Phase-averaged Particle Image Velocimetry Measurements Research Articles

Experimental investigation of the transonic buffet cycle on a supercritical airfoil

Transonic buffet behaviour of the supercritical airfoil OAT15A was investigated experimentally at flow conditions Ma=0.7 and alpha =3.5^circ , using schlieren and particle image velocimetry (PIV). The general behaviour of the buffet cycle was characterised with short-exposure schlieren visualisation and phase-averaged PIV measurements. A spectral analysis showed that the shock oscillation occurs with a dominant contribution at 160 Hz (St = 0.07, in good agreement with the literature) and between 25 and 55 % of the chord of the airfoil. Proper Orthogonal Decomposition (POD) was applied to the PIV data to extract the main modes connected with buffet. It is found that the first three most energetic modes capture around 65 % of the total fluctuating kinetic energy. The first and the third modes have a main frequency peak at 160 Hz and are well representing the separated area and the shock oscillation. The second mode was, instead, associated with an asymmetrical behaviour of the separated area and of the shear layer and displays a main peak at 320 Hz, being double the main buffet cycle frequency. Finally, it was shown that by using the 11 most energetic POD modes, an accurate reduced-order model (ROM) is obtained, which when subtracted from the instantaneous velocity fields allows the visualisation of the small-scale structures present in the flow, such as the upstream travelling waves (UTWs) and the vortex shedding in the separated area near the trailing edge. The analysis allowed to estimate the velocity of the UTWs, obtaining values in good agreement with the literature. In contrast, the analysis of the vortex dynamics in the trailing edge area revealed that vortices shed at the shock foot, which convect downstream in an area detached from the airfoil surface, cannot be considered responsible for the creation of UTWs in view of the mismatch in frequency of the two phenomena.Graphic abstract

Control of Separated Flow Using Actuated Compliant Membrane Wings

Membrane wings exhibit several aerodynamic advantages at low Reynolds numbers, passively adapting to flow conditions and delaying stall to significantly higher angles of attack relative to rigid wings. Rigid wings, on the other hand, often rely on active flow control mechanisms to achieve high angles of attack, injecting momentum to induce vortex roll-up in the shear layer. Active flow control for membrane wings is limited by the flexible nature of the wing surface. However, it is possible to achieve active flow control with a membrane wing by using a dielectric elastomer actuator as the membrane material. In this work, the performance of a sinusoidally actuated membrane wing is characterized for a range of actuation frequencies, freestream velocities, and angles of attack. Lift measurements show lift enhancement of up to 20%. Time-resolved membrane kinematics are used to calculate the coefficient of added momentum of the actuator, and the effect on the flow field is shown using phase-averaged particle image velocimetry measurements. Dynamic mode decomposition is used to show vortical structures being shed from the leading edge in phase with actuation, and comparisons are made with synthetic jet literature to show the similarities to separated flow control techniques used on rigid wings.

The role of the separation point in streamwise vortex-induced vibrations

Structures in crossflow are susceptible to vortex-induced vibrations (VIV) when the vortex-shedding becomes synchronised with the structural vibration. Strategies to control VIV often include modifying the separation point on the cylinder surface, such as adding helical strakes, although there remains disagreement regarding the mechanism by which these work. We explore the role of the separation point on VIV acting in the streamwise (drag) direction, by performing high-speed Particle-Image Velocimetry (PIV) measurements of the wake and the structural displacement of a range of cylinders with different cross-sectional shapes, including circular (no fixed separation points), equilateral triangles (fixed separation points) and elliptical cylinders (which act as an intermediate case). None of the non-circular cylinders are found to exhibit VIV, despite having approximately the same experimental conditions (mass ratio, structural damping, Reynolds number range, etc.) as the circular cylinders, which undergo VIV. The phase-averaged PIV measurements of the near wake of the circular cylinder are used to calculate the separation angle throughout the shedding cycle for different wake modes, and it is shown that all the wake modes that are associated with VIV require a periodic movement of the separation point. In contrast, the variation in the separation angle was negligible for the von Kármán vortex street observed behind near-stationary circular cylinders and for all non-circular cylinders. The experiments illustrate the great sensitivity of the wake mode and streamwise VIV to modifications of the separation point and demonstrate that even a moderately elliptical cylinder (major to minor axis ratio of 1.54) is sufficient to completely suppress VIV.

Volumetric intake flow measurements of an IC engine using magnetic resonance velocimetry

Magnetic resonance velocimetry (MRV) measurements are performed in a 1:1 scale model of a single-cylinder optical engine to investigate the volumetric flow within the intake and cylinder geometry during flow induction. The model is a steady flow water analogue of the optical IC-engine with a fixed valve lift of \(9.21\) mm to simulate the induction flow at crank-angle \(270^{\circ }\)bTDC. This setup resembles a steady flow engine test bench configuration. MRV measurements are validated with phase-averaged particle image velocimetry (PIV) measurements performed within the symmetry plane of the optical engine. Differences in experimental operating parameters between MRV and PIV measurements are well addressed. Comparison of MRV and PIV measurements is demonstrated using normalized mean velocity component profiles and showed excellent agreement in the upper portion of the cylinder chamber (i.e., \(y \ge -20\) mm). MRV measurements are further used to analyze the ensemble average volumetric flow within the 3D engine domain. Measurements are used to describe the 3D overflow and underflow behavior as the annular flow enters the cylinder chamber. Flow features such as the annular jet-like flows extending into the cylinder, their influence on large-scale in-cylinder flow motion, as well as flow recirculation zones are identified in 3D space. Inlet flow velocities are analyzed around the entire valve curtain perimeter to quantify percent mass flow rate entering the cylinder. Recirculation zones associated with the underflow are shown to reduce local mass flow rates up to 50 %. Recirculation zones are further analyzed in 3D space within the intake manifold and cylinder chamber. It is suggested that such recirculation zones can have large implications on cylinder charge filling and variations of the in-cylinder flow pattern. MRV is revealed to be an important diagnostic tool used to understand the volumetric induction flow within engine geometries and is potentially suited to evaluate flow changes due to intake geometry modifications.

Tethered cube stabilization by means of leading-edge DBD plasma actuation

An experimental investigation was carried out to assess the effectiveness of active flow control as a means for suppressing oscillations of a tethered cube. Two experimental configurations were considered: a static configuration involving surface pressure and particle image velocimetry (PIV) flow field measurements and a dynamic, tethered, configuration. Corner-mounted, dielectric barrier discharge plasma actuators were employed at the leading-edges and were pulsed at reduced frequencies of order one and at varying duty cycles. On the static configuration, actuation changed the direction of the side-forces and virtually eliminated yawing-moment excursions. Surface pressure and flow field measurements showed that control of separation bubbles on the surfaces, as well as control of the separated shear layer, were responsible for these effects. Phase-averaged PIV measurements elucidated the mechanism whereby actuation severs the leading-edge vortex that subsequently sheds downstream. For the tethered cube, actuation dramatically reduced the yawing motions, particularly when the momentum coefficient exceeded 0.3 %. Drag reduction, based on the deflection of the cube, was estimated to be approximately 12 %, consistent with the static data. Reduced frequency and duty cycle had a marked effect on control effectiveness.

Self-excited oscillations of a high-speed impinging planar jet

This paper examines aeroacoustic tone generation of a high-speed planar gas jet impinging normally on a flat, rigid surface. Experiments are performed over the complete range of subsonic jet velocities for which tones are generated, and over the complete range of impingement distance for which tones occur for a single nozzle thickness of h=3mm. The behavior of the planar impinging jet case is compared to that of the axisymmetric case and found to be significantly different, with tones being excited at much larger impingement ratios and lower flow velocities. The acoustic tones have been found to be generated by both symmetric and anti-symmetric jet instabilities, coupled with resonant acoustic modes occurring between the nozzle and plate surface. The nature of the flow instabilities has been investigated using phase-averaged particle image velocimetry measurements. The frequency behavior of the resulting tones is predicted using a simple feedback model, which allows the identification of the various shear layer modes of the instabilities driving tone generation. Finally, a thorough dimensionless analysis is performed in order to quantify the system behavior in terms of the appropriate length and velocity scales.

Flap Vortex Management Using Active Gurney Flaps

An experimental study was conducted to assess the applicability of limited-span Gurney flaps for altering the flap-edge vortex characteristics of a swept constant-chord half-model with a commercial aircraft airfoil and high-lift system (slat and flap). The strategy employed was based on the well-known vortex roll-up relations between the span loading and the wake that were extended and modified to include configurations with flapped wings. Phase I of the study involved the testing of static Gurney flaps mounted at the trailing edge and flap edge, in which different flap heights and fractions of flap span were evaluated. Data acquisition included particle image velocimetry measurements downstream of the flap edge, six-component load measurements, and surface pressure measurement on the main element and in the flap-edge region. The Gurney flaps produced significant variation of the vortex centroids, up to 5.3% of the semispan (13.1% of chord), with corresponding small changes to lift and drag coefficients. Phase II of the study involved the evaluation of dynamically deployed Gurney flaps, in which phase-averaged particle image velocimetry measurements were made of the vortex trailing the flap. This was motivated by the desire to periodically perturb the vortex centroids, with a view to exciting wake instabilities and thus triggering a faster decay. When driving the Gurney flap and hence perturbing the vortices at wavelengths shorter or longer than the wingspan, the authority over the vortex centroids did not materially change.

Morphology of the forced oscillatory flow past a finite-span wing at low Reynolds number

A study of the morphology of the vortical skeleton behind a flapping NACA0030 wing with a finite aspect ratio of 3, is undertaken. The motivation for this work originates with the proposal that thrust can be efficiently produced by flapping aerofoils. The test condition corresponds to a Strouhal number of 0.35, Reynolds number, based on aerofoil chord, of 600 and an amplitude of flapping, equal to the chord length of the wing. This test condition corresponds to the optimal thrust-producing case in infinite-span flapping wings. This study investigates the effect of wing three-dimensionality on the structure of the wake-flow. This is accomplished here, by quantitatively describing the spatio-temporal variations in the velocity, vorticity and Reynolds stresses for the finite-span-wing case.Preliminary flow visualizations suggest that the presence of wingtip vortices for the three-dimensional-wing case, create a different vortical structure to the two-dimensional-wing case. In the case of a two-dimensional-wing, the flow is characterized by the interaction of leading- and trailing-edge vorticity, resulting in the formation of a clear reverse Kármán vortex street at the selected test condition. In the case of a three-dimensional-wing, the flow exhibits a high degree of complexity and three-dimensionality, particularly in the midspan region. Using phase-averaged particle image velocimetry measurements of the forced oscillatory flow, a quantitative analysis in the plane of symmetry of the flapping aerofoil was undertaken. Using a triple decomposition of the measured velocities, the morphological characteristics of the spanwise vorticity is found to be phase correlated with the aerofoil kinematics. Reynolds stresses in the direction of oscillation are the dominant dissipative mechanism. The mean velocity profiles resemble ajet, indicative of thrust production. Pairs of strong counter-rotating vortices from the leading- and trailing-edge of the aerofoil are shed into the flow at each half-cycle. The large-scale structure of the flow is characterized by constructive merging of spanwise vorticity. The midspan region is populated by cross-sections of interconnected vortex rings.