream turbulence is investigated by juxtaposing the current and former results. The stereo-scanning PIV measurements at Reynolds numbers up to 60,000 elucidate the three-dimensional character of the vortical structures, which evolve at the downstream end of the separation bubble. It is shown that the same typical structures are formed, e.g., the c-shape vortex and the screwdriver vortex at each Reynolds number and angle of attack investigated and the occurrence of these patterns in relation to Λ-structures is discussed. To evidence the impact of the freestream turbulence, these results are compared with findings of former measurements.     

An Experimental Investigation of the Separated-Flow Transition Under High-Lift Turbine Blade Pressure Gradients

Laminar boundary layer separation, shear layer transition and reattachment have been experimentally investigated on a flat plate installed within a double contoured test section designed to produce an adverse pressure gradient typical of Ultra-High-Lift turbine profiles. Measurements have been performed for the Reynolds number range 70,000 < Re < 200,000, typical of real engine operation. Profile aerodynamic loadings as well as boundary layer velocity profiles have been measured to survey the separation and transition processes. Particle Image Velocimetry measurements allowed the visualization of vortical structures induced by the shear layer instability. Spectral analysis of hot-wire velocity data has been adopted to identify the characteristic frequencies of the phenomena. Distinct energy peaks, associated with the Kelvin–Helmholtz waves generated in the shear layer over the separation bubble, appear in the spectra. In particular the evolution along the shear layer of the energy contents at the characteristic frequencies of the phenomenon has been analyzed. Two frequency ranges have been identified in which the instability waves are amplified within the shear layer over the stagnation area. The inviscid Kelvin–Helmholtz instability is the main mechanism that drives transition, but it starts to be relevant only after that lower frequency oscillations are amplified and reach the saturation.     

Large Eddy Simulation of a Controlled Diffusion Compressor Cascade

In this research a Controlled Diffusion (CD) compressor cascade stator blade is simulated at a Reynolds number of ??700,000, based on inflow velocity and chord length, using Large Eddy Simulation (LES). A wide range of flow inlet angles are computed, including conditions near the design angle, and at high negative and positive incidence. At all inlet angles the surface pressure distributions are well-predicted by the LES. Near the design angle the computed suction side boundary layer thickness agrees well with experimental data, whilst the pressure side boundary layer is poorly predicted due to the inability of LES to capture natural boundary layer transition on the present grid. A good estimation of the loss is computed near the design angle, whilst at both high positive and negative incidences the loss is less well predicted owing to discrepancies between the computed and experimental boundary layer thickness. At incidences above the design angle a laminar separation bubble forms near the leading edge of the suction surface, which undergoes a transition to turbulence. Similar behaviour is noted on the pressure surface at negative incidence. At high negative incidence contra-rotating vortex pairs are found to form around the leading edge in response to an unsteady stagnation line across the span of the blade. Such structures are not apparent in time-averaged statistical data due to their highly-transient nature.     

PIV measurements in a weakly separating and reattaching turbulent boundary layer

A high Reynolds number flat plate turbulent boundary layer was studied in a wind-tunnel experiment using particle image velocimetry (PIV). The flow is subjected to an adverse pressure gradient (APG) which is designed such that the boundary layer separates and reattaches, forming a weak separation bubble. With PIV we are able to get a more complete picture of this complex flow phenomenon. The view of a separation bubble being composed of large scale coherent regions of instantaneous backflow occurring randomly in a three-dimensional manner in space and time is verified by the present PIV measurements. The PIV database was used to test the applicability of various velocity scalings around the separation bubble. We found that the mean velocity profiles in the outer part of the boundary layer, and to some extent also the Reynolds shear-stress, are self-similar when using a velocity scale based on the local pressure gradient. The same can be said for the so called Perry–Schofield scaling, which suggests that the two velocity scales are connected. This can also be interpreted as an experimental evidence of the claimed relation between the latter velocity scale and the maximum Reynolds shear-stress.     

Low Reynolds number aerodynamics of free-to-roll low aspect ratio wings

The aerodynamics of thin, flat-plate wings of various planforms (rectangular, elliptical and Zimmerman) have been studied in free-to-roll experiments in a wind tunnel. Non-zero trim angles at low angles of attack, self-induced roll oscillations with increasing angle of attack and even autorotation in some cases were observed. The rectangular wings with round leading-edge had non-zero trim angles at low incidences due to the asymmetric development of the three-dimensional separation bubble at these low Reynolds numbers. With increasing angle of attack, the bubble increases in length and once reattachment is lost, large amplitude roll oscillations develop. The Strouhal number of the roll oscillations is of the order of 10⁻², which is in the same range as those expected for small aircraft experiencing atmospheric gusts. Velocity measurements revealed that variations in the strength of the vortices drove the rolling motion. At the mean roll angle, because of the time lag in the strength of the vortices, an asymmetric flow is generated, which results in a net rolling moment in the direction of the rolling motion.     

Airfoil boundary layer measurements at low Re in an accelerating flow from a nonzero velocity

The effects of an accelerating freestream from a nonzero velocity on the transitional separation bubble characteristics were investigated quantitatively. Hot wire anemometry was used to determine the boundary layer velocity profile repsonse to the acceleration at selected chordwise locations on a Wortmann FX 63-137 airfoil at 7 ° angle of attack. Both positive and negative accelerations were studied from base chord Reynolds number of 100,000 and 150,000, respectively. The purpose of this experiment was to verify the trends witnessed in previous research concerning a sinusoidally oscillating freestream velocity by uncoupling the accelerating and decelerating boundary layer effects. The experimental results indicate that as a result of a freestream acceleration, the separation bubble position shifts in the direction opposite to the chordwise direction it would move for a quasi-steady velocity change. The transition location was more responsive to the acceleration than was the separation position. This supports the oscillating freestream experiment conclusions.This research was supported by the U.S. Navy Office of Naval Research under contract N00014-83-K-0239     

Flow development upstream of a fence

The effect of Reynolds number on the flow development upstream of a rigid, non-porous, static fence is investigated experimentally. The flow field is measured using time-resolved, two-component particle image velocimetry at Reynolds numbers based on fence height of 18000, 36000, and 54000. The results show that a laminar separation bubble forms upstream of the junction vortex at the base of the fence. The mean extent of the bubble decreases with increasing Reynolds number, with mean separation moving downstream and mean reattachment moving upstream. In the aft portion of the bubble, shear layer vortices form and are shed at scaled frequencies and wavelengths that are comparable to laminar separation bubble shedding in low Reynolds number airfoils and flat plates with an imposed adverse pressure gradient. The strong periodicity of the associated coherent structures and the proximity of shear layer roll-up relative to the fence should be taken into consideration in the relevant designs due to potential implications to structural loading. A simple flow separation prediction model combining inviscid fence flow solution with Thwaites’ method is introduced and shows good agreement with the experimental results for the Reynolds number range considered.     

Detection of flow separation and reattachment using shear-sensitive liquid crystals

Coatings of pure chiral nematic liquid crystals are known to change colour under different levels of surface shear stress. In this study, the liquid crystal was used to provide information about flow separation and reattachment on both a two-dimensional aerofoil and a delta wing. The tests were carried out at a free-stream velocity of 28 m/s and a number of incidence angles. The Reynolds numbers based on the central chord length of the models were 200,000 and 270,000 for the aerofoil and delta wing models, respectively. The study showed that locations of boundary layer separation and reattachment can be identified from spatial variations in the surface colour; the agreement between the results and those obtained using surface oil flow was good. Issues relating to interpretation of the crystal colour pattern and the limitation of this technique in detection of flow separation were also discussed.     

High-fidelity numerical simulation of the flow field around a NACA-0012 aerofoil from the laminar separation bubble to a full stall

In the present work, large eddy simulations of the flow field around a NACA-0012 aerofoil near stall conditions are performed at a Reynolds number of 5 × 10⁴, Mach number of 0.4, and at various angles of attack. The results show the following: at relatively low angles of attack, the bubble is present and intact; at moderate angles of attack, the laminar separation bubble bursts and generates a global low-frequency flow oscillation; and at relatively high angles of attack, the laminar separation bubble becomes an open bubble that leads the aerofoil into a full stall. Time histories of the aerodynamic coefficients showed that the low-frequency oscillation phenomenon and its associated physics are indeed captured in the simulations. The aerodynamic coefficients compared to previous and recent experimental data with acceptable accuracy. Spectral analysis identified a dominant low-frequency mode featuring the periodic separation and reattachment of the flow field. At angles of attack α ≤ 9.3°, the low-frequency mode featured bubble shedding rather than bubble bursting and reformation. The underlying mechanism behind the quasi-periodic self-sustained low-frequency flow oscillation is discussed in detail.     

An experimental study on boundary layer transition detection over a pitching supercritical airfoil using hot-film sensors

In the present work, experimental tests are conducted to study boundary layer transition over a supercritical airfoil undergoing pitch oscillations using hot-film sensors. Tests have been undertaken at an incompressible flow. Three reduced frequencies of oscillations and two mean angles of attack are studied and the influences of those parameters on transition location are discussed. Different algorithms are examined on the hot-film signals to detect the transition point. Results show the formation of a laminar separation bubble near the leading edge and at relatively higher angles of attack which leads to the transition of the boundary layer. However, at lower angles of attack, the amplification of the peaks in voltage signal indicate the emergence of the vortical structures within the boundary layer, introducing a different transition mechanism. Moreover, an increase in reduced frequency leads to a delay in transition onset, postponing it to a higher angle of attack, which widens the hysteresis between the upstroke and downstroke motions. Rising the reduced frequency yields in weakening or omission of vortical disturbances ensuing the removal of spikes in the signals. Of the other important results observed, is faster movement of the relaminarization point in the higher mean angle of attack. Finally, a time–frequency analysis of the hot-film signals is performed to investigate evolution of spectral features of the transition due to the pitching motion. An asymmetry is clearly observed in frequency pattern of the signals far from the bubble zone towards the trailing edge; this may reflect the difference between the transition and relaminarization physics. Also, various ranges of frequency were obtained for different transition mechanisms.     

FLOW CHARACTERISTICS AROUND AN INCLINED ELLIPTIC CYLINDER IN A TURBULENT BOUNDARY LAYER

The flow characteristics around an inclined elliptic cylinder located near a flat plate were investigated experimentally. The axis ratio of the elliptic cylinder was AR=2. The pressure distributions along the surface of the cylinder and the flat plate were measured by varying the angle of attack of the elliptic cylinder. The velocity profiles behind the cylinder were measured using hot-wire anemometry. When the angle of attack varies, the peak pressure location on the windward cylinder surface moves towards the rear edge of the cylinder, while that on the leeward surface moves towards the front edge of the cylinder. The vortex-shedding frequency also gradually decreases, defining a critical angle of attack for each gap ratio. The location of the minimum pressure on the flat plate surface moves downstream for positive angles of attack, while it moves upstream for negative angles of attack. Negative angles of attack cause a greater disturbance in the boundary layer near the wall compared to positive angles of attack. This shows that the separated wall shear layer from the boundary layer and the lower shear layer of the cylinder wake are strongly merged compared to other cases.     

Effect of Local DBD Plasma Actuation on Transition in a Laminar Separation Bubble

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.     

High-fidelity simulations of moving and flexible airfoils at low Reynolds numbers

The present paper highlights results derived from the application of a high-fidelity simulation technique to the analysis of low-Reynolds-number transitional flows over moving and flexible canonical configurations motivated by small natural and man-made flyers. This effort addresses three separate fluid dynamic phenomena relevant to small fliers, including: laminar separation and transition over a stationary airfoil, transition effects on the dynamic stall vortex generated by a plunging airfoil, and the effect of flexibility on the flow structure above a membrane airfoil. The specific cases were also selected to permit comparison with available experimental measurements. First, the process of transition on a stationary SD7003 airfoil section over a range of Reynolds numbers and angles of attack is considered. Prior to stall, the flow exhibits a separated shear layer which rolls up into spanwise vortices. These vortices subsequently undergo spanwise instabilities, and ultimately breakdown into fine-scale turbulent structures as the boundary layer reattaches to the airfoil surface. In a time-averaged sense, the flow displays a closed laminar separation bubble which moves upstream and contracts in size with increasing angle of attack for a fixed Reynolds number. For a fixed angle of attack, as the Reynolds number decreases, the laminar separation bubble grows in vertical extent producing a significant increase in drag. For the lowest Reynolds number considered (Re _c  = 10⁴), transition does not occur over the airfoil at moderate angles of attack prior to stall. Next, the impact of a prescribed high-frequency small-amplitude plunging motion on the transitional flow over the SD7003 airfoil is investigated. The motion-induced high angle of attack results in unsteady separation in the leading edge and in the formation of dynamic-stall-like vortices which convect downstream close to the airfoil. At the lowest value of Reynolds number (Re _c  = 10⁴), transition effects are observed to be minor and the dynamic stall vortex system remains fairly coherent. For Re _c  = 4 × 10⁴, the dynamic-stall vortex system is laminar at is inception, however shortly afterwards, it experiences an abrupt breakdown associated with the onset of spanwise instability effects. The computed phased-averaged structures for both values of Reynolds number are found to be in good agreement with the experimental data. Finally, the effect of structural compliance on the unsteady flow past a membrane airfoil is investigated. The membrane deformation results in mean camber and large fluctuations which improve aerodynamic performance. Larger values of lift and a delay in stall are achieved relative to a rigid airfoil configuration. For Re _c = 4.85 × 10⁴, it is shown that correct prediction of the transitional process is critical to capturing the proper membrane structural response.     

Characteristic Regimes of Transitional Separation Bubbles in Unsteady Flow

This experimental investigation deals with transition phenomena of a separated boundary layer under unsteady inlet flow conditions. The main purpose of this investigation is to understand the influence of the rotor-stator interaction in turbomachinery on the subsequent, highly loaded boundary layer. The research project is divided into two phases. In the first phase, which has been completed recently, only the variation of mean velocity caused by upstream blades was simulated in the experiments while the free-stream turbulence intensity was retained at a constant low level. The experiments are carried out in an Eifel-type wind tunnel to investigate the laminar separated boundary layer of a flat plate under oscillating inlet conditions. The adverse pressure gradient, similar to that of turbomachines, is generated by the contoured upper wall. The unsteadiness is produced by a rotating flap located downstream of the test section. The reduced frequency, the amplitude and the mean Reynolds number are varied to simulate the conditions prevailing in turbomachines. In addition to the Kelvin–Helmholtz instability of the separated shear layer, a lower frequency instability was observed. This is frequently referred to as `free shear layer flapping' and results in two distinctly different ways of re-attachment, depending primarily on the Reynolds number. For low momentum thickness Reynolds numbers at the separation point, large-scale vortices locked to the frequency of the unsteady main flow are identified. They originate nearly at the top of the separation bubble and are ejected downstream. A fully turbulent boundary layer develops after these vortices mix out. For higher Reynolds numbers, transition is completed within a short length of the free shear layer and there-attachment region. The characteristic momentum thickness Reynolds number separating these two regimes in unsteady flow is about 125. The Strouhal number (reduced frequency) does not appear to have any significant effect. Based on the experimental results, this behaviour is discussed in some detail. This revised version was published online in July 2006 with corrections to the Cover Date.     

Reynolds number effects on leading edge vortex development on a waving wing

The waving wing experiment is a fully three-dimensional simplification of the flapping wing motion observed in nature. The spanwise velocity gradient and wing starting and stopping acceleration that exist on an insect-like flapping wing are generated by rotational motion of a finite span wing. The flow development around a waving wing at Reynolds number between 10,000 and 60,000 has been studied using flow visualization and high-speed PIV to capture the unsteady velocity field. Lift and drag forces have been measured over a range of angles of attack, and the lift curve shape was similar in all cases. A transient high-lift peak approximately 1.5 times the quasi-steady value occurred in the first chord length of travel, caused by the formation of a strong attached leading edge vortex. This vortex appears to develop and shed more quickly at lower Reynolds numbers. The circulation of the leading edge vortex has been measured and agrees well with force data.     

Flow structure on a three-dimensional wing subjected to small amplitude perturbations

Small amplitude angular perturbations, of the order of one-half degree, can substantially modify the flow structure along a three-dimensional wing configuration, which is quantitatively characterized using a technique of high-image-density particle image velocimetry. Excitation at either the fundamental or the first subharmonic of the spanwise-averaged instability frequency of the separating shear layer from the stationary wing nearly eliminates the large-scale separation zone along the wing at high angle of attack. The physics of the flow is interpreted in terms of time-mean streamlines, vorticity and Reynolds stress, in conjunction with phase-averaged patterns of instantaneous vorticity. Distinctive vorticity patterns occur along the leading edge when the time-averaged separation zone is minimized.